86
ANALYTICAL CHEMISTRY
(98) Schweitxer, G. K., “Radioactive Tracer Techniques,” Xea York, D. Van Nostrand Co., 1949. (99) Shaw, P. F. D., and Collie, C. H., J . Chem. Soc., 1949, 1217. (100) Siri, W., “Isotopic Tracers and Nuclear Radiations,” New York, McGraw-Hill Book Co., 1949. (101) Smales, A. A., and Brown, L. O., Chemistry &. Industry, 1950, 441. (102) sue, P., Compt. Tend., 229, 878 (1949). (103) Sue, P., “Dix ans d’application de la radioactivitb artificielle,” Paris, Soc. Ed. Sci., 1948. (104) Sue, P., and Saeland, E., Bull. soc. chim. France, 1949, 437. Jr., iVucZeonics. 5 , KO.6, 4 (105) Taylor, T. I., and Havens, W.W., (1949). (106) Ibid., 6, No. 2, 66 (1950). (107) Ibid., 6, No. 4, 66 (1950). (108) Tompkins, P. C., Wish, L., and Burnett, W. T., Jr., Ax.4~. CHEM.,22, 672 (1950). (109) Wang, P. K. S., Rev. S c i . Instruments, 21, 816 (1950). (110) Way, K., Fano, L., Scott, A I . R.. and Thew, K., Satl. Bur. Standards, Circ. 499 (1950).
(111) Weigl, J. W., and Calvin, hl., J . Chem. Phys., 17, 210 (1949). (112) Wiener, M., and Yagoda, H., Rev. Sci. Instruments, 21, 39 (1950). (113) Williams, R. R.. Jr., “Principles of Nuclear Chemistry,” Kea. York, D. Van Nostrand Co., 1950. (114) Wilson, D. W.,“Preparation and Measurement of Isotopic Isomers,” Ann Arbor, hlich., J. W. Edwards, 1947. .Vatwe, 164, 183 (1949). (115) Winteringham, F. P. W., (116) I’affe, L., and Justus, K. >I., J . Chem. Soc., 1949 (Suppl. Issue KO.2), S341. (117) Tagoda, H., “Radioactive Measurements with Nuclear Emulsions,” New York, John Wiley 8: Sons, 1949. (118) Yankwich, P. E., Aiv.4~.CHEM.,21, 318 (1948). (119) Yankwich, P. E., and Calvin, hl., J . Chem. Phya., 17, 109 (1949). (120) Zimen, K. E., 2. S a l u ~ f o r s c h .4a, , 95 (1949). (121) Zuber, K., H e h . Phy8. Acta, 21, 365 (1948). (122) Ibid., 22, 112 (1949). RECEIVED November 6, 1950.
Polarographic Theory, Instrumentation, and Methodology JAMES J. LINGANE Harvard University, Cambridge 38, Mass.
T
HE polarographic literature continues to expand rapidly,
as indicated by the histogram in Figure 1. The present rate of growth is about 200 publications per year. During the past few years the proportion of applied to theoretical papers has increased, indicating that polarography is coming of age and gaining increasing acceptance in practical analysis. Thi- review covers the years 1948 to 1950. Access t o the most recent literature is facilitated by the comprehensive bibliography prepared by Leeds & Xorthrup Co. ( $ 0 )which covers the polarographic literature up to 1950 and is available gratis on request. This bibliography lists over 2200 titles in chronological sequence, and its usefulness is enhanced by subject and author indexes. An equally comprehensive bibliography up to 1949 has also been prepared by Semerano ( 4 3 ) . The well-known bibliographic series in the Collections of Czechoslovak Chemical Cornmirnzcations is being kept up to date by Heyrovsk9 by periodic additions ( I S ) . The monograph literature of polarography is enriched by the new book, “Polarographische Arbeitsmethoden,” by van Starkelberg ( 5 2 ) . This very well written 478-page monograph is intended primarily as a practical manual, and it should serve this purpose admirably. In addition, the book emphasizes fundamental theory to a much greater degree than its modest title indicates. An especially useful feature is a 130-page concluding chapter which comprises a selected list of about 1200 polarographic papers with bricf abstracts of each.
which takes this curvature into account. 25” is
This new equation at
where the various quantities are expressed in the customary units (15). This equation differs from the IlkoviE equation by
2oo
57 05
t
t
25
100
75
THEORY
50
Dsusion Current. The fact that the familiar Ilkovi6 equation is not wholly adequate has been recognized for some time. In particular, it does not account for the observed variation of the diffusion current “constant” id/Cm2/3t1/6with the characteristics (rate of mercury flow and drop time) of the dropping electrode (24). Recently Strehlow and von Stackelberg ( 5 7 ) and Lingane and Loveridge (66)independently demonstrated that the faultiness of the IlkoviE equation lies in its neglect of the curvature of the electrode surface, and they developed a modified equation
25
0 I920 1925 1930 1 9 3 5 1 9 4 0 1945 I ! 50 Figure 1. Rate of Growth of Polarographic Literature
V O L U M E 23, NO. 1, J A N U A R Y 1 9 5 1
87 of the lead line because t'he diffusion coefficients of the two substances are closely similar. The observed slopes of both the lead and zinc lines agree within experimental error with the t.heoretical slope, confirming the correctness of the value 39 for A . The observed diffusion current of lead ion is about 8% smaller than the ideal infinite dilution value computed from Equation 1. This is entirely logical because the actual diffusion coefficient of lead ion in 1 M potassium chloride must be smaller than at infinite dilution. I n most cases observed diffusion currents n.ill agree somewhat better with the values computed by the original IlkoviE equation than with those computed from Equation 1, when ideal infinite dilution diffusion coefficients are used to conipute the theoretical values. This, of course, simply reflects a compensation of errors, because the neglect of the electrode curiature in the original IlkoviE equation tends t o compensate for the fact that actual diffusion coefficients are smaller than infinite dilut.ion values. -4dditional evidence for the validity of Equation 1 exists in data obtained by Loveridge ( 2 6 ) for the influence of the pressure on the dropping mercury on the diffusion current. According to the IlkoviE equation
3.7
1 1 1 1 1 1 1 .a
0.2
1
0.6
t1/8/m1
id
=
X.h1/2
(2)
1.4
/S
Figure 2. Variation of id/Cm2/st'/s with Capillary Characteristics According to Lingane end Loveridge (25): 1. Lead ion in 1 M ootassium chloride containinn 0.01% eelatin. 0.01% gelatin. 2. '-Fetraamminozinc Tetraamminozinc ion in 1 M ammonium chloride1 M ammonia eontaining 0.0lqo gelatin. 3. Theoretical line for lead ion according to Equation 1 with R 3 39.
-
t,he term in parentheses, which is the correction for the curvature of the electrode surface. According to Strehlow and von Stackelberg, the theoretical value of constant A is 17 a t 25", whereas Lingane and Loveridge concluded it is 39. K i t h dropping electrodes of the characteristics usually used the ratio t1/6/m1/S is close to 1, and since D112is of the order of 3 X 10-3 with most substances, the correction factor has a value ranging from about 1.05 to 1.10. I n other words, because of the curvature, the diffusion current (integrated average over the drop life) is 5 to 10% greatkr than it would be a t an expanding plane electrode of the same area. Equation 1 predicts that the quantity idjCrn2/3t1P should increase linearly with increasing values of the rat'io t 1 / 6 / m 1 / 3 , corresponding to increasing drop time and decreasing rate of flow of mercury. This is the observed effect, as shown by the data in Figure 2 obtained by Lingane and Loveridge. The rapid increase of id/Cm2/3t1/ea t values of t1.'6/m1/3 smaller than about 0.7 (drop times smaller than about 1.5 seconds) results from stirring by the rapidly forming drops n-ith consequent enhanced convective trdnsfer of the reducible subst,ance to the electrode surface and disturbance of the diffusion layer. Strehlow and von Stackelberg attribute this stirring t o tangential movement of the mercury at t,he inner surface of the interface produced by the internal flow of mercury from the capillary orifice down through the center of the drop to its bott,om and then upward along its surfacc. They conclude that this 8piilefeX.t is operative only below a certain critical drop time of the order of 1 second when grlat,in is present, but it persists to a longer drop time in the absence of gelatin. Tangential movement of the surface of rapidly forming drops has also been discussed by Frumkin and Levich ( 1 0 ) and by Kryukova (16) in connection with polarographic maxima. The dashed line, 3 in Figure 2 , is the theoretical line for lead ion according to Equation 1, computed using the ideal infinite dilution diffusion coefficient of lead ion and the value 39 for A . The theoretical slope of the zinc line is virtually the same as that
where the constant k is a function of the geometry of the dropping electrode capillary, and h is the vertical distance between thc mercury level in the reservoir and the tip of the dropping electrodr~ corrected for the back pressure due to the interfacial tension a t the mercury-solution interface. This relation follows from the fact, verified by Loveridge, that m is directly proportional to h but t is inversely proportional to h. On the other hand, Equation 1 predicts that id/h1I2should not be constant with various values of h for a given capillary but should follow an equation of thc form
(3) where constants k and b are characteristic of the particular capillary. Experimental values of id/h'/* obtained by Loveridge with lead ion with many different capillaries show a small but reproducible decrease with increasing values of h as predicted tiy Equation 3 down to drop times at which the stirring effect conws into play. Strehlow and von Stackelberg ( 5 7 ) presented data, obtained with thallous ion and cadmium ion in 0.1 AI potassium chloride containing 0.01% gelatin, which also verify the correctness of the form of Equation 1. These investigators concluded that a valuc. of 17 for the constant A fitted their results better than the value 39. However, the precision of their measurements, especial1)with cadmium ion, is such that this conclusion does not s e m justified. I t is evident from Figure 2 that a value of 17 definiteljis much too small to fit the data of Lingane and Loveridge. I n t,he experiments of Strehlon- and von Stackelberg the ratio of the concentration of supporting electrolyte to that of the reduciblt, ion \vas rather small-50 in the ritse of thallous ion and only 23 with cadmium ion-and hence the limiting currents they measured probably included a significant contribution from electrical migration. 1-alues of id/Cm2/3t1/6only slightly to the right of the minimum (Figure 2 ) will tend to be too large because of the hrginning of the stirring effect anti this tends to make the observrtl slope too small. In other n-ords, in dSciding on the best observed slopc relatively little !wight should be given to point,s near tht. minimum. 11eites and &kites (SI)reportrd that the id/Cm2/3t1/6 values of cadmium ion in 0.1 111 pot'assium chloride-0.1 M hydrochloric acid-0.01% gelatin increase with increasing drop time between about 1.5 and 6 seconds as expected from Equation 1, but pass. through a flat maximum and d ~ c w a s csignificantly a t drop times between about 5 and 12 seconds. Their data for bigmuthy1 ion in 1 .If nitric acid shon- a similar rffect,, except that in this case the value of i d / C r n 2 / 3 t ' / 6 begins to increase rapidly again at drop
88
ANALYTICAL CHEMISTRY
tin1c.s above about 11 seconds. I n the absence of gelatin the idlCm2/3t116values of both cadmium and bismuth showed very little variation over a range of drop times from about 4 to 13 seconds. Addition of 0.01% gelatin increased the diffusion currents over the drop time range from about 1.5 to 6 seconds. Meites and Meites concluded that no single equation accounted for the diffusion currents they observed with and without gelatin present. Without gelatin their data correspond better with the original IlkoviE equation t,han with Equation 1, but when 0.01 % gelatin is present their data are in accord with Equation 1 up to a drop t'ime of about 6 seconds. The writer believes that this indicates closer approach to the condition of diffusion control assumed in JCquation 1 when gelatin is present than when it is absent. In a further study ( 3 2 ) \kites and Meites determined the id/Cm2/3t1/6values of silver ion and cadmium ion in 0.1 Af potassium chloride, and iodate ion in 0.1 M potassium chloride0.1 M hydrochloric acid, with 0.009% gelatin present in all cases. In these instances Meites and Meites concluded that Equation 1 was valid a t drop times between approximately 1.5 and 6 seconds, and they observed an average value of A of 31.5 * 4.6. Above a drop time of about 6 seconds (or tl/6/m1/a greater than 1.2 to 1.4) the id/Cm2/at1/6 values showed the same anonialous decrease described above. It has not been established wlir,ther this decrease, and the concomitant appearance of the flat maximum in t'he id/C1n~l3t~/~versus t1/6/m1/3 curve, is real. There is reason to suspect that it may be only apparent, and a reflection of the galvanometric measuring technique. Lingane and Loveridge ( 2 4 )demonstrated that the apparent diffusion current (average of the galvanometer oscillations) is constant and independent of the ratio of galvanometer period to drop time, provided this ratio is greater than about 1. This was confirmed by Taylor, Smith, and Cooter (59),who also presented evidence that the average of the oscillations of a long period galvanometer corresponds very closely to the true integrated average current derived from oscillographic measurements. However, a t very long drop times which approach and even esceed the period of the galvanometer, as in the experiments of bleites and Meites, the average deflection of the latter may be significantly smaller than the true average current. Whatever its explanation, this effect is more of academic than practical importance because it occurs a t drop times much larger than ordinarily used in practical measurements. Strehlow and von Stackelberg derived the following equation for tJheanodic diffusion current of a dropping amalgam electrode: (4)
erably a t least 0.5 M ) , the variation of the diffusion rurrent "constant" id/Cm2/3t1/' originally defined (68) on the basis of the original Ilkovi6 equation normally ail1 not esceed about *2%. When an accuracy of better than +2% is required the true diffusion current constant from Equation 1
should be used. Current-Time Curves during Drop Life. Steghart (6~9, hlcKenzie (SO), and Taylor, Smith, and Coot.er (69) studied the current-time relation during the growth of individual drops and concluded that the current-time curves deviate considerably from the sisth-order parabola predicted by the IlkoviE equation. Taylor, Smith, and Cooter (59) employed a recording cathode ray oscillogrnph, a drop time of about 3.5 seconds, and a solution composcd of 3 millimolar cadmium ion in 0.1 Mpotassium chloride containing 0.01% gelatin. When they attempted to fit their data to an equation of the form it = k t n (where according to the original Ilkovii: equation n = l / ~ )they found that no single value of n applied over the entire drop life. From 0.1, 0.5, 1, and 2 seconds to the end of the drop life (ca. 3.5 seconds) the observed values of n were seriatim 0.31, 0.249, 0.227, and 0.186. During the lat,ter half of the drop life the current-time curve does approach closely to a sisth-order parabola, but when the drop is very young the current is abnormally small and increases more nearly with the one-third power of t'he time. Airey and Smales (2) reported that the relation id = kt116 was obeyed with an accuracy of about *2.5% from 0.75 second to the end of the life of a 3.9 second drop. In agreement with the investigators quoted above they observed that the current during the iriitinl stages of drop growth was smaller than corresponds to a sisth-order parabola. The dropping electrode they used was of a type with an enlarged orifice of 0.1 mm. n-hich produced a freely falling drop time of 15 seconds, but which was vibrat.ed by an electromagnetic pulsator to cause reproducible disengagement of the drops after 3.9 seconds. Evidently this type of dropping electrode provides conditions which correspond more closely to those assumed in the IlkoviE equation than does a conventional dropping electrode. The divergence of the current-time curve from a pure sixthorder parabola is in qualitative accord with the equation for the instant'aneous current given by Lingane and Loveridge ( 8 5 )
+
(6)
+ Pt'l'
(7)
it = 709nD1/2Cm2/3t116 31,560 nDCm1/3t1/3 which differs in form from Equation 1 only by the minus sign before the second term in parentheses, which results from the outward diffusion of the metal in the amalgam. Strehlow and von Stackelberg concluded that B is about 30. They tested this equation using data obtained a t various m and t values with 0.0139 molar cadmium amalgam in 0.1 ill potassium chloride containing 0.01% gelatin. Their plots of id/Cm2/3t1/6 versus tl/B/m1/3 show much more curvature of the left branches than the corresponding curves obtained in the reduction of metal ions, but they do tend to approach a straight line of negative slope at the larger values of tl/6/m1/3 as predicted by Equation 4. Better agreement with Equation 4 probably could be obtained with a smaller amalgam concentration and larger concentration of supporting electrolyte; in the esperiments of Strehlow and van Stackelberg the supporting electrolyte concentration was only seven times larger than that of the amalgam, which is too small to provide a condition of complete diffusion control of the current. The applicability of standardized diffusion current constants in practical analysis is not very seriously influenced by this new information. Under practical analytical conditions with normal dropping electrodes of drop time between about 2 and 5 seconds, with a small amount (0.005 to 0.01%) of gelatin present, and with adequately large concentrations of supporting electrolyte (pref-
which has the form it
=
cut'/6
whose curve is intermediate between a sixth-order and thirdorder parabola. Meites and Meites (38) demonstrated that the current-time curve obtained by Taylor, Smith, and Cooter does approach that predicted by Equation 7 aft,er about the fmt 1 second of the 3.5-second drop, but Equation 6 does not account for the abnormally small current during the early life of the drop. The abnormally small early current is partially caused by the fact that the rate of mercury flow is not constant during the drop life, but is smaller a t the beginning because the back pressure created by the interfacial tension is much larger when the drop is very small. However, the observed discrepancy is too large to be accounted for entirely by this effect. Airey and Smales suggested that the abnormally small early current is caused by the young drop emerging in that part of the solution whose concentration has been depleted by the reaction a t the preceding drop. The average current during the drop life to which both the original IlkoviE equation and Equation 1 refer corresponds to the quantity of electricity associated with each drop divided by the
89
V O L U M E 23, NO. 1, J A N U A R Y 1 9 5 1 drop time. Because only 8. small part of the total quantity of electricity accumulates during the earlier stages of drop formation, the tot$ quantity is relatively uninfluenced by the ahnormally sniall current during the very early life of the drop. Consequently, the exponent o f t for the average current remains close to l/e.
Taylor, Smith, and Cooter found that the ratio of the graphically integrated average current during the drop life t o the maximum current was 0.81, which is significantly smaller than the value 6/7 or 0.857 predicted by the IlkoviE equation. Shulman, Battey, and Jelstis (45) also observed a ratio of average t o maxiEquamum ourrent that was several per cent smaller than tion 1 predicts that the ratio of the average to maximum current should follow a relation of the form
”,.
i a - 607a ~i,. 709a
+ 23,670b + 31,560b
and bemuse the second term in the denominator is larger than that in the numerator the ratio should he slightly smaller than 6/,. IIowever, the observed ratio is considerably smaller than expcoted from Equation 8.
Figure 3.
Sargent M a n u a l Polamgraph
Smith (50) has reported experiments with a dropping electrode of extraordinarily long drop time (16 seconds t o 8 minutes), obtained by using 8. restricted capillary with a very large orifice (0.216 mm.). With such very slowly forming drops current-time relationships at constant potential, as well as current-voltage curves over the life of a single drop, can he studied with ordinary polarographic equipment. According to Smith, the currenb time curve with these very large slowly forming drops follows a second-order parabola. it = k t ‘ / 2 , indicating that the diffusion conditions are very different with such vcry slowly forming drops than with the drops normally used. Reaction Rate Theory Applied to Polarographic Waves. Eyring, Marker, and Kwoh (9)derived 8 system of equations t o interpret irreversible polarographic waves. The reversible half-wave potential, overvoltage, and free energy of activation can be evaluated from experimental data. Brdicka ( 3 )has reviewed the interpretation of limiting ourrents t h a t are controlled both by diffusion and by the rate of a reaction at or very near the surface of the dropping electrode. Tamamushi and Tanaks. (58)have also discussed equations for the polarographic wave on the basis of kinetic considerations and their conclusions are in accord with current accepted views. Influence of Madmum Suppressors on Wave Characteristics. That gelatin and other maximum suppressors can profoundly influence polarographic wave characteristics has long been recog-, niaed. Suppression of diffusion currents and displacement of
half-wave potent,ials by escwively large (grra,terthan ea. 0.01%) cancontmtions of gelatin offer a familiar esample, and the most recent literature on spplied polarogrrtphio analylysis reveals an increaqing appreciation of the fact that gelatin and other maximum suppressors should he employed judiciously in only the minimal concentrations required to suppress actual maxima. However, until the recent study of Meites, Meites, and Colichman (38) there has been little systematic investigation of the divers effects of maximum suppressors. These authors observed that addition of gelatin, Triton X-lW, methyl red, and other suppressors produces virtually no pffect on the drop time up to B certain sharply defined concentration (“polarographic critical concentration”) hut that Iaiger eoncentrations of the suppressor cause the drop time to decrease nearly linearly with increasing suppressor eancentrrttion. Meites Meites, and Calichman associate this polarographic critical concentration with the classical “critical concentration for micelle formation.” .4ccording to Meites et al., the effects of bhhe maximum s u p pressors they studied fall into six classes according to their type of influence on half-wave potentials m d diffusion ourrents, whether they muse the division of an orginally single wave into two parts, and their effect on the degree of reversibility of the electrode reactions. Although the causcs of these effects are still obscure, Meites et al. reject the idea of complexation between the suppressor (such as gelatin) and metal ions on the stoichiometric grounds that the concentration of the suppressor is too small. They also raise objections to the postulate that the action of the surfaeeaetivo upp pressor involves its adsorption on the electrode surface. Hoaever, Meites et al. observed that pronounced color changes occur when gelatin and various athor suppressors are added to certain metal ion solutions, and i t would he difficult to account far this if complexation did not take place. Influence of Drop Time on Half-Wave Potential. On the basis of Equation 1 Strehlow and vow Stackclberg (.57) predicted that the half-wave potential in metal ion reduction should show a very slight shift to a more negative value with increasing drop time. They also predicted that the internal stirring of thc amalgam within the drop (Spiilefekekt) should diminish the amalgam concentration a t the drop surface and thus augment this shift. With thallous ion Strehlow and van Stackelherg experimentally demonstrated that the negative shift amounts to about 18 mv. when the drop time was increased from 1.5 to 7 seconds, and with cadmium ion the shift was only 8 my. over the same range of drop time. Assuming that these data are typical, the effect evidently is small enough to he neglected in mast practical work. INSTRUMEWATION
Manual Instruments. Many polarographic measurements can he made inst about as conveniently with relatively inexpensive manual instruments as with recording instruments, especially in routine analyses where it is onlynecessary t o measure the diffusion current a t a single value of the applied electromotive force. Manual instrumentation is also very usoful in amperometric titrations. This has prompted the appcarrtncc of two new commercial manual instruments. The instrument manufactured by E. E. Sargent and Co., Chicago, is shown in Figure 3. The voltage applied to the dropping electrode cell is regulated by a precision 10-turn potentiometer provided with an integrating scale which permits readings t o 0.1%. This bridge is powered by dry batteries and the total voltage across i t is read on the 3volt voltmeter (accuracy 1% of full scale) in the center of the panel. The current is indicated by an enclosed galvanometer on the 30-em. translucent scale above the panel, whioh is correctly curved t o eliminate tangent error. An Ayrton shunt provides te? different galvanometer sensitivities ranging from 0.006 to 6 mcroamperes per mm. (It would be more convenient if this ked-step shunt were replaced by a precision 10-turn potentiameter, so that the galvanometer sensitivity could he adjusted to
ANALYTICAL CHEMISTRY
90 discrete integral values to facilitate conversion of scale divisions to microamperes.) The potentiometer a t the extreme right side of the panel may he used to send either an additive or opposing current into the galvanometer circuit to provide either up-scale compensrttion (1 times full scale) or down-scale compensation (9 times full scale). A 250,000-ohm resistor, which plugs into the cell lead terminal, is supplied for calibrating the galvanometer by impressing a k n a m voltage across the resistor, and computing the current via Ohm's law, and comparing with the galvanometer deflection. The instrument is fairly priced a t about one fifth the cost of a recording polarograph. The manual instrument manufactured by the Cambridge Instrument Co., Grand Central Terminal, New York, N. Y . , is shown in Figure 4. A potentiometer slide-wire is used to adjust the e.m.f. applied to the cell, and the total voltage across this bridge may he precisely adjusted against a self-contained standard cell. To eliminate batteries the bridge is powered by B rectifier operated from the alternating current line. The electrolysis current may either he read directly on the galvanometer, or alternatively by means of a self-contained potentiometer in terms of the iR drop across a standard resistor, using the galvanometer a a. null point indioator. A shunt is provided for stepwise selection of the galvanometer sensitivity. Galvanometer damping and counter current (compensation) adjustments are incorporated in the instrument. Delahay (5) described an electronic instrument specifically designed for measurement of wave heights by the technique of Petering and Daniels (M), wherehy the wave height is taken as the difference in current readings a t two fixed voltages preceding and following the wave.
of the substance in question. However, this is not strictly true because Aiobad. is related by C hy AiDb.ii.
Figure 4.
Cambridge Voltamoseope
Manuel instrument for polerographio m e ~ s ~ v c m c n f a
A sr%-itchingarrangement is provided for the convenient selection of three pairs of adjustable applied voltages from a set of six varisble potentiometers, so that the heights of as many as three successive waves can be measured. Ruggedness and pore ability Bre xhieved by measuring the Cnrlent in terms of the i~ drop Bcross one of set of different resistors in series with the the i~ drop being amplified by a one-stageamplifier ,vhose is indicated by panel.type microitmmeter, B~~~~~~~~ by rectified alternating current to are power the bridge, The instrument is well suited to amperemetric titrations, and it can also be used to meBSurea complete current-voltage curve. ~
~
As originally proposed, the Petering-Daniels technique &ssume8 that the difference in the observed currents, Ai,&., a t the two fixed applied voltages is a direct measure of the concentration, C,
=
bC f A ~ B
($1)
where the constant k is dafncrl by the Ilkovid equation and Ais is tho difference in background current at the two applied voltages. The hackground current may he the residual current of the supporting electrolyte alone, or it may include the diffusion current of some other substance whose wave precedes that of the substance being determined. In general, is not zero, and thercfore correction for it must he made. Evaluation of this correction is simple when the background current is entirely a residual ourrent. When the background current includes the reduction current of another substance, Ai, can he approximated from the relation i,[(t./t,)'P -11 whom i, is the background current a t the first potential and tl and tz arc tho drop times a t the two potentials. Visible Recording Polarographs. The Leeds & Northrup Electro-chemograph shown in Figure 5 is 3r visible recording polaxograph. This new instnlmcnt has been designed and engineered exceptionally well and its pcrformance characteristics are cscellent' After amplification the current is recorded on a Speedomax potentiometer recorder. The rurrent amplification is amomplished without introducing a significant iR drop in the cell circuit, and a variety of current sensit,ivitcs may be selected. Provision is made for several degrees of rerorder damping with remsrkahly little distortion of the wave form. A convenient arrangement is provided for accurately adjusting t'he bridge voltage against R self-contained Weston dandard rcll, which enables the instrument to provide accurate and veliahle values of half-rave potentials. The polarogram ma," he recorded with either increasing or decreasing applied voltage, and several starting voltages and ranges bridge may be selected. The instrument incorporates a precisely regulated rectifier unit for the direct
V O L U M E 23, NO. 1, J A N U A R Y
1951
current bridge supply which eliminates batteries and enables complete operation from IL 110-volt alternating current line. The Voltamograph of the Cambridge Instrument Co., shown in Figure 6, is a visible recording polamgraph. The current is recorded in terms of the amplified iR drop aoross a standard resistance by a pen-ariting galvanometer. The CUIrent sensitivity is variable in steps from 0.0025 to 2.5 microamperes per nun. Provision is made for precisely st.andardiaing the bridee voltaee sminst s self-contained standard cell. and three rsiaes of b i d & voltwe, +0.2 t o -0.7, +0.4 to lt1.4, and +0.8-to +2.8, G e provided. The rectilinear chart i s only 8 cm. on the current axis and 17 cm. on the voltage axis, which is smaller than desirable for precise mensurementa.
91
Because this method of amplificetion produces a direct current output whose polarity is fixed regardless of the polarity of the original iR drop, the recorder deflects in the.same direction regardless of the direction of current flow in the cell. Aside from the method of recording, the instrument utiliees more or less conventional circuitry. Miiller (S4) described the "polarographic scanner" shown in Figure 8,which can be assembled in about 1 hour using standardized precision mechanical components available from Servomechanisms, Inc., Old Country and Glen Cove Roads, Mineola, N. Y .
pola;og&& bridge. ' A revolution counter couded" t o the Helipot indicates the applied voltage directly. The current is recorded in terms of the iR drop across a standard resistor by an auxiliary 0- to 2.5-mv. Brown Eleetronik potentiometer recorder, and any desired current sensitivity may be obtained by using a precision Beckman Helipot potentiometer as a variable standard resistor. Pedestals mounted below the table itecommodate line and motor reversing switches, potentiometers, and rbeost& for span adjustment and sensitivity control. In addition to the recording potentiometer, the other auxiliary equipment needed is a voltmeter to read the total bridge voltage (the recordin potentiometer could be uscd for this purposr also), and dryaatteries to pouer the bridge.
Figure 6.
Camhridge Voltamograph
The visible recording polarograph manufactured by the Radiometer Co., Copenhagen, Denmark, is shown in Figure 7. This instrument employs 5 pen-xwiting milliammeter as the recording element. The iR drop generated by tho electrolysis current in a precision resist.or is converted to an alternating current signal by a vibrating reed. The alternating current signal is amplified, converted back to dim,!. current by afull-wave cuprous oxide rectifier, and then prexented to the recording milliammeter.
Figure 8.
polarographic Scanner
Ascordin:: to MZller (34) sasembled fmm Servomeohanism., Ino., preoision meehnoionl componenfa
Figure 7.
Visible Recording I'olarograph Manofactored by Radiometer, Copenhagen
Automatic m-Measurer. I n conwction with the URC of standardized diffusion current constants (B)i t is most convenient to have a means of automatically measuring the rate of flow of mercury, rn, from the dropping electrode. An improved version of the device originally described by Lingane (ZS) has been developed by the Leeds & Nortbrup Go., Philadelphia, and is shown in Figure 9. The timing device comprises a revolution counter driven by a synchronous motor, whose operation is governed via B low voltage relay circuit by the movement of the mercury past three tungsten contacts sealed into the dropping electrode stand tube. The instrument is precise and accurate to within a few t,entbs of 1%. The timing unit also may he used as a stopclock for measuring the drop time. Special Dropping Electrodes. Tsimmergakl(60) recommended an electrode similar to that of Riches ( 4 1 ) but provided with an
92
ANALYTICAL CHEMISTRY
electromagnetic tapping device to dislodge the mercury drops. This technique is intended to obtain more uniform drop size than that obtainable by natural dislodgement of fully formed drops. Enforced dislodgment of mercury drops from a conventional type of dropping electrode capilluy has also been recommended by Skohets and Ka,vetsld ( 4 7 ) , who employ a small glass "hoe" attached to the dropping electrode in such a way that the blade is under the forming drop. When the growing drop comes in contact with the blade it separates from the oapillary orifice. It is claimed that this device eliminates current oscillations and maxima in the ourrent voltage curves.
less. They demonstrated that the action of the vibrator does not cause any appreciable distortion of the current-time curve, provided the amplitude of the vibration is kept small and sell damped. The use of multiple tip dropping electrodes has been discussed anew by Payne (ST), who recommends the use of six electrodes with individual drop times between 2.0 and 3.0 seconds. Stankcvi8nSky (55)also employed a six-oapillary electrode. Bricker and Furman (4)reported unfavorable experience with multiple tip electrodes, and the writer is inclined to agree that the attrartive potentialities of such electrodes are difficult to realize in practice. Cells. Gawron i l l ) recommended the use of a viscow membrane to scparate the anode and cathode compartments of a polarographic cell. The iR drop across bhe membrttno is negligibly small. Laitinen and Burdett (171 . . desimed the cell in Firmre 10. which permits measurements while the solution is continuously stirred by a stream of inert gas. ~
~
~
~~
Aberrations of the current due to stirring are prevented by surrounding the dropping electrode capillary with 8 double glass mantle, slotted in such a. way that concentration equalization between the inner compartment and the bulk of the solution is quickly attained when the outer solution is stirred by a stream of inert gas. The other novel feature is the large sintered-glass disk in the false bottom of the cell, which disperses the purging gas stream into very h e bubbles and thus leads to very rapid removal of dissolved oxygen (ea. 30 seconds). This cell is especially convenient for amperametric titrations. Smith (49) developed a cell for general use which incorporates a thermostated jacket for temperature control, an external anode with a free liquid junction, provision for deoxygenation of the test solution in the absence of mercury, and a means for determining the capillary characteristics. METHODOL4lGY
Figure 9.
Leeds & Northrup Automatic rn-Measurer
Airey and h a l e s ( 8 ) thoroughly investigated electricd and mechanical means of causing reproducible disengagement of mercury drops from the dropping electrode. The purpose of their study was t o devise a practical method of enforced synehronimtion of the drop times of two dropping electrodes in differential and derivative polarographic techniques. They developed an electromechanical pulsator unit to which both dropping electrodes are affixed, whose periodic vibration shears the drops from the electrode tips very reproducibly. Airey and Smales used eleetrodes of the Riches type (41) with an enlarged orifice of 0.1 mm. and a normal drop time of about 15 seconds, but set the vibrator to obtain enforced drop times of the order of 4 seoonds or
Derivative Polarography. This type of measurement, which WBS originated by Heyrovskj. (le),utilizes the .derivative di/dt m. E of the current-voltage curve. The ordinary wave is thus replaced by a maximum or peak located a t or very near the halfwave potential. The primary advantage in principle over eonventioual polarography is that a very small concentration of % substance can be determined in the presence of a large concentration of Some other more easily reducible substanoe, 80 that (agzin in principle) preliminary chemical separation is more or less eliminated, The Heymvskjr technique achieves differentiation of the current-voltage curve by employing two dropping electrodes in the aame cell with a constant small voltage difference maintained across them as the total voltage is increased. The reoording galvanometer is placed across the two electrodes in such a way that it responds to the difference in the two currents. A fundamental drawback to any derivative or differential technique that employs two droppingelectrodes, evcn though their characteristics a~ closely matched, is that the drop times periodically go into and out of phase, and the derivative curve is distorted by the consequent periodic variation in the magnitude of the current oscillations. Heyrovskj. recommended the use of two streaming mercury electrodes of conFtant a x a to overcome this difficulty. Airey and S m l e s (8) devised a synchronization technique in which the two dropping electrodes are mounted together on an electromechanical vibrator which causes reproducible simultaneous disengagement of the drops. The latter authors also developed improvements in the Heyrovskjr derivative circuit, and presented a thorough discussion of various fundamental aspects of both derivative and differential polarography. The application of synchronized dropping electrodes in differential polarography has also been discussed by Stankoviansky
(64). A derivative technique which utilizes electrical differentiation
V O L U M E 23, N O . 1, I A N U A R Y 1 9 5 1
93
of the current-voltage curve, and thus requires only a single conventional dropping electrode, has been described by Leveque and Roth (21). They employed the circuit shown schematically in Figure 11, whose fundamental operational principle was originally discussed by Delahay (6). Because of the high capacity electrolytic condenser, C1 (2000 If), in series with the recording galvanometer, G, the latter records only the rate of change of the potential drop across R1, and hence the rate of change, di,/dt, of the electrolysis current as the applied e.m.f. is increased a t a constant rate. Because condenser C1 does Figure 10. Polaronot pass constant current, the graphic Cell galvanometer remains a t zero According to Laitinen and before and after polarographic Burdett ( 1 7 ) wave but deflects to a maximal valbe a t the half-wave point. The adjustable resistances, R1 and RS (whose sum is kept constant a t 1200 ohms), constitute an Ayrton shunt for regulating the galvanometer sensitivity. Condenser Cz serves to damp the galvanometer oscillations; its optimum capacitance is not specified by Ifiveque and Itoth, but prcsumably it should be such that the time constant, R,CS ( R , being the internal resistance of the galvanometer), is of the order of the drop time. IVith a sufficiently large value of the quantity C‘I(RI R? [to),t h maximal galvanometer deflection i p given tiy
+
i,
+
In hi. c+rc*uit,like that of Heyrovskg and Forejt, the horizontal deflection o n the cathode ray tube is proportional to the alterto the cell and the current is registered as means of the amplified ZR drop across a the cell. .4 conventional dropping electrode is used. The alternating voltage applied to the cell has a saw-tooth pattern with a quiescent period between linear voltage sweeps, and the frequency is adjustable from 5 to 1000 cycles per second. According to Delahay, the quiescent period or “dead time” between the voltage sweeps allows for the complete reoxidation of t’he reduced substance formed a t the electrode surface during the preceding sweep. The anodic current from this reoxidation decays to zero before the next sweep, and thus there is no distortion of the cathodic figure by an anodic current. The ratio of the quiescent period to the sweep period is adjusted from the order of unity at low frequencies (up to about 50 cycles per second) to a much larger value a t high frequencies (up to 1000 cycles per second). The quiescent period must be relatively longer the higher the frequency, in order to maintain sufficient time for complete reoxidation.
dic ClR, - d4
1,cvc~queand Roth found that this rclation was valid-i.e., that the value of C1(R1 Rz R,) has little or no effect on the maximal value of i,, when CI(R1 Rz R,) was varied between about 3.5 and 7.5 seconds. Presumably the drop time in these experiments x i s not greater than about 3 seconds. When the e.m.f. applied to the dropping clectrode cell is increased a t a constant rate, di,/dt (and hence io) goes through a masimum a t the half-wave potential. Because the value of i, a t the half-wave potential is directly proportional to the concentration of the reducible ion, the maximal value of i, should be directly proportional to concentration. This was experimentally verified by Leveque and Roth. The maximal value of i, is also dircct,ly proportional to the rate of application of the applied e.m.f., and hence the sensitivity can he altered by varying d E / d t a.s ~ v c l as l R1 and RI. Because its reproducible functioning depends on a uniform rate of application of the applied e.m.f., the polarographic bridge must be driven a t it uniform rate. Using an amnioniacal solution containing cadmium and nickel, Leveque and Roth demonstrated the utility of their circuit for determining a small amount of a substanm in the presence of a much larger amount of a more easily reducible substance; with a hundredfold excess of cadmium the small differential maximum of the nickel was clearly developed immediately folloiving the relatively huge cadmium maximum. By simply placing a single-pole single-throw switch in parallel with C1 the Leveque-Roth circuit can be used either for differential polarography (switch open) or for conventional polarography (switch closed to short out Cl), Vogel and Riha (61) credit Heyrovskj. as the originator of the derivative circuit employed by Leveque and Roth, and they present several examples of its application. Oscilloscopic Polarography. The increasing literature on this subject reveals two fundamentally different lines of approach.
+ +
One of these employs a periodically recurring voltage sweep of relatively great frequency, and i t is particularly valuable for theoretical studies of the specific rates of electrode reactions arid for determining their degree of reversibility or irreversibility. Heyrovskj. (14) has recently reviewed the principles and applications of this technique. The other utilizes a relatively slow v o l t age sweep with the purpose of obtaining a figure on the cathode ray tube screen, whose characteristics will approach as closely a8 possible to conventional polarograms. This second technique appears to be more useful than the first for practical quantitative analysis, but less powerful for studying the rates of electrode reactions. Dehhay (7‘) discussed some fundamental aspects of oscilloscopic polarography and designed a cirouit which permits very high rates of potential change (5 to 1000 volts per second).
+ +
vious investigators that with stationary electrodes the current increases rapidly a t first, passes through a maximum, and then gr,idually decays to a steady value, but that steady currents are
Figure 11. Leveque and Roth Circuit (21) for Derivative Polarography
Provision is made for photographic recording on 35-mm. film, which is an evident improvement over earlier techniques. An exposure time of’ 20 seconds is used, so that 5 to 10 traces corresponding to various ages of successive drops are recorded. The waves are somewhat similar to ordinary polarographic waves, except that they exhibit large peaks due to the rapid rate of voltage change. Only the largest peak, corresponding to the maximal area of the mercury drop, is used for measurement. The magnitude of this “maximal peak current” is about twenty times larger than the ordinary diffusion current, and it is directly proportional to the concentration of the reducible substance. The maximal peak current is influenced by the drop time, the frequency of the m e e p generator, and the amplitude of the voltage sweep. Equations for the maximal peak current for a reversible electrode reaction have been derived by Randles (40) and Sevcik (44), and Delahay ( 7 ) has shown that the equations of both these authors have the same form
this can be suppressed by a trace of methylcellulose. Laitinen and Syman showed that the dropping mercury electrode functions as an “electron electrode” in liquid ammonia
A N A L Y T I C A L CHEMISTRY
94
I
= Iznalagl/aCv‘l~a/3t2/a
(11)
here n and D have their usual significance, C is concentration in moles per liter, m is rate of mercury flow in grams per second, 2 is the drop time, and is the rate of change of the voltage across the cell in volts per second. According to Randles the theoretical value of constant k is 2344, but according to Sevcik it is 1852. Delahay systematically tested this relation and confirmed the predicted linear relation betxveen Z and C. The quantity ( ~ n t ) ~ is / 3 proportional to the maximal area of the mercury drop and is independent of the pressure on the dropping mercury. (‘onsequentlv the foregoing equation predicts that Z should be independent of the pressure or mercury head. Delahay found that this was very nearly true in cases where the electrode reaction proceeds reversibly. When the electrode reaction is irreversible Z sho\T s large variations with the pressure, apparently because the specific rate of the electrode reaction, rather than diffusion, hecomes the current-controlling factor. 4
L!
of small inevitable variations of drop time from drop to drop. This has been solved most ingriiiously by Snowden and Page (5i),by using the sharp decreased signal which results when thc drop falls to actuat.e a time de1a.v relay which instructs the sweep generator when to begin its next sweep. The Snowden and Page circuit is shown schematically in Figure 12. (The original paper contains c-oml)lctecircuit diagrams and specificat,ions of all componcxnts.)
Figure 12. Principle of Snowden and Page (51) Circuit for Oscillographic Polarography
The sweep generator applies a linear voltage stveep from +0.5 to -2.5 volts us. the saturated calomel electrode during the last 0.1 to 0.5 second of the drop life. Compared to other oscilloscopic techniques the rate of change of applied voltage with time is relatively small (6 to 30 volts per second); this keeps t h r charging current from being intcrferingly large, and also minimizes the influence of the specific rate of the electrode reaction so that m-ell developed current-voltage curves can be obtained with irreversible electrode reactions. The current through the dropping electrode cell (which is of the conventional type with a drop time of 3 to 5 seconds) is measured by placing a resistor in series with the cell, amplifying t’he resulting iR drop by thcx “vertical amplifier,” and applq-ing thc amplified signal to the vert,ical plates of the cathodc ray tube. The sweep voltage applied across the c d l is presented to the horizontal plates of the cathode ray tube after amplification by the “horizontal amplifier,” so that the single current-voltage curve which appears on the screen shows increasing applied voltage from left to right on the abscissa and increasing rurrent on the ordinate like :I conventional polarogram. Beginning a t a time near t,he end of the drop life when the voltage slv-eep is being applied, the cycle of events is as follows. T h e n the drop falls, the sharp decrease in the amplified in drop across the resistor in series with t,he cell actuates the elwtronic time delay relay labeled “delay gate” which promptly switches the sweep generator off the cell. Simultaneously the delay gate operates the “blanking relay” Thich switches the cathode of the cathode ray tube to a voltage sufficient to displace the spot, off the screen during the several-second delay period. This protects the screen from burning by the steady spot, and by maintaining a dark screen between voltage sweeps it facilitates photography of the image by eliminating the need for close synchronization of the camera shutter and voltage sweep. After a predetermined time (ivhivh can be adjusted up to 5 seconds with a prectiaion of better than *l%) the del:+gate deactivates the blanking relay and simultaneously snitches on the sweep generator. The caycle thcn repeats.
Delahay also confirmed the predicted linear relation between I and v 1 / 2 in cases of reversible electrode reaction, but demonstrated that Z does not increase as rapidly Kith increasing rate of potential change as the Randles-Sevcik equation predicts when the electrode reaction is irreversible. Delahay observed that addition of gelatin to a zinc solution greatly diminished the maximal peak current, and 0.05% gelatin wiped out the peak entirely and produced a flat estended wave. This is a clear indication that gelatin decreases the specific rate of the electrode reaction, and it helps to explain the deleterious action of large amounts of gelatin in ordinary polarography. I n oscilloscopic polarography uith alternating applied voltage, or when many voltagr. sweeps are applied during the drop life, the several current-voltage figures that appew on the oscilloscope screen reflect the incrrasing area of the electrode. Randles (39) and Airey ( 1 ) demonstrated that this can be overcome, and a single current-voltage figure corresponding closely to a conventional polarogram obtained, when only a single linear voltage sweep of short duration is applied to each drop during the last 0.1 to 0.5 second of its life. The voltage sweep is applied near the end of the drop life because the rate of change of area with time is smaller the older the drop. K i t h these conditions the situation closely approximates what i t would be if a solid spherical electrode of constant area were used. Quantitative interpretation of the current-voltage figure is thereby greatly facilitated, and the advantages of a mercury electrode whose area is constantly renewed are retained. The chief problem, of course, is the provision of an automatic means of precisely timing the brief voltage sweep so that it will almyays begin a t the same age of the drop regardless
The function of the “voltage clipper” is to prevent the applicd voltage from exceeding -2.5 volts LIS. the saturated calomel electrode when the time of an occasional drop exceeds the average drop time, and thus to prevent the deterioration of the capillary orifice of the dropping electrode and consequent irregular dropping which result when an excessively negative potential is applied to the dropping electrode. The voltage sweep always begins at a fixed time after the beginning of drop formation--e.g., a t 2.80 seconds for an average drop time of 3.00 seconds and continues linearly until the potential of the dropping electrode reaches -2.5 volt’s. The voltage clipper then maintains the potential constant a t -2.5 volts until the drop falls. Because part of the total voltage applied by the sweep geneutor is dissipated as a variable iR drop across the resistor in series n i t h the cell, the voltage across the cell it,self will not increase linearly with time, even though the sweep generator functions perfectly linearly. The “compensator circuit” corrects this aberration by supplying an additional voltage to the circuit, so that the voltage increase across the cell remains perfectly linear. This assures that, the voltage axis of the cathode ray tube figure will be directly proportional to the potential of t,he dropping electrode, prevents the apparent shift of the vave position with changing concentration (current) which otherwise occurs, and enables the instrument to measure half-11-ave potentials with a reproducibilitj- of 10.01volt when a 5-inch cathode ray tube is used. The current-voltage figures obtained with this circuit comespond closely to conventional polarograms, and Snowden and Page state that the half-wave potentials agree with those observed by the conventional technique in the cases they studied. It is to be expected, howevcr, that some discrepancy should be
I
I
SWEEP
CATHODE-RAY
fG&GKil AMPLIFIER I
F I1 FH$E&fiZ/ AHPLlF IER
FOLLOWER
1
V O L U M E 23, NO. 1, J A N U A R Y 1 9 5 1 observed if the electrode reaction is very slow. The chief difference from conventional polarogranis is the appearance of a rounded maximum at the top of the wave. This is simply due to the decay with time of the concentration of the electroactive substance. The situation corresponds to what would be observed if an electromotive force were suddenly applied to a stationary spherical elect'rode of constant area, as discussed by Kolthoff and Lingane ( l j ) , and theoretically the current should decay to a coiistant steady state value. Under the present conditioris sufficient time is not available to attain the steady st'ate, and helice the decay continues a t a decreasing rate until the reduction potential of t,he next substance is reached. Snowden and Page demonstrated that t,he magnitude of the iiiarimum current is directly proportional to the concentration of the reducible substance, and concentrations could be determined \vith an accuracy of about + 2 % over the range from about 0.2 t o several millimolar. For very small concentrations the finite \vidth of the cathode-ray t'race is a limiting factor, but this is st trivial difficulty which doubtless can be overcome. These authors also demonstrated that the maxima could be eliminated by elect r i d integration of the curves ticfore presentation to the cathode ra). tube: but &s they point out this merely makes the curve more familiar appearing and has no particular advantage. Snowden and Page also demonstrated that the current-voltage curve could be differentiated before presentation to the cathodelay tube, to yield a derivative curve showing sharp peaks a t the half-wave potentials instend of the usual plateaued waves. In addition to performing all the functions of a conventional polarograph, the Snowdeii and Page polarograph is uniquely useful for following the rates of r:tpid reactions which involve polarographically determinable substances, and the authors present several examples of this application. .i prophet's prescience is not needed to appreciate the practical significance of these developments, and it is to be hoped that instrument manufacturers xvill not allow the Snowden and Page in-trument, to molder overlong i n t,lie limbo of the research litcwture. Solid Microelectrode Current-Voltage Curves. The influence oi such I'srtors as tht, rate I){ application of the applied e.ni.f., clcctrotk area, and stirring, oil automatically recorded currentvoltage curves with platinum niicroelectrodes has been studied by Rogers, Miller, Goodrich, and Stehney (&), using 0.5 millimolar silver ion in 0.1 M potassium 11itrat.e. The curves recorded with stationary electrodes generally show rounded maxima or peaks duc to the decay with time of the concentration gradient a t the electrode surface, and the m:ignitude of the peak increases with increasing rate of application of the e.1n.f. to the cell. The current peaks usually did not' appear when a rotated platinum niic~roelectrodewas used, presuma1,ly because of t.he rapidity with which a steady rt,nte of diffusion is attained. L-nder controlled conditions the precision of concentration deterininations Lxi-et1 on the measurement of the wavr heights was somewhat poorer than with tht, dropping electrode, but the technique hah some useful potentialities for studying certain reductions and osihtions which occur at too positi potential for use of the dropping mercury electrode. The use of solid microelectrodes i n polarography has been irivcxstigat'edby Skobets, Bercnblyum, and Atamanenko (@), and they especially recommend a rotating anialgamated silver electrod(. (1 sq. cm. in area). This is claimed to be nearly as satisfilcto1,y as a dropping mercury rlectrode and to permit deterniiiiLitiona of metal ions tlo\vn to a concentration of 10-4 '11. Skobets, Turov, and Ryabokon (48)studied current-time curves of cadmium ion ],eduction in 0.1 Af potassium chloride using a ~ iamalgamated silver globule electrode and a platinum wirr microelectrode. They confirmed t'he observations of previous investigators that with stationary electrodes the current increases rapidly a t first, passes through a maximum, and then gradually decays to a steady- value, but that steady currents are
95 quickly established when the microelectrode is rotated. The same investigators also recommended the use of an elevated temperature (60" C.) as an aid in accelerating the attainment of steady current with solid microelect,rodes, and in eliminating the marimum when curves are recorded automatically. Lyderseri (29) advocated the use of a platinum microelectrode of very small surface to hasten the attainment of steady state sq. currents. He recommended an electrode of about 2 X cm. area, prepared by sealing a platinum wire into a glass tuhe and then cutting it off flush with the end of the tube. Lyalilcov and co-workers (27, 28) devised a dipping microelw trode which consists of a plat'inum needle surrounded by an openend glass mantle, with an inert gas passing through the mantle tube and escaping in bubbles from its lower open end. The platinum microelectrode comes into contact with the solutioii each time a gas bubble escapes, and thus is periodically dipped into and isolated from the solution. Olson, Brackett, and Crickard (36) developed a novel technique of using a stationary platinum microelectrode for the detrrmination of oxygen uptake by yeast suspensions. Their circuit periodically polarizes the microelectrode cathodically, and thcii anodically, with an intermediate quiescent period. Polarography in Liquid Ammonia. Systematic investigation. of liquid ammonia as a polarographic solvent have been carriwl out by Laitinen in collaboration with Yyman (18)and Shoemaker ( l g ) , with most interesting r e s u h . LIore or less conventional cells were employed and solution* were prepared by condensing purified ammonia into the cell at -36" C. All measurements were made in a thermostat at -36.0" + 0.2" C. At this t'emperature the vapor pressure of ammonia (665 mm.) is far enough below atmospheric pressure so that the handling of the solutions presents no great difficulties. The c,hoice of supporting electrolyte is somewhat of a problem due to the limited solubilities in liquid ammonia of salts comprising sufficiently difficultly reducible cations. Tetraal kylammoniuni salts were found to be fairly satisfactory. Using a saturated solution of tetrabutyl ammonium iodide (0.0057 M ) as supporting electrolyte, Laitinen and Sgman' observed that the alkali metal ions undergo reversible reduction to the metallic state and produce well developed diffusion currents which obey the IlkoviE equation. The half-wave potential? against a lead -0.1 -Vlead nitrate reference electrode are lithium, -1.67 volts; sodium, -1.31 volts; potassium, -1.24 volts; rubidium, -1.21 volts: and cesium, -1.15 volts. The observed diffusion currents of 0.001 'iJ lithium, sodium, rubidium, and cesium ions averaged 10 to 12% larger t'han the values predicted by the IlkoviE equation, and with potassium ion the observed value was 2% high. This \v\'its logically at,tributed to the fact that supporting electrolyte concentration was too small (0,0057 M) completely to eliminate the migration current. The diffusion coefficients of the alkali metal ions in liquid ammonia at -36" are about' five times larger than in aqueous niedia at. 25", and correspondingly the diffusion current constants are two to three times greater. Laitinen and Shoemaker concluded that mercuric ion in liquid ammonia is reversibly reduced t'o the metal, and that mercurous ion is unstable in this solvent and disproportionates to mercuric ion and mercury. The potential of a mercury pool anode i i independent of the concentration of nitrate, chloride, iodide, or ammonium ions and depends only on the mercuric ion concentrat'ion, indicating t8hatthe corresponding mercuric salts are all soluble, and in the case of ammonium ion that n o appreciable solvolysis to form species such as HgSH2' occurs. The same authors recommended thallous ion as a pilot ion to defme the polarographic potential scale in liquid ammonia. The reduction wave of thallous ion shows a rounded maximum, but this can be suppressed by a trace of methylcellulose. Laitinen and Xyman showed that the dropping mercury electrode functions as an "electron electrode'' in liquid ammonia
96
ANALYTICAL CHEMISTRY
when the cation of the supporting electrolyte is nonreclucible. Under this condition the cathodic reaction is the dissolution of electrons from the electrode. Polarography in Fused Salt Media. This field of polarography is being pioneered by Steinberg and Nachtrieb (35, 6 6 ) with highly interesting results. The solvent melt used in their most recent work (66) is the ternary eutectic composed of 30 nio!e 70 lithium nitrate, 17 mole % sodium nitrate, and 53 mole % potassium nitrate (melting point 120" C.). Polarograms of various heavy metal salts were obtained a t 160" =t1' using a conveiitional dropping mercury electrode. The melt is highly conducting (the cell resistance was as small as with aqueous solutions) and the niclt itself serves as supporting electro1yt.e. The potential of the dropping electrode was measured against a calomel electrode in the same melt, and to furnish chloride ion for this reference electrode 0.5 mole of potassium chloride wa.3 added per 100 moles of the ternary salt. Nickel nitrate, lead nitrate, cadmium nitrate, and zinc nitrate, in concentrntions ranging from about 1 to 18 millimolar, produce well developed normal reduction waves corresponding t o reduction of the niet,al ions to the metallic stat.es (amalgams). 111 all cases the reductions proceed reversibly, as shown by the agreement of the observed reciprocal slopes of the plots of log i/(& -i) us. Ed.$. with the theoretical value (0.043 volt a t 160" for a 2electron reduction), and even more convincingly in the cases of lead and cadmium by the fact that dropping amalgam electrodes of these metals yield well developed anodic waves whose halfwave potentials are the same as those of the cathodic waves of the two metal salts. The half-wave potentials compared to the values in aqueous solution are Xi+Ella volt (nieltA
El/%volt (aqueous)
-0,361 -1.1
Pb--0.473 -0.405
Cd-*
Zn-&
-0.549 -0.586
-0.880 -1.01
the values for aqueous solution being in reference to the aqueous saturated calomel electrode. The half-wave potentials are, of course, not directly comparable because the relation between the calomel electrode potentials in the two solvents is unknown. However, the values in the melt are remarkably close to the aqueous solution values, except for nickel. The reversible reduction of nickel ion in the melt is in marked contrast to its highly irreversible reduction in aqueous medium. The diffusion currents in the melt are directly proportional to concentration, and apparently in agreement with the IlkoviE equation. The diffusion currciit constants are some\+hat more than twice as small in the melt at 160" as in aqueous solution a t 25 ', because of the greater viscosity, and correspondingly much smaller diffusion coefficients, i.n the former solvent. Steinberg and Xachtrieb observed t.hat pot produces a poorly defined reduction wave with four steps, somewhat analogous to the stepwise reduction of chromate ion in aqueous solution. Cobaltous chloride yields a double wave which Steinberg and Nachtrieb attributed to oxidation of the cobalt to the + 3 state by the nitrate melt with subsequent stepwise reduction to the +l state and the metal. Potassium bromate shows no reduction wave, and the same is true of barium nitrate, uranyl nitrate, praseodymium nitrate, indium sulfate, and thorium nitrate. Steinberg and Nachtrieb were unable to obtain significant polarograms with dropping electrodes composed of molten lead or bismuth in a lithium chloride-potassium chloride eutectic at 500", or with molten silver in molten sodium chloride a t 1000" C. Polarography of fused salt media with a periodically dipped platinum microelectrode was investigated by Lyalikov and Karmaain ( d g ) , who used molten potassium nitrate and observed reduction waves for cadmium, copper, and nickel salts. Linearity of wave height and heavy metal salt concentration wa.s obtained only at relatively lov concentrations (about lo-* mole per mole of potassium nitrate). Phosphate (present LLS sodium
orthophosphate) was reported to yield a reduction wave in molten potassium nitrate. The decomposition potentials of several metal chlorides in molten mixtures of aluminum chloride and sodium chloride have been measured polarographically by Delimars'kiI, Skobets, and Berenblyum (8). LITERATURE CITED
(1) Airey, L., Analyst, 72, 304 (1947). 12) Airev. L.. and Smales. A. h..Ibid.. 75. 287 (1950). f3j Brdicka, 'R., Proc. Iiatevi. i'ong. P u r e and'Applied Chem. ( L o n d o n ) , 11, 345 (1947). (4) Bricker, C . E., and Furman, N. H.. .Ix\L. CHEM.,20, 1123 (1948). (5) Delahay, P., Ibid., 21, 1425 (1949). 16) Delahay, P., A n a l . C h i m . A c t u , 1, 19 (1947). (7) Delahay, P., J . Phy9. & Colloid Chem., 53, 1279 (1949); 54, 402. 630 (1950). (8) Deiimadkii, Yu' K., Skobets, E. RI., and Bcrenblyum, L. S., J . Phys. Chem. (U.S.S.R.),22, 1108 (1948). (9) Eyring, H., hIarker, L., and Kwoh, Ting-Chang, J . P h y s . d. Colloid Chon., 53, 1453 (1919). (10) . . Frumkin, A. and Levich, V , J . P h y s . Chem. (U.S.S.R.), 21, 689, 953, 1183. 13:35 (1947). (11) Gawron, O., AS.AL.C H E W 22, . 614 (1950). (12) Hcyrovsk$, J., C h f m . L i s t y , 40, 222 (1946); A n a l y s t , 72, 229 (1947) (13) Heyrovskg, J.. Collections Ctcchoslor. Chem. Commun., 12, 156 (1947); 13, 481 (1948); 14, 569 (1949). Inter% Congr. Pure and d p p l i e d Chem. (14) Heyrovskj., J., PTOC. ( L o n d o n ) , 11, 481 (1947). (15) KolthofF, I. AI., and Lingane, J. J., "Polarography," pp. 26-9, New York, Interscience Publishers, 1946. (16) Kryukova, T. A., J . P h y s . Chem. (U.S.S.R.), 21, 365 (1947); Zavodskaya Lab., 13,511 (1948). (17) Laitinen, H. A., and Burdett, L. IT.,. ~ N A L . CHEM.,22, 833 (1950). (18) Laitincn, H. A,, and Nyman, C. J., J . Am. Chem. SOC.,70, 2241, 300% (1948). (19) Laitinen, H. A., and Shoemaker, C. E., Ibid., 72, 663 (1950). (20) Leeds & Sorthrup, Philadelphia, Bibliography E-90(1). (21) Leveque, 51.P., and Roth, F., J . chim. phys., 46, 480 (1949). (22) Lingane, J. J.. IXD.Ex;. C H E Y .As.LI.. . ED.,15, 588 (1943). (23) Ibid., 16, 329 (1944). (24) Lingane, J. J., and Loveridge, R. A., J . Am. Chem. Soc., 66, 1425 (1944); 68, 395 (1946). (25) I b i d . , 72, 438 (1950). (26) Loveridge, B. A , Ph.D. thesis, Harvard University, 194i. (27) Lyalikov, Yu S., and (>laser, R. I., Zavodskaya Lab., 15, 909 (1949). (28) Lyalikov, Tu S., and Karmszin, V. I., Ibid., 14, 138, 144 (1948). (29) Lydersen, D., Acta Chern. Sccirtd., 3, 259 (1949). (30) McKenaie, H. A , , J . Am. C'hem. Soc.. 70, 3147 (1948). (31) Meites, L., and Meites, T., I b i d . , 72, 3656 (1950). (32) Ibid., in press. (33) Meites, L., Meites, T., and Colichman, E. L., I b i d . , in press. (34) Muller, R. IT. A x . 4 ~CHEM., . 22, 76 (1950). (35) Sachtrieb, S . H., and Steinberg, M., J . Am. Chem. Soc., 70, 9013 (1948). (36) Olson, It. A,, Brackett, F. S., and Crickard, R. G., J . Gen. Physiol., 32, 681 (1949). (37) Payne, S.T., Anal. C h i m . A c t a , 3, 686 (1949). (38) Petering, H. G., and Daniels, F. J., J . Am. C h e m SOC., 60, 2796 (1938). (39) Randles, J. E. B., Faraduy SOC.Discussions, 1, 19 (1947); Analyst, 72, 301 (1947). (40) Randles, J. E. B., Truns. F a m d u y Soc., 44, 327 (1948). (41) Riches, J. P. R., iYnfzwe, 157, 520 (1946). (42) Rogers, L. B., Miller, H. H., Goodrich, R. B.. and Stehney, A. F., ANIL. CHEM.,21, 777 (1919). (43) Semerano, G., "Bibliografia Polarografica," La Ricerca Scientifica, Supplement A, Vol. 19 (1949). (44) Sevcik, A , , Collection Czechoslov. Chem. Commun., 13, 349 (1948). (45) Shulman, J. H., Battey, H. B., and Jelatis, D. G., Reu. Sci. Instruments, 18, 226 (1947). (46) Skobcts, E. M., Berenblyum, L. S., and Atamanenko, S . N., Zauodskaga Lab., 14, 131 (1948). , (47) Skobets, E. hf., and Kavetskii, N. S.,Ibid., 15, 1299 (1949). (48) Skobets, E. >I., Turov, P. P., and Ryabokon, V. D., I b i d . , 14, 772 (1948); 15, 912 (1949). (49) Smith, G. S., A n a l y s t , 75, 215 (1950). I
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Characterization of Organic Compounds ROBERT L. PECK Merck & Co.. Znc.,Rahuuy, S. J .
C
HA11ACTERIZATIOS of pure organic compounds niay be achieved today with rapidity and certainty. The continued introduction and development of new reactions and techniques have so extended the scope of the organic chenlist that only the most complex substances are a t present beyoiid the possibility of direct structural elucidation. The chemical and physical properties which constitute the criteria for recognition, identification, and proof of structure of organic compounds may be obtained a t the present time with relative rapidity and with relatively small quantities of sample. Developments in micro and semimicro techniques, applications of organic reactions to small scale operation, introduction of new procedures for evaluation of purity, adaptation of instrunients for small scale physical determinations, and t,lie ingenuity of analytical chemists and those engaged in characterization work are all in part responsible for the present scope of the field (98). Insight into the current trend of thinking is provided by a recent article (40)which surveys the course of analyticrtl chemistry over the past quarter of a century. During the past year or so, additional reactions, derivatives, and techniques of interest have been reported. The present review considers a number of these developments. It is noted, howewr, that methods of characterization may be found in a considerable numP,r of fields of study and as incidental observations made in connection with investigations whose major objective is not primarily the development of characterization procedures (2.i). Accordingly, this review is limited to procedures and reactions which :m considered to be representative. Many useful studies may be omitted here either because of overlooking material submerged in reports of primary interest from other points of view or because of lack of time. The material reviewd is, as before (98), taken up in three sections-namely, a section dealing with methods for deterinination of purity, one on techniques concerned with determination of physical properties, and one on procedures which concern the organic chemistry of the compounds being ch:tracterized. PURIlY
Procedures which are designed to evaluate the purity of an organic compound are generally considered to comprise an attempt a t separation and a test of identity. Chromatographic procedures fulfill these specifications, as do phase-solubility measurements and countercurrent distribution. The now well-known technique of countercurrent distribution, which has found many applications in determination of purity as well as in small scale separation work, has recently been reviewed (Sf). By means of a new all-glass apparatus of 200 tubes, the usefulness of this technique has been considerably extended (Sf). Advantages of the new apparatus include better visibility of solutions, greater ease of removal of sample from a
single tube for examination a t any position in tlw systr’rn, and greater versatility as regards solvent systems. The “solubility temperature” has been suggested as a criterion for characterization of pure compounds as well as of niixtures (104). This is similar to determination of consolute tcmperature, and is considered to be a convenient property to dvtermine, if sufficient quantities of material are available. Enip1o)-nient of the “critical mixing temperature” for rccognition and determination of sniall amounts of organic liquids has been descrihed (46). Fractional sublimation of organic compounds on removable transparent films (66) presents an attractive, small scale procedure for determination of purity of minute amounts of sublimable substances. The procedure permits uninterrupted sublimation and separation of several components in one operation. Removal of the sublimate by pceling off the strip facilitates ready separation of the several fractions for qualitative examination. Ion exchange resins show pronlise of far greater application in purity determination (ff6) as well as in separations related thereto (22, 61). The characterization of ribonucleotides by means of a rrsin is an elegant procedure providing separation arid identity data (26). It has been remarked that before long exchange resins will be used for srparation of most compounds of acidic or basic nature, leaving only the relatively neutral compounds as suhject for srparation by the orthodox adsorption chomatography . Charcoal chromatography has provided a useful evaluation of purity of sugars by means of a modification of the Tiselius technique (126). S e w streak reagents for location of zones of colorless substances in extruded chromatographic columns have been developed (80). Paper chromatography continues to receive much attention. hlentioii niay be made of procedures for separation and identification of mixtures of keto acids (88),of flavanol-3-glycosides (50), nucleic acids (80,lf4),purines, pyrimidines, and derivatives (81, 64), fats and fat-soluble pigments (SY), phosphoric esters (60), aromatic aldehydes and lignin degradation products ( I S ) , various phenolic compounds ( 4 2 ) , and miscellaneous antibiotics (100). Many of these methods include procedures for quantitative determination of the substances of interest. The last-mentioned procedure provides a very useful method for rapid recognition of previously known antibiotics, especially when combined with observations on physical properties and chemical reaction behavior on a micro scale. Its application to screening work is obvious. Similar applications for groups of compounds other than antibiotics are becoming apparent. A recently reported simple device for handling large numbers of two-dimensional paper chromatograms (32) would appear to solve one problem confronting investigators in this field. Photometric procedures for quantitative determinations of zones have been studied further (107). Further applications of radioactive tracers, development of a radioactivity
