Review of Fundamental Developments in Analysis
Polarographic Theory, Instrumentation, and Methodology David N. Hume Massachusetts lnsfitute of Technology, cambridge, Mass.
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HIS review, the sixth in the series, is essentially a continuation of its immediate predecessor (72). I n the twoyear period surveyed (late 1959 to late 1961) the volume of papers appearing has continued to increase. The annual bibliographies founded by Heyrovski (68) and Semerano (205) continue to be of great assistance to anyone attempting to follow the literature. The Heyrovskjl compilation is no longer a supplement to the Collection of Czechoslovak Chemical Communications but is published separately. Two journals of interest to polarographers have appeared since the last review: Of particular importance is the Journal of Electroanalytical Chemistry published by Elsevier. The abstract section, which provides prompt and thorough coverage of polarographic and other electroanalytical papers, should be of value to everyone working in the field. It is regrettable that there are so many minor errors-e.g., in the spelling of authors’ names-but this is probably the inevitable price of getting the literature abstracted and into print in the shortest possible time. Electrochemica Acta published by Pergamon should also be mentioned. A considerable number of general reviews appear each year, mostly addressed to specialized scientific or linguistic groups, which are not of interest to the general reader. A few are of particular timeliness or value, however, and should be mentioned here, notably the excellent review on direct current polarography by Niirnberg and von Stackelberg (165) and surveys of recent developments by von Stackelberg and Schmidt (220) and Llilner (164. Heyrovskj’s Nobel Lecture was printed in Science (69). Among recent books, note should be made of the three-volume collection of papers given at the Second International Congress of Polarography in Cambridge, England, 1959, edited by Longmuir (I%%), and of the volume on oscillographic polarography by Heyrovsk? and Kalvoda (70). The perennial problem of nomenclature, symbols, and sign convention has not been left in peace, and the results of the efforts of a number of individuals
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and international committees have appeared in print. 1Clilazzo (152) has reviewed the problems involved and with others (153) has compared the differences among various proposals. The Commission on Nomenclature and Electrochemical Definitions of the CITCE and the IUPAC subcommittee on symbols and terminology in electrochemistry have issued a report which has appeared in more than one publication (241, 2.42). Some of the proposals, notably the elimination of the term potential and its replacement by tension, are not likely to be adopted by English-speaking chemists overnight. The report of a three-man (Delahay, Charlot, and Laitinen) committee on classification and nomenclature of electroanalytical methods, published in French (42) and in English (41) is more likely to be influential. I n an attempt to resolve confusion in terminology and develop a logical basis of nomenclature, the committee has used IUPAC symbols and attempted to organize current practice with a minimum introduction of new terms. They suggest that the term polarography be restricted to methods involving electrodes of varying area, and that another term such as voltammetry be used for electrodes where the area is fised. The term oscilloscopic polarography is criticized on the ground that much practice does not involve the use of an oscilloscope, and they suggest, for example, that single sweep oscillographic polarography be called instead potential sweep chronoamperometry. The proposals in the report, in this reviewer’s estimation, are worthy of careful consideration. CLASSICAL POLAROGRAPHY
Instruments and Apparatus. New commercially available instruments for polarography continue to appear. The Nesco Instrument Co. of Costa Mesa, Calif., markets a n inexpensive recording polarograph of conventional design. Interscience (not the publishing company) of Richmond, Va., offers a compact and versatile device which they call the Electroscan. Acting as a source of controlled current or voltage, it can be used either for voltage
or current scanning polarography, or for coulometry. An auxiliary recorder is required. The Cambridge Instrument Co. of London offers a general purpose instrument for use with any suitable recording potentiometer, and a Univector attachment to convert a direct current polarograph for alternating current applications. This unit uses Jessop’s phase detector principle and the makers claim a 10- to 100-fold increase in sensitivity by its adoption. Jarrell-Ash in Sen-tonville, Mass., now acts as agent for the Yanagimoto, PA-101, an automatic instrument for a.c. and d.c. polarography made in Japan. The manufacturer claims that it is also adaptable for chronopotentiometric studies. The unit, which they call the Polarlog, is furnished with integral recorder and all accessories a t a price competitive with the better American direct current polarographs. An automatic, ax., bridge-polarograph, the RP-3, is also made by Shimadzu Seisakusho in Kyoto, Japan (198). Blervyn Instruments, St. John’s, Xorking, Surrey, England, markets the Barker square wave polarograph under the name Rlervyn-Harwell, available in the U.S.A. through the Instrument Corp. of America, Baltimore, Lid. Widespread use of the instrument is liable to be deterred by the price which is in the vicinity of $15,000. Southern Analytical Instruments in England is reported to be bringing out a version of Barker’s pulse polarograph. Atlas Werke A.-G. in Bremen manufactures an instrument which they call the Tast polarograph and which registers the current only during a short period toward the end of the drop life. Thereby are eliminated the large current fluctuations and the effect of the very considerable charging component at the beginning of drop growth. Metrohm A.-G. in Switzerland offers an electrode stand with a drop time controller designed to give a very rapid dropping rate for use with the technique which they call rapid polarography, described later in this review. The article by Schmidt (198) may be consulted for a general discussion of contemporary polarographic apparatus, principally of European design and construction.
Noncommercial efforts in the direction of improving direct current polarographic apparatus are largely centered upon the problem of eliminating errors due to iR drop, particularly in high resistance systems. Arthur et al. (5) describe a polarograph using an X-Y recordcr nith a cathode folloiver in the voltage measuring circuit. This instrument records both current and voltage fluctuations throughout the life of each drop. Although effective and actually providing more information than the conventional polarograph, it has the disadvantage of requiring an X-Y recorder and the current-voltage curves obtained, particularly for high resistances, are unconventional in shape. An alternative approach which involves the use of operational amplifiers to control the potential of the dropping electrode a t the desired value with respect to the reference electrode, regardless of iR drop in the cell, is proving t o be very popular. An increasingly large percentage of the voltammetric research reported, particularly in ANALYTICAL CHEMISTRYand in recent papers given a t American Chemical Society meetings, has been performed on apparatusdesignedalong the lines of that originally described by DeFord (40) and Kelley, Jones, and Fisher (92). The latter authors have recently described an improved apparatus utilizing an electronic scan in the polarizing unit as w l l as operational amplifiergoverned potential control and linear residual currcnt compensation. They report that noise in the voltage scan is much 1css n-hen the scan is performed electronically than when the conventional inultiturn potentiometer slidewire is employed, and this lowering of noise lewl is of particular importance in high sensitivity polarography (93). This reviewer's experience with a similar electronic-scan controlled-potential polarograph constructed in his own laboratory has been entirely satisfactory. The unit is reliable and convenient t o operate, and it is suggested that a unit based on these principles is well wrthwhile whether the cell resistance is high or not. Arthur and Vander Kam (6) have described an electronic iR compensator which can be attached to a conventional polarograph. The circuit involves a stable 1:l amplifier which takes as its input signal a measure of the iR drop and feeds its output in series with the polarograph, thereby giving a plot of current us. actual. rather than applied, potential. It shows entirely normal polarograms even with nonaqueous solutions, having resistances of several megohms. Papoff, Grifone, and Zuliani (170) have described a pen-recording polarograph with a wide range of scanning rates. Drake and Johnston (50) pro-
pose a semiautomatic polarograph in which current readings are plotted manually on a chart coupled with the voltage scan. The apparatus is said to be convenient for instructional purposes. Barker (11) and Nash (160), respectively, have been awarded patents for a residual current compensation circuit and a device for effectively damping current oscillations by inhibiting movement of the recording pen during rapid changes of current. The most important development in dropping electrode design derives from an important study by Cooke, Kelley, and Fisher (35), who studied the influence of the capillary on the residual current noise in high sensitivity polarography. They obscrved that capillary noise was the chief limiting factor in the reproducibility of very small limiting currents. The authors suggrst drawing down the tips of capillaries so that the drops form a t a constricted orifice. With this arrangement, a severalfold improvement in background noise was observed. A propos of noise in polarographic currents, the obwrvations of Southworth et al. (219) on the behavior of the dropping electrode at very high negative potentials are worth mentioning. These authors noted that a t potentials greater than -1.9 volts os. S.C.E., solution traveled up the capillary, reaction took place in the capillary, and disruption of the mercury drop took place with intermittent tiny geysers of mercury rking and diqpersing small droplets throughout the solution. These observations appear to supply the explanation for the erratic behavior which many observers have noted a t high negative potentials. Cozzi and Desideri also studied the effect of capillary shape and other characteristics which influence the ratio i/rn2'9 t1I4. They concluded that the Il'koviE equation was satisfactory over a considerable range if the capillary were properly formed and the galvanometer corrected (37). Laforgue-Kanzer (114) made high-speed motion pictures of dropping electrodes to which mere applied low and very low frequency alternating currents. Mechanical perturbations induced by the electric current were observable. Lyubimova and Sochevanov (134) studied the deformation of polarographic waves a t high concentrations. They concluded the effects observed t o be caused partly by the shape of the polarograph cell partly by the capillary. They report that a low resistance capillary consisting of a very short length (10 t o 12 mm.) of capillary fuse to a longer and wider tube with a platinum contact above the narrow portion, when used in conjunction with a low resistance cell, is capable of giving accuracies of about i2% a t concentrations as high as 0.25M. Lockwood (125) has described a device
for synchronizing two dropping mercury electrodes through detachment of the drops by a solenoid action. Tamamushi, Momiyama, and Tanaka (269) have done further studies on the characteristics of the rapidly dropping electrode and have recommended it for use in flowing systems, the current being relatively insensitive to flow in the solution. The biennium brings forth its usual harvest of new polarographic cells. Milner et al. (165) hare described a cell with an arrangement for catching falling drops in a cup and removing them so that they do not contaminate the quiescent mercury anode. Parkhurst (171) gives details for the construction and use of an ingenious microcell for the deterniination of metals in biological material. The sample is weighed directly into the cell where it undergoes combustion, solution, and dilution before the polarogram is taken. A graduated side-arm, which is an integral part of the cell, makes it possible to measure the volume without the necessity of any transfers. Roe and Nyman (193) have described a threecompartment polarograph cell with a removable bridge. von Sturm (2.26) has designed a microcell with an external calomel reference electrode, and Landry (117) has suggested the design for a low-resistance saturated calomel reference electrode for use in H cells. Blaedel and Strohl (87) have described a cell for the continuous analysis of flowing streams and Mancy and Okun (141) have provided a cell for the determination of oxygen in gas streams. Mairanovskii and Titov (139) have described a number of cells, salt bridges, and dropping electrodes used in Russian laboratories. The type of capillary with a small glass spade attached for releasing the drop a t the same point in its life each time is much favored. Among special-purpose cells, those of Shimojima (211) for attachment to a vacuum system (the application was in liquid sulfur dioxide) and of Schaap, Conley, and Schmidt (19'7) for polarography under pressures up t o 10 atmospheres deserve particular mention. Luck (133) recommends the use of a rotating sample table for the determination of large numbers of samples. Half-Wave Potentials. Schaap (196) has derived a general equation for the polarographic wave applicable t o solvents of low dielectric constant. The effect of ion pair formation on halfwave potential is taken into consideration, and a summation of terms including formation constants of ion pairs is introduced. TIEek (243) has continued his studies on the relation between electronic structure and the polarographic behavior of inorganic substances. His results show that the theoretical equations for the determination of the activation energy in various VOL. 34, NO. 5 , APRIL 1962
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electrode processes are not yet refined to the point of having general validity. Stromberg (223, 22'5) on the basis of his slow discharge-ionization theory for electrode processes has derived a relationship correlating the irreversible half-wave potential of reduction of metals with the drop time of the electrode. A plot of the irreversible halfwave potential against logarithm of drop time gives a straight line with a slope equal to one half the slope of the wave. The method has been used for studies of manganese reduction at the dropping electrode (225) and the behavior of the titanium(1V)-titanium (111) couple (2%). Koepp, Wendt, and Strelom (104) have proposed a n ingenious technique for the comparison of potentials in differing solutions. They make use of the reversible potential of the ferrocene-ferrocinium ion couple which shows a striking independence of solvent effects in a wide variety of solvents. The results are applicable not only to polarography but to electrochemistry in general. The ferrocene suggestion appears t o be a more practical one than that of Nelson and Iwamoto (161), n-ho suggested t h e 4,7- dimethyl - 1,lO-phenanthroline ion half-wave potential for comparison of solvents and experimental evaluation of liquid junction potentials among different solvents. Currents. Los a n d Murray (127-129) have undertaken further to refine the Il'koviE equation on the basis of a very careful experimental and theoretical treatment of all the known experimental variables. Current-time studies on individual drops allowed appraisal of depletion effects and the hitherto neglected factor of variability of m during the life of the drop. The resulting equation gives an improved fit, but a t the cost of considerably increased complexity. Deviations toward the end of the drop life suggest that convection may be becoming significant even with the dropping electrode. The theoretical relationship between polarographic currents and electrode reactions has continued to interest a considerable number of workers. Weber and Kouteckj. (109) and Keber, Kouteckg, and Koryta (248) have derived equations for limiting currents influenced by the adsorption of an electroinactive component on the drop. DraEka (48) has considered the effect of electrostatic repulsion in the double layer and &ek, Koryta. and Kouteckf (33) have examined the situation when the electroactive species is formed in a second-order reaction from two electroinactive species. Kouteck9 and Koryta (108) have given a general mathematical theory of polarographic kinetic currents and Zhbranskji (254) has derived and verified relations describing the kinetics of electrode proc174R
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esses involving complexes. Yatsimirskii and Budarin (2'5s)have shown that it is possible to determine equilibrium constants for the formation of complexes by determination of polarographic catalytic currents. The depcndence of the catalytic current on the concentration of substances involved in the complex formation reaction n ith the catalyst itself makes the determination possible. Nurnberg, van Reisenbeck, and von Stackelberg (164) have shown that by measuring the dependence of currents upon drop time, i t is possible to determine reaction rates. I n this manner dissociation and recombination rates of weak acids were determined polarographically. Good results were obtained with the second dissociation steps of phosphoric and sulfurous acids, and the third step of citric acid. Mairanovskii (138) has studied the influence of catalj tic evolution of hydrogen on the shape of the polarographic wave and Riddiford (192) has deduced relations for steady-state conditions a t electrodes where complex reactions are taking place. Koryta (107) has studied the kinetics of polarographic reductions of compleves where both the free ion and the complex ion may be being reduced simultaneously. In collaboration n i t h Biernat (26), the same n-orker studied the deposition of manganese from various complexes and observed the interesting result that although the reduction of manganous ion to manganese amalgam n as reversible, the amalgam quickly changed to a less active form. The rate constant of the inactivation reaction ivas determined from the polarographic data. Nkmec (162) has made a study of the determination of average current with a moving coil galvanometer and shoned that equal areas rather than equal galvanometer excursions above and below should be used to define the position of the average line. Instantaneous Current - Time Curves. T h e examination of instantaneous current time curves a t individual drops has begun t o attract theoretical as n ell as practical interest. Weber (247) has examined several types of instantaneous polarographic currents and developed equations assuming spherical diffusion and control, both by diffusion and a slom- electrode process. Kapulla and Berg (89) discuss the relationship of current-time curves of kinetic currents to the half-wave potential of the reversible reaction and describe a switching apparatus to prevent the depletion effect a t the dropping electrode. The use of instantaneous current-time curves for the study of depletion effects by Los and Murray (127) has already been mentioned. KPlta and Smoler (113) studied the effect of capillary-active substances on
electrode processes a t the dropping electrode and developed a simple formula for evaluating the extent of electrode coverage in the initial phases of the drop life from current-time data. Peizker (172) attributed some deformations in current-time curves to the effect of ohmic resistance. KBta and Smoler (112) applied current-time curves t o the study of the deposition of metals from relatively concentrattd solutions (up to 0.2M) n i t h drop times varying between 2 and 50 seconds. If no crystallization of the metals takes place in the amalgam a t the drop surface, the current time curves follow the theoretical values, but with long drop times and concentrated solutions it is possible to see nith the microscope thc formation of metal crystals at the surface of the amalgam and a corresponding distortion of the current-time curves. The effects were observed nith thallium and silver. Tamamushi, Aloniiyama, and Tanaka (229) made current-time studies with the rapidly dropping electrode, finding the limiting current t o be controlled both by diffusion and convection in potential ranges where the mercury siirface n-as in motion. Mechanisms of Electrode Reaction. Studies on the mechanism of electrode processes have continued t o be popular and the interested reader is advised t o consult the e x e l l e n t review by Reinmuth (188). Classification of the work is difficult inasmuch as many of the studies involve alternating current polarography and other electrodes, but an effort has been made to segregate in this section those investigations which have particular significance for the dropping electrode and conventional direct current polarography. consideration of double layer eflects is one of the principal avenues of approach toward electrode reaction mechanisms and kinetics. Apparatus for measurement of the double layer capacity on the dropping mercury electrode has been described by LovreEek and Jendra'si6 (132), and a highly accurate bridge for measurement of the impedance of the metal electrolyte interface a t microelectrodes has been described by Gesteland and Howland (59). Grahame and Parsons (63) have endeavored to elucidate the structure of the double layer for a mercury electrode in a potassium chloride solution and analyzed the capacitance into contributions from the difTuse and the inner double layers. Studies on the structure of the electrochemical double layer have also been reported by Brodowsky and Strelom
(W.
l l a t s u d a and Delahay have been particularly active in the field of relating double layer structure with the kinetics of electrode reactions. These
authors have discussed a double pulse galvanostatic method for measuring transfer coefficients (149) and compared i t ‘VI ith relaxation methods for measuring faradaic impedance (147). hlatsuda (146) has determined the magnitude of the double layer effect on electrode processes n-ith a preceding chemical reaction 3s a function of the rate constant of the preceding chemical reaction and developed z general equation applicable to the derivation of transition times in chronopotentiometry and limiting current in polarography. Delahay and Kleinerman (49) tnktx into consideration the effect of the elrctrode itself on the electrode processes by its influence upon double layer. Thcy studied the electrode effect by comparing the dropping mercury electrode n ith a dropping thallium amalgam, the composition of 1%-hich could be changed over a considerable range, thereby pumitting changes in the point of zero charge. They concluded that specific. electrode effect must be considered in any kinetic study. Asada. Delahay, and Sundaram (7) evplaincd the failure of double layer corrections in electrode kinetic calculations in terms of a local field effect due to ions of the same sign as the depolarizer ions being present in the double Iityer. Hurnitz (7.3) has also evaluated the influmce of double layer effects on kinetics of rlcctrode processes preceded by a clicmical reaction. Baticle and Thourenin (14) describe a method for study of evchange currents a t electrodes by a simple process, and Reinmuth and Rogers (189) have shown it feasible to obtain transfer coefficients in the electron transfer controlled waves by a derivative technique even TT hen accuratr estimation of the limiting current is not poqsible. Smit and Wijnen (816) have applied a cyclic potential step method and a cyclic current step , 250) for determination of exchange currents and transfer coefficients. Laitinen (116), in a discussion of methods of studying t h e electroanalytical chemistry of surface monolayers, points out the utility of double-layer capacity measurements. The rectification of alternating current by an electrode reaction as a consequence of an aqymmetry of the current voltage characteristics with respect t o equilibirum potential-a phenomenon named the redoyo kinctic effect by its discoverer.. D o ~ s and Agarwal, but more commonly called faradaic rcctification by western workers-is being given consideration as a method for the study of electrode processes, particularly the kinetics of fast reactions. Matsuda and Delahay (248) developed equations for study of electrode kinetics, pointing out the possibility of correcting for double layer cffects. Delahay, Senda, and Weis (46) developed a
general matliematical theory for faradaic rectification involving two types of control: either the total mean current density, or the mean electrode potential. I n a later paper (45) the same authors developed methods for calculating transfer coefficients and exchange current densities either with or without correction for double layer structure, and concluded that the method should be applicable t o the study of the kinetics of very fast reactions because interference by double layer capacity is avoided and t h e influence of cell resistance minimized. Senda, Imai, and Delahay (206) discuss in addition a double pulse method to shorten the time for double layer charging in rectification voltage measurements. Reddy (178) has given a simple picture of faradaic rectification, and Rangarajan (177) has derived a general equation for the phenomenon and pointed out that Barker’s radiofrequency polarography represents a special case. METHODOLOGY
Alternating Current Polarography. The use of alternating current superimposed upon t h e conventional direct current scan has been one of t h e most popular lines of investigation during the period covered by this review. Quite a p a r t from t h e large number of applications, which are beyond t h e scope of this review, there have been many studies of the theory and its application to elucidation of the kinetics of reactions and mechanisms of electrode processes, The review by Bauer (18) is an excellent starting point for anyone wishing to become familiar with the descriptive aspects of the technique. Bauer and Elving (20) have summarized important factors involved in alternating current polarography and evaluated the accuracy of the data obtainable. Bauer (19) has given a clear account of the effect of frequency, diffusion coefficients, and transfer coefficients upon the height and shape of the alternating current polarogram. Bauer and Elving (81) have pointed out the utility of phase angle measurements in the study of tensammetric phenomena where a n alternating current polarogram results from the adsorption of surface-active species on the electrode. Schmidt and r o n Stackelberg (199) made a study of the influence of ohmic resistance and double layer capacity on the peak currents in alternating current polarography. Some very practical hints on the use and operation of Miller’s device for adapting the Sargent Model XXI for alternating current operation have been given by Head (65). H e points out the importance of correct cell design and suitable spacing of dropping electrode
and pool. Tsfasman (236) describes a technique which he calls vector polarography and an apparatus, the vector polarograph, which does not appear to be different from the conventional alternating current polarograph with phase discrimination. Another polarograph in which the faradaic and capacitative currents are separated by a phase sensitive bridge has been described by Gorelkinskii, Grinman, and Kozlov (62). Ishibashi, Fujinaga, and Saito (77, SO) have modified Breyer’s alternating current polarograph with an auxiliary resistance-capacitance circuit which compensates for the resistance and capacitance of the cell and counterbalances the residual current. ‘Vl’alker, Adams, and Alden (246) have overcome one of the most annoying limitations of alternating current polarography by using operational amplifiers to control both the direct current scan and superimposed alternating current potentials. Although conventional apparatus gives very large changes in peak current due to cell resistance, it was found that with controlled potentials, resistances of over 100,000 ohms resulted in no change either in current or phase angle. Bauer and Elving have discussed the theory of alternating current polarography in a series of papers. h’ew equations have been derived using less restrictive assumptions than previously ( 1 6 ) , improved methods for measurement of phase angle developrd ($@, and the theoretical equations tested experimentally by Bauer, Smith and Elving (24). The effect of frequency dependency has been examined (23), and the conclusion drawn that previous equations were invalid because of insufficiently rigorous assumptions. Reinmuth and Smith (190) have severely criticized these conclusions, bringing forward evidence t h a t the prior n-ork was rigorous while t h e assumptions involved in the derivations by Bauer and Elving were incorrect, leading even to violations of the first and second laws of thermodynamics. Bauer’s reply (15) and Reinmuth and Smith’s counterreply (191) have a polemical trenchancy seldom seen in these days of multiply-revien-ed manuscripts. Sluyters (214) and Sluyters and Oomen (215) discuss the use of alternating current polarography for the measurement of rate constants. double layer capacities, and diffusion coefficients. Matsuda, DeIahay, and Kleinerman (150) compare alternating current methods with galvanostatic and potentiostatic methods for the study of the kinetics of discharge of metal ion complexes with a preceding chemical reaction. Studies on electrode reactions involving slow steps have been described by Tamamushi and Tanaka VOL. 34.
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(230) and by Reddy and Doss (179). Kalvoda (86) has compared alternating current polarography with oscilloscopic and classical polarography by the study of a series of compounds adsorbed on the mercury electrode. Comparable results were found by all of the methods. The application of second harmonic alternating current polarography has been further developed by Smith and Reinmuth (618), who point out that because the double layer charging process corresponds to a n approximately linear circuit element and the faradaic process is somevhat nonlinear, the contribution to higher harmonic alternating current components is predominantly faradaic. Theory shows that for small amplitudes, the first and second harmonic alternating current polarograms should be directly proportional to the first and second derivatives of the classical direct current polarogram. Circuit details are given of an apparatus to measure the second harmonic component, and it was shown that half-wave potential, peak potential for the first harmonic, and zero current potential for the second harmonic agreed very well. Second harmonic polarography appears to be as accurate as alternating current polarography and has the advantage that the base current is very low. Bauer (17) has also studied alternating current harmcnic polarography and noted similar advantages. He points out t h a t t h e background current may be 5% rather than 75% of the peak as in conventional alternating current polarography. Juliard (83, 84) has demonstrated the effectiveness of alternating current voltammetry a t platinum electrodes. Using a rather rapid d.c. scan (about 2.5 volts per minute) between - 1.0 and $1.5 volts us. the S.C.E. and 30-millivolt r.m.s. 60-cycle alternating current superimposed, good and characteristic peaks appear for the deposition of metals such as silver and lead and the oxidation of iodide or the reduction of iodine. Surface effects are observed to be important, and it is suggested that the peaks may have their shapes determined more by chemical changes at the surface than by transfer of material through the diffusion layer. Walker, Adams, and Juliard (246) have extended voltammetry with alternating current to graphite rod and carbon paste electrodes. Smith and Reinmuth (117) have extended Juliard’s technique by use of an electronic phase-sensitive detector, which allows rejection of the charging current while retaining the major part of the faradaic current. Very high sensitivity wis obtained, 10-siZf cadmium being determinable to rtlyo repeatability. The authors pointed out the potential application to kinetic studies.
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The square wave modification of alternating current polarography continues to attract attention. Yasumori (666) has described an instrument for square wave work and discussed the theory involved. Von Sturm and Ressel (668) investigated the influence of foreign electrolyte concentration in square wave polarography. Using the Harwell type of instrument, they examined several of the reversible and irreversible systems and reported that a considerable increase in sensitivity for irreversible reactions was obtainable without interference from reversible couples if low foreign electrolyte concentrations were maintained. Ton Sturm ($97) elsevhere has compared the precision and accuracy of regular and square wave polarography with respect both to reproducibility of halfwave or peak potentials and the current sensitivity obtainable. Kaukewitsch and Von Sturm (91) in applying polarographic methods to inorganic trace analysis have made a comparison of conventional polarography with the square wave technique. Oscilloscopic Polarography. This term has in the past been applied to two radically different techniques: the multisweep approach introduced b y Heyrovsk? and favored by t h e Czech polarographers, and t h e socalled single-sweep or triangular potential scan approiich. It has been suggested (41) that the term oscilloscopic not be applied to the latter inasmuch as the oscilloscope is seldom used for this type of measurement as now done. The term chronoamperometry is suggested instead. Davis and Shalgosky (39) have discussed dropping mercury electrode chronoamperometry (which they call cathoderay polarography) using a potential sweep from negative to positive instead of the usual direction. They point out that for oxidation waves, this is more convenient and gives greater sensitivity for elements reduced near zero potential. The basis for this observation lies in the fact that the larger current obtained at the high negative beginning potential is better for triggering the sweep when the drop falls. Kaufman, Loveland, and Elving (90) described an improved apparatus for chronoamperometry and Seaborn (603) suggested a circuit for compcnsating for the effect of cell capacity. Valenta and Volke (938) discussed the principles and applications of triangular sweep current voltage curves in the study of electrode processes and in qualitative and quantitative analysis. Valenta and Titwicki (239) applied the technique to the study of catalytic waves. Mann (149) has studied chronoamperometry at solid electrodes using a stepped voltage sweep. Called staircase voltammetry, this technique seeks to combine the advantages
of square wave polarography with rapid sweep polarography. Kalvoda has reviem-ed recent literature on the theory, applications, and progress in classical oscilloscopic polarography (85). The same author has developed a semiconductor probe-device for measuring the depths of incisions on oscilloscopic curves (86). I n another study (81)he has investigated the possible use of oscilloscopic polarography for the determination of nonreducible substances, such as alcohols, which adsorb on mercury electrodes and affect the capacity of the double layer. Incisions due to adsorption and desorption are visible on the curves of dE/dt as a function of E. The capacity effect was seen to be related to solubility with very sharp incisions if a precipitate were formed on the surface of the electrode. Some differentiation between different alcohols was found possible. The relationship of this technique to tensammetry is evident. Skobetz and Shapiro (613) have published an improved circuit for use with silver amalgam electrodes in oscilloscopic polarography while Moln8r and Bir6 (157) have proposed an electronic device for recording oscilloscopic polarograms by means of a single alternating current cycle, which allows the properties of clean electrodes in undepleted solutions to be examined. Other Modifications. Kolf (261) described a technique which he calls “rapid polarography,” involving a controlled, rapidly dropping electrode and a scan rate about five times that of normal direct current polarography. An electromechanical dislodger is used to give drop times between 0.2 and 1 second. The wave shapes obtained were very much like those in ordinary polarography. The advantages are said to be that the current is independent of potential, that there is a very considerable diminution in the current oscillations, that better separation is obtained between consecutive waves. and of course the time involved in taking polarograms is cut to a small fraction of what is usual. The depletion effect is greatly diminished and maxima of the second kind are not observed. Apparatus for carrying out rapid polarography is commercially available. The technique appears to be of considerable practical significance and deserves further investigation. The technique known as Tast polarography has been described again by Elbel (61). This technique, based on a development by Strelow in 1957, involves measuring the current only during a short interval in the latter part of the drop life. The result of this technique is a considerable diminution of the background current due to elimination of the charging component in the early part of the drop
life. This permits an increase in the sensitivity by a factor of about tenfold. The currents are independent of potential along the plateaus and the current oscillations are much smaller than in conventional polarography. The apparatus is commercially available. Pulse polarography has been proposed by Barker and Gardner (12, I S ) as a n improvement over square wave polarography. As in Tast polarography the current is registered only during a bricf interval toward the end of the drop life. I n one modification the current resulting from a relatively small voltage pulse is examined; this gives peaks of the shape normally associated with alternating current or derivative polarography. Alternatively, the pulses may be of steadily increasing magnitude, giving a current voltage curve analogous in shape to a conventional polarogram. Because the rapid charging a t the beginning of the drop and the capacitative current a t the beginning of the pulse is eliminated, the sensitivity of the method is very high. The apparatus, unfortunately, is rather complex. Incremential polarography is another related technique. Glickstein and Auerbach and their associates (8, 60) have described the principle and given details of the apparatus and its application. At a fixed point in the life of each drop, the voltage applied is stepped up by an increment and the current is measured. The principle is somewhat similar to Tast polarography, for again the current is measured only in the latter part of the drop life. A difference amplifier can be used to compare successive drop currents and give a derivative curve. The apparatus is so arranged that a derivative current or total current may be plotted, or both plotted simultaneously. The linearity and resolution reported are very good and the sensitivity is superior to conventional polarography. Jiickel (82) performed derivative polarography with a rapidly dropping electrode and conventional R-C differentiating circuits. The curves obtained were distorted and differed considerably from the theoretical shape. VOLTAMMETRY
Mercury
WITH
OTHER
Electrodes.
ELECTRODES
Simmonin
(212) described a mercury electrode, the
surface of which is refreshed once or twice a second by drops of mercury from a capillary hanging above it. The surface of the electrode is about 1 mm. in diameter; no maxima are observed, and the sensitivity is variable by varying the size of the electrode. The author observes that care is needed in its manipulation. Penketh (174) has described a mercury pool electrode for
oscilloscopic polarography, the surface of which is renewed between voltage sweeps by overflowing. Good reproducibility is claimed. Rooney (194) has also proposed a pool renen-ed by overflow for oscilloscopic and conventional polarography. Gokhshtein (61) uses a mercury pool electrode for oscilloscopic polarography, particularly anodic stripping, and recommends it for use on flowing systems. Okinaka and Kolthoff (166) have used the rotated dropping mercury electrode for the study of irreversible waves and the measurpment of kinetic currents. In comparison with the conventional dropping mercury electrode, the rotating electrode shows a greater tendency for irreversibility. Copper reduction in perchlorate medium which is reversible a t the dropping electrode, for example, is irreversible at the rotated electrode. Expressions were derived for kinetic currents a t the rotated dropping mercury electrode based on the same reaction-layer concept as used with the conventional dropping mercury electrode. Bersier, Bersier, and Hiigli (25) have proposed a mercury electrode which contacts the solution through a membrane of alumina on which the electroactive species may be adsorbed. Hanging Drop Electrodes. The relative simplicity, both experimental and theoretical, of the hanging mercury drop has made this electrode exceedingly popular, particularly in applications involving anodic stripping voltammetry. Reinmuth (183)has developed the theory of stripping voltammetry with spherical electrodes, considering both linear potential scan and chronopotentiometry. Shain and Lewinson (207) have discussed the concentration distribution of metals deposited from the preelectrolysis step. With this and Reinmuth’s treatment of stripping processes, theoretical calculation of current voltage curves is possible. Shain and Martin (208) studied reversible, and Shain, hfartin, and Ross (209) irreversible processes under conditions of electrolysis with constant potential, taking into consideration the spherical shape of the electrode. Delahay, Oka, and Matsuda (44) used the potentiostatic method with the hanging drop to study kinetics of chemical reactions preceding charge transfer and found that with the hanging drop the method was applicable even with reactions too fast for polarographic studv. Stromberg and Stromberg (624) have reviewed ultramicro determination by stripping voltammetry on stationary mercury electrodes comparing the hanging drop with four other electrode forms. Kemula, Goerlich, and Kowalski (100) noted troubles in the plating step when oxygen was present. As would be expected, the previous discharge of oxygen can in-
terfere nith the plating of cadmium and zinc by precipitation of these metals as hydroxides a t the surface of the drop. Kemula, Galus, and Kublik have studied the effect of various metals present in the hanging mercury drop, describing the results in a series of interesting papers. They first observed that the presence of gold interfered with the stripping of cadmium and zinc, from which they concluded that a goldzinc intermetallic compound is formed (9’7).Further work on the influcnw of gold showed that even traces-0.1 to 0.001% of gold in the mercury-could have quite striking effects. The tffect of gold on zinc and cadmium was confirmed, but it was found that thallium and lead were not affected (99, 101). The hanging drop electrode was later used to demonstrate the existence of complex intermetallic compounds in mercury by the influence of one metal on the stripping behavior of another. Tin and nickel were found to influence each other, and nickel and zinc. The authors suggested that the existence of 1:1 compounds between nickel and tin, and nickel and zinc vas indicated (94, 98). Kemula and Galus also claimed evidence for the formation of amalgams of iron, cobalt, and nickel m-ithin the hanging drop if the deposit were allowed to age (96). Differences in the behavior of a hanging mercury drop and a platinum sphere covered a i t h a thin layer of mercury were observed and the differences explained by assuming the formation of a platinum amalgam (109). Underkofler and Shain (237), however, mention that in their experience with mercury drops hung on platinum points they never noticed interferences due to platinum, contrary to the observations of Kemula et al. A microcell for voltammetry with hanging drop electrodes in very small volumes has been described by Underkofler and Shain (237). Micka has used a hanging mercury drop on a silver base for oscilloscopic polarography (151). Solid Electrodes. The theory of stationary electrode polarography has been further developed by Reinmuth (18‘7) who has obtained explicit solutions for the equations to replace the approximations of previous workers. Reinmuth has also studied the behavior of reversible systems in stationary electrode polarography and shown that the current’s independence of scan rate a t a fixed potential at the foot of the irreversible wave allows a rapid and simple distinction between irreversible and reversible systems (189). Morgan, Harrar, and Crittenden (158) have measured half-wave potentials with cylindrical electrodes. The mechanical simplicity of a cylindrical electrode makes its application favorable. Current-potential relationships VOL 34, NO. 5, APRIL 1962
177 R
were found to deviate only slightly from linear diffusion a t small electrolysis times. Hamelin (64) has determined polarization curves on monocrystalline zinc and found differences for different crystal faces exposed. This work should have considerable bearing on the theory of electrode processes. Mituya and Obayashi (156) have studied the behavior of platinum electrodes under conditions of anodization in acidic solutions. With the aid of neutron-activated platinum electrodes they were able to show no dissolution or disintegration of the electrode under moderately strenuous conditions. Cohen, In-amoto. and Kleinberg (34) have suggested the use of the rotated platinum electrode for the study of comples formation, applying it to voltammetry of the mono-oxalato silver ion. Panchenko (169) has done derivative polarography with rotated solid electrodes in molten salts. Loveland and Dimeler (131) have applied the rotated platinum electrode to anodic voltammetry in acetonitrile. Working with hydrocarbons they have obtained satisfactory current voltage curves as far out as f1.8 volts us. the S.C.E. Frumkin and coworkers (55) have proposed the use of a rotated platinum disk electrode and found i t suited to the investigation of intermediate products formed in electrode reactions. Pleskov (176) gives a method of studying separately two electrochemical processes taking place simultaneously at an electrode. K i t h the rotating disk, processes governed by diffusion give currents proportional to the square root of angular velocity while those kinetically controlled do not. Bardin, Lyalikov, and Temyanka (10) have recommended the micro disk electrode for general polarographic work. Leontiev and Fedotov (120) have described an automatic apparatus for polarography with the vibrated platinum electrode. Cozzi and Desideri (38) have utilized the so-called bubbling platinum electrode in which a platinum disk dipped in the solution is periodically covered and uncovered by a bubbling gas. Current voltage curves are obtained which resemble greatly those produced by a dropping mercury electrode. Sawyer and Interrante (195) have used solid electrodes to study the electrochemistry of dissolved gases. The behavior of oxygen a t platinum, palladium, nickel, gold, silver, tungsten, tantalum, copper, zinc, cadmium, and lead electrodes has been determined. The effects of supporting electrolyte and electrode preconditioning were determined, and it was shown that the latter can be very important. Osterlind (168) described an electrode which could be connected to the artery of a living animal for continuous recording of oxygen in blood. Krog and Jo178 R
ANALYTICAL CHEMISTRY
hansen (111)developed a Teflon-covered platinum microelectrode for intravascular oxygen determination. The electrode could be introduced by a catheter into the heart of a dog. Strkfelda (822) made studies of stationary rotated and vibrating platinum electrodes in flowing solutions. As u-ould be expected, it was found that the rate of flow was relatively unimportant for rapidly moring electrodes. Elving and Smith (53) have made an exhaustive study of the factors involved in the prcparation and reproducibility of graphite clectrodcs. They found preivetting n ith a wetting agent to be of great help in obtaining reproducibility. Olson and Adams (16'7) have made a critical study of the carbon paste electrode and its application to anodic roltammetry. Electrodes made from finely polvdered graphite and either il'ujol or bromonaphthalene were found to be satisfactory. Potential scans up mere attainto + l . 2 volts us. the S.C.E. able and the repeatability of the area mas about 5 6 % over a period of days. It was observed that sometimes the behavior of the electrode varied with the carbon used. Voorhies and Davis (244) performed coulometry with a carbon black electrode. It would seem worthwhile to investigate the use of carbon black for anodic stripping work in dilute solutions, the adsorptive capacity of the carbon black making it attractive for this type of application. hlueller and Adams discussed criteria to be applied to the behavior of inert electrodes and concluded that boron carbide is a highly desirable electrode material (159). Techniques for fabrication of the electrodes were given, and they were found to be good for analytical applications in both organic and inorganic systems. The electrode is highly inert with a very small residual current, peak potentials are reproducible, and peak currents repeatable to *170. Von Stackelberg, Vielstich, and Jahn (221) have used a rotated lead disk electrode in studying the kinetics of electrode processes. A plot of limiting current divided by the square root of angular velocity against limiting current is horizontal if the electrode reaction is diffusion-controlled ; othermise the lines have slopes depending upon the kinetics. The method was applied to a study of the third dissociation step of citric acid. Kolthoff and Sambucetti (106) have done voltammetry n ith the rotated aluminum wire electrode. The application is mainly in the determination of fluoride. MacNevin and Wilson (137) have observed a very puzzling effect with aluminum electrodes in acetic acid solutions containing some fluoride. When two otherwise identical electrodes are placed in such a solution, it is found that the smaller electrode acts as an anode with
respect to the larger. The phenomenon is repeatable and appears to be connected with the oxide layer on the aluminum electrodes. Shain and Perone (210) have done anodic stripping analysis of iodide with silver electrodes, and Laitinen and Chao have studied the stability of condensed monolayers of palmitic acid on gold electrodes (116). As would be expected, the behavior is dependent upon the surface state of the electrode. Chronopotentiometry. Chronopotentiometry has been one of the most active fields of study in electroanalytical chemistry during the last two years. The analytical applications are relatirely fen, b u t a great deal of use has been made of the technique in the study of electrode kinetics. Reinmuth (181) has written a basic paper on the fundamentals of interpreting chronopotentiograms. Four diagnostic criteria are given: linearity and slope of a log transition time function against potential; E1 4 us. current density behavior; Elid us. concentration; and the ratio of transition time of the reverse scan to transition time of the forward scan. These criteria are applied to 15 representative cases for interpretation in terms of reaction type and kinetics. I n another paper (184) Reinmuth discusses chronopotentiometric transition times and their interpretation where the electrode reaction is preceded by adsorption or chemical kinetic complications. Diagnostic criteria are given for distinguishing various types of electrode mechanisms. For example, variation of current in chronopotentiometry is analogous to the variation of the square root of mercury head in polarography for distinguishing diffusion and kinetic control. Testa and Reinmuth (235) have continued the theoretical examination of chronopotentiograms by considering a system with three consecutive steps which may be chemical or charge-transfer, in any order. Peters and Lingane (175) have developed a theory describing the behavior of cylindrical electrodes in chronopotentiometry. The resulting equation works for a single constituent but becomes too complex for successive steps, thereby necessitating empirical calibration, As the authors point out, the choice is between a simple theory and a n inconvenient electrode, or a simple electrode and a n inconvenient theory, Lingane (182) has checked the fit of the theory by studies on the chronopotentiometry of ferric ions and hydrogen ions. The effect of solid electrode configuration and the magnitude of the transition time on the accuracy of chronopotentiometry has undergone a careful study by Bard (9). He looked at plane electrodes in various positions, protected
and unprotected from convection, and varied his transition times from the millisecond range to 300 seconds. He concluded that for analytical purposes a horizontal plane electrode shielded to prevent convection currents being set up as the result of density changes during electrolysis was the most favorable. Relatively long (10 to 60 seconds) transition times are recommended for analytical applications, the double layer capacity and surface roughness effects being more significant a t shorter times. Relatively short transition times are recommended only if convection is not avoidable. Reinmuth (186) has also given consideration to distortions due to double layer and surface roughness effects. A semiquantitative discussion is given and methods of minimizing the disturbances suggested. Surface roughness seems not to be as serious a factor as has been presumed. Reinmuth also showed that the technique of enhancing the sensitivity of a constituent by prereduction of a more noble constituent is not as practical a procedure as once believed. He gives a new empirical graphical method of estimating transition time which in practice is more satisfactory than any of the methods commonly used (185). Anson (a)has criticized some of Reinmuth's discussion on the effect of double layer and surface roughness factors, to which criticism Reinniuth has replied (186). Lingane (121) has studied chronopotentionietry with platinum and gold anodes and finds that the effect of surface oxidation of the electrodes is very important in the oxidation of ohalic acid. He has also contributed further evidence that platinum oxides participate generallj- in redox reactions taking place a t platinum surfaces (123). Anson, too, has observed the influence of surface oxidation of platinum electrodes and pointed out that the behavior of the ferric-ferrous couple indicates that the oside films show decided aging effects (1). The usefulness of chronopotentiometry for the study of the electrochemical behavior of adsorbed substances on platinum electrodes has been very clearly demonstrated by ilnson (3, 4 ) . He shom-ed that adsorbed ferric and ferrous ions are very slowly desorbed from platinum electrodes and will give chronopotentiograms even though the electrodfs have been transferred to solutions containing no iron. I t is of interest to note that the behavior of iodine adsorbed on platinum indicates it to be present as iodine atoms. This proinising technique will undoubtedly find great application. The use of chronopotentiometry m-ith current reversal has been developed by DraEka (47') for the study of the kinetics of electrode reactions. With various coworkers he has studied the effect of
bimolecular irreversible chemical back oxidation of the products of the polarization (64) and of succeeding reactions of higher orders (49). The same technique has been developed independently by Testa and Reinmuth (231), u-ho pointed out the value of the technique for the preparation and study of unstable systems in situ. The theory is relatively simple for current reversal chronopotentiometry and provides new methods for calculation of the rate constants of irreversible chemical reactions fo1loLving reversible electrocheniical reactions (231, 232). Similar treatments hare been developed by King and Reilley (103) and by Furlani and hlorpurgo (0'6,57). Hurm-itz and Gierst (7'6) and Hurivitz (7'8, 7'4) hare examined rather carefully a variation of chronopotentiometry in which the imposed current changes with the square root of time. Details for construction of the apparatus were given, the theory developed for simple reductions and for electrode reactions involving a series of steps, and applications made to spherical, plane, and cylindrical electrodes. The technique has the attractive feature that for direct discharge processes, transition times are proportional to concentration. Testa and Reinmuth (234) have given consideration to chronopotentiometry with step functional changes in the current during electrolysis, The situation has been examined for increasing currents, decreasing currents, and for reversal of current during electrolysis. They conclude that there is probably no analytical advantage to this approach, but it may be useful in kinetic studies. Bon-ers et al. (29) used both constant current and linear current scan chronopotentiometry ITith membrane electrodes. They point out the advantages of a cellophane or similar membrane to control diffusion conditions and eliminate convection at plane electrodes. Ishibashi et al. (79) have done chronopotentiometry a t the slowly dropping mercury electrode. They claim it to be useful analytically, especially for the determination of traces in the presence of large amounts of more easily reducible materials. In amoto, Adams, and Lott (81) did chronopotentiometry on a drop scale for increased sensitivity. Bruckenstein and Sagai (31) described a new technique in which a mercuric salt is added t o the solution and mercury plated continuously with other metals onto a platinum electrode. The amalgam is then subjected to chronopotentiometry for determination of the metals by stripping from the amalgam. hlather and Anson (145)have done chronopotentiometry in acetic anhydride solutions, and Liu has used a platinum electrode in molten lithium potassium sulfate eutectic. Kemula and Galus (95) have used chrono-
potentiometry to study the kinetics of formation of intermetallic compounds in mercury. By this means they were able to determine the rate constant of formation of duZn. Van Sornian (240) has utilized a liquid bismuth electrode for determination of traces of various metals in bismuth by chronopotentiometry with fused lithium chloridepotassium chloride a t 450' C. Laitinen (116) discussed chronopotentiometry as a method of studying the electrochemical behavior of surface monolayers. Liu (124) has done chronopotentiometry in molten lithium-potassium sulfate. MISCELLANEOUS
A three-dimensional surface showing the relationships among various timedependent voltammetric techniques with microelectrodes has been developed by Reinmuth (180). Analogous to the Reilley, Cooke, and Furman surface describing the time-independent, current-voltage-concentration relations, intersections of the surface with appropriately oriented planes represent polarography, chronopotentiometry, and constant potential voltammetry. New techniques suggested by the model include linear current scan chronopotentiometry and normalized chronopotentiometry in which the increase of current proportional to t 1 I 2 leads to transition times directly proportional to concentration. I n the area of nonaqueous solvents, Sellers and Leonard ($04) have used N-methylacetamide, a solvent of interest because of its very high dielectric constant, for polarography and found it useful for both organic and inorganic systems with the dropping mercury electrode. Larson and Iwamoto (118) examined the utility of various nitrile solvents, comparing acetonitrile, propionitrile, benzonitrile, phenylacetonitrile, and acrylonitrile. Schober and Gutmann (201) investigated ethylenediamine, benzyl chloride, acetic anhydride, morpholine, and phosphorus osychloride as polarographic solvents. I n a later paper (200) they did an extensive study on anhydrous ethylenediamine and ammonia as polarographic solvents. Hubicki and Dabkoivska (71) investigated polarography in liquid ammonia above its critical temperature, utilizing the fact that lithium perchlorate with 4 moles of ammonia forms a liquid essentially stable a t room temperature. Schaap, Conley, and Schmidt (197) were able to perform polarography in liquid ammonia a t 25' C. with the aid of a high pressure cell system which should be applicable to low boiling liquids in general. Elving, Markowitz, and Rosenthal (62) found many difficulties in attempting to do voltammetry in liquid sulfur dioxide. The D.M.E. is unstable and VOL. 34, NO. 5, APRIL 1962
179 R
no satisfactory supporting electrolyte is known. Larson, Iwamoto, and A d a m (119) proposed a reference electrode for polarography in acetonitrile and point out the problems involved in attempting to relate nonaqueous half-wave potentials to the aqueous saturated calomel electrode. Applications of polarographic techniques in molten salts have continued to be popular. Hills and Oxley (67') have discussed the theory and practice of polarography in melts using both gas-flushed and stationary solid electrodes with the lithium chloride potassium chloride eutectics. Panchenko (169) has done derivative polarography with solid rotated electrodes in the same melt, and Topol and Osteryoung (935) have studied the polarography of bismuth a t platinum and tungsten electrodes in molten bismuth chloride at 240 to 350' C. The process studied was the anodic osidation of dissolved bismuth metal. Rlaricle and Hume (143) described tungsten in Vycor electrodes and applied them to polarography in molten sodium-potassium chloride. Christie and Osteryoung (Sg) have applied the DeFord-Hume technique for determination of formation constants of complexes to molten nitrate media. A depression of maxima by less than monomolecular layers of insoluble substances on a mercury drop was observed by Kolthoff and Okinaka (105). Mercurous iodide was found to suppress completely ma.xima both of the first and second kind and behave like an anionic surfactant, making some reductions more and others, less reversible. MacNevin and Steele (156) studied the motion of solutions about drops during maxima and found a correlation between the type of solution movement and the charge on the drop with respect to the electrocapillary maximum. Krjukova (110) likewise studied maxima, comparing the characteristics of the maxima of the first and second kinds. Schwabe and Mai (202) examined the effect of monovalent alcohols on maxima of the first kind. Ishibashi, Fujinaga, and Sat0 (781used the rapidly dropping mercury electrode and the behavior of the abnormal wave of copper in perchloric acid as aids in the study of the characteristics of maxima and maximum suppressors. They reported polyacrylamide as being the most satisfactory of the maximum suppressors which they studied. Millicoulometry, which Delahay, Charlot, and Laitinen (41) now propose be called polarographic coulometry, has been further developed by Cover and Meites (56),who suggest the use of a pilot ion technique and the simultaneous observation of the change in limiting current of the two species involved. Magek (144) has described a 180 R
ANALYTICAL CHEMISTRY
simple three-electrode microcell for millicoulometry, in which a graphite working anode is employed to avoid polarization of the reference pool. AIacNevin and Moorhead (155) were successful in their efforts to make a practical dropping gallium amalgam electrode and found reversible behavior potassium thiocyanate. in 7 . 5 X Nikolaeva and Palm (16s) studied the electrocapillarity curves of mercury containing various amounts of copper as a function of the electrolyte. The electrocapillary maximum for such a n electrode is more positive than that of pure mercury, as mould be expected. Delahay and Kleinerman (43) used thallium amalgam electrodes containing up to 31% thallium, resulting in changes of the electrocapillary maximum by as much as -0.4 volt lyith respect to the dropping mercury electrode. Heus and Egan (66) were able to do polarography in molten lithium, potassium chloride a t 450' C. using a dropping bismuth cathode. Waves were obtained for lead, zinc, and cadmium, the results being analogous to those obtainable with the dropping mercury electrode in aqueous media. Maki and Geske (58, 140) showed the feasibility of using electron spin resonance spectroscopy for the direct study of radicals produced electrolytically a t a mercury pool electrode. Blaedel and Todd (68) devised a n elegant method for continuous determination of amino acids being eluted from a chromatographic column. The solution was contacted with copper phosphate which gave copper in solution equivalent to the amino acids and permitted direct and continuous polarographic registration. Peiaker (1Y3) employed the dropping electrode for preparative work on a microscale. Love and Greendale (1S0) utilized i t to remove radioactive tellurium from solution as a n amalgam. Determining the radioactivity per drop as a f m c tion of potential, it becomes possible to determine current-voltage curves without measuring current and to obtain polarograms of vanishingly low concentrations.
Vander Kam, R. K., ANAL.CHEW33, 488 (1961). (6) Arthur, P., Vander Kam, R. II., Ibid., 33, 765 (1961). (7) Asada, K., Delahay, P., Sundaram, A. K., J . Am. Chem. SOC. 83, 3396 (1961). \ - - - - I
(8) Auerbach, C., Finston, H. L., Kissel, G., Glickstein, J., X s . 4 ~ . CHEX 33, 1480 (1961). (9) Bard, A. J., Ibid., 33, 11 (1961). (10) Bardin. M. B.. Lvalikov. Tu. S.. Ternyank& V. S., Zhu; Anal. Khim. 14; Te 24 (1969). . 1) I ) Barker, G. C., Brit. Patent 860,130 (June 26, 1957). -2) 2) Barker, G. C., Gardner, 9. W.,
Atomic Energy Research Establ. (Gt. Brit.) C/R 2297 (1961). .3) 3 ) Barker, G. C., Gardner, A. W., Z. anal. Chem. 173, 79 (1960). -4) 4) Baticle, A. M., Thourenin, Y., Compt.
rend. 248, i94 (1959). -5) 5) Bauer. H. H.. BXAL. CHEX 33. ' 1803 (1961). (16) Bauer. H. H.. J . Elechoanal. Chem. 1 , 2(1959).' (17) Ibid., 1,256 (1960). (18) Ibid.. o. 363. (19) Ibid.; i, 66 (1961). (20) Bauer, H. H., Elving, P. J., - 4 ~ s tralian J . Chem. 12, 335 (1959). (21) Ibid., p. 343. (22) Bauer. H. H.. Elvinrr. P. J..' J . ilm. ' Chem. s i c . 82, 2091 (1960). (23) Bauer, H. H., Elving, P. J., J . Electroanal. Chem. 2 , 53 (1961). (24) Bauer, H. H., Smith, D. L., Elving, P. J., J . Am. Chem. SOC.82,2094 (1960). (25) Bersier, P., Bersier, J., Hugli, F., Helv. Chim. Acta 43,478 (1960). (26) Biernat, J., Koryta, J., Collection Czechoslov. Chem. Communs. 25, 38 (1960). (27) Blaedel, W. J., Strohl, J. H., ANAL. CHEM.33, 1631 (1961). (28) Blaedel, JV. J., Todd, J. W.,Ibid., 33. 205 (1961). (29) 'Bowers, R. C., Ward, G., Wilson, C. ~
hl., DeFord, D. D., J . Phys. Chem.
65, 672 (1961). (30) Brodowsky, H., Strelow, H., Z. Elektrochem. 64, 891 (1960). (31) Bruckenstein, S.,Sagai, T., ANAL. CHEM.33. 1201 (1961). (32) Christie, J. H., O&eryoung, R. .4., J . 4 m . Chem. SOC.82, 1841 (1960). (33) Ciiek, J., Korvta, J., Kouteckg, J., '
Collection Czechodov. Chem. Communs.
24, 3844 (1959). (34) Cohen, S. H., Iwamoto, R. T., Kleinberg, J., J . Am. Chem. SOC.82, 1844 (1960). (35) Cooke, W.D., Kelley, hI. T., Fisher, D. J., ANAL.CHEJI.33, 1209 (1961). (36) Cover, R. E., hleites, L., Anal. Chim. Acta 25, 93 (1961). (37) Cozzi, D., Desideri, P. G., Contributi
teoricz e Sperimentali d i Polarograja Vol. IV, Suppl. to Ricerca sei. 29, 165
ACKNOWLEDGMENT
This work was supported in part by the U. S. Atomic Energy Commission under Contract AT(30-1)-905.
LITERATURE CITED
(1) Anson, F. C., ANAL.CHEX. 33, 934 (1961). (2) Ibid., p. 1438. (3) Ibid., p. 1498. (4) Anson, F. C., J . Am. Chem. SOC.83, 2387 (1961).
(5) Arthur, P., Lewis, P. A., Lloyd, N. A.,
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