Exam ination of Absolute and Comparative Methods of Polarographic Analysis JOHN KEENAN TAYLOR, National Bureau of Standards, Washington, D . C.
Comparative methods of polarographic analysis consist essentially of comparing the heights of waves obtained for the substances to be determined with those found for solutions of known composition. By the absolute method, the concentration of an electrolyzable ion is correlated with its diffusion current and the characteristics of the dropping-mercury electrode employed. The advantages and limitations of several methods of these two types are dis-
cussed. Comparative methods minimize many of the experimental uncertainties but require numerous calibration experiments for their application. By the absolute method, calibrations are almost eliminated but experimental conditions must correspond to the boundary conditions for which the IlkoviE equation was derived. When this requirement is satisfied, the accuracy of the absolute method approaches that of the best comparative technique.
T
HE polarographic method of chemical analysis owes its inception and early development to Heyrovskg and his 'students at Prague, who studied the behavior during electrolysis at the dropping-mercury electrode of a number of cations, an~ Q Mand , nonelectrolytes in many kinds of media. As a result of these studies it was shown that the half-wave potential is charaoferistic of a given ion in a given supporting electrolyte, while the height of the step or wave on the current-voltage curve is, for a given electrode, directly proportional to the concen-tration of the reducible material. All the early work made use of the polarograph as an interpolative device in that the determination depended upon the comparison of the heights of the waves found for standard and unknown solutions. The discovery by Kemula (9) that the height of a wave is influenced by the rate of flow of mercury from the dropping electrode led IlkoviO (8) to develop a theoretical expression id = knDl/2Cm2/3t1/6
elimination or compensation. Although numerous articles have appeared in which descriptions of procedures for polarographic analysis are given, no comprehensive and critical discussion of the advantages and disadvantages of the various methods available has been published. This paper describes a number of types of polarographic methods and shows what factors should be controlled for their precise utilization. ABSOLUTE METHOD
The absolute method of polarographic analysis makes use of the IlkoviE equation for the interpretation of the polarograms and the calculation of the concentration of the electrolyaable ion. In its strictest application all terms in the equation should be evaluated by direct experimental measurement. However, as already pointed out, the unavailability of precise values for D, the diffusion coefficient, limits the application of the general equation. The use of the diffusion-current constant of Lingane (16)
for the relation of the limiting current, id, to the mass of mercury, m, emitted from the capillary in unit time, the drop time, t, of t h e electrode, the diffusion coefficient, D, and the concentration, C,of the reducible ion, the valence change, n, of the electrode
permits the diffusion current to be correlated with the experimental constants of the system under investigation and is in this sense a quasi-absolute method. The discussion that follows is concerned entirely with the advantages and limitations of this procedure. The advantages of using diffusion-current Constants are several. Only a single calibration experiment is required to establish a value for I , whiah can then be used, along with the measured characteristics of the electrode, to determine the concentration of the reducible ion. The saving in time and labor arising from the reduction in the number of calibration experiments may be considerable, especially when small numbers of determinations or a large variety of measurements are required. In addition, reporting diffusion-current constants is the only reliable means of making an experimental procedure directly available to ,a potential user without the necessity of performing standardization experiments for himself. For utilization of diffusion-current constants, it is obviously necessary to control the experimental conditions closely, since any errors will be propagated into the final result. The physical quantities that must be measured or known for eacli analytical determination are rate of flow of mercury from the electrode, the drop time, the diffusion-current constant, and the diffusion current. In practice the last-mentioned quantity is, of course, obtained from the wave height and the electrical constants of the
process, and k which contains theoretical constants and proportionality factors. This equation has been verified both experimentally ( 4 1 0 , 18) and theoretically (80)by a number of investigators and has become the fundamental law of theoretical polarography. Its development permitted, for the &st time, the magnitude of wave heights to be predicted and correlated with the characteristics of the electrode and solution. The general application of this equation has been retarded somewhat by the unavailability of precise values for the diffusion coefficient of the electrolyaable ion under the exact conditions of the experiment. The utili* of the IlkoviE equation can be greatly increased by following a proposal of Lingane (15). Since n and D are constants for a given electrolyzable ion in a given medium at a given temperature, and k is a constant independent of the system investigated, he suggested that these three be combined into one constant, I , such that
which can be readily determined from a single calibration experiment. The concentration, C, can be obtained by substituting known values for I and the experimental values for id and the characteristics of the electrode in this equation. The realization of the ultimate precision of any analytic method depends upon recognition of the sources of error and their
368
'
JUNE 1947
3-69
Table I. Tolerances in Control of Conditions to Ensure Precision of j=2% in Values of Z or C 1. Standardsolutions (calibration): 6 = , ~ 0 . 2 % Mass flow of mercury, m: 6 = *l%,If a. Head of mercury is controlled to *0.3% b . Temperature of cell is controlled t o *0.5’ C. c. Temperature of roomis controlled to * 5 O C. d . Calibration is performed i n a polarographically similar solution e. E r r o r in determination of ,m 0.5% 3 . Drop time, t : 6s *2%,if a . Drop time is determined a t half-ware potential b. E r r o r i n clocking is f 1% 4. Diffusioncurrent. id: 6 = * l . 5 % . i f a. Correctionfo;re3iduaI curreni.1 b. Temperature o i cell is c o n i r n l l r e . Calibration of electrical ec d. Size of wave caI
2.
-
=~=0.57~
+z
4 + ?
00148 x IO-’M
v)
z 0 u
3 0
I
2
3
4
5
6
7
DROP-TIME , S E C
Figure 1. Dependence of Diffusion-Current Constant of Cadmium upon Drop Rate of Electrode and Concentration of Suppressor
apparatus, after proper correction for the residual current is applied. In a recent review ( 4 ) the physical factors influencing the various terms in the IlkoviE equation mere analyzed and a set of tolerances were established which permit a precision in the determination of values for diffusion-current constants or of the concentration of electrolyzable ion to within 1 2 % . These tolerances, summarized in Table I, are conservative and readily realizable. By using the convenient apparatus suggested by Lingane (14) for the determination of m, and simply clocking the drop time of the electrode, precise values of the characteristics of the capillary are readily obtained. Without undue hardship it is thus possible to reduce the uncertainties of absolute polarographic methods to about those inherent in the measurement of the diffusion current. The use of the absolute method of analysis is based upon a t least three assumptions, the failure of any one of which would vitiate the applicability of the procedure. It is assumed that the experimental conditions correspond precisely to the boundary conditions for which the IlkoviE equation was derived, that the diffusion-current constant is independent of small variations in the oomposition of the electrolyte, and that no irregularities occur in the behavior of the solution. Since an understanding of the limitations imposed by these assumptions is essential for precise use of the absolute method, they will be discussed in some detail. The IlkoviE equation assumes that the limiting current which is responsible for the formation of waves is determined by the rate of diffusion of the electrolyzable ion from an inexhaustible supply in the solution to the surface of the electrode where it is zero because the ions are removed as rapidly as they arrive. This cur-
rent is augmented by the residual current composed of the 80called condenser current and any faradayic current due to simultaneously reducible substances ( I d ) , and correction is usually made for this. In addition the waves are often complicated by more or less pronounced maxima caused by abnormally l a e concentrations of the electrolyzable substance a t the electrode surface produced by forces other than diffusion. The suppression of these maxima is imperative if the absolute method is to be employed. It has been shown that errors of many per cent in t h e values of diffusion-current constants can result from the presence of maxima which are not readily recognizable (3). Such is the case, for example, when cadmium is electrolyzed in neutral solutions of potassium chloride, where a study of diffusion-current constants as a function of the drop time of the electrode was required to show the presence of maxima. The results of such a study (3) are shown in Figure 1, which also demonstrates the effectiveness of gelatin in suppressing maxima and inducing conformance to the boundary conditions of the IlkoviE equation. Studies in which nickel, bismuth, and thallium (3) were used have given similar results. If the logarithm of the minimum drop time above which constant values for the diffusion-current constant are obtained, designated as the critical drop time, is plotted with respect to the logarithm of the concentration of gelatin in the solution, a linear relationship is found to exist which is independent of the concentration or nature of the reducible ion. Preliminary experiments have shown that the relationship depends on the size of the drops produced by the electrode, but the quantitative relationship has not been determined as yet (3). Large amounts of suppressor should not be used indiscriminately, since an excess may decrease the size of waves, especially in dilute solutions of reducible ion (3, 15). Although there is no completely reliable simple test for determining when maxima have been suppressed, two methods have been suggested which may have practical value under certain conditions. One criterion mentioned by Heyrovski. ( 7 ) , is that the current-voltage curve should be parallel to that of the supporting electrolyte except, of course, in the region where the wave is developed. A similar rule-namely, the near parallelism of the segments of the curve immediately preceding and succeeding the wave-has also been found reliable in a number of instances ( 3 ) . The uncertainties arising from the occurrence of maxima and their suppression make it imperative that experimental conditions be such t h a t the application of the equation is valid, not only when values for diffusion-current constants are determined but also for their use. There are two general methods of measuring diffusion currents, and the estimated values may depend upon the method employed. The first, which has been designated as the “exact method” (11), consists in determining the residual current from a separate experiment and subtracting this from the total diffusion current of the substance in question. Under the second type may be considered the several graphical methods f h i c h have been proposed for approximating the magnitude of i d ( 2 , d l ) . Of these, the method which seems most satisfactory (24) consists in extrapolating the portion of the curve preceding the wave and measuring the distance between this extrapolated line and the curve after the diffusion current has been fully developed (11). Indeed, under ideal conditions, the exact method reduces to this procedure if the wave is in the range where the residual current is a linear function of the applied voltage. The dependence of the diffusion-current constant upon the method used for the estimation of id is illustrated by the data of Table 11. Appropriate quantities of the pure metals were dissolved along with that amount of zinc which would have been present had there been used 1 gram of alloy of the specified zinc content. The s u p porting electrolytes were 1 molar sodium hydroxide in the case of lead and 1 molar in hydrochloric acid and 4 molar in ammonium chloride in the case of tin, as recommended by Lingane for t h e
V O L U M E 19, NO. 6
370 Table 11.
The data were obtained by dissolving 1 gram of spelter (National Bureau of Standards sample 108; O.O470j, lead; 0.09201, cadmium) in acid, evaporating to dryness, and dissolving in the appropriate volume of supporting electrolyte. The large variation in the values of I is no doubt due to changes in the values of the diffusion coefficients of the ions in the presence of the large
Diffusion-Current Constants for Lead and Tin
Concentration of Metal
Concentration Zinc in Sampleof
MilLimoles/lite7
%
0.095 0.096 0.128 0.137 0.977
27 0 40 0 0
Observer IExtrapolation Observer 1 2
Lead 3.31 3.28 3.33 3.41 3.45 A%'. 3 . 3 6 * 0 . 0 6
3.39 3.37 3.34 3.43 3.50 3.41
* 0.05
Observer IExaot Observer 1 2 3.37 3.35 3.41 3.43 3.49 3 . 4 1 -k 0 . 0 4
3.35 3.31 3.39 3.43 3.49 3.39
f
0.05
Tina
of zinc ion. I t is thus very important t o specify the size of sample to be determined, especially if small increases in its size change significantly the ionic enBv. 0.04 vironment of the ion to be determined. Second wave used, after correction for contribution of diffusion current of lead present. A third assumption made in applying the absolute method of polarographic analysis is that the solutions are perfectly analysis of copper-base alloys (16). The diffusion currents were stable and show no anomalous behavior. This assumption is so estimated by two observers, and the values calculated for the often valid that it is frequently ignored as a source of error. It diffusion-current constantsare listed in Table 11, The values of is well known that some metals in dilute aqueous solutions are adare evidently independent of the amount of zinc in the solutions. In the case of lead, both observers got the same values for I by sorbed by the glass walls of the vessels in which they are contained. the two methods of measurement, within the experimental error. a consequence, too low results would be obtained when small For tin, however, both observers obtained lower values by the quantities Of these ions are determined* In a recent paper extrapolat,ion method. In addition, the average values found by smith, Taylor, and Smith ($3) show a rather striking effect that each method do not agree as well as in the case of lead. The POlarographic waves for lead are well defined and it is easy to detercan take place when dilute solutions of certain ions are allowed to mine the magnitude of the diffusion current. I n the case of tin, stand in contact with mercury and air in neutral solutions of Pothe definition is not as good and the values of I depend upon the tassium On prolonged exposure, waves Of W e r d criterion used by the observer for estimating the wave height. metals were found to decrease from their initial value and apThus, it is very important when using the absolute method to ascertain that the measurement of wave heights yield true values proach zero height. The reactions which are probably responfor diffusion currents. sible for this behavior are 0.168 0.164 0.219 0.231 1.705
27 0 40 0 0
3.18 3.14 3.12 3.11 3.23 3.16
3.37 3.23 3.40 3.12 3.48 3.32 + 0.12
3.32 3.39 3.32 3.35 3.50 3 . 3 8 * 0.06
In another paper (ZQ),the author points out that some polarographic waves-e.g., cadmium in a supporting electrolyte of 0.1 N nitric acid-cannot be interpreted by the IlkoviE equation. This is the case when two or more ions present in a given supporting electrolyte have half-wave potentials sufficiently close together so that the diffusion current cannot be properly developed. The absolute method is, of course, not applicable under this condition. In the derivation of the IlkoviE. equation it is assumed that the mercury drop is perfectly spherical and that the rate of flow is constant throughout its life. Any departure from these conditions would require somewhat different exponents f o r m and/or t . Lingane and Loveridge (19) have pointed out that a better agreement with experimental data is found when the exponent for m is made 0.64 rather than the theoretical value of a t least in one case. While the errors involved are small a t low values of m, they become increasingly important for fast-dropping electrodes.. Again, the necessity for verification of the applicability of the equation under f i e experimental conditions prevalent is emphasized. The second assumption, that the diffusion-current constant does not vary over the small changes in composition of the electrolyte of the cell due to differences in composition of the sample to be anlayzed, is usually valid. However, the size and composition of the sample taken for analysis can have significant influence upon the value of the diffusion-current constant in certain cases. An example of this is found in the determination of lead and cadmium in impure zinc (spelter). The sample of zinc is dissolved in hydrochloric acid, evaporated to dryness, and dissolved in a supporting electrolyte about 0.1 N in hydrochloric acid and 1 N in potassium chloride with 0.02y0 gelatin to suppress maxima (1). To increase the sensitivity of the method a large quantity of the sample may be dissolved in a small volume of supporting electrolyte. The diffusion-current constant calculated in the usual manner varies markedly with the size of sample, as shown in the data of Table 111.
and
3.38 3.50 3.43 3.39 3.53 3 . 4 5 * 0.06
MClz 2HC1
+
+ 2HzO M(OH)*+ 2HC1 + 2Hg HgzCl2 + HzO
'/202
where M represents the metal in question. It was found that the waves of lead, copper, zinc, nickel, and cobalt show significant decreases after several hours' standing in contact with mercury and air. In these cases it would be unwise to use the common mercury pool as anode if the cells are to be prepared any considerable time in advance of electrolysis.
Table 111. Diffusion-Current Constants for Lead and Cadmium as a Function of the Size of Sample Used concentration of Zinc i n Supporting Electrolyte Q./ml. 0
0.05 0.10 0.20
IPb
I Cd
3.79 3.40 3.09 2.50
3.46 3.02 2.77 2.35
COMPARATIVE METHOD
Those methods in which the polarograph serves merely to show the degree of correspondence of waves obtained with a known and an unknown solution are designated as comparative. By their utilization many of the difficulties inherent in the absolute method may be eliminated or minimized. The precision of such methods increases, the greater the correspondence of the cgmposition of the unknown and known and the physical condition of the analysis. The limiting factors are obviously the reproducibility of the waves and the precision with which the intercomparison of wave heights may be made. The latter is, of course, nearly independent of the method of measurement, provided similar procedures are used for both the unknown and the standard solution.
J U N E 1947 rl number of methods have been described which may be classified as comparative. Some of these are briefly reviewed below and the precautions n-hich should be observed for their most precise utilization are pointed out.
Wave Height-Concentration Plots. This method is perhaps the simplest to use. Solutions of several concentrations of the ion in question are prepared, the composition of the supporting electrolyte being the same as that in which the unknown is situated. The heights of the waves obtained are measured in any convenient manner and plotted as a function of the concentration. The unknown is run and measured exactly as the standard, and the concentration is read from the curve. Indeed the graph may be so drawn that the percentage composition of the constituent sought is read directly from the curve. A portion of one of the standard solutions should be run along with each subsequent analysis, and any deviation of this standard from the calibration should be applied as a correction factor to the determination. In this way minor differences in temperature, drop time of the electrode, and other deviations from the conditions of the calibration experiment may be readily corrected. The method is strictly empirical, arid no assumptions except correspondence n-ith the conditions of calibration are made. Although it frequently is true, the wave height need not be a linear function of the concentration. It is subject to several limitations, however. The precision may fall sharply if the control of the electrode, the temperature, and composition of the solution is not sufficiently precise. For best results the unknown should be bracketed by standard solutions run concurrently, so that the analysis becomes a simple interpolative process. As a result the preparation and analysis of calibrating solutions can become laborious. However, in analyses of the highest precision where extra labor is justified no other polarographic method gives higher precision. Pilot-Ion Method. Relative diffusion currents of ions in the same supporting electrolyte are independent of characteristics of the capillary electrode and, to a close approximation, independent of temperature. This is the basis of a method described by Forche (6) in which the analysis depends on determining the relative wave heights of the unknown ion and some standard ion added to the solution in known amount, the ratio for known amounts of the two ions having been determined by a previous calibration. Holvever, it has had limited application, primarily because only a small number of ions are available to act as pilot or reference ions. The main requirement for such an ion is that its half-wave potential should differ by a t least 0.2 volt from the unknown or any other ion in the solution with which it might interfere. Khen a single unknown is present this is not too difficult to satisfy, but in complex mixtures which are most frequently encount,ered, there is seldom sufficient “room” t o introduce additional waves. This is a true comparative method, however, and is capable of very high precision wherever applicable. Methods of Standard Addition. Two procedures may be used whereby additional amounts of the ion to be determined are added to a solution for determining the relation between the wave height and concentration. In the first, the polarogram of the unknown is recorded, after which a known volume of a standard solution of the same ion is added to the cell and a second polarogram is taken ( 5 , 1 7 ) , From the magnitudes of the two waves, the known amount of ion added, and the volume of the solution before and after the addition, the concentration of the unknown is readily calculated. The second procedure consists in adding a known quantity of the substance sought to the unknown before processing-for eximple, adding a standard solution to the beaker containing an alloy to be analyzed-and subjecting this sample to the same treatment as the unknown. The amount of the unknoivn is then determined in the manner indicated above. Both procedures give excellent results, since the standardization and analysis are performed under closely similar conditions. The precision of the first method is limited by the imprecision
371 with which the volume of solution in the cell is known. The material added should be contained in a medium of the same composition as the supporting electrolyte, so that the latter is not altered by the addition. The assumption is made that the wave height is a linear function of the concentration in the range of concentrations employed. This is usually valid, especially if t’he change produced by the addition is not too large. Best results would appear to be obtained when the wave height is about doubled by the addit,ion of the known amount of material ( I S ) . The high precision which may be attained in most cases outweighs the disadvantage resulting from the additional analyses that must be performed. When large numbers of determinations are being made the limited number of calibration experiments required, \Then prorated among the total, viould not seriously increase the time and labor involved in an individual :malysis. Use of Standard Samples. This method makes use of standard analyzed samples for cont’rol and calibration purposes. Although this is general practice for the classical procedures in many laboratories, it appears to have been little used by polarographers. Essentially, samples of known composition are run along with the material to be analyzed and a comparison of wave heights may be used to give the result of the determination. Conversely, when ot,her calibration experiments are used, a “right answer” for the standard sample lends confidence to the analysis of the unknown.. The use of standard samples for calibrations makes the determination independent of the characteristics of the electrode, the temperature of the solution, and the method of measuring wave heights. Since no additions are made, the errors in analyses depend mainly upon the certainty viith which the standard is known, the precision of the intercomparison of wave heights, and the degree of correspondence of known and unkno-m. The method, of course, presupposes that samples of known composition closely duplicating those to be analyzed are available. The Kational Bureau of Standards distributes a large variety of standard samples (32), the compositions of which are similar to many technical materials. These have been used in the laboratory of the writer both to calibrate and to serve as checks on analyses. T h e r e one of these is not suitable, samples from other sources which have been checked by careful polarographic work or by the more laborious classical methods can be used for control purposes. The use of standard samples is highly recommended for the control of routine analyses. * Semicomparative Method. The methods just described require for the highest precision that the intercomparisons be performed under the same experimental conditions and, preferably, consecutively. However, the number of calibrations may be kept to a minimum by observing the experimental conditions for each determination (temperature and capillary constants) and using the IlkoviE equation to make any small correction required to bring the observations back to the conditions of standardization. This procedure amounts essent’ially to the determination of “quasidiffusion-current constants” which may ,differ from their true values because the currents are not diffusion-limited, the wave heights are not measured to permit the determination of diffusion currents, and the constants of the current-measuring system are not accurately known. Any errors involved in the use of these quasi-constants become small when a single observer performs both the calibration and the determination rrith the same instrument and when the conditions of the analysis are not allowed to deviate too far from those of the standardization. The exact limits cannot be stated, however, since they depend upon the system being investigated.
.
SUMMARY AND CONCLUSIONS
Polarographic methods of analysis have been described in which diffusion currents are interpreted in terms of the concentration of the reducible ion by means of the IlkoviE equation.
372
V O L U M E 19, N O . 6
These are shown to be absolute measurements and any uncertainties in the calibration of the galvanometer or any errors in the measurement of the constants of the electrode are propagated into the final result. Moreover, the experimental conditions must be controlled closely to ewure conformance with the boundary conditions for which the I l k o d equation was derived. The resulting requirement that the limiting current be diffusioncontrelled (maxima absent) is difficult to realize experimentally and no simple criterion for proving its attainment is available. The above-mentioned difficulties are circumvented to a large e f e n t by using comparative methods, a number of which have been described in the literature. These can be so carried out that the determination is reduced to a comparison of wave heights for a known and unknown solution. Uncertainties in the values of the constants of the apparatus and errors cawed by nonideality of the waves cancel out to a large extent, so that the over-all accuracy is limited mainly by the reproducibility of the waves, the precision of the intercomparison of wave heights, and any uncertainty in the value of the standard solution. It is recommended, therefore, that comparative methods be used not only for routine determinations but where analytical results of the highest precision are desired. LITERATURE CITED (19 Am. Soc. Testing Materials, Polarographic Determination of Lead and Cadmium in Slab Zinc, A . S . T . M . Specification E 40-46T. (2) Borcherdt, G. T., Meloche, V. R., and Adkins, H., J . Am. Chem. SOC.,59,2171-6 (1937).
(3) Buckley, F., and Taylor, J. K., J . Research S a t l . B u r . Standards, 34,97-114 (1945). (4) Buckley, F., and Taylor, J. K., Trans. Am. Electrochem. Soc., 87,197-212 (1945). (5) Cranston, H. A , , and Thompson, J. B., ISD. ENG.CHEM.,ANAL. ED.,18,323 (1946). (6) Forche, H. E., Mikrochemie, 25, 217-24 (1938). (7) Heyrovskg, J., “Polarographie,” p. 264, Vienna, Julius Springer, 1941. (8) IlkoviE, D., Collection Czechoslov. Chem. Commun., 6 , 498-513 (1934). (9) Kemula, W., Congr. intern. p u r a q u i m . applicada, 9th Cong., M a d r i d , 1934,297-303 (1935). (10) Kolthoff, I. M., and Lingane, 3. J., Chem. Rev., 24, 1-94 (1939). (11) Kolthoff, I. M., and Lingane, J. J., “Polarography,” p. 57, New York, Interscience Publishers, 1941. (12) I b i d . , p . 111. (13) Ibid., p. 252. (14) Lingane, J. J., IND.ENG.CHEM.,ANAL.ED.,14, 655 (1942). (15) Ibid., 15,583 (1943). (16) Ibid., 18,429-32 (1946). (17) Lingane, J. J., and Kerlinger, H., Ibid., 13, 77 (1941). (18) Lingane, J. J., and Kolthoff, I. M., J . Am. Chem. SOC.,61, 82534 (1934). (19) Lingane, J. J., and Loveridge, B. A . , J . Am. Chem. SOC., 68, 396 (1946). (20) MacGillawy, D., and Rideal, E., Rec. trav. chim., 56, 1013-21 (1937). (21) Muller, 0. H., J . Chem. Education, 18, 322 (1941). (22) Natl. Bur. Standards, Circ. C-398 (1944), (23) Smith, E. R., Taylor, J. K., and Smith, R. E., J . Research iVat2. B u r . Standards, 37, 151-5 (1946). (24) Taylor, J. K., IND.ENG.CHEST., ANAL.ED.(in press). P R E ~ E N Tbefore E D t h e Division of Physical a n d Inorganic Chemistry, Symposium on Polarography, Electrolysis, a n d Overvoltage, at the 110th Meeting of the AMERICAN CHEXICAL SOCIETY.Chicago, Ill.
Polarographic Determination of Sodium or Potassium in *Various Materials J. R. WEAVER A m LOUIS LYKKEN, Shell Demlopment Company, Emerydle, Calif.
Rapid polarographic methods are described for the determination of sodium or potassium in a variety of materials consisting essentially of silica, alumina, ferric oxide, calcium salts, copper salts, and organic matter. These procedures are particularly useful for samples containing 0.001 to 3% of sodium; generally, the accuracy is =t3$70 of the amount of sodium present in the sample and the precision is better than *3qo of themean value.
T
HE polarographic technique has certain characteristics which allow it to be used with considerable advantage for the determination of sodium and potassium in a variety of materials (1, 8, 8, 9, 11). In the absence of interfering elements, it has the advantage of being direct and very rapid; with the possible exception of the spectrographic or flame-photometer methods, it is probably the fastest of all the common methods for the determination of these elements. The polarographic methed is especially adapted to the determination of small concentrations where the chemical methods often fail or give inaccurate results. Tke methods described below utilize general fundamental polarographic operations but they contain certain variations in the preparation of the sample solution; the procedure consists of obtaining a solution of the sample free of interfering (reducible) ions and determining the current-potential curve (polarogram) using a dropping mercury cathode and an aqueous tetraethylammonium hydroxide electrolyte. A previously prepared calibration curve serves to relate the height of the wave produced to the concentration of the sodium or potassium.
APPARATUS
The polarograph should give clear, reproducible polarograms even when operating a t high sensitivity. The polarographic cell assembly should operate a t a reasonably constant temperature ( 1 0 . 5 ’ C.). The apparatus used b the authors consisted of a photopen recording polarograph (7r and a unitized dropping mercury cell assembly (7) provided with temperature control and operated a t 25’ C. Pyrex vessels were used for all dilute, noncorrosive solutions. Vessels made of platinum, Vycor glass, or fused silica were used wherever there was danger of contamination by chemical attack on Pyrex. A special glass air-jet (Eigure 1) was used to purge the flasks of fumes as an aid to evaporation of sulfuric acid solutions. It is designed to avoid contamination and to minimize loss by spattering. ELECTROLYTE
All reagents were selected from C.P. stocks found to contain a minimum of alkali metals, and were stored under conditions that minimize contamination from dust’or corrosion of the container.
