The instrument is very simple and is easily installed in the laboratory. The instrument demands almost no manipulation, except for switching the galvanometer off and on between successive determinations. Higher accuracy is obtained than in polarographic determinations, in the same ranges of concentrations (10). The results obtained with the galvanometer deflection method were accurate to within ‘ a relative error of =ko.77c for the range of concentration of to 2.5 X 10-3 mole of silver per liter (Tables I and 111), and +l.5yo for 10-6 to 2.5 X 10-3 mole per liter. At the lowest concentration of 5 X 10-6 mole per liter, the error of determination was 5yC (Table 111). The addition of potassium cyanide to an ammonia-ammonium chloride supporting electrolyte caused a high increase in blank reading as a result of the cathodic depolarization of the
+
dropping electrode. This unusual hehavior will be discussed a t a later date. ACKNOWLEDGMENT
The authors are indebted to Avraham Baniel and the Board of Directors of Israel Minjng Industries for enabling them to conduct and publish this research. The helpful criticism of Leonard Shorr is also gratefully acknowledged. LITERATURE CITED
(1) Cave, G. C. B., Hume, D. N., ANAL CHEM.24, 588 (1952). (2) English, F. L., Ibid., 22, 1501 (1950). (3) Israel, Yecheskel, Analyst 83, 432 (1958). (4) Israel, Yecheskel, Bull. Research Counn2 Israel 6A, 184 (1957). (5) Kolthoff, I. M., Lingane, J. J., ,‘Polarography,” 2nd ed., Vol. 1, p. 14, 299-301, Interscience, New Yorf, 1952. (6) Kolthoff, I. >I Miller, ., C. S., J . Am. Chem. SOC.62, 2171 (1940). ( 7 ) Kolthoff, I. M., Watters, J. I., IND. ENG.CHEM.,ANAL.ED.15,8 (1943).
(8) Linhardt, F., Chem. 2isty 46, 136 (1952). (9) ,Meites, Louis, “Polarographic Techniques,” pp. 69-70, 250-95, Interscience, New York, 1955. (10) Milner! G. W. C., “The Principles and Ap hcations of Polarography and
Other dectroanalytical Processes,” pp. 9, 211, Longmans, Green, London,
1957. (11) Shcherbov, D. P., Zavodskaya Lslt. 18,899(1952). (12) Spalenka, M., Sbornik menndrod. polarog. sjezdu praze, 1st Congr., Pt. I, 600 (1951). (13) Tomicek, O., Cihalik, J., Dderal, J., Simon, V., Zyka, J., C h m . li&y 46, 710 i1952). (14) Voriskova, M., Collection Czechmloz . Chem. Communs. 1 1 , 580 (1939). (15) Willis, J. B., J. Am. Chem. SOC.66, 1067 (1944).
(16) Wilson, L. D., Smith, R. J., ,4NAL. CHEM.25, 334 (1953). (17) Wilson, R. F., Daniels, R. C., Ibid., 27,904 (1955). (18) Wise, W. S., Chemistry & Industry 1948,37. RECEIVED for review January 10. 1958. hccepted December 2, 1958.
Rapid Polarographic Determination of Low Concentrations of Mercuric Ion YECHESKEL ISRAEL’ Israel Mining Indusfries, Hoifa, Israel
b A rapid polarographic method was used for the determination of low concentrations of mercuric ion in various supporting electrolytes, with measurement of the galvanometer deflection at a constant applied voltage. Determinations were rapid and accurate, and required simple instrument manipulations. A reversible wave was obtained involving a 2-electron reduction in a 2M sodium chloride supporting electrolyte. This was checked from a plot of log [Hg++] vs. the apparent ET/,. The value of applied voltage required to attain a constant current varied significantly with concentrato mmole of mertions of curic ion per liter, thus providing a method of determination in this concentration range.
K
and Miller investigated the polarographic behavior of mercurous and mercuric ions, showing that both ions produce well defined diff usion-controlled reduction waves. OLTHOFF
Preaent address, Coates Chemical Laboratory, Louisiana State University, Baton Rouge 3, La.
When an internal mercury anode is used, the waves start from a.n applied e.m.f. of zero. The diffusion currents of completely dissociated mercury salts are reached before oxygen reduction begins (2). Although the reduction potential of mercury in complex-forming solutions is more negative, most of these complexes still precede the reduction of many other elements. This is desirable for the development of rapid methods of determination, especially in routine work. I n previous work, a rapid polarographic method was used for the determination of low concentrations of silver in the presence of interfering elements (1). The same method was adapted for the determination of low concentrations of mercuric ion in various neutral or alkaline supporting electrolytes. The reduction waves o b tained showed maxima that were suppressed by the addition of gelatin. Dissolved oxygen interfered, reducing a t the various voltages used for the measurements of galvanometer deflection. Instead of inert gas being bubbled through the solution, sodium sulfite
was added to remove the effect of dissolved oxygen and thus avoid unnecessary waste of time. EXPERIMENTAL
The same experimental technique was used as described previously (1). The following reagents were prepared: 5M sodium chloride, 2.5M potassium chloride, 5M ammonium chloride, 5M ammonium sulfate, 3M manganese sulfate, Dead Sea end brine saturated with isoamyl alcohol, 1.5M sodium sulfite, 0.5% gelatin in 0.1% hydrochloric acid, and a standard solution of mercuric chloride. The Dead Sea end brine contained the following ions, grams per liter: calcium, 47.3; magnesium, 90.4; potassium, 0.7; sodium, 17.0; chloride, 349; and bromide, 12.2. I n the polarizing unit, the total volb age from storage batteries applied on the terminals of the potentiometer, through a variable resistance, was 1 volt. A wire-wound Micropot potentiometer was used, having a 10-turn 360” scale, 100 divisions per turn. which enabled the reading of 1 mv. The constant applied voltage of measurement was about 0.20 volt more negative than the apparent half-wave potential (4) measured for a relatively VOL 31, NO. 9, SEPTEMBER 1959
1473
low concentration of mercuric ion (5 X 10-2mM). Both an H-cell saturated calomel electrode (3) and an internal mercury pool electrode were employed. OF MERCURIC ION
'
Polarographic study of the general behavior of mercuric ion was carried out in a base solution containing 2M sodium chloride, 0.01% gelatin, 0.15M sodium sulfite, and 0.2 to 0.5 mmole
Table 1.
of mercuric ion per liter (Figure 1). Polarographic determinations were made in the same base solution containing 1 ~ to - ~5 mmoles of mercuric ion Der liter (Table 1). ,, and measurements of the galvanometer deflection were made a t a constant applied voltage of LOAvolt US. S.C.E. _. To find the most appropriate voltage for measurement in this supporting electrolyte, the apparent half-wave potential (4) for various concentrations of
Calibration Data for Determination of Mercury
Base solution. 2 M NaCl, 0.01% gelatin, and 0.15M NanSO, Constant applied voltage. -0.40 volt us. S.C.E., m1/3 t 1 / 8 = 2.471 mg.%f3 set.-'/* c, %++, Deflection, id, mM Pa. Ira. idle Deviation, 5.000 $0.16 34.67 34.63 6.926 2.000 13.95 13.91 6.955 +0.58 8.710 8.668 6.934 1.250 +0.2& 1.OOO -0.32' 6.942 6.900 6.900 -0.66 0.500 3.477 3.435 6.870 -0.28 0.250 1.766 1.724 6.896 0.200 1.431 1.389 6.945 ,+0.44 0.125 +0.65 0.9120 0.8700 6.960 0.050 -0.22 0.3870 0.3450 6.900 0.010 -2.8 0.1092 0.0672 6.720 -8.0 0.005 0.0738 0.0318 6.360 Blank ... 0.0420 ... ... From average 6.915 pa. per mmole per liter.
For a very low concentration range of mercuricion, such as 1 0 - 3 t ~10-*mM, the voltage a t which a constant current is attained was plotted us. concentration (Figure 2) to find whether quantitative and/or qualitative determinations are feasible in this range. DISCUSSION
In the various supporting electrolytes reported (Table 111), mercuric ion produced reduction waves rising from the zero current and forming part of a reversible anodic-cathodic wave. Therefore, the reduction wave had no lower plateau, and the diffusion current, in each case, was equal to the height of the upper plateau from the zero current (after blank correction). The ratio of anodic to cathodic portions of the composite wave is very large,
.
4.5
0
-=
3.6 2.7
Q
L
1.8
L
3
0.9
mercuric ion was determined. A graDh
-0
vo1.t v s . s.c.e.
c,E$+,
x,a
pa.
Sb
l2 6.911 o.0204 l2 o'0684 0.0034 Arithmetic mean of diffusion current. Standard deviation, S =
'.Oo0
o'olo a
*
Table 111.
the method, replicate samples of-two different concentrations of mercuric ion (1.0 and 0.OlmM) were tested (Table 11),and the standard deviation for each concentration was calculated. Various other supporting electrolytes were studied to ascertain their suitability for polarographic determination of mercuric ion (Table 111).
WIW~Q~,
aom g h - 0 . -
Constant Apparent EUP Voltage us. S.C.E., us. S.C.E., Volt Volt
c.
0.3
ANALYTICAL CHEMISTRY
idb,c Cm2/3t1/6
Error,
-0.200
-0.40
2.80
k0.7
-0.206
-0.40
3.24
f0.6
-0.216 -0.234
-0.42 -0.43
3.16 2.92
k0.7 f0.5
-0.193
-0.40
2.55
*0.5
-0.208
-0.40
2.46
fO.8
2.95
k0.8
-0.33 -0.225 Concentration of Hg++was 6 X IO-BmM. b m. per mmole per liter per mg.2/a sec.-l'* c The range of concentrations of Hg++ was 5 X lo-* to 2 m M .
1474
Drop rate different from that used in Table I. Base solution: 2M NaCI, 0.01% gelatin, 0.1 5M N02S03. Concentration of Hg ++, mM: a. Blank d. 0.4 b. 0.2 e. 0.5
Characteristics of Polarographic Determinations of Mercuric Ion in Various Supporting Electrolytes
Base Solution 2 M NaCl,O.Ol% gelatin, 0.15M N&SOa 0.5M KCl,O.Ol% gelatin, 0.15M Na,SOJ 0.5M "&OH, 0.25M NHIC1, 0.01% gelatin, 0.15M NanSOt 0.6M NarSOa,0.01% gelatin Diluted Dead Sea end brine (1:5), 0.01% gelatin, 0.08M ?&SO3 Diluted Dead Sea end bnne saturated with isoamyl alcohol (1:5),O.Oi% gelatin, 0.08M Na&Q 0.3M MnS04, 1M NHICI, IM NsoSOI
Figure 1 . Polarographic determination of mercuric ion
320 400 v o I t a g e , m v . v s . s.c.e.
260
Figure 2. Applied voltage measurements a t constant current (0.05 pa.) for very low concentrations of mercuric ion
depending upon the concentration of the anions present in the base solutions capable of depolarizing the mercury anode. This is a unique case in which the electrode constitutes the reduced form, and hence the apparent halfwave potential, obtained from the reduction wave alone, is not identical with the true half-wave potential of the composite wave. The apparent E l / , plotted us. log [Hg++], in a 2M sodium chloride supporting electrolyte, produced a straight line with a slope of 31 mv., in excellent agreement with a %electron reduction. The apparent E l / , became more negative by decreasing the concentration, an important, factor to be considered in polarographic determinations. Thc galvanometer deflection measurements must be carried out a t a suitable constant voltage corresponding to the upper plateau. The voltage chosen was approximately 0.2 volt more negative than the apparent El/, obtained for low concentration of mercuric ion (5 x 10-2mM) in the various supporting electrolytes (Table 111). Mercuric ion was best determined with a manual polarograph (polarometer), with the galvanometer deflection measured a t the limiting plateau. This deflection was corrected for the blank to obtain the diffusion current and it served for quantitative determinations. The advantages of this method were described previously
(0. Both the accuracy and precision
were tested in a 2M sodium chloride supporting electrolyte (Tables I and 11). The results were also satisfactory when other supporting electrolytes were investigated (Table 111). Both the Dead Sea end brine alone and the Dead Sea end brine saturated with isoamyl alcohol were used to demonstrate the possibility of using the medium in which mercuric ion is being tested as the supporting electrolyte itself. A supporting electrolyte composed of manganese sulfate, ammonium chloride, and ammonium sulfate w a s also tested. This medium proved suitable for the determination of low concentrations of mercuric ion in manganese sulfate solutions. Also, the use of O.BM sodium sulfite alone as a supporting electrolyte was found suitable; it appeared that sodium sulfite not only removed dissolved oxygen, but also formed a stable comples nith mercuric ion, possibly NazHg(SO&, generally increasing the solubility of mercuric ion in certain supporting electrolytcs, such as the ammoniaammonium chloride, and causing a negative shift in the apparent halfwave potential in this medium. The lowest detectable concentration with the polarographic method was 5 X 10-3 mmole of mercuric ion per liter; a t lower concentrations the variations from blanks were insignificant, but the plot of the voltage a t which a constant current is attained-e.g., 0.05 Ma.-vs. concentration (Figure 2) yielded significant differences in the range of
concentrations of 10-3 to 10-2mM. The determination of mercuric ion in this concentration range was thus possible. The mercury pool anode wa5 also suitable for the determination of mercuric ion in the various supporting electrolytes that were used for the saturated calomel electrode. The voltage variance due to concentration changes of mercuric ion was negligible, and the apparent half-wave potential of reduction was more positive in each case. The use of a constant voltage of -0.25 volt was suitable for all the supporting electrolytes involved in this work. ACKNOWLEDGMENT
The author thanks Avrahani Baniel, Israel Mining Industries Laboratories, for permission t o conduct and publish this research, Bruno Paschkes and Karol Jusskiewicz for assistance in some of the experiments, and Jaacov hIashall for valuable criticism. LITERATURE CITED
( I ) Israel, Yecheekel, Vromen, Avrsham, ANAL.CHEM.31, 1470 (1959). (2) Kolthoff, I. M., Miller, C. S., J. A n . Chem. SOC.63, 2732 (1941). (3) Meites, Louis, “Polarographic Techniques,” pp. 69-70, Intersciencc, New York, 1955. ( 4 ) Wattera, J. I., Mason, J. G., J. A m . Chem. SOC.78, 285 (1956).
RECEIVED for review December 15, 1958. Accepted June 10, 3959.
Controlled-Pote ntia I and Derivative PoI a rogra ph M. T. KELLEY, H. C. JONES, and D. J. FISHER Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tenn.
b
Controlled-potential
polarography
has been exploited in the versatile controlled-potential and derivative polarograph. An amplifier continuously forces the potential of the polarized electrode with respect to the reference electrode to equal that of the linearly increasing control potential independently of circuit and of cell resistances. Controlled-potential polarography is especially suitable for high resistance media or for high current polarography in aqueous media where polarograms on ordinary instruments suffer distortion, because of iR drop in the electrolyte. The polarograph can be used for the analysis of irreversible and reversible species at yery low concentrations. It is possible to record the instantaneous
currents, the successive peak currents, the successive average currents, or the time derivative of the polarographic wave as a function of the potential of the polarized electrode with respect to the solution. Regular and derivative polarograms are presented that illustrate the advantage of controlledpotential over conventional polarography.
C
polarography, beset by many experimental problems, has led to more refined methods of analysis such as those of controlledpotential and desivative polarography. ONVEKTIONAL
PROBLEMS DUE TO iR LOSSES AND KINETICS
Regular Pdarography. The pres-
ence in a conventional polarographic circuit of the resistances of the solution, dropping mercury electrode capillary, measuring resistor, and salt bridge results in the potential of the polarized electrode with respect t o the solution being different from the potential applied from the polarograph. With aqueous solutions, solution, capillary, and salt bridge resisb ances totaling as high as 1000 ohms can usually be tolerated in regular polarography, because a t a cell current of 10 pa, this would represent an error of only 10 mv. in the potential of the polarized electrode. Square wave polarographs, however, require that t,he total cell, capillary, and salt bridge resistance be maintained a t about 50 ohms or less (6). The required high VOL. 31, NO. 9 , SEPTEMBER 1959
1475
