Rapid Polarographic Determination of Low Concentrations of Silver in the Presence of Interfering Elements YECHESKEL ISRAEL' and AVRAHAM VROMEN Israel Mining lndosfries, Haifa, Israel
A procedure for determining low concentrations of silver in the presence of copper, as well as other interfering elements, uses a supporting electrolyte composed of ammonium hydroxide, ammonium chloride, and potassium cyanide. A simple manual polarograph (polarometer) was employed for measuring the galvanometer deflection, with constant voltage applied at the limiting current plateau of the reduction wave of silver. The results were more accurate than those obtainable by conventional polarographic procedure and determinations required simple and rapid instrument manipulation. Thallium interfered, developing a diffusion-controlled wave. Anomalous behavior of potassium cyanide was observed in copper-free ammoniacal base solution with cathodic depolarization of the dropping electrode.
S
investigators have used complex-forming supporting electrolytes for polarographic determination of silver ion ( I , 9, 12, 13). However, few methods are satisfactory for determination of low concentrations of silver, especially in the presence of interfering elements. Simplified polarographic methods have been used to measure the diffusion current a t constant applied voltages for the quantitative determination of materials in routine work ( I , 2, 4, 7 , 9 , 11,16, 18). Recording and manual polarographs were employed and some of the methods (1, I, 4) yielded better accuracy than conventional procedures (6). In the course of this investigation, a rapid polarographic procedure for the determination of low concentrations of silver in the presence of various interfering ions was developed. The supporting electrolyte used was composed of 0.5M ammonium hydroxide, 0.25M ammonium chloride, suitable concentrations of potassium cyanide, and 0.005% EVERAL
* Present address, Coates Chemical Laboratory, Louisiana State University, Baton Rouge 3, La. 1470
ANALYTICAL CHEMISTRY
gelatin. One per cent sodium sulfite was added to remove the effect of dissolved oxygen instead of using inert gas. The behavior of the supporting electrolyte was also studied. EXPERIMENTAL
Materials. Analytical grade reagents and double-distilled water were used. Ordinary grade gelatin was found suitable. A purified, triple-distilled, and degreased mercury was used for the dropping electrode. When the mercury pool electrode cell was applied, purified, distilled, and degreased mercury (3)was used. The following reagents were prepared: 5M ammonium chloride, 0.5% gelatin in 0.05% hydrochloric acid [to prevent bacterial contamination ( S ) ] , 10% sodium sulfite, 1M potassium cyanide, a standard solution of silver nitrate, and a standard solution of cupric nitrate. Grade A volumetric flasks and pipets were used. Apparatus. POLAROMETER. A polarometer (manual polarograph) was constructed, similar to that of Kolthoff and Lingane [Figure XVI-2 (6)], which used a Hartmann-Braun galvanometer containing five sensitivity stages. Each scale division equaled 0.00397 #a. a t the highest sensitivity. As the zero of the galvanometer a t the highest sensitivity was somewhat unstable, a zero check before each determination was necessary. The potentiometer was linearly wirewound, having a 270" scale with 7.5 mv. per scale division. A 100-fif. electrolytic condenser was connected in parallel to the galvanometer for damping. The instrument was grounded in a suitable manner to avoid the effect of electrostatic fields of the thermostatic bath and surroundings. I n this instance, the grounding of the dropping electrode contact was suitable. The average of maximum and minimum readings of the galvanometer deflections was taken a t a constant applied potential, corresponding to the plateau of the limiting current. The potential chosen was about 0.15 volt more negative than the half-wave potential. POLAROGRAPH. For general investigations, a Tinsley pen-recording polarograph, Type MK/14, as used. The quick drive of the automatic potentiom-
eter (2 inches per 1.0 volt) was applied and the polarogram was recorded as required with full damping and no countercurrent. The zero current line was plotted with the electrodes disconnected, and its intersection with the rising anodic-cathodic wave was used for interpreting the cathodic wave of silver. CELLS. Investigations were carried out with both an agar-bridge saturated calomel electrode and an internal mercury pool electrode The cell was thermostated to 25" rt 0.2" C. The temperature of the sample was measured in every determination and corrected (2% per 1" C.) to 25" C. The dropping capillary values, m and t, were determined a t the same constant voltages as those applied on the cell for galvanometer deflection measurements. DEVELOPMENT OF PROCEDURE
Determination of Silver. IN AMMONIA-AMMONIUM CHLORIDE SUPPORTING ELECTROLYTE. A base solution containing 0.5M ammonium hydroxide, 0.25M ammonium chloride, O.OO.!ic/, gelatin, and 1% sodium sulfite was used for polarographic determinations of standard silver solutions (Table I). These measurements were made a t a constant voltage of -0.35 volt us. S.C.E. (half-wave potential = -0.18 volt, Table 11). Studies were also conducted on the interference of metallic ions in this medium. I N AMMONIA-AhfMOXIUM CHLORIDE AND POT.4SSIUM CYANIDE SUPPORTINQ ELECTROLYTE. To eliminate the interference of copper(I1) in the ammoniaammonium chloride supporting electrolyte, varying amounts of potassium cyanide (0.01, 0.02, and 0.05M) were added. Polarographic determinations of standard silver solutions were carried out in the presence of 100 mg. of copper per liter (Tables I1 and 111). Mensurements were made a t suitable constant voltages which depended on the concentration of potassium cyanide (Tsble 11). When the determination of standard silver alone, in the absence of copper, was attempted in this medium, a marked, unstable increase, about twen-
-~
~~
~
tyfold, in the blank deflection was observed. Therefore, low poncentrations of silver ion could not be determined, and even the accuracy in determining high concentrations was inadequate. This current increase due to the presence of potassium cyanide could be eliminated, however, by the addition of a t least 50 mg. of copper per liter (Table IV). Silver was determined in potassium cyanide in the presence of 100 mg. of copper per liter and thus the determination of lower concentrations of silver was made possible (Table 111). To evaluate the precision of the method, replicate samples of three different concentrations of silver ion (1.0, 0.01, and 0.005mM) were tested (Table V). The standard deviation for each concentration was calculated using the formula, S =
&&
where S equals the standard deviation, N equals the number of determinations, and d equals X i - x , X , being
~
~~~
~
~~
~~
~
Table 1.
Calibration Data for Determination of Silver in the Absence of Copper Base solution. 0.5M ”,OH, 0.25M “&I, 0.005% gelatin, and 1% NaaOa Constant applied voltage. -0.35 volt us. S.C.E., m2/at1’6= 2.032 mg.*Iaset.+*
c,*
Deflection, pa.
id,
pa.
2.000 1.500 1.000 0.500 0.400 0.200
a
8.151 8.068 6.190 6.107 4.114 4.031 2.104 2.021 1.717 1.634 0.889 0.806 Blank 0.083 ... From average, 4.0575pa. per mmole per liter.
Table II.
id/C
Deviation,e yo
4.034 4.071 4.031 4.042 4.085 4.030
-0.58 f0.33 -0.65 -0.38 +0.68 -0.68
...
~
..
Characteristics of Polarographic Determinations of Silver in the Absence and Presence of Potassium Cyanide
Base Solution5
Ei/zlb
Constant Voltage, Volt us. S.C.E.
ml/J t u 8
idc
Cm2/Jt1/6 Error,d % 1 -0.18 -0.35 2.032 1.996 f0.7 2 -0.44 -0.60 2.017 2.189 zkO.5 3 -0.48 -0.65 1.978 2.258 4 -0.55 -0.70 2.055 2.077 i0:3 a 1. 0.5M NHIOH, 0.25M NH4C1,0.005% gelatin, and 1% NsnSO,. 2. 1 plus 100 mg. of Cu++ per liter and 0.01M KCN (total concentration of KCN). Excess is a proximately 0.004M, within limit of concentkations described in procedure. 3. 1 pyus 100 mg. of Cu++per liter and 0.02M KCN (total concentration of KCN). 4. 1 plus 100 mg. of Cu++per liter and 0.05M KCN (total concentration of KCN). * Containing also 100 mg. of silver per liter. a pa. per mmoleper liter per mg.2!3sec. -112. For range of silver concentrations of 2 X 10-4 to 2 X 10-3M.
the individual value of diffusion current and the arithmetic mean of replicates. Interfering Ions. IN AMMONIA- Table 111.
Volt
Mg.9” Seg,-l/P
Calibration Data for Polarographic Determination of Silver in the
AMMONIUMCHLORIDESUPPORTING Presence of Copper ELECTROLYTE. Besides copper, goldBase solution. 0.5M”@H, 0.25M NH4C1, 0.005% gelatin, 1% NaaOa, 0.01M KCN,. (I11 and I) and chromium(V1) also and 100mg. of Cu++perliter
interfered in this medium, producing Constant applied voltage. -0.60 volt us. S.C.E., m W / 6 = 2.017 mg.5/asec.-1/8 reduction waves very close t o that of Deflection, id, silver ion (Table VI). Platinum(1V) Ira. pa. id/C Deviation,b Yo interfered in a similar manner only 2.500 11.14 11.03 -0.11 4.412 when an inert gas was bubbled through 2.000 8.915 -0.34 4.402 8.804 the solution instead of sodium sulfite 1.250 5.608 -0.43 5.497 4.398 4.436 1 .Ooo 4.547 4.436 $0.43 being added. The presence of sodium 2.310 0.500 2.199 -0.43 4.398 sulfite even in ammoniacal solutions re0.250 1 213 1.102 -0.20 4.408 duced platinum(IV), eliminating its in0.100 0.556 0.445 4.450 +0.75 terference. The anodic waves of iron0.050 0.222 0.333 4.440 $0.52 0.025 0.112 0.223 +1.40 4.480 (11) and vanadium(IV), which were 0.155 0.010 4.400 0.044 -0.38 developed at more negative potentials 0.132 0.005 -4.90 0.021 4.200 than the cathodic wave of silver (Table 0.111 Blank ... ... ... VI), also interfered in polarographic dea Total concentration of KCN. terminations, being typical of the mised b Average reading 4.417 pa. per mole per liter, potentials effect (6). Palladium(I1) (16, 1 7 ) , rhodium(II1) (I@, cobalt(II1 and 11), and thallium(1) did not interfere, Table IV. Influence of Varying Conhaving more negative reduction potenTable V. Precision of Polarographic centrations of Copper tials (Table VI). Method IN AMMONIA-AMMONIUM CHLORIDE Base solution. 0.5M “,OH, 0.25M AND POTASSIUM CYANIDE SUPPORTIKG NH,Cl, 0.005% gelatin, 1% NazS03,and Base solution, 0.5M NHIOH, 0.25M 0.005M excess KCN ELECTROLYTE. A study of the effects NHdCl, 0.005% gelatin, 1% NaZSOa, exConcentration of Ag+. 2.000 mM cess‘0.005M KCN, and 100 ma. - of Cu++ of various metallic ions in a supporting Constant applied voltage. -0.60 volt per liter electrolyte containing 0.02M potassium us. S.C.E. Constant applied voltage. -0.6 volt cyanide revealed a number of interfering us. S.C.E., m2/3t1/6= 2.045 mg.2/a sec. Concn. ions. Gold(III), vanadium(V), and of c u + +, id, Mg./L. pa. CdIC5 C, chromium(V1) produced reduction 2,. Ag +I waves very close to that of silver ion. ... 11.18 5.590 mM N pa. S 25 9.740 4.870 Platinum(1V) behaved as in the absence 50 9.092 4.546 of potassium cyanide and interfered 0.0133 1.000 10 4.505 100 9.020 4.510 0.00105 0.010 10 0.0450 in the absence of sodium sulfite. These 500 9.020 4.510 0.00134 0.005 10 0.0214 ions interfered only when present in 1000 8.990 4.495 2000 8.990 4.495 their higher valencies and were easily a Arithmetic mean of diffusion current. reduced by addition of sodium sulfite a pa. per mmole of Ag+ per liter. to silver solutions acidified with sulfuric VOL. 31, NO. 9, SEPTEMBER 1959
1471
acid cooled to a temperature of 15' t o 20" C. At higher temperatures metallic silver w a s formed, especially after the addition of ammonium hydroxide. The redissolution of metallic silver, caused by the subsequent addition of potassium cyanide, was not complete. The presence of manganese(I1) caused an increase in the current deflection that was not diffusion-controlled when the solution contained 0.02 to 0.2M potassium cyanide. This interference was eliminated by addition of an excess
Tuble VI.
Half-Wave Potentials of Various ions" Eli2
Xon
us.
S.C.E., Volt
b
f
-0.18 -0.46 T1+ -0.50 -0.50 cu++ -0.22 NRd C,s + -0.20 -0.42 Pt4+ -0.18 -0.46 .ill+++ -0.18 -0.45 AU + -0.18 co+++ -0.53 N R d Co++ -1.29 -1.42e Mn++ -1.66 -1.6W Fe++ (-0.34)s NRd VS+ -0.96 -0.47 V4 (-0.32)s ... Pd++ -0.73 KRd Moa+ NRd NRd NRd Sn++ NRd Cd++ -0.81 -0.83 Xi++ -1.10 -1.44 Zn++ -1.35 -1.42h a Only the first oxidation or reduction wave reported. In supporting electrolyte of 1M NH40H and 1M NHE1 (9, . , 10,, 13, 14. .. present work). In base solution of 0.5M ",OH, 0.25M NH4C1, 0.005% gelatin, 1% Na2S03,excess 0.005M KCN, and 100 mg. of Cuff per liter; also, 200 mg. - of each ion per liter. Not reducible. 0 See text. Within solubilitv limit. Parentheses indicate anodic wave. Only ammoniacal complex in excess is reducible. Ag +
+
of 0.002 to O.OO5M potassium cyanide after the disappearance of the blue color of cupric ammonia complex. In this case, the presence of 500 mg. of manganese(I1) per liter did not interfere (Table VII). Cobalt(I1) also interfered, causing a n increase in the current deflection that was not diffusion-controlled. When excess 0.002 to 0.005M potassium cyanide was used, the presence of 10 mg. of cobalt per liter did not interfere (Tables VI1 and VIII). However, the blank deflection was slightly higher. Thallium(1) interfered with the determination of silver ion, producing a diffusion-controlled reduction wave (Table VI). This method could be used for the quantitative determination of thallium in the absence of silver (Table IX) . The presence of considerable amounts of cadmium, nickel, zinc, molybdate, iron, lead. indium, arsenic, tin, and palladium did not interfere with the determination of silver when an excess of 0.005714 potassium cyanide was used (Tables VI, VII, and VIII). The half-wave potentials obtained for these ions in this medium (200 mg. per liter each) are reported (Table VI). A procedure was developed for the determination of low concentrations of silver in the presence of the above elements in which only thallium(1)
I
~
Table VII. Polarographic Determination of Silver in the Presence of Various Ions
Table VIII. Polarographic Determination of Low Concentration of Silver in the Presence of Various Ions
Base solution. Same as in Table VII. Concentration of Ag+. 0.01OmM. Ion rldded
Mg./L. ... ~ t 4 +Vb+,'Cr6+, , 100 Au+++,Pd++ each Cd++, Xi++, Zn++, 100 Fe+++,Pb++ each Au+++,Mn++ loo each\ 10 co++ As+++,Sn++, In+++, 100 ?\lob each
id,
pa. 0.0445 0.0440
0,0450 0.0450 0.0435
+
Base solution. 0.5M NH40H, 0.2544 NHIC1, 0.005% gelatin, 1% N & & , excess 0.005M KCN, and 100 mg. of
Cu++ per liter Concentration of Ag+. 1.OOOmM Ion Added
Mn++ co++ Cd++ Zn++
V6+ Cr6+ ptA;+++J Pd+)+ Pt4+ Au$++ 4+
c&+,Mn+S-, Cn++ --
Cd++ Ni++ Zn++ Fe$++Pd++ +++ &++ AsIn+;+, M$+ 1472
Mg./L. ... 500 10 1500 500 2oo each ] : :e
_i n_
ZdJ
pa. 4.510 4.520 4.525 4.510 4.505 4.495 4.525 4.500
2oo
4.510
each
ANALYTICAL CHEMISTRY
Base solution. 0.5M NHrOH, 0.25M NH4C1, 0.005% gelatin, 1% Na808, excess 0.005M KCN, and 100mg. - of Cu ++ per liter Const,ant applied voltage. -0.65 volt US. S.C.E., m2/3t1/6 = 2.050mg.e/3sec.-1'* , l
v,
T1+
Deflect'ion, id, id/Ca pa. pa. 5.384 5.264 5.264 1.0 2.746 2.626 5.252 0.5, 1.168 1.048 5.240 0.2 Blank 0.120 ... ... a pa. per mmole of T1 per liter.
&
2oo'
each
Table IX. Polarographic Determination of Thallous ion
and higher concentrations of cobalt (I11 and 11) interfered. PROCEDURE
To a 50-ml. volumetric flask containing the silver solution acidified with 1 ml. of normal sulfuric acid, enough standard copper(I1) was added to make its final concentration at least 100 mg. per liter. Sodium sulfite stock solution (0.5 ml.) was added to the sample, cooled to a temperature of 15" to 20" C., to reduce gold(II1) , platinum(IV), vanadium(V), and chromium(V1). Concentrated ammonium hydroxide was added to make the final concentration approxi-mately 0.5M. The followine stock solutions were then added: 2.5-ml. of ammonium chloride, 0.5 ml. of gelatin, and 5.0 ml. of sodium sulfite. The ammonia complex of copper thus formed was titrated, with a diluted stock solution of potassium cyanide (1 to 10), t o the disappearance of the blue color, and an excess of 1.0 to 2.5 nil. was added. The solution was diluted to volume and an aliquot was transferred to the electrolytic cell. The galvanometer deflection wm recorded at -0.6 volt vs. the saturated calomel electrode or -0.2 volt vs. the mercury pool electrode. CONCLUSIONS
Complexes of silver in both ammonia-ammonium chloride and ammonia-ammonium chloride-potassium cyanide supporting electrolytes produced reduction waves beginning at the zero current line. The reduction wave of silver in the presence of potassium cyanide, developed a t the half-wave potential us. the S.C.E., was more negative than in the ammonia-ammonium chloride medium alone, and depended on the concentration of cyanide ion (Table 11). With the mercury pool reference electrode, the cyanide ion did not cause any considerable shift in the half-wave potential of the silver wave and the galvanometer deflections were measured at -0.2 volt for all cyanide concentrations. The potential of the mercury pool electrode was affected by the electrolysis medium. I n the presence of cyanide ion its potential became more negative, compensating for the negative shift of the silver reduction wave. Otherwise, this reference electrode behaved similarly to the saturated calomel electrode. Silver was best determined using a manual polarograph (polarometer) , measuring the galvanometer deflection at the limiting current plateau. This deflection was corrected for the blank to obtain the diffusion current and served for quantitative determinations. This method has the following advantages : Only one reading of the galvanometer is required.
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
