ANALYTICAL CHEMISTRY
588 rent (corrected for blank) w i t h the calibration chart gives a quantitative measure of methacrylonitrile in original sample. Tests for interfering substances showed that moderate amounts of isobutyronitrile (equimolecular) do not affect either the halfwave potential or the diffusion current. Even a 13to 1 molecular ratio of isobutyronitrile-methacrylonitrile caused only a 6% depression in diffusion current and no effect was observed on the half-wave potential. a,8-Unsaturated aldehydes do not interfere because they are reduced a t niuch more positive potentials than niethacrylonitrile ( I , 3 ) . *4crylonitrile was found to interfere, but, a very limited study indicatcd that it can also be determined polarographically in the absence of methacrylonitrile. Acrylonitrile appears to be slightly easier to reduce (about 50 mv.) and to give a slightly higher diffusion current. Time did not permit further investigation of this problem. Removal of oxy-
gen prior to analysis is unnecessary. However, quantit'at ive det,ermination of methacrylonit,rile should be conducted soon after the nitrile is dissolved. Xoticeably lower diffusion currents were obtained aft,er samples had stood for 4 hours. ACKNOWLEDGMENT
The author is grateful to the Allied Chemical and Dye Colp. for permission to publish this work. LITERATURE CITED
(1) Fields and Blount, J . A m . Chein.
SOC.,70,930 (1948). (2) Lingane and Kolthoff, Ihid.. 61, 825 (1939). (3) Moshier, IND.ENG.CHEM.,Axax.. ED., 15, 107 (1943). RECEIVEDfor review Soveinber 8. 1950. Accepted July 13, 1951. h e sented before the Division of Analytical Chemistry at t h e 118th Meeting of the AMERICANCHEYICALSOCIETY,Chicago, Ill.
Polarographic Determination of low Concentrations of Silver G. C. B. CAVE' AND DAVID N. HUME Laboratory f o r Nuclear Science and Engineering and Department of Chemistry, Massachusetts Znstitute of Technology, Cambridge 39, Mass.
K T H E course of determining the solubility of silver thiocyanate in potassium nitrate-pot'assium thiocyanate mixtures, i t was necessary t o develop methods for determining small amounts of silver in the presence of relatively large amounts of thiocyanate. By the application of unusual precautions it was possible to do the determination polarographically over a range of 5 X 10-eM to 0.01 M silver with a precision such that duplicate samples agreed nithin 0.5% except, at the very lowest concentrations. A Sargent Model XXI polarograph with a conventional dropping electrode and H-cell assembly, thermostated a t 30' f 0.1' C., was used. The numerical value of m*/3t1/Gatthe potential of the saturated calomel electrode was 1.68. Repurified nitrogen was used to remove oxygen. The supporting electrolyte was a mixture 1 M each in potassium nit,rate and potassium thiocyanate, no maximum suppressor being required. All measurements were made a t -0.400 volt PS. the saturated calomel electrode, the potential being set with the aid of an auxiliary potent,iometer. The diffusion current was recorded for several minutes a t this potential and the average of the maximum pen excursions was determined. Because silver, even in thiocyanate medium, is more noble than mercury, the half-wave potential of t.he silver reduction is more positive t.han the dissolution potential of mercury and therefore is not observable.
1 .If both in potassium nitrate and thiocyanate, i t became evident that the degree of uniformity of the individual spikes waa dependent on the applied voltage. If the applied voltage were such that the residual current was anodic or essentially zero, the individual spikes were erratic-e.g., variations of 0.02 pa. at - 0.20 volt. With small cathodic residual currents, however, the variations were much less-0.001 pa. a t - 0.4 volt. The working potential should be chosen to give as small a cathodic residual current as is feasible.
Table I.
Calibration Data for Determination of Silver
Residual Diffusion Ag Current, Current, Sensitivity. Concn., Ma Mm. h1m.b pa/Mm. 0.999 x 10 -5 3.9 14.3 0,003 3.9 41.2 0.003 2.998 x 10 -5 3.9 66.2 4.997 x 10-5 0.003 133.1 0.999 x 10-4 3.9 0.003 135.5 4.997 x 10 - 4 0.9 0.015 135.2 0.03 0.4 0.999 x 10-3 135.0 0.15 0.1 4.997 x 10 - 8 134.9 0.9993 X 10 - 2 0.0 0.3 Mean of all 8. ed/C = 4088,c = 79.0,or 1.9%. Mean of runs 3 t o 8. id/C = 4049,c = 26.2,or 0.65%. B y dilution of stock 0.9993 X 10-2 M solution. b Mean values of triplicate runs, corrected for residual current +
Run SO.
id/(?.
pa./M
No unusual difficulties were observed in the determination of concentrations greater than M , but a t extremely low concentrations, with correspondingly high inst>rumentSensitivity settings, a number of ordinarily minor instrumental variables became significant. The immediate problem was to extend the lower limit of concentration to 5 X 10-8 31 silver while maintaining t,he error a t not over & 5 % . The solution was achieved through ~tcareful study of the factors that affect the constancy in the height of the individual "spikes" (pen excursions for the individual drops) at constant applied voltage. I t vias found that grounding the water bath, the metal framework holding accessories in the thermostat, and finally the dropping electrode itself eliminated several sources of fluctuat,ions in t,he diffusion current. Further undulations of about f 0.006 pa. in the diffusion current tracings were shown to he due to high atmospheric humidity and were satisfactorily eliminated by working in an air-conditioned room or keeping the recorder interior dry with a large charge of desiccant. It was also found advisable to use no damping on the polargraph; otherwise a t high recorder sensitivities (0.003 pa. per nini.) the recorded diffusion currents became completely steady only after 5 or 6 minutes. During measurements of the residual currents for solutions
. I short study was made of the effect of drop time on the rcproducibility of the diffusion current. The reproducibility appeared to be independent of drop time between 4 and 8 seconds a t low concentrations of silver, but in 10-2 Jf solutions the diffusion current was erratic unless the drop time was greater than 5 qeconds. The concentration of 5 X 10-6 fir appears to be near the practical lower limit of the method using conventional equipment. This correqonds to a diffusion current of 0.020 pa. after correction for a residual current of 0.012 pa. For adequate reproducibility at this level, the composition of the supporting rlectrolyte had to be constant within + l o % and the potential of the dropping electrode. JTithin fl mv. The sensitivity and reproducibility of the method a l e indirated by the calibration data in Table I I t is seen that above 3 x 1 0 6 AI, the deviations from constancy of i d / C are very small, and even a t the lowest calibration concentration the error is only of the order of 5%. By interpolation in the empirical calibration curve it was found possible to determine the concentration of silver in solutions 5 X 10-6 M with an accuracy of +5%.
1 Present address, Provincial Department of Mines, Victoria, B. C . , Canada.
RECEIVED for review August 16,1951. Accepted November 5, 1951. Work supported in part by Atomic Energy Commission.
