A linear change in Eli?for the reduction of the azomethine group was obtained for seb-era1 compounds and the data shown in Table 11. As is apparent from Table I, the reduction in nonaqueous media for the >C=N-
group
resulted
in
an
additional wave which indicated a more complex reaction. The electrode processes of this class of compounds are currently under investigation utilizing controlled potential electrolysis. A comparison of solvents for the polarography of the benzodiazepines is presented in Figure 3. The most satisfactory definition of the waves was
obtained using hydrochloric acidmethanol as the solvent and linearity of diffusion current with concentration was obtained. ACKNOWLEDGMENT
The authors thank Leo Sternbach and the Roche Chemical Research Department for the compounds used in this investigation and Miss S. Andriola for assistance in preparing the drawings. LITERATURE CITED
( 1 ) Breyer, B., Buchanan, C. S., Duewell, H., J . Chem. SOC.1944, 360. (2) Brezina, M., Zuman, P., “Polar-
ography,” p. 323 ff., Interscience, New York, 1958. (3) Brinkmann Instruments, Westbury, N. Y., distributor. (4) Kolthoff, I. AI., Elving, P. J., “Treatise on Analytical Chemistry,” Part I, Vol. 4, Chap. 46 by L. Meites, p. 2303 ff.. Interscience. New York. 1963. (5) Kolthoff, I., Lingane, J. J., “Polarography,” p. 809 ff., Interscience, New York, 1952. (6) Lingane, J. J., Swain, C. G., Fields,
AI., J. Am. Chem. SOC.65, 1348 (1943). (7) Lund, H., Acta Chem. Scand. 13, 24967 (1959). (8) Oelschlager, H., Arch. Pharm. 296, 396 (1963). (9) Smith, 11. L., Elving, P. J., J . Am. Chem. SOC.84, 1412 (1962). (10) Ibad., p. 2741. RECEIVED for review February 11, 1964. Accepted June 22, 1964.
Precise Method for Assay of Potassium Iodate by Comparison with (Primary Standard) Arsenic Trioxide SAMUEL M. TUTHILL, ROBERT S. SPRAGUE,’ and WILLIAM C. STOECKER2 5439, Sf. Louis, Mo. 63760
Mallinckrodf Chemical Works, P . 0. Box
b A method is described for the precise assay of potassium iodate by direct comparison with a National Bureau of Standards certified primary standard arsenic trioxide. A slight excess of potassium iodate is added to the arsenious acid in 7.5M hydrochloric acid. The excess potassium iodate is then titrated potentiometrically with 0.02N sodium arsenite solution. The precision of an individual result is &0,04% relative qt the 9570 confidence level, resulting in a precision of =tO.O2% relative for the mean of three results. The method is accurate to 0.03% relative. Assay results confirm earlier findings and assumptions that potassium iodate is a satisfactory primary standard.
P
OTASSIUM IODATE has long been an accepted primary standard in oxidation-reduction work. Kolthoff and Stenger ( 7 ) recommend it highly for standardizing thiosulfate solutions in iodometry, and for preparing standard iodate solutions determinately for use in the Andrew-Jamieson procedures. They also recommend it for standardizing hydrochloric acid solutions. Stenger ( 1 1 ) , in discussing the requirements for primary redox standards, states that 1 Present address, Department of Chemistry, Lehigh University, Bethlehem, Pa. 2 Present address, Malllnckrodt Chemical Works, lJranium Division, P. 0 BOX 4 i 2 , St Charles, Mo.
1994
ANALYTICAL CHEMISTRY
potassium iodate is an excellent primary standard which can be used in a variety of ways. The only important disadvantage noted was its low equivalent weight, but this objection can be partially overcome by preparing solutions determinately, and then aliquoting. He also was concerned about the possible undetected presence of rubidium, Others who have recommended potassium iodate as a primary standard are Kolthoff (4) and Nutten (9). The most definitive work establishing potassium iodate as a primary standard is that of Kolthoff (3-5). This work involved the following indirect comparisons of potassium iodate with other primary standard substances: potassium iodate, potassium dichromate, oxalic acid dihydrate, iodine, and potassium bromate for standardizing a 0 . W sodium thiosulfate solution; potassium iodate, arsenic trioxide, and sodium oxalate for standardizing a 0.1N potassium permanganate solution; and potassium iodate, sodium carbonate, and sodium tetraborate decahydrate for standardizing a 0.1N hydrochloric acid solution. I n making these comparisons, a volumetric technique was used which involved 100-ml. titration volumes measured to ~ 0 . 0 2ml. Potassium iodate gives normalities comparable to those obtained from the other primary standards within 0.03yGrelative, except in the case of the dichromate and iodate
standardizations of the sodium thiosulfate solution. Here the difference of 0.06% relative may be a result of the slow reactions encountered when dichromate is used to oxidize the iodide ion (6).
Milstead ( 8 ) , however, reported results a t variance with these findings. He determined the normality of a 0.l.Y thiosulfate solution by titration with a determinately prepared iodine solution, which was also checked against National Bureau of Standards primary standard arsenic trioxide. When this thiosulfate solution was checked against reagent grade potassium iodate, the normality was 0.2y0relative less than that obtained by standardization against iodine. Recrystallization of the iodate did not improve the situation, but rather worsened it slightly. It was believed, without definite experimental proof, that sodium iodate impurity in the potassium iodate was responsible for the difficulty. I n view of the fact that no reports have been found in the literature in which potassium iodate was compared by direct reactions with a recognized primary standard (in this case, arsenic trioxide), it was felt that it would be worthwhile to establish conditions for carrying out such a compariqon with high precision (ca. &0.027G relative). Such a method would permit an assay (or purity factor) to be calculated for potassium iodate, and would eliminate the question of the purity of the potas-
sium iodate in any future work with this substance. Impurities in Reagent Grade Potassium Iodate. T h e sample of Mallinckrodt potassium iodate analytical reagent (sample CVR) used in this investigation did not receive any special preparative or purification treatment. By application of thLe usual impurity tests applied to reagent grade potassium iodate (IO), the following impurities were found to be as indicated below. Insoluble matter Sitrogen compounds ( a s X ) Iodide ( I ) Sulfate (Sod) Heavy metals (as Pb) Iron (Fe)
