The polarographic behavior of 1,1′-dimethyl- and 1,1′-dibenzyl-4,4′-bipyridinium dichloride has been investigated over the pH range of 1 to 13. Reversible reduction waves are obtained for the first single-electron step for both compounds. The second step is electrochemically irreversible but evidence is presented to indicate that the second step is fleetingly reversible at the dropping mercury electrode for methyl viologen but not benzyl viologen. Minor waves due to adsorption of the reduced viologen on the surface of the mercury have been observed.

from 20 to 40 ml. The average deviation from the theoretical weight was 0.1% for precipitates that were washed with hydrochloric acid solution.The factor 2.890 converts weight of thallous sulfate to tetraphenylarsonium chlorothallate. The weight of thallous perrhenate is converted to the equimolecular mixture of tetraphenylarsonium perrhenate and tetraphenylarsonium chlorothallate by the factor 2.998. Thallium was not separated from rhenium but was simultaneously precipitated with tetraphenylarsonium chloride. SUMMARYThallium may be quantitatively determined by precipitation from hydrochloric acid solutions containing the element in the trivalent state. The oxidation of the thallous ion may be accomplished with hydrogen peroxide or any other effective oxidizing agent which introduces no interfering substance. The precipitating reagent is a water solution of tetraphenylarsonium chloride. The amount of excess of this reagent is not critical. The precipitate obtained should be washed with hydrochloric acid solution to prevent hydrolysis, and dried in an oven at 110°C. LITERATURE CITED(1) Chrétien and Longi, Bull. soc. chim., 11, 241 (1944).(2) Feigl, "Qualitative Analysis by Spot Tests," 2nd ed., p. 93, New

The reduction of diazotized aromatic amines has been observed to occur at the dropping mercury electrode. The reduction is irreversible and the polarographic waves are too complex to treat with electrochemical equations. Nevertheless distinct waves whose heights vary directly with concentration are obtained. One wave of the reduction occurs at a potential well below that of the azo or nitro groups or nitrite ion. Therefore, it is possible to use the dropping mercury electrode with an applied potential of 0.3 volt vs. the saturated calomel electrode as an indicator for the measurement of the concentration of diazotized amines. In this manner, amperometric coupling titrations, diazotization rates, coupling rates, etc., may be studied. The new technique is more generally applicable than either the gasometric or colorimetric methods of measuring the concentration of diazotized amines. In certain cases, such as measuring the rapid formation of an insoluble azo compound, the polarographic technique is the only practical method available.

Presented before the Division of Biological Chemistry, at the 111th Meeting of the American Chemical Society tlantic City,, N. J. Approved for publication as Paper No. 268 of the Journal Series of the North Carolina Agricultural Experiment Station.

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