W. A. Drushel — Potassium as the Cobalti-nitrite. 433 



Art. XL. — On the Volumetric Estimation of Potassium as 

 the Cobalti-nitrite j by W. A. Drushel. 



[Contributions from the Kent Chemical Laboratory of Yale Univ. — clxvi.] 



The use of sodium cobalti-nitrite for the qualitative detec- 

 tion of potassium is well known and its use as a quantitative 

 reagent has been described by R. H. Adie and T. JB. Wood,* 

 whose results are fairly accurate and favorably comparable 

 with results obtained by the platinic chloride gravimetric 

 method. In the process worked out by these investigators a 

 solution of a potassium salt containing the equivalent of 0*5 

 per cent to 1 per cent of K 2 Q is acidified with acetic acid and 

 precipitated by an excess of sodium cobalti-nitrite. The mix- 

 ture is allowed to stand at least a few hours, preferably over 

 night, and is then filtered through a perforated crucible fitted 

 with an asbestos felt. The precipitate is washed with 10 per 

 cent acetic acid. According to Sutton it is important that the 

 precipitation should be made in a solution containing the equiv- 

 alent of 0*5 per cent to 1 per cent of Iv 2 0, since in solutions 

 of lower concentration the precipitate comes down in a condi- 

 tion in which it is apt to run through the filter in washing. 

 The precipitate is then decomposed by boiling in dilute sodium 

 hydroxide, and the cobalt is removed as the hydroxide by fil- 

 tration. The nitrites, which are a measure of the potassium 

 in the precipitate, are estimated by titrating with standard 

 potassium permanganate. Adie and Wood found by analysis 

 that the composition of the precipitated potassium salt is rep- 

 resented by the formula K 2 NaCo(JN0 2 ) 6 .H 2 0. According to 

 their method a cubic centimeter of strictly 5- potassium per- 

 manganate is equivalent to 0*000785 grm. K 2 0. 



The object of this investigation was to determine the best 

 conditions for precipitating and filtering the potassium cobalti- 

 nitrite, and to shorten the work of estimating the potassium by 

 oxidizing directly with potassium permanganate without the 

 preliminary decomposition of the precipitate and removal of 

 cobalt recommended by Adie and Wood. In a series of pre- 

 liminary experiments the precipitated cobalti-nitrite was oxi- 

 dized by an excess of potassium permanganate, the excess of 

 permanganate reduced by standard oxalic acid, and the 

 remaining oxalic acid titrated to color. In this treatment triva- 

 lent cobalt is reduced to the bivalent condition, and from the 

 formula of potassium sodium cobalti-nitrite it would appear 

 that the oxygen thus made available should be equivalent to 

 one-twelfth of that necessary to oxidize the nitrites. The 

 results of these experiments are given in Table I. 



*Jour. Chem. Soc, lxxvii, 1076. Sutton's Vol. Anal., 9th ed., p. 62. 



