W. A. Drushel—Cobalti-Nitrite Method. 329 



Art. XXXYI. — The Application of the Cobalti- Nitrite 

 Method to the Estimation of Potassium in Soils ; by W. 

 A. Deushel. 



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



In a previous paper* from this laboratory it was shown that 

 potassium may be estimated with a fair degree of accuracy by 

 precipitating it as potassium sodium cobalti-nitrite in a solution 

 acidified with acetic acid and oxidizing the precipitate with 

 standard potassium permanganate. In the same paper the 

 applicability of the method to the estimation of potassium in 

 commercial fertilizers was shown by a series of experiments. 



In the method as previously worked out an excess of con- 

 centrated sodium cobalti-nitrite solution acidified with acetic 

 acid is added to a neutral solution of a potassium salt, and the 

 mixture is evaporated to a pasty condition on the steam bath. 

 After cooling, the residue is stirred up with sufficient cold 

 water to dissolve the excess of sodium cobalti-nitrite. The 

 precipitate, consisting of K 2 NaCo(N0 2 ) 6 .H 2 0, is filtered on a 

 rather close asbestos felt in a perforated crucible and well 

 washed with cold water, or preferably with a half saturated 

 sodium chloride solution. The precipitate and felt are trans- 

 ferred to an excess of standard N/10 or N/5 potassium perman- 

 ganate which has been diluted to about ten times its volume and 

 heated nearly to boiling. If particles of the precipitate stick 

 persistently to the walls of the crucible and cannot be removed 

 with a spray of water, the crucible is put into the permangan- 

 ate solution. After stirring for a few minutes the solution is 

 gradually acidified with 5 c,a3 to 20 cm3 of dilute sulphuric acid, 

 and the oxidation is allowed to go to completion, a process 

 which seldom requires more than five minutes. If no parti- 

 cles of the yellow precipitate settle out on standing a minute, 

 the oxidation may be considered complete. The hot solution 

 is then bleached by running in a measured amount of standard 

 oxalic acid, containing 50 cm3 of concentrated sulphuric acid per 

 liter. The solution after bleaching is titrated to color with 

 standard permanganate in the usual manner. 



In this process the cobalt in the molecule is reduced from 

 the trivalent to the bivalent condition and not reoxidized, 

 consequently from the molecule of the potassium sodium 

 cobalti-nitrite we find 0*000857 grm. K 2 equivalent to l cm3 of 

 strictly N/10 potassium permanganate. This factor of course 

 must be corrected for any variation in the normality of the 

 permanganate solution used. 



-This Journal, xxiv, 433, 1907. 



