562 Drushel — Estimation of Potassium in Animal Fluids. 



nitric-sulphuric acid mixture. In the presence of protein 

 oxidation by bromine, or by nitric acid alone, finishing the 

 ignition in the latter case with a little concentrated sulphuric 

 acid was found more satisfactory. 



For the volumetric estimation the ignited residue was 

 treated with a few drops of acetic acid and a little water. To 

 this solution an excess of sodium cobalti-ni trite was added and 

 the mixture evaporated nearly to dryness. The residue was 

 cooled, treated with cold water, filtered on asbestos and well 

 washed with a half saturated sodium chloride solution. The 

 precipitate was oxidized by an excess of hot standard potas- 

 sium permanganate, the solution bleached by an excess of 

 standard oxalic acid, and titrated to color with permanganate. 



For the gravimetric controls the calcium, phosphoric acid 

 and iron (in the case of the blood) were removed either before 

 or after the oxidation and ignition. The residue of sulphates 

 was dissolved in a little hydrochloric acid and water, and the 

 potassium estimated as the chlorplatinate, taking care to wash 

 the precipitate with alcohol, Grladding's reagent and again 

 with alcohol. 



Conclusions. 



The necessity of allowing the cobalti-nitrite mixture to stand 

 from 12 to 20 hours to complete the precipitation, suggested 

 by Gilbert, A die and Wood, and others, is avoided by evapo- 

 rating the mixture nearly to dryness. The removal of the 

 cobalt from the precipitate before oxidation with permanga- 

 nate is unnecessary, since. the cobalt is reduced and not reoxi- 

 dized in the titration process, and since with proper dilution 

 its color does not interfere with the end point. 



For small amounts of potassium fairly accurate results are 

 obtained by using the permanganate factor calculated from 

 Adie and Wood's formula for potassium cobalti-nitrite. Sutton 

 has suggested that more accurate results are secured by obtain- 

 ing a factor empirically from a pure potassium salt. All the 

 results from this and previous papers were obtained, however, 

 by using the theoretical factor calculated from the formula of 

 Adie and Wood, their analyses of potassium sodium cobalti- 

 nitrite having been verified by the analysis of a carefully 

 prepared salt. 



The chief sources of error in the method appear to be in the 

 slight solubility of the potassium sodium cobalti-nitrite, being 

 one part in 25,000 to 30,000 parts of water at room temperature, 

 and its tendency to include traces of sodium cobalti-nitrite. 

 These sources of error tend in opposite directions, resulting 

 usually in a positive error, which by proper washing of the 

 precipitate may be kept within fair limits. 



The method requires less time and labor than the chlorplat- 

 inate method, and is applicable in the presence of substances 

 which form no insoluble cobalti-n it rites and which neither 

 oxidize oxalic acid nor reduce potassium permanganate. 



