57O AGRICULTURAL ANALYSIS 



for the filter ash is unnecessary because the ash is completely 

 dissolved by the treatment received. The platinum sponge 

 which is collected in the crucible in this way is removed in case 

 it does not adhere to the sides and the crucible is then ready for 

 the next operation. 



481. Sources of Error in the Platinum Method. In the com- 

 parative work done in the determination of potash by the mem- 

 bers of the Association of Official Agricultural Chemists there 

 has been noted, from year to year, marked differences in the data 

 obtained by different analysts. Such differences often are due 

 to personal errors, or a failure to accurately follow the directions 

 for manipulation. Sometimes, however, they are due to sources 

 of error in the processes employed. In the platinum method these 

 sources of error have been long known to exist. Chief among 

 these is the remarkable facility with which potash becomes in- 

 corporated with the precipitates of other bodies. The character 

 and magnitude of some of these errors have been studied by 

 Robinson. 50 



Many precipitates occlude potash and hold it so firmly that 

 it cannot be washed out with hot water although the potash 

 compounds present in the precipitate are perfectly soluble. It 

 appears to be a kind of molecular adhesion. Barium sulfate 

 has this property of attaching potash molecules in a high degree, 

 and ferric and aluminic compounds only to a slightly less extent. 

 To reduce the losses, consequent on the conditions just men- 

 tioned, to a minimum, the sulfuric acid and earthy bases should 

 be very slowly precipitated, with violent agitation, at a boiling 

 temperature. 



Another source of loss in the platinum method arises from the 

 use of a solution of ammonium chlorid for washing the potassium 

 platinochlorid precipitate. There is danger here, not only of 

 the solution of the impurities present in the precipitate, but also 

 of a double decomposition by means of which some ammonium 

 40 Journal of the American Chemical Society, 1894, 16 : 364. 



