ON THK ESTIMATION OP POTASH AND PlIOSrHOKlC ACIO. 37 



tion of sodium sulphate, which is not readily conveitible into sodio-platinic 

 chloride, but must be converted into chloride by double decomposition with 

 barium chloride — a tedious and unnecessary process, and one liable to lead to 

 error in any than very skilled hands. 



"Sulphate of potash made from kelp is an excellent example of the kind 

 of salt, as it usuaEy contains about 20 per cent, of sodium sulphate in the 

 form of the double salt 3K^ SO^ + Na, SO^. 



" It was with the view of obtaining a process for the estimation of potas- 

 sium by platiuic chloride, directly, in these compounds, that the process we 

 employ was originated, and it has stood the test of practice for 15 years. It 

 has been objected to our process that the potassio-platinic chloride is soluble 

 to an appreciable extent in platinic-chloride solution ; but our experience goes 

 to show that in the circumstances of a potassium determination, as above 

 described, the results obtained are accurate. As an instance of this, we may 

 mention that a German firm, supposing that we must necessarily get too high 

 results, handed to us for analysis, in the usual commercial way, a sample of 

 muriate of potash. We found and reported, as the result of the only trial made, 

 99-95 per cent, of chloride of potassium, and were afterwards informed that 

 the sample consisted of pure chloride of potassium, prepared and sent in order 

 to test our process. A further instance will suffice to show the exactness of 

 this mode of estimating potassium in presence of sodium compounds in quan- 

 tity. A portion of a carefully mixed and pounded sample of muriate of potash, 

 of which we had made previously a complete analysis as usual by this method, 

 was forwarded by our client to Dr. Fresenius, unknown to us, with the request 

 that he would spare no pains to arrive at the truth regarding the relative 

 proportion of potassium and sodium salts which it contained. The following 

 are the results of the respective analyses : — 



W. T. & C. Fresenius. 

 per cent. per cent. 



Chloride of potassium 88-50 88-86 



Sulphate of potash -13 



Chloride of sodium 8-46 8-39 



Sulphate of lime -18 -22 



Chloride of magnesium -50 -47 



Insoluble -23 -23 



Water 1-80 1-83 



99-80 100-00 



Potash 55-97 56-10 



Equal to chloride of potassium 88-65 88-86 " 



With reference to Mr. Tatlock's method and the above analyses, Dr. 

 Fresenius writes : — 



"I must object to his washing the precipitate with chloride of platinum. 

 He dissolves by doing so a small quantity of chloride of platinum and potassium ; 

 and you see that he makes the chloride of potassium 0-21 per cent, lower than I. 

 This discrepancy, however, will scarcely ever be greater. To make sure 

 not to keep any chloride of sodium along with the chloride of platinum 

 and potassium, I first extract the chloride of platinum and sodium with 

 spirits of wine of 80 degrees, and then wash the chloride of platinum and 

 potassium with a few cubic centimetres of water drop by drop; then I evapo- 

 rate this solution, adding a little chloride of platinum, treat the small preci- 



