CRITICAL EXPERIMENTS FOR DETERMINATION OF POTASSIUM, ETC. 593 



free of sulphate ; the residue (chloroplatinate of potassium plus sal-ammoniac) 

 is dried, ignited in hydrogen, the platinum collected as before, and weighed. 

 In this case, of course we have the option of determining the chloride of 

 potassium extracted by water from the ignited residue, either quite directly by 

 evaporation, &c, or indirectly by determining its chlorine. 



When one of us, some years ago, was commissioned by the Challenger 

 Authorities to carry out exact analyses of a large number of samples of ocean- 

 water, he inquired critically into the several known methods which might have 

 been used for the determination of the small proportion of potassium present 

 in sea-water salts, and found that Finkener's was the only one which afforded 

 a fair approximation to the truth. To render his results susceptible of 

 subsequent correction, he brought the Finkener method for his purpose into 

 a definite form, regarding which it may suffice here to state the main points as 

 follows : — # 



100 c.c. of sea-water are weighed, evaporated to near dryness, and the salts 

 made into normal sulphates ; these are dissolved in water, any sulphate of 

 lime, &c, is filtered off, and in the filtrate, the potassium is determined by 

 Finkener's method (form A) by means of a quantity of chloroplatinic acid 

 containing 200 mgs. of platinum, i.e., about twice the calculated quantity. 



A number of analyses of synthetically prepared mixtures showed that the 

 platinum- weight, when multiplied by K 2 -f- Pt = 94 -f- 198 = 04747, gave 

 results about 1 per cent, short of the potash used (as chloride of potassium). 

 When the factor, calculated from Seubert's atomic weight for platinum 

 (Pt = 194-8) and Stas' value for K 2 was used (i.e., the factor 0-48386)t the 

 results were about by 001 of their value too high. We may state, in passing, 

 that it was this observation which gave the start to the experiments reported 

 above as Nos. I. to VII. in the First Series. 



After the " Challenger " analyses had been reported, we again determined, 

 for our own satisfaction, what degree of exactitude would have been attained 

 if we had separated out the potassium and sodium as chlorides (free of 

 calcium and magnesium), and then applied what we are in the habit of calling 

 " Fresenius' Method," because it is the one recommended for mixtures of the 

 two alkyl-chlorides in his handbook of analysis. 



Leaving the errors involved in the elimination of the lime and magnesia on 

 one side, we prepared a mixture containing very nearly the weights of chloride 

 of potassium and sodium present in 100 c.c. of ocean- water (that of the former, 

 of course, adjusted exactly), and analysed it by means of the following 

 method : — 



*For details, see "Challenger Memoirs," Physics and Chemistry, vol. i. (appendix), p. 233 et seqq.; 

 see also body of Memoir, pp. 12 et seqq. 

 f New factor _ i.aiq2 

 Old factor 



VOL. XXXIII. PART II. 4 S 



