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XXV. — Critical Experiments on the Chloroplafinate Method for the Determination 

 of Potassium, Rubidium, and. Ammonium; and a Redetermination of the 

 Atomic Weight of Platinum. By W. Dittmar and John M' Arthur. 



(Read 18th July 1887.) 



The analytical methods referred to in our heading are infected with 

 numerous sources of error, amongst which until lately the uncertainty of our 

 knowledge of the combining constant " Pt " played a prominent part. This 

 uncertainty, it is true, has been removed to a great extent by Seubert's 

 investigation " Ueber das Atomgewicht des Platins," Liebig's Annalen (for 

 1881), vol. ccvii. p. 1 et seqq. The value 194*8* for Pt, which he ultimately 

 adopted, falls in well with his analyses of the chloroplatinates of potassium 

 and ammonium, and there can be no doubt that his chloroplatinates were close 

 approximations to the ideal substances. Hence his atomic weight 1948 must 

 be admitted to be nearer the truth than the value 198 of Andrews, which, 

 until lately, was so generally employed by chemists in the calculation of their 

 analyses. But it does not follow that in, for instance, the determination of 

 chloride of potassium "as metallic platinum," the factor 2KC1 : Pt = 07657, 

 calculated from Seubert's Pt, gives a more correct result than even the factor 

 075252, which follows from K = 39 : CI = 35*5, and Pt = 198. The expe- 

 rience of practical analysts might almost be said to point the other way. A 

 forcible illustration is afforded by Finkener's test-analyses in support of his 

 own form of the chloroplatinate method for the determination of potassium. 

 Finkener separates out the potassium as chloroplatinate (in admixture with 

 the sulphates of the foreign metals present), and weighs it as platinum. 

 In reducing his results, he used Andrews' value, Pt = 198 (or one not 

 far removed from it), and obtained close approximations to his syntheses. 

 Had he calculated with Seubert's number, 194 8, his results, if reduced to 

 K 2 0, would have been too high by nearly 2 per cent. All this, of course, 

 goes no hair's-breadth towards invalidating Seubert's result ; it only shows 

 that those analytical factors which are, by theory, equal to K 2 : Pt ; 

 K 2 : PtCl 6 K 2 , &c, must be determined directly by standard experiments; 

 and separately for the several methods. This is what we have attempted (in 

 a limited sense) to do. But a purely empirical determination of these factors 

 by even the most exact experiments would have been of little use. If, for 



*Let us state at once that in this memoir all atomic weights are referred to = 16. For the 

 atomic weights of potassium, ammonium, and chlorine we adopt Stas' numbers, as calculated by Lothar 

 Meyer and Seubert ; K = 39-136; NH 4 = 18-056; 01 = 35-455. 



VOL. XXXIII. PART II. 4 O 



