608 PROFESSOR DITTMAR AND MR JOHN M'ARTHUR ON 



0*8 grm. (we calculate from the NaCl number) as surplus chloroplatinic 

 acid.] Some 5 c.c. more of the choride of platinum solution are mixed with 

 the residue (which, in the case of NaCl, will produce some 6 c.c. of a 17 

 per cent, solution) ; the whole is stirred well, and set aside in a cool place for 

 at least an hour, with occasional stirring. The precipitate is then thrown on a 

 very small filter, the basin rinsed out with about 15 drops of the platinum 

 solution, and the precipitate on the filter washed with 16-24 drops more. 

 The basin, and filter and contents, are then washed with the smallest possible 

 quantity of alcohol of 95 per cent, (by weight or volume % ; we used 95 per cent, 

 by weight), and dried at 100° C. The precipitate is transferred as far as 

 possible to a tared capsule, and further dried until it assumes a distinct orange 

 colour. The filter with the remnant of precipitate adhering to it is incinerated, 

 and the residue calculated as Pt + K 2 C1 2 . The weight of the chloroplatinate of 

 potassium, multiplied by 0*3056, gives the weight of chloride of potassium to be 

 determined. [The factor is calculated from CI = 35 -457; K = 39137; Pt = 197'19.] 



Whenever in the following we state that an analysis was executed according 

 to " TatlocFs directions" these directions were followed closely as above given, 

 except that we allowed ourselves to recover the small quantity of chloro- 

 platinate sticking to the filter, by dissolving it off in hot water, and evaporating 

 the solution to dryness in the tared crucible intended to receive the main 

 quantity, and that we continued the drying process at 100° until the weight 

 became constant. 



In the case of a substance rich in sulphates, Tatlock recommends to add 

 a quantity of pure chloride of sodium. This rule, however, is obviously based 

 on the misapprehension that " platinum solution " is one of PtCl 4 # while it 

 really is one of PtCl 6 H 2 . Yet the chloride of sodium may do good by 

 substituting acid sulphate of soda for the H 2 S0 4 liberated, and besides, 

 by displacing some of the HC1 in the surplus PtCl 6 H 2 . 



Of the various sources of error involved in Tatlock's method, the most 

 obvious is the appreciable solubility of chloroplatinate of potassium in water, 

 and aqueous liquids generally. We therefore, at an early stage of our 

 investigation, determined the 



Solubility of the Chloroplatinate 



in the following reagents : — Five small flasks were tared, each charged with 0'2 

 grm. of chloroplatinate of potassium, and a convenient volume of the respective 



* We used to be under this erroneous impression ourselves until some three years ago, when we 

 analysed a carefully prepared platinum solution (which had been specially freed from extra hydro- 

 chloric acid) for chlorine and platinum. It contained very nearly 6 x CI for 1 x Pt, which, by the way, 

 is in accordance with an old analysis of " chloride of platinum," quoted in Gmelins's handbook as 

 having been made by Vauquelin. 



