578 PROFESSOR DITTMAR AND MR JOHN M'ARTHUR ON 



II. On the Composition of Chloroplatinate of Potassium. 



Second Series of Experiments. 



Chloroplatinate of potassium, as has been long known, is liable to contain 

 water, and indeed in most cases does contain water, so intimately combined 

 with the rest that it cannot be completely expelled at even 150°. That this 

 water should all be present as such, as a mere enclosure within the crystals, is 

 difficult to believe ; it is more likely to be present, at least partly, in the form 

 of hydroxyl, functioning as part of the loose chlorine in the ideal substance. 

 From certain observations of Seubert's, indeed, it appears that whenever 

 chloroplatinate of potassium is recrystallised from hot water, part of the chlorine 

 passes into solution, and is replaced, of course, by its equivalent of hydroxyl or 

 oxygen. That this exchange should take place only in hot, and not at all in 

 cold, solutions, is by no means probable. Any chloroplatinate is liable to be 

 thus contaminated, and as long as its purity is not proved, and quite apart 

 from any free chloride of potassium, or surplus platinum in this form or that, 

 which may adhere to it, must be looked upon as a (mixed) oxychloride of the 

 general formula PtC] (e _ 2 j / )O y . K 2 + ^H 2 0, where y of course is a fractional 

 number, and x may be greater than y, because the salt may contain combined 

 water in addition to the water present as hydroxyl. To be able to make a 

 direct and complete analysis of a " chloroplatinate," we must have methods for 

 the direct determination of the water and of the oxychloride- oxygen. 



The determination of the ivater presents no difficulty; it indeed is so easy that 

 we wonder that Seubert did not effect it with his chloroplatinate of potassium, 

 and thus remove the cloud of uncertainty which hangs over those of his 

 calculations of the atomic weight of platinum which are based on the ratio of 

 platinum to non-platinum in the chloroplatinate. All that is required is, from 

 a known weight of substance, contained in a porcelain boat standing in a 

 combustion-tube, to expel what goes off at a dull red heat, to remove the 

 liberated chlorine by passing the volatile products through a spiral of red-hot 

 sheet silver, placed in the exit-end of the combustion-tube, to collect the thus 

 purified water in a tared chloride of calcium tube, and weigh it. The water 

 determinations referred to in the above reports were carried out in this 

 manner.* To avoid the uncertainties arising from the use of cork joints, the 

 exit end of the combustion-tube was drawn out to quill size, and the entrance 

 end of the chloride of calcium tube joined on by means of a piece of india- 

 rubber, in such a manner that only a narrow line of the latter was exposed to 

 the out-going vapours. This joint, during the whole of the operation, was 



* The water detenu ination in Experiment III. forms an exception, in tins sense, that the substance 

 was heated in a current of nitrogen, and the products filtered through a stopper of copper gauze (instead 

 of metallic silver). 



