592 PROFESSOR DITTMAR AND MR JOHN M'ARTHUR ON 



chloroplatinate of potassium, and may be adopted as being virtually his final 

 result. Combining it with our own number 195*50, we have Pt = 195*46, or 

 rather 195 5 (because the uncertainly on either side is more than ± 01), as 

 being at present the most probable value of the constant. 



The true number, we mean a number ranking in probable precision with, 

 say, Marignac's number for chlorine, will, we hope, be determined one day, 

 but if so, it must be derived from other experiments than analyses of chloro- 

 platinates, which are clearly unfit for the purpose. 



If, instead of searching for the true atomic weight, we want the quasi 

 " constant," which tells us how much platinum is associated with 2KC1 parts 

 of chloride of potassium, in a chloroplatinate produced in the ordinary methods 

 of analysis, even our value is too low by about half a unit. So at least we must 

 conclude from our experiments on Finkener's and on Tatlock's form of the 

 chloroplatinate process for the determination of potassium. 



These experiments are detailed in the next following section. 



III. Finkener's and Tatlock's Methods of Potash Determination. 



Finkener's Method. 



This method is not so widely known in this country as it ought to be, we 

 therefore begin by shortly explaining it. Assuming, for the sake of greater de- 

 finiteness, that the substance to be analysed is a mixture of chlorides and sulphates 

 of potassium, sodium, and magnesium, it is dissolved in water, and the solution 

 mixed with a quantity of sulphuric acid sure to be sufficient for converting 

 all the foreign oxides into sulphates ; a quantity of platinum solution, a little 

 more than equivalent to the potassium to be determined is now added, and, if 

 necessary, so much water that the chloroplatinate precipitate produced is dis- 

 solved in the next operation, which is to heat the mixture on a boiling water 

 bath. The solution produced is evaporated on a water-bath to the consistence 

 (after cooling) of a magma. This is allowed to cool, mixed with ether-alcohol,""'' 

 and allowed to stand, well covered, until the precipitate has settled completely. 

 The precipitate, — a mixture of chloroplatinate of potassium and the sul- 

 phates of the foreign metals, — is washed with ether-alcohol, to be worked up 

 in one or other of the following two ways : — (A) The precipitate is heated 

 in hydrogen gas to dull redness, so as to reduce the chloroplatinate to 

 Pt + 2KC1, the product treated with water (then with hydrochloric acid, if 

 necessary, and again with water) ; the residual platinum ignited and weighed, 

 to be calculated into potassium. (B) The precipitate is lixiviated, as quickly as 

 possible, with concentrated (cold) solution of sal-ammoniac, until the filtrate is 



* " Ether alcohol," in connection with Finkener's method, always means 1 volume of anhydrous 

 ether and 2 volumes of absolute alcohol. 



