602 PROFESSOR DITTMAR AND MR JOHN M'ARTHUR ON 



minutes for the alcohol and then twenty-five minutes for the alcohol and ether 

 suffice. In two test analyses (of a " 95 per cent." salt, vide infra) we used 

 this shorter mode, and found that the errors in the chloride of potassium 

 amounted to only +0-09 and -f-0'94 mgr., for about 753 mgs. to be deter- 

 mined. The mixture is filtered, and the precipitate washed with ether-alcohol. 

 To prepare it for " recrystallisation," it is washed finally with plain ether, and 

 allowed to dry. 



3. The " recrystallisation" of the mixture (PtCl 6 K 2 + ^R 2 S0 4 ) obtained in 

 the Finkener process is necessary, in general, for the removal of chloro- 

 platinates, but addition of sulphuric acid in this subsidiary operation does no 

 good. 



4. The alcohol and the ether must be absolutely free of ammonia; we always 

 distilled them with a little phosphoric acid before use. 



' 5. With the generality of substances, the sal-ammoniac form of the process 

 offers no advantages over the straight-forward determination of the platinum in 

 the "recrystallised" precipitate (of sulphates and PtCl 6 K 2 ). 



6. The determination of the platinum is best effected by reduction with 

 hydrogen in the wet way. 



In our final series, the Finkener and the Tatlock processes were worked side 

 by side of one another, in this sense, that for every mixture analysed by means 

 of one of the processes, a substantially identical mixture was analysed by the 

 other. Yet we prefer, meanwhile, to detail our test experiments on the 

 Finkener process first, and by themselves. 



For the preparation of the mixtures to be analysed, we used the following 

 materials: — 



(1) Standard solutions of chloride of potassium made from perchlorate. 

 The solutions were standardised synthetically by weight, and, immediately 

 after their preparation, quantities containing the desired amounts of salt were 

 weighed out into so many bottles, which were marked, and kept ready for 

 use. 



(2) Chloride of sodium solution, prepared from potassium-free salt, and 

 standardised volumetrically. 1 c.c. contained 81 94 mgs. of dry salt. 



(3) A sulphate of magnesia solution, prepared from pure (alkali-free) magnesia 

 (MgO)"" by solution in a very slight excess of standard sulphuric acid, and 

 diluting to a definite volume. 1 c.c. contained 1-539 mgs. of MgO. 



(4) A standard solution of sulphuric acid, made from distilled acid. 1 c.c. = 

 47-87 mgs. H 2 S0 4 . 



For the preparation of a mixture for analysis, one of the portions of chloride 

 of potassium solution (see (1)), was mixed with measured volumes of solutions 

 (2) and (3), and in general (4). The resulting solution was then Finkenerised 



* For mode of preparation, see " Challenger Memoir," p. 16. 



