§ 44. POTASSIUM. 47 



magnesic chlorides also, and highly concentrated, add 

 platinic chloride in excess, until the liquid has a bright 

 yellow color, and evaporate the mixture nearly to dryness 

 over the water-bath, with care not to heat the water quite 

 to boiling. 



Pour alcohol of 84" |„, mixed with '|g its volume of 

 ether, over the residue, let stand several hours in a well 

 covered vessel, with occasional stirring, transfer the in- 

 soluble double chloride to a dried and weighed filter, 

 wash it with alcohol and ether mixed as above directed, 

 dry at 100° C, and weigh. 



If great accuracy is required, evaporate the filtrate 

 from this first portion of the chloride nearly to dryness, 

 at a temperature not above 75° C, after addition of some 

 Avater and more platinic chloride, and some sodic chlo- 

 ride if but little of this is supposed to be present, and 

 treat this almost dry residue with the mixture of alcohol 

 and ether as above ; if a second quantity of insoluble 

 chloride is thus obtained, collect it on a filter, wash, dry, 

 and weigh it, and add the amount so found to the first 

 quantity. 



The salt contains IG"]^ of potassium. 



If the quantity of the precipitate is quite small, less 

 than 0.03 grm. or thereabouts, it is better to collect it on 

 a small filter, incinerate the filter, add a little pure oxalic 

 acid to the cooled residue, cover the crucible, and ignite 

 again gently at first, and more strongly afterwards ; after 

 this ignition nothing but platinum and potassic chloride 

 remains ; dissolve out the salt by washing the residue 

 with water until the washings give no turbidity with ar- 

 gentic nitrate, and dry, ignite, and weigh the platinum. 



d. In some cases, as in the analysis of wood-ashes, 

 potassium or potassa may be determined by a volumetric 

 process, which consists in ascertaining the amount of a 

 solution of sulphuric acid of known strength, that is re- 



