§ 93. QUANTITATIVE METHODS. 159 



action, it should be evaporated to dryness and tlie residue 

 treated with oxalic acid again. 



(3.) According to Stohmann {Fresenius's Zeitschrift, 5, 

 306), potassium may be separated out at once by platinic 

 cliloride from a solution containing only alkalies and alka- 

 line earths. 



Having precipitated the sulphuric acid completely as 

 above in the boiling solution of about 10 grms. of the 

 substance, filter out the precipitate if the quantity of it is 

 large ; if but small, let it remain in the liquid ; dilute the 

 liquid, when cool, to 1000 c.c. and mix the whole thor- 

 oughly together. To 100 c.c. of the clear solution add an 

 amount of platinic chloride containing about 2 grms. of 

 the metal, evaporate the mixture nearly to dryness, and 

 proceed as directed for the separation of potassium and 

 sodium by platinic chloride (§ 46 h). The method is based 

 upon the fact that the double chlorides of calcium, barium, 

 and magnesium, and platinum, are soluble in water and 

 alcohol, as well as the double chloride of sodium and 

 platinum. 



H. Reparation oi phosphoric acid alone. 



(1.) Evaporate the hydrochloric acid solution, contain- 

 ing no great excess of iron over the phosphoric acid, to 

 dryness on the water-bath, to eliminate silica, moisten the 

 perfectly dried residue with about 2 c.c. of concentrated 

 hydrochloric acid, and, after a while, add about 10 c.c. of 

 concentrated nitric acid (Sp. Gr. = 1.2) for every 0.15 

 grm. of phosphoric acid supposed to be present, dilute 

 with water, filter if necessary, and wash the residue of in- 

 soluble silica; evaporate the filtrate and washings nearly 

 to dryness, dissolve the residue in about half as much 

 concentrated nitric acid as was added before, and proceed 

 to precipitate phosphoric acid with amnionic molybdate 

 (§ 61 h). [Fresenius's Zeitschrift, 4, 404.) 



(2.) To the solution of the phosphate add ferric chloride 

 in slight excess over the phosphoric acid, if there is not 



