MICROCHEMICAL REACTIONS OF POTASSIUM 329 



assured before the test can be considered conclusive of the pres- 

 ence of potassium. 



Salts of sodium form sodium chloroplatinate Na 2 PtCl6 • 6 H 2 0, 

 a quite soluble salt crystallizing in yellow triclinic prisms, having 

 an extinction angle of about 22 degrees, and usually exhibiting 

 brilliant polarization colors. It is seldom that well-formed, 

 distinct crystals can be obtained, the result generally being an 

 aggregate of imperfectly developed crystals. The salt is soluble 

 in even strong alcohol, so that the addition of this reagent will 

 not cause the separation of crystals, but evaporation is hastened. 



The chloroplatinates of potassium, rubidium, cesium and am- 

 monium are isometric. That of glucinum, which is also obtained 

 when evaporation is practiced, is tetragonal. Lithium forms a 

 very soluble chloroplatinate similar to that of sodium. 



Precautions. 



If salts of ammonium are present, or suspected of being 

 present, place a little of the material to be tested on platinum 

 foil, moisten with water, dry and ignite carefully, until all the am- 

 monium salts have been driven off. Dissolve a portion of the 

 residue in water, with the addition of a little hydrochloric acid 

 if necessary; transfer to a glass slide, and test; then again ignite 

 the remainder of the residue and test again. 



The reagent should never be employed, even though freshly 

 prepared, without first testing it by evaporation to ascertain 

 whether octahedral crystals are deposited, since potassium may 

 have been extracted from the containing vessel, or ammonium 

 absorbed from the air. In making the reagent from metallic 

 platinum it must be borne in mind that the acids employed may 

 contain salts of potassium or ammonium, or both. 



When the potassium salt consists of a compound other than 

 the chloride it is always best to evaporate repeatedly with strong 

 hydrochloric acid before applying the platinum reagent. 



EXPERIMENTS. 



a. Test as above KC1, NaCl, NH4CI. 



b. Test a phosphate, a sulphate, and a tartrate of potassium. 



c. Test K 2 S0 4 in the presence of much H 2 S0 4 . 



