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 Na2PtCl6 • 6 H2O, 
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 KCl, NaCl, NH4CI. 
b. Test a phosphate, a sulphate, and a tartrate of potassium. 
c. Test K2SO4 in the presence of much H2SO4. 
