The instability of ammonium hydroxide is used as a means for ‹detecting ammonium-ion› in its salts. The latter are treated with some strong base, such as sodium or calcium hydroxide, and the ammonia, liberated by the decomposition of its hydroxide, is recognized by its odor or by the more sensitive test of its action on moist litmus. The delicacy of the test is dependent on the conditions expressed in the equilibrium equation (p. 160). Sodium and potassium resemble each other so profoundly in the chemical behavior of their compounds, that they are recognized, and separated from each other, by ‹physical› methods. A very simple physical test is based on the color of their heated vapors, the color imparted to the nonluminous bunsen flame by the introduction and volatilization of their salts. The sodium flame is so intense that, if sodium is present in any quantity, the color of its flame easily masks the faint color of potassium vapor. The color of the flame is best examined, in such a case, with the aid of a spectroscope, in which the light emitted by the two elements may be readily recognized side by side, or with the aid of cobalt glass, which absorbs the sodium light. [p162] The ions of the two metals may be separated and identified by means of difficultly soluble salts. There are so few of these in the case of both ions, that recourse must be taken to the salts of comparatively uncommon acids. ‹Potassium chloroplatinate› K_{2}PtCl_{6}, precipitated by the addition of chloroplatinic acid H_{2}PtCl_{6} to concentrated solutions of potassium salts, gives very satisfactory results, both in qualitative and in quantitative work. The acid tartrate, KHC_{4}H_{4}O_{6}, the picrate, KC_{6}H_{2}N_{3}O_{7}, and the cobaltinitrite, K_{3}Co(NO_{2})_{6},[332] are difficultly soluble and are sometimes used to ‹identify potassium-ion›. The corresponding ammonium salts are also difficultly soluble and resemble the potassium salts, and ammonium-ion must therefore be removed, as stated above, if present, before any of these precipitates may be used for the identification of potassium-ion. In the case of sodium-ion, recourse is taken to the salt of a still more uncommon acid; ‹pyroantimonate of sodium›, Na_{2}H_{2}Sb_{2}O_{7}, 6 aq., is sufficiently difficultly soluble and characteristic to be used as a means of ‹identifying sodium-ion›.