Even should the more soluble salt be precipitated first from a solution containing, say, equal concentrations of barium and strontium ions, ‹it could not remain in equilibrium with the supernatant liquid and would be converted into the less soluble one, before equilibrium was reached in the system›. We can follow similar relations, experimentally, by using precipitates of different colors. Silver chromate Ag_{2}CrO_{4} is an intensely red precipitate, that is rather difficultly soluble in water (‹exp.›); a liter of water dissolves[338] 0.0252 gram or 8E−5 mole at 18°. The concentration of the silver-ion in the saturated solution is then 0.00016 mole.[339] Silver chloride AgCl, a white salt, is still less soluble in water, a liter of water at 18° dissolving 0.00134 gram or 1E−5 mole, and the concentration of the silver-ion in the saturated solution is, therefore, only 1E−5 mole, as compared with 1.6E−4 in the saturated silver chromate solution. If a mixture of potassium chromate and potassium chloride, containing approximately equal (0.01 molar) concentrations of the two salts, is prepared and silver nitrate solution added, drop by drop, to the mixture, the first ‹permanent precipitate› is the white silver chloride (‹exp.›). However, as the silver nitrate solution strikes the surface of the liquid, a red precipitate of the chromate, mixed with chloride, is momentarily seen, where the silver nitrate temporarily produces a ‹local excess› of the precipitant. But the red precipitate disappears rapidly and gives way to the white precipitate of the less soluble chloride. The quantitative relations, which may be developed with the aid of the principle of the solubility-product (see below), are such that, if little chromate is used, it may serve as an [p166] ‹indicator to determine quantitatively the moment when all the chloride, within the limits of allowed quantitative error, is precipitated›, the first ‹permanent› tinge of pink (solid Ag_{2}CrO_{4}), mixed with the yellow color of the solution, being used as the indication that the precipitation of the chloride is complete. Potassium chromate is used as a favorite indicator in quantitative analysis, for this purpose. The quantitative relations[340] for the precipitation may be developed as follows: For a supernatant liquid in which a precipitate of silver chromate just appears permanently, together with the chloride, ‹i.e.› for the condition of saturation with both silver salts at 18°, we have K_{AgCl} = [Ag^{+}] × [Cl^{−}] = (1E−5)^2 = 1E−10 and K_{Ag_{2}CrO_{4}} = [Ag^{+}]^2 × [CrO_{4}^{2−}] =  (1.6E−4)^2 × (8E−5) = 2E−12, and therefore: [Cl^{−}]^2 / [CrO_{4}^{2−}] = (K_{AgCl})^2 / K_{Ag_{2}CrO_{4}} = (1E−10)^2 / 2E−12 = 1 / 2E8.