1 2 3 4 5 6 7 AgNO_{2}. 100 p. Ag-Acet. 100 p′. acetate. [Ag^{+}]. K_{S.P.}. 0. .... 0.0603 70.8 0.0427 0.0427 0.00182 0.061 82.0 0.0417 64.0 0.0267 0.0767 0.00204 0.119 78.4 0.0341 58.6 0.0200 0.1142 0.00227 0.239 74.0 0.0195 51.7 0.0100 0.1809 0.00182 «Applications in Analysis.»—A few instances of the application of this relation in analysis follow. The determination of the sulphate-ion is based on the precipitation of barium sulphate from solutions of sulphates. The solubility of barium sulphate in water at 18° is 0.0023 gram, or 0.0023 / 233 = 1E−5 mole per liter. In such an extremely dilute solution, the salt may be considered to be completely ionized, and the value of the solubility-product constant is found from K_{S.P} = [Ba^{2+}] × [SO_{4}^{2−}] = (1E−5)^2, or 1E−10. Now, the amount of sulphate-ion left in solution, which would be about one milligram per liter of the aqueous solution, may be reduced by the use of a small excess of the precipitant, barium chloride. An excess of as little as 0.2 gram or 0.001 mole of BaCl_{2} per liter would increase the concentration of the barium-ion about one hundredfold, and barium sulphate would be precipitated, until the concentration of the sulphate-ion had been reduced about one hundredfold. The loss of the sulphate-ion is thus reduced to approximately 0.01 milligram per liter. In passing, we may ask what the approximate loss of dissolved nonionized barium sulphate would amount to. The value of the ratio ([Ba^{2+}] × [SO_{4}^{2−}]) : [BaSO_{4}], representing the ionization of barium sulphate, is unknown for the extreme dilution represented [p148] by the saturated solution. If we assume it to be roughly of the order 2000 : 1,[309] the solubility of nonionized barium sulphate at 18° would be roughly 0.05 milligram per liter. As a rule, then, in the absence of complicating conditions,[310] an excess of the precipitant promotes the complete precipitation of an ionogen. «Washing of Precipitates.»[311]—When barium sulphate has been brought on the filter and the excess of precipitant is to be washed out, then, as the excess of barium chloride is removed, the sulphate becomes more soluble again. It is advisable, therefore, to wash the precipitate as effectively as possible with a very small volume of water—as a rule, the water is used in a very fine stream or is applied drop by drop.