not known from direct experiment, but in these cases the molecular conductivity at infinite dilution can be ascertained, as also the degree of dissociation which is set equal to that of some closely allied salt (here, silver nitrate) at the same dilution. The concentration of a given solution . of silver chloride can be estimated from the conductivity, and if a saturated solution be used we obtain the solubility. The solubilities of several difficultly soluble salts, such as silver chloride, silver bromide, silver iodide, barium sulphate, calcium sulphate, strontium sulphate, and various silicates have been determined in this way by Kohlrausch and Eose (;?) much more exactly than could be done by ordinary analysis.
At high dilution all salts when applied in equivalent quantity conduct almost equally well. Consequently when' it is desired to find approximately how many gram-equivalents of salt are present in a water {e.g, a well water), this can be done simply by determining the resistance.
Application of the Electrometer as an Indicator.— Behrend {3) introduced this instrument for the titration of mercury. Suppose we have two solutions of mercurous nitrate over mercury in two beakers, and that the concentra- tion of one of the solutions is known {e.g, O*l-normal), whilst that of the other is to be found. The two solutions are connected by a syphon tube containing mtric acid, and two platinum wires dip into the mercury in the vessels — the wires are fused through glass tubes, so that only the ends remain free. The platinum wires are connected with the poles of an electrometer; and the electromotive force of the concentration element thus constructed is measured with the electrometer. To the solution whose concentration is to be determined a standard solution of a chloride, e.g, potassium chloride, is added, and the mercury is precipitated as calomel. This causes the electromotive force to change slowly until almost all the mercury has been thrown out of solution. The logarithm of the concentration, the value of which determines the electromotive force, then changes very
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