Page:Text-book of Electrochemistry.djvu/148

 electrolytic conductor in the corresponding arm of a Wheat- stone bridge circuit. Further, any residual error arising from difTerential polarisation is eflfectively drowned by the employment of high potentials and high resistances.

The arrangement of the Wheatstone bridge circuit is as follows: One arm of the bridge is formed by the long- column electrolytic ceU^ Cy in series with which is a resist- ance, iZ, forming the second arm. In parallel with these is the short-column electrolytic cell, c, and an adjustable resistance box, r ; these together form the third arm of the bridge, whose remaining arm consists of a resistance = B. If r be adjusted till there is no deflection of the galva- nometer, the same current is traversing each cell, presumably producing, at all events approximately, the same polarisation, and r is equivalent to the resistance of a column of the electrolyte equal to the difference between the long and short colunms. From the value of r the specific conduc- tivity can be calculated. The voltage used is about 30, and the adjustable resistance about 20,000 ohms.]

Experimental Results. — Experiments carried out by the Kohlrausch method show that pure water has only a very inappreciable conductivity. When increasing amounts of an electrolyte are added to the water, the conductivity gradually increases, and finally reaches a maximum, provided that the solubility of the substance permits of reaching a sufficiently high concentration ; as the concentration is further increased, the conductivity falls, and for pure electrolytes, e.g. hydro- chloric acid or acetic acid, it has about the same value as for pure water. The observed conductivity consists of two factors, namely, that of the water used in making the solution, and that due to the dissolved electrolyte. The former is generally caused by dissolved impurities such as salts, ammonia, or carbon dioxide, and is only to a very slight extent due to the real conductivity of the pure water ; in this connection it is of little theoretical interest. In practice, a correction is introduced by subtracting from the conductivity of the solution that of the water used as solvent. The

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