Page:Text-book of Electrochemistry.djvu/149

 134 CONDUCTIVITY OF ELECTROLYTES. chap.

conductivities of the electrolytes are thus obtained, some examples of which are given in the table on p. 135.

The table contains the specific conductivity k of sodium chloride, and the equivalent conductivities X of NaCl, KCl, NaNOs, CHaCOOK, iK2S04, iMgS04, HCl, iH2S04, CH3COOH, and NHs- The numbers given refer to the temperature 18®.

If we consider, in the first place, the values of k for sodium chloride, it is apparent that at high dilution these are almost proportional to the concentration, i.e. almost exactly inversely proportional to the volume v, in which 1 gram-molecule is dissolved. For acetic acid and ammonia, and for all weak acids and bases, k rises much more slowly with increasing concentration. At higher concentrations of all electrolytes the increase of k is less than proportional to the con- centration ; for potassium chloride the deviation is least. In some cases the maximum conductivity is reached with- in the limits given, e.g. with sulphuric acid, acetic acid, ammonia, hydrochloric acid, and magnesium sulphate. In the neighbourhood of this maximum the conductivity is almost independent of the concentration, so that it is easy with these electrolytes to prepare a solution of a particular conductivity for the determination of the capacity of the resistance cell. For this purpose sulphuric acid, or magne- sium sulphate, is usually employed.

The best conducting solution of sulphuric acid contains 30 per cent, by weight of H2SO4, and has the specific gravity 1-223. The maximum specific conductivity ic^^ig = 0'7398, and an error of 0005 in the specific gravity reduces this by about 0004. Vessels of small capacity are standardised by means of a dilute solution of potassium chloride.

For ^0 -normal KCl, k^s = 0011203 ; for ,,V I, » K,8 = 00023992.

The regularities at high dilution are, however, much more striking when we consider the molecular conductivities. It has already been pointed out that an increase in the value of

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