2o8 Conductivity of Aqueous Solutions. Part VII. 



At 18, in the solutions of ammonium chloride and sodium acetate, 

 the ionization of water is so small that the above mentioned hydrolysis 

 correction entirely disappears ; on the contrary, the conductance of the 

 added base or acid itself must be subtracted. This correction is cal- 

 culated by the mass-action law to be 



10-^bCb (AnH4 + Ac0 (A NH4 + Aoh) 



Ms 



for the ammonium hydroxide in the ammonium chloride solution, and 



10~ g KaC& - - (Ah + A Ac) for the acetic acid in the sodium acetate 



solution ; where Kb (or A'a) is the ionization constant of the base (or 

 acid), Cb (or Ca) is the concentration of the added base (or acid) in 

 equivalents per liter, Anh 4 , Aci and Aoh (or AN a , Aa c and Ah) are the 

 equivalent conductances of the respective ions, and l is the specific con- 

 ductance of the salt in the mixture. 



The effect of the excess of acid or base upon the ionization of the salt 

 remains to be considered. In almost all cases this is negligible, as is 

 apparent from the smallness of the correction for its conductance ; but 

 in the dilute solutions of ammonium chloride and sodium acetate at 18, 

 the concentration of the common ion from the added base or acid is 

 sufficient to diminish appreciably the ionization of the salt itself, so that 

 the conductance obtained by subtracting that of the base or acid is not 

 the true conductance of the salt at the concentration in question. How- 

 ever, no correction was applied for this, since these 18 measurements 

 were made only to show whether any contamination or change had taken 

 place in the solution during the heating. 



The concentration is diminished in the case of the more volatile solutes 

 by the volatilization of a small amount of the solute. In the case of 

 acetic acid at 218, Noyes and Cooper (section 49, Part V) have already 

 shown that the concentration is not appreciably affected by vaporization 

 into the small vapor-space of 2 or 3 c.cm. usually present. The total cor- 

 rection to be applied to the concentration for the vaporization of both 

 water and solute was directly determined for ammonia at 218 and 302 

 and for acetic acid at 306 by measuring the difference in conductance 

 produced by increasing the vapor-space from 2 c.cm. to 30 or 50 c.cm. 

 Without describing the details of the experiments or of the calculation, 

 the results may be stated. It was found that the correction to be made 

 on the concentration per cubic centimeter of vapor-space in the case of 

 ammonium hydroxide solutions is 0.025 per cent at 218 and 0.12 

 per cent at 302 or 30G, and in the case of the acetic acid solutions is 

 0.05 per cent at 306. Thus the correction is negligible at 218 for the 

 ammonium hydroxide just as for acetic acid, and is small for both sub- 

 stances even at 306. 



