64 
Proceedings of the Royal Society of Edinburgh. [Sess. 
VI. — The Precipitation of Salts by the Corresponding Acids. By 
Irvine Masson, M.Sc. (1851 Exhibition Scholar of the University 
of Melbourne). Communicated by Professor James Walker, F.R.S. 
(MS. received December 16, 1912. Read same date.) 
It is well known that salts such as the chlorides of the alkali metals are 
less soluble in water containing the corresponding acid than in water alone ; 
and for sparingly soluble salts, in weakly acid solution, the decrease of 
solubility has been shown to follow quantitatively the course predicted 
from simple ionic notions. With concentrated solutions of strong electro- 
lytes, however, the causes which operate are still obscure ; and, indeed, much 
has yet to be discovered as to the facts in such cases. There may be said 
to be two methods of studying the isothermal phenomena in question. The 
first, which has received attention from numerous observers, consists in de- 
termining the solubility of a salt in water containing the acid in varying 
concentrations. This may be termed the “ solubility ” method. The second 
method seeks to ascertain the minimumconcentration of aqueous acid which 
must be used in order to cause deposition of salt, when the acid is added in 
small quantity to the saturated aqueous salt solution. This we shall refer 
to as the “ precipitation ” method ; it was adopted by Gibson and Denison, 
and formed the subject of a communication by them to this Society ( Proc ., 
vol. xxx., 1910, p. 562). 
The objects of the present paper are : first, to show the manner in which 
the results of the two methods are quantitatively related and may be com- 
pared, and secondly, to inquire how far the conclusions arrived at by users 
of the two different methods are in accord. 
The change in the solubility of a given salt in water containing varying 
concentrations of the corresponding acid can be plotted, using solubility and 
acidity as co-ordinates. The data available include numerous determina- 
tions by Engel at 0° ( Compt . rend., 1886, 102, 619 ; 1887, 104, 433 ; Ann. 
Chim. Phys., 1888 [vi.], 13, 132), and also measurements carried out by the 
author for certain chlorides in hydrochloric acid and nitrates in nitric acid, 
at 30° {Trans. Chem. Soc., 1911, 99, 1132). In the last-mentioned paper it 
is shown that the whole course of the curve is, in each case, expressed by 
the empirical equation 
da a 
~db= k « +k b 
(i) 
