HYDRATION I N SOLUTION.
63
HYDRATION IN SOLUTION AS THE CAUSE OF CERTAIN SOLUBILITY INFLUENCES. B Y C. S HUDSON. Received October 28, 1908.
The recent address b y Prof. Victor Rothmund’ on reciprocal solubility influences and the published discussion by Professor Abegg which follows it lead the writer to present the following criticism : Lowenherzz has found that the solubility of anhydrous sodium sulphate at j z o is decreased considerably b y the presence of urea in the solutions, b u t the solubility of sodium sulphate decahydrate a t the same temperature is slightly increased by the urea. From these facts, according to Rothmund’s3 formula for the reciprocation of solubility influences, it is to be concluded that the solubility of urea will be increased in solutions of sodium sulphate decahydrate and decreased in those of the anhydride, but as the solutions of these two crystalline forms of the sulphate are identical in their physical and chemical properties their actions on the solubility of urea cannot be different as demanded above. There is here a direct contradiction between the formula of Rothmund and the experiments. This exaiiiple is not an exceptional or isolated one, for it can be shown, when these solubility influences are considered from the point of view of the hydration in solution of the dissolved substances, that the influence of foreign substances in the neighborhood of the transition temperatures of all hydrates is necessarily such as to lead in all cases to similar contradictions of Rothmund’s formula. I n a discussion oE this question in 1901, Professor Abegg? gave a suggestion of the cause of these solubility influences which explains them in a very simple manner as due to the existence of hydrates in the solutions, and does not lead t o such contradictions as have been mentioned above in the case of hydrated substances. In 1906 I published an article on hydration in solution6 in which this simpler view was again suggested ant1 developed niathematically ; as this work gixres a detailed exp1:in:~1 i o i i u p o i i tlit.r~notl~t~aniic. grouiirls oE t lie solubility influences wliich Prof. Rothrnund has discussed in his address, I take this occasion to repeat its main argument, in the hope t h a t light may be thrown on the cause of the solubility changes. A t the time t h a t the article was published I was not acquainted with the fact t h a t Professor Abegg had sug2. Elektrochem., 14, 532-4 (1908). Z . physik. Chem., 18, S j (189j). 2. Elektvochem., 7, 675-7 (1901). Z . physik. Chem., 40, 612 (1902). The general statement of the formula is, “If the solubility of a substance A is decreased (or increased) by the addition of B, then in turn the solubility of B must be decreased (or increased) by the addition of A.” 2. Elektrochem., 7 , 677-8 (1901). Physic. Rev., 23, 37-81 (1906).
64
GENERAL, PHYSICAL AND INORGANIC.
gested before me in a general way this cause of these solubility influences and i t accordingly gives me pleasure now to recognize his prior work. B!- aid of the accompanying diagram consider the solubility curl-es of a h!-drate and its anhydride, differing in composition b y I I molecules of water, and having at 7' a transition temperature. .\t 7' the aqueous vapor pressure of a mixture of , the trio solids is equal lj?. theory t o the vapor pressure of their common saturated solution. If now a foreign substance be dissolved in this saturated solution, its 1-apor pressure becomes less' and consequently the transition teniperature in the presence of the new solution becomes reduced t o 7.'. The solubilit\- curves of the hydrate and anhydride in the solution of the foreign substance must by theory cross at T', just as they cross similarly at 7 in the pure solvent. I n order, however, t h a t they map cross at 7" it is necessar!. that the solubility of a t least one of the two solids be changed by the presence of the foreign substance. U p to this point the considerations are the same as have been advanced by H . Goldschniidt,? who takes the further view t h a t the solubility of the anhydride is miaffected, froni which it follows necessarily t h a t t h a t of the hydrate must be increased b y the foreign substance. I t seems t o the writer preferable, however, t o follow the more general view t h a t the solubilities of both hydrate and anhydride are affected and in the article mentioned this vie\\. is developed. If the true solubilit!, of the anhydride be written Sg,ant1 the concentration of the hydrate which is in equilibrium in t h e pure solution with this amount of the anhydride be C,, the experimental soliibility of the anhydride as shown hp the curve S; may be written S z Sg T C,. S o w the ratio3 C,/Sa - const. ( p ) " , where p is the vapor pressure of the solution. and accordingly if p is decreased by the additioii oi a foreign substance to t h e solution the ratio Chis: niust decrease. Since however S$ is fixed b y the presence of anhydride as solid phase C, must decrease to CJL'. The experimental solubility of the anhydride in a solution of the foreign substance is thus S'; SG C',, and S'; c, and c’h2
