Salts, vapour pressure saturated solutions

TABLE 17. Salt VAPOUR PRESSURES (mm Hg) OF SATURATED AQUEOUS SOLUTIONS IN EQUILIBRIUM WITH SOLID SALTS Temperature % Humidity 10" 15" 20" 25" 30" at 20" ... 

We shall now assume that it is possible to have a system in equilibrium composed of the various phases at a specified temperature and total pressure. This will be characterised by certain definite relations between the compositions of the phases (for example, a solid salt, saturated solution, vapour of the solvent). Let 77, T = total pressure, and temperature, of the system. n = number of components (cf. 84). r = phases ... 

In the above investigation Q (x, T) has the significance of a heat of dilution, i.e., it denotes the heat absorbed when more solvent is added to a solution of concentration x. If we consider a solid salt which is dissolved in a solvent, Q (x, T) has the significance of a heat of solution. If we consider a saturated solution we recover the case treated in 132. The vapour pressure p of the solution is now a function of temperature alone, since the concentration of a saturated solution is, at a given pressure, completely defined by the temperature, and it alters only very slightly with change of pressure. 

VAPOUR PRESSURES (mm Hg) OF SATURATED AQUEOUS SOLUTIONS IN EQUILIBRIUM WITH SOLID SALTS... 

The saturated solution exhibits the remarkable phenomenon of having two boiling-points, 126° C. and 255° C., a property characteristic of some other readily soluble salts, exemplified by the nitrates of sodium, potassium, silver, and thallium. It is due to an increase with rise of temperature in the vapour-pressure of the solution up to a maximum greater than the atmospheric pressure, further rise of temperature being accompanied by a diminution in the vapour-pressure of the solution as the composition of the system tends to approximate to that of the solid phase.12... 

The lines correspoial to the following states a v salt, vapotir, ancryohydric point, only with the reservation that it is the cryohydric point, not for atmosph( .ric priisstire, hut for the maximum pressure of the saturated solution. 

Applying these results to the system water-salt, we see that when salt solution and water vapour are in equilibrium the temperature only can be varied at will, and that if we fix the temperature, the concentration of the solution (solubility) and the vapour pressure of the saturated solution are determined. 

At all other temperatures the solution can only be saturated vdth respect to two salts, viz. the two simple salts, or one of the simple salts and the double salt. The concentration of the solution and the vapour pressure are then determined by the... 

Or, again, take the case of pure benzene on the one hand and a saturated solution of benzene in water on the other, both systems being at the same temperature A saturated solution of benzene is necessarily in equilibrium with pure liquid benzene itself because of the fact of saturation The conclusion to he drawn from the thermodynamic criterion considered is, that under these conditions, (8A)TV = o, and therefore, if we imagine one mole of benzene transferred from the pure benzene to the saturated solution, the work must be zero That is, there must be the same vapour pressure over the pure benzene as there is over its saturated solution in water, the vapour in each case being benzene vapour In the case of a hydrated salt on the one hand and the saturated solution of the salt on the other, the conditions are more complex We shall consider this point in Chap X in connection with the application of the Phase Rule to two component systems... 

Temperature, Vapour Pressure over the Saturated Solution of CoC126H20 Dissociation Pressure of the Hexahydrate and Dihydrate, Solid Salts... 

It will be observed that at the transition temperature the vapour pressures are identical In the above case we meet with the pecukanty that the vapour pressure over the saturated solution actually decreases in the neighbourhood of the transition temperature, 1 e the vapour pressure curve becomes retroflex This is due to a large increase in solubility of the hexahydrate near its transition temperature, a great solubility necessarily depressing the vapour pressure Naturally, no such effect is observed in the case of the solid salts... 

Deliquescence.—As is evident from Fig. 72, salt can exist in contact with water vapour at pressures lower than those represented by OAMF. If, however, the pressure of the vapour is increased until it reaches a value lying on this curve at temperatures above the cryo-hydric point, solution will be formed for the curve AMF represents the equilibria between salt—solution— vapour. From this, therefore, it is clear that if the pressure of the aqueous vapour in the atmosphere is greater than that of the saturated solution of a salt, that salt will, on being placed in the air, form a solution it will deliquesce. 

On evaporating the solution of a salt in an open vessel, therefore, salt can be deposited only if at some temperature the vapour pressure of the saturated solution is equal to the atmospheric pressure. This is found to be the case with most salts. In the case of aqueous solutions of sodium and potassium hydroxide, however, the vapour pressure of the saturated solution never reaches the value of i atm., and on evaporating these solutions therefore, in an open vessel, there is no separation of the solid. Only a homogeneous fused mass is obtained. If, however, the evaporation be carried out under a pressure which is lower than the maximum pressure of the saturated solution, separation of the solid substance will be possible. 

On heating Na2S04,ioH20 a point is reached at which the dissociation pressure into anhydrous salt and water vapour becomes equal to the vapour pressure of the saturated solution of the anhydrous salt, as is apparent from the following measurements the differences in pressure being expressed in millimetres of a particular oil —... 

The vapour pressure of the different systems of sodium sulphate and water can best be studied with the help of the diagram in Fig. 75, The curve ABCD represents the vapour-pressure curve of the saturated solution of anhydrous sodium sulphate. GC is the pressure cuiwe of decahydrate -f- anhydrous salt, which, as we have seen, cuts the curve ABCD at the quadruple point, 32 6°. Since at this point the solution is saturated with respect to both the anhydrous salt and the deca-... 

In connection with the vapour pressure of the saturated solutions of the anhydrous salt and the deca-hydrate, attention must be drawn to a conspicuous deviation from what was found to hold in the case of one-component systems in which a vapour phase was present (p. 40). There, it was seen that the vapour pressure of the more stable system was always lower than that of the less stable in the present case, however, we find that this is no longer so. We have already learned that at temperatures below 32 6° the system decahydrate—solution— vapour is more stable than the system anhydrous salt—solution— vapour but the vapour pressure of the latter system is, as has just been stated, lower than that of the former. At temperatures above the transition point the vapour pressure of the saturated solution of the decahydrate will be lower than that of the saturated solution of the anhydrous salt. 

With regard to sodium sulphate heptahydrate, the same considerations will hold as in the case of the decahydrate. Since at 24 the four phases heptahydrate, anhydrous salt, solution, vapour can coexist, the vapour-pressure curves of the systems hydrate—anhydrous salt— vapour (curve EB) and hydrate—solution— vapour (curve FB) must cut the pressure curve of the saturated solution of the anhydrous salt at the above temperature, as represented in Fig. 75 by the point B. This constitutes, therefore, a second quadruple point, which is, however, metastable. 

Vapour Pressure. Quintuple Point.—In the case of Glauber s salt, we saw that at a certain temperature the vapour pressure curve of the hydrated salt cuts that of the saturated solution of anhydrous sodium sulphate. That point, it will be remembered, is a quadruple point at which the four phases sodium sulphate decahydrate, anhydrous sodium sulphate, solution, and vapour, can coexist and is also the point of intersection of the curves for four univariant systems. In the case of the formation of double salts, similar relationships are met with and also certain differences, due to the fact that we are now dealing with systems of three components. Two cases will be chosen here for brief description, one in which formation, the other in which decomposition of the double salt occurs with rise of temperature. 

A saturated solution of NH4NO3 in liquid NH3 (which has a vapour pressure of less than 1 bar even at 298 K) dissolves many metal oxides and even some metals nitrate to nitrite reduction often accompanies the dissolution of metals. Metals that form insoluble hydroxides under aqueous conditions, form insoluble amides in liquid NH3, e.g. Zn(NH2)2. Just as Zn(OH)2 dissolves in the presence of excess hydroxide ion (equation 8.24), Zn(NH2)2 reacts with amide ion to form soluble salts containing anion 8.11 (equation 8.25). 

To determine partial pressure of any organic compormd i is needed the information about its solubility or solubibty coefficient in ground water and nonpolar solution. These values for fresh water may be found in Handbook of physicochemical properties and environmental2006. For accormting for the effect of mineral salts of water solutions should be used equation (2.290). Solubility of nonpolar compormds declines with increase in salinity. For instance, Sechenov coefficient in normal conditions is equal for aniline 0.130, for phenol - 0.133, and for benzene and nitrobenzene - 0.166 (Sergeyeva, 1965). The saturated vapour pressure and solubility parameters for a number of organic compounds are listed in Table 2.32. As a rule, saturated vapour pressure is provided in mm Hg, more rarely in Pa or atmospheres (1 mm Hg = 133.3224 Pa or 1.3332-10 bar). 

Salt Solubility (g/lat20°C) Water vapour pressure of saturated solution (mmHg) ... 

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