Salt Effect in Liquid-Liquid Equilibria of Acetic Acid-Water-MIK System
A. Suryanarayana and N. Khan
Dept. of Chemical Engg

The effect of Ammonium chloride, Potassium sulphate and potassium chloride on the distribution of acetic acid between MIK and water at 350c is reported. Distribution data of the three quaternaries have been determined at salt saturation and basic ternary in absence of salt at that temperature. The binodal curve of the system is also presented. The simple equation of setschenov is not able to correlate the distribution data for the salt effect in this system. All the three salts studied are found to have salting out effect for Acetic acid in varying degrees. An empirical equation of the form:y = x /(a +b * x) is correlating the data. Setchenov equation is obtained when b is zero. Here x= XS and y=ln(X0/X).

The presence of a salt can either raise or lower the distribution coefficient of a solute in liquid-liquid equilibria. The distribution coefficient of a solute is defined as the ratio of Composition of solute in solvent(MIK) rich phase to Composition of solute in diluent (water) rich phase. These phenomena are known as salting out or salting in respectively. Up to now, no general equation is formulated to explain the salt effect in liquidliquid equilibria. Explanations are given for individual systems (7). Empirical correlations of setschenov (4), Eisen and joffee(2) and of extended setschenov model (8) are available in literature. Theory is available to obtain the solubility data of ternary liquid systems using NRTL method and UNIFAC method without salt and with salt. This salt method is used in extraction of metals.

In the present work, the simple relation of Setschenov is tried to correlate the experimental data but not working satisfactorily.

Most of the papers on salt effect correlated the experimental data by empirical equations in extraction using compositions or relative volatilities in distillation.

Acetic acid water cannot be separated out cheaply by ordinary distillation, so liquid-liquid extraction is used. To improve separation in extraction, a suitable salt can be used. Earlier papers show, phenol (pollutant) can be removed from aqueous streams by this salt effect. Some data, correlations, uses and explanations of salt effect in both aqueous and non-aqueous ternary systems are given in literature (1-8).

Experimental Materials
Analytical grade acetic acid, succinic acid, sodium hydroxide, Potassium sulphate, Potassium chloride,distilled water, Ammonium chloride, and MIK are used.

Sodium hydroxide solution is standardized using standard succinic acid solution with phenolphthalein as indicator. Standard succinic acid solution was prepared by weighing accurately and dissolving it in a known volume of distilled water. Weight percent of acetic acid in each layer was estimated by titrating a weighed amount of solution against standard sodium hydroxide solution with phenolphthalein as indicator. The results are reproducible with a maximum error of 1 %( volumetric analysis).

For X 0,Y0 values: Accurately weighed amounts of acetic acid, water and MIK were mixed in different amounts to obtain various X0, Y0 values without salt. The mixture was stirred by a magnetic stirrer for at least three hours in stoppered conical flasks. The layers were settled and separated in a separating funnel. A sample of around one gram from each layer was weighed accurately in an electrical balance and then titrated with standard sodium hydroxide solution. For X-Y values at saturation of the salt, accurately weighed amounts of acetic acid, water and MIK were mixed in different proportions, as were done to obtain X0, Y0 data without salt. Then weighed amounts of salt were taken and from this small amounts were added at a time to the acetic acid-water-MIK mixture. The mixture was stirred using a magnetic stirrer and addition of salt was continued until no salt was seen at the bottom of the stoppered conical flask. The amount of salt added for saturation was beginning salt minus the remaining salt. acetic acid content was determined as above taking weighed amount of each layer and titrating with standard sodium hydroxide solution using phenolphthalein indicator. The temperature was around 350C. Lot of time was given for dissolving the salt in solution for saturation points and for settling of layers, for reproducibility and accuracy. When some observations were not following certain trend, then those observations were repeated for confirmation.

Table-1 contains the solubility data acetic acid water - MIK system (Binodal curve data). Table-2 contains the distribution data of acetic acid, water and MIK system without salts. Table-3 contains the distribution data of acetic acid, water and MIK ternary with Ammonium chloride salt at saturation . Table-4 contains the distribution data of acetic acid, water and MIK ternary with Potassium shate salt at saturation. Table-5 contains the distribution data of acetic acid, water and MIK ternary with Potassium Chloride salt at saturation. Table-6 contains the salting out constant values for the acetic acid-MIK-water system. Fig 1 contains bimodal curve and Fig-2 contains distribution curves. The salt is present in varying amount in both phases. Knowing the amount of salt dissolved in total solution (both layers) and the amount of salt dissolved in

aqueous layer(determined by evaporation), the amount of salt present in MIK layer is obtained by difference. These will give percentages of salt in both layers. Weight percentages of acetic acid in the two layers having salt are calculated by volumetric analysis. Later these percentages are converted to salt free basis (Knowing the weight percentages of salt present in the layers). Weight percent of salt at saturation on salt free basis means : weight of salt/weight of salt free solution expressed in percentage at equilibrium. An empirical equation of the type of: y=x/(a+bx) fits the data for the three salts used. "a" is the slope obtained by potting: 1/y versus 1/x where x= Xs and y= Ln (Xo/X) and "b" is intercept.


It is observed that the empirical equation of Setschenov ln( X0/X ) = KsX s cannot correlate the data for ternary systems when distribution coefficient (Y0/X0) is greater than 0.2. Here in this system, the distribution coefficient varies from 0.60 to 1.

An empirical equation of form, ln(X0/X) =Xs/(a+bXs) (ASN equation) is correlating the data. Separation can be achieved with less number of stages using salt in ternary systems. The theoretical explanation for salt effect can be adsorption theory, since adsorption type equation is working here. This empirical correlation of ln(X0/X) = Xs/(a+bXs) may work for all systems and for all salts. When b=0,it becomes Setchenov equation(at low distribution coefficients).

DISCUSSION This salt work is useful in pollution control and to separate components using cheap solvents and with less number of stages. The salt alters the solute composition in extract phase or in raffinate phase due to adhering of a solvent to salt molecules. The salt which has more Y/X value and greater than one is preferred.

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