A survey of reported compositions along the solubility curve and compositions of coexisting phases in equilibrium for the system nitromethane + formic acid + water is given in Table 1.
Solubility curve
The system nitromethane + formic acid + water forms a miscibility gap of Type I, Treybal.3 Only one binary pair of components, nitromethane + water, is partially miscible. The data for this system were previously compiled and critically evaluated.4 The recommended value for the solubility of nitromethane in water at 293.2 K is x1" = 0.0345; and at 313.2 K, x1" = 0.0426. The solubility of nitromethane in water at 293.2 K reported in Zhuravlev and Maslovskay1 (0.0900 mass fraction; 0.0284 mole fraction) and at 313.2 K in Maslovskaya2 (0.1000 mass fraction; 0.0318 mole fraction) are rejected because they are inconsistent with recommended value from Savonov et al.4
Solubility data obtained by the titration method for the ternary system nitromethane + formic acid + water were reported in two references. Data of Maslovskaya,2 which were reported at 313.2 K, indicate a smaller miscibility gap than the data at 293.2 K reported in Zhuravlev and Maslovskay.1 Thus, with increasing temperature the mutual solubility of the components increases, Fig. 1 . The data for the nitromethane + formic acid + water system are consistent with the data for the nitromethane + acetic acid + water system measured and reported by Maslovskaya in the same paper.2 Experimental errors were not reported, but are estimated by the evaluator to be ± 0.005 mass fractions. Solubility data at 293.2 K and 313.2 K (water-rich and nitromethane-rich branches together) are described by an algebraic equation in the form:
x2 = a + b·x3 + c· (x3)2 + d· (x3)1/2 + e· (x3)3/2.
This smoothing equation gives a smaller standard deviation than any other tested equation. The equation parameters (a, b, c, d, e) were calculated by the least squares method and presented in Table 2. Binodal curves in this system were calculated and compared separately for solubility data reported in Zhuravlev and Maslovskay1 and Maslovskaya.2
The solubilities calculated by the proposed equation are presented in the Fig. 1 and in Table 3. Experimental solubility data, Zhuravlev and Maslovskay1 and Maslovskaya,2 are presented in Fig. 1 for comparison.
Liquid-liquid equilibrium (LLE) data
LLE data for the ternary system nitromethane + formic acid + water were reported in Maslovskaya.2 Composition of each phase was calculated from the index of refraction-composition curve of saturated mixtures prepared at the time of the saturation curve measurements. The tie lines at 313.2 K cover the whole range of the miscibility gap and are nearly parallel with one another and to the tie line between nitromethane and water. Data reported at 293.2 K and at 313.2 K, Maslovskaya,2 are in agreement. Experimental errors were not reported in the papers, but are estimated by the evaluator to be ± 0.005 mass fractions.
Recommended or tentative data
Solubility and liquid-liquid equilibrium data measured by different methods are in agreement with one another. Experimental data on solubilities at T/K = 293.2 K and 313.2 K, Zhuravlev and Maslovskay1 and Maslovskaya,2 are presented in Fig. 1. The ternary solubility data, 1,2 were used to obtain equation parameters of the binodal curve.
In view of the inaccurate results obtained in Zhuravlev and Maslovskay1 and Maslovskaya2 for the binary nitromethane + water system, the evaluator considers the ternary system data to be tentative and concludes that new studies on the ternary system would be of value.