Chr. Balarew

Thermodynamics of formation of carnallite type double salts

The solubility of the systems RbCl−MgCl2−H2O, RbBr−MgBr2−H2O and CsBr−MgBr2−H2O have been investigated by the physicochemical analysis method at 25°C and formation of carnallite type double salts was established. The Pitzer model is used to calculate the thermodynamic functions needed to plot the solubility isotherms of the systems. The results obtained are in good agreement with the experimental data. The free energies of formation of carnallite and bromcarnallite type double salts from simple salts and the standard molar energies of formation are estimated.

Investigation of the aqueous lithium and magnesium halide systems

The solubilities of the system LiBr−MgBr2−H2O have been investigated at 25°C and 50°C. It is established that the system is of a simple eutonic type. Pitzers model is used for calculating the thermodynamic functions needed for plotting the solubility isotherms of the systems LiX−MgX2−H2O (X=Cl, Br) at 25°C. According to calculations made, the Gibbs energy of formation of LiCl·MgCl2·7H2O from simple salts is ΔrG°m=−2.01 kJ-mol−1, while the value ΔfG°m=−2748 kJ-mol−1 corresponds to formation from the elements.

Solubility and Crystallization in the System MgCl2–MgSO4–H2O at 50 and 75°C

The system MgCl2–MgSO4–H2O has been investigated experimentally and modeled thermodynamically according to the Pitzer method at 50 and 75°C. It was found that, even when seemingly all requirements for reaching the stable thermodynamic equilibrium are fulfilled, the crystallization of higher hydrates as metastable phases is possible, and cannot be avoided in each crystallization field of a stable lower hydrate of magnesium sulfate. Crystallization of MgSO4 · x H2O (x = 1, 4, 6) and MgCl2 · 6 H2O at 50°C and of MgSO4 · H2O and MgCl2 · 6 H2O at 75°C as stable phases has been observed. Three metastable crystallization fields of MgSO4 · x H2O (x = 4, 6, 7) have been detected at 50°C and two of MgSO4 · x H2O (x = 4, 6) at 75°C. The results obtained and the contradictions existing in the literature with respect to the solubility and the crystallizing solid phases are discussed in terms of the crystal structures.

Effect of temperature on the solubility of carnallite type double salts

A method based on Pitzers model has been used for calculation of the solubilities of carnallite type double salts crystallizing in the systems MeX−MgX2−H2O (Me=Li, NH4, K, Rb, Cs; X=Cl, Br). The solubility of congruently soluble double salts was also determined experimentally at 25–75°C. The results from studies of the solubility diagrams of ternary carnallite type water-salt systems over a wide temperature range are summarized. It is found that the width of the crystallization region for each of the salts can be explained by the relative differences in the solubilities of the ternary solution components (MeX, MgX2·6H2O and MeX·MgX2·6H2O) and by a change of temperature, and by the effect of temperature on the solubility. A method is proposed for calculating Pitzers ternary parameters of interionic interaction (ΘMN and ΨMNX) on the basis of experimental data for the solubility in water of the double salts crystallizing in the corresponding ternary water-salt systems.

Calculation of the Gibbs energy of mixing in crystals using Pitzer's model

Pitzers model has been applied to the thermodynamic study at 25°C of a series of systems of the type MeX-Me′X-H2O, MeX-MeX′-H2O (where Me, Me′=K, NH4,Rb and Cs; X, X′=Cl, Br and I), MeSO4-Me′SO4-H2O (where Me, Me′=Mg, Ni, Zn and Co). The integral Gibbs energy of mixing Gmix and the excess Gibbs energy of mixing GE(s) are calculated, and simplified correlation equations are proposed for their calculation. The results obtained are compared with experimental data from the literature and with values calculated using various theories. The rational activity coefficients of the mixed crystal components are estimated. The energy of phase transition of one structure into another has been estimated for the isodimorphous mixed crystals in the system MgSO4−CoSO4−H2O.

Relation between the crystal structures of some salts of the type Me(OCOCH3)2 · nH2O and their ability to form mixed crystals or double salts (Me2+ = Mg, Ca, Mn, Co, Ni, Cu, Zn, Cd)

Abstract An attempt is made to find the relation between the crystal structures of some salts of the type Me (OCOCH 3 ) 2 · n H 2 O ( Me 2+ = Mg, Ca, Mn, Co, Ni, Cu, Zn, Cd) and their ability to form mixed crystals or double salts by taking into account the difference in the ground-state configurations of the metal (II) ions. Such a treatment is based on the theoretical argument that the formation of isomorphous and isodimorphous mixed crystals occurs when the admixed ion may assume the coordination environment of the substituted ion in the crystal structure of the host salt. Double salts are formed mainly between the acetates of the d 5 , d 10 and p 6 metal ions, i.e., for ions that allow strong angular deformations of the coordination polyhedra or when at least one of the metal ions meets this condition so that acetate bridge bonding may occur.

Sea-water solubility phase diagram. Application to an extractive process

The main goal of this presentation is to tentatively forecast or improve an extractive process. The sea-water system is described using a graphical representation of the quinary ion-pair system Na+, K+, Mg2+/Cl−, SO42-//H2O. The operations required by extraction or purification of a sea-water component are simulated by a path through the solubility diagram.The procedure is applied to isothermal evaporation of Black Sea water at 25 °C, and the balance of the crystallization sequence is determined. In the same way, the cooling of residual brine, after precipitation of pure sodium chloride, is studied.

Calculation of the free Gibbs energy of phase transitions using solubility data. 1. The system Na 2 SO 4 -Na 2 Se0 4 -H 2 O at 15 °C: Stable and metastable

The solubility isotherms of the systems Na2SO4 ·10H2O–Na2SeO4·10H2O–H2O and Na2SO4·7H2O–Na2SeO4 ·7H2O–H2O have been investigated at 15 °C. It is established that discontinuous series of mixed crystals are formed in both systems. Two methods of calculation of the free Gibbs energy of phase transition at the interruption point of the solubility diagrams are used: (i) on the basis of the composition of the two mixed crystal types, which are in equilibrium with the saturated solution in the eutonic point of the system, and (ii) from the distribution coefficients between each of the mixed crystal phases and their saturated solution, using both experimentally obtained values and calculated distribution coefficients for ideal isomorphic mixing. The data found by the two methods exhibit very good agreement. It is established that the free Gibbs energy of the phase transition in the case of decahydrates is lower than in the case of heptahydrates. This fact is considered as a confirmation of the buffering action of water molecules with respect to the distortion of the crystal structures of the pure salts provoked by the formation of mixed crystals.

The double salt RbCl · NiCl2 · 2H2O

Abstract A study of the RbClNiCl 2 H 2 O system revealed the existence of the double salt RbCl · NiCl 2 · 2H 2 O at 50 and 75°C. In the temperature range 25–75°C the congruently soluble salt is 2RbCl · NiCl 2 · 2H 2 O. The salt RbCl · NiCl 2 · 2H 2 O does not appear at 25°C. It shows a relatively narrow crystallization field at 50°C, which is considerably broadened at 75°C. The thermal behavior of RbCl · NiCl 2 · 2H 2 O and 2RbCl · NiCl 2 · 2H 2 O has been studied by DT analysis.

Thermogravimetrical study on some crystal hydrates of metal(II) benzoates

Abstract The following salts are studied thermogravimetrically: Ca(C6H5COO)2·H2O, Mg(C6H5COO)2· 4H2O, Mn(C6H5COO)2·H2O, Co(C6H5COO)2· 4H2O, Ni(C6H5COO)2· 4H2O, Cu(C6H5COO)2· 3H2O. It is found that up to 200°C the crystal hydrates lose their crystallization water and form corresponding anhydrous salts. The TG and DTG curves show that with the exception of Mn(C6H5COO)2·2H2O, the dehydration of the metal benzoates takes place gradually. The anhydrous salts are characterized by IR-spectra. The thermal decomposition of the anhydrous salts begins at temperature higher than 220°C and the corresponding metal oxides or metals are formed.