István J. Zsigrai

Thermochromic complex compounds in phase change materials: Possible application in an agricultural greenhouse

The possibility to combine two physico-chemical phenomena is studied in this work in order to attain simultaneous control of temperature and light intensity in a greenhouse: (a) isothermal heat storage in phase change materials and (b) thermochromic behavior of complex compounds dissolved in the phase change materials. Two binary mixtures melting near the ambient temperature were studied: (a) 0.925 Ca(NO3)2·4.06 H20+0.075 CaCl2·6.11 H2O melting at 35.6°C and (b) 0.9 CH3CONH2+0.1 Ca(NO3)2·4 H2O, melting at 27.7°C. The melting temperature, the enthalpy of fusion and the heat capacity of both mixtures were determined from DSC measurements. Both mixtures showed to be suitable solvents for the formation of consecutive cobalt(II) chloride complexes. The compositions of the complex compounds were adjusted in each mixture so that the absorption spectra in visible spectral range exhibited low absorbance at the melting temperature and a pronounced increase of the absorbance and/or change of color with the increase of temperature in the range relevant for passive solar energy receivers. The combined latent heat storage and the outstanding reversible change of the optical properties of the dissolved complex compounds with temperature is proposed to be applied in solar heated agricultural greenhouses in areas with fluctuating climate conditions. The increase of absorbance with temperature acts as an auto-regulated shading protection from overheating.

Determination of organic bases in non-aqueous solvents by catalytic thermometric titration

Catalytic thermometric titrations have been developed for bases (brucine, diethylaniline, potassium acetate and triethylamine) in acetic acid by continuous and discontinuous addition of the standard solution and automatic temperature recording. The determination of weak bases, e.g., antipyrine, unsuccessful in acetic acid by catalytic thermometric titration, has been achieved by using nitromethane or acetic anhydride as solvent. Catalytic thermometric titrations were also performed by coulometric generation of hydrogen ions for the determination of micro amounts of weak bases in a mixture of acetic anhyride and acetic acid.

Electronic Spectra and Stability of Cobalt Halide Complexes in Molten Calcium Nitrate Tetrahydrate

The complex formation between cobalt(II) and chloride and bromide ions in molten calcium nitrate tetrahydrate at different temperatures has been studied by a spectrophotometric technique. Addition of halide ions to cobalt(II) nitrate solution in calcium nitrate tetrahydrate caused a pronounced shift of the absorption maximum toward lower energies and a large increase of absorption intensity, indicating a change from octahedral to a tetrahedral co-ordination. The change of co-ordination depends on temperature and halide concentration. Stability constants for the [Co(NO3)4]2−, [Co(NO3)2X2]2− and [CoX4]2− complexes at 40 and 70 ◦C and the corresponding species spectra are reported.

emf Studies of silver nitrate-potassium nitrate melts

The activity coefficients of silver nitrate in binary melts of AgNO3-KNO3 were determined by emf measurements in a formation cell consisting of a silver and a nitrate electrode, in a temperature range of 513-583 K and in the concentration range allowed by the melting point of the mixture. Using a polynomial expression of the excess chemical potential, the excess Gibbs function and the characteristic constants of the investigated binary molten salt system were calculated. The excess molar entropy was calculated by combining the excess Gibbs function with calorimetric data on the excess enthalpy of mixing from the literature. The excess chemical functions were represented by orthogonal series involving Legendre polynomials, which enabled a comparison of the characteristic constants for different binary molten salt systems. The conformal ionic solution theory has been tested with respect to its capability to predict the excess chemical potentials of a binary additive system.

Coulometric thermometric titration of halides in molten calcium nitrate tetrahydrate

A method for coulometric thermometric precipitation titrations of chloride, bromide and iodide in molten calcium nitrate tetrahydrate at 55 degrees with coulometrically generated silver ions has been developed. The change in temperature during the titration is followed with the aid of a thermistor bridge coupled to a recorder. To minimize the temperature effect of the passage of current through the melt, two thermistors are connected in opposition in the bridge, with one in the anodic and the other in the cathodic cell compartment. Amounts of 62-80 mumole of halide have been determined with relative error below 0.4% and relative standard deviation less than 2.7%. The relative error in determination of 40 mumole of iodide was + 2%.

Thermodynamics of metal complex formation in aqueous melts of calcium dinitrate–ammonium nitrate. Part I. Cadmium(II) chlorides

The formation constants of [CdBr]+ and CdBr2 in melts of Ca[NO3]2·a[NH4][NO3]·xH2O (a= 1.5—1 1.5,x= 0—8), in the range 323–433 K, have been determined from e.m.f. measurements in suitable concentration cells. The results are discussed in terms of a previously developed model based on the quasi-lattice concept of aqueous melts.

THERMODYNAMICS OF CADMIUM HALIDE COMPLEX FORMATION IN ACETAMIDE - CALCIUM NITRATE TETRAHYDRATE MELT

Abstract The complex formation between cadmium(II) ions and chloride and bromide ions in the melt of 0.8CH 3 CONH 2 0.2Ca(NO 3 ) 2 ·4H 2 O has been studied at six temperatures between 30 and 80°C. The formation constants of the complexes CdX + and CdX 2 (X=Cl, Br) were determined from emf measurements by means of Ag/AgX electrodes. The thermodynamic parameters ΔH mn o and ΔS mn o for the stepwise complex formation Cd m X n −1 2 m − n + X − ← → Cd m X n 2 m − n have been estimated. The results are ΔH 11 o = 4.2 kJ mol −1 . ΔS 11 o = 52.4 J K −1 mol −1 , ΔH 12 o = 3.3 kJ mol −1 , δS 12 o = 40.7 J K −1 mol −1 for the chloride complexes and ΔHi 11 o = −3.3 kJ mol −1 , ΔS 11 o = 33.0 J K −1 mol −1 , ΔH 12 o = −2.5 kJ mol −1 , ΔS 12 o = 30.4 J K −1 mol −1 for the bromide complexes. The parameters are compared with the literature data for the same process in dilute aqueous solutions and in some hydrated salt melts.

Calculation of phase equilibria of ternary additive molten salt systems with a common anion

Phase diagrams for ternary additive systems LiF + NaF + KF, LiF + NaF + CaF2, LiF + NaF + SrF2 and NaF + KF + SrF2 have been calculated a priori using a set of readily available physical parameters for each component. The method of calculation is based on a simple ion-interaction model developed for binary molten salt systems and now extended to ternary additive salt mixtures. Calculated and experimentally derived phase diagrams are compared and discussed.

Metal Complex Formation in Melts of Acetamide-Ammonium Nitrate-Water Mixtures, Part I. Cobalt(II) Chloride Complexes

The complex formation between cobalt(II) and chloride ions in molten NH4NO3・2.61H2O, NH4NO3・CH3CONH2・1.61H2O and NH4NO3・2.61CH3CONH2 has been investigated. Absorption spectra of cobalt(II) chloride containing variable amounts of ammonium chloride were recorded at 45 to 60 °C. In the absence of chloride, the solutions show spectra typical for octahedral co-ordination of cobalt(II). Addition of chloride caused a shift of the absorption maximum toward lower energies and an increase of the molar absorption coefficient with increasing chloride concentration. The position of the absorption maximum and the intensity of the absorption indicate tetrahedral or severely distorted octahedral co-ordination. The stability constants for [Co(NO3)4]2−, [Co(NO3)2Cl2]2− and [CoCl4]2− complex formation in NH4NO3・2.61CH3CONH2 are reported.

Thermodynamics of Lead(II) Halide Complex Formation in Calcium Nitrate Tetrahydrate -Acetamide Melts

Abstract The complex formation between lead(II) ions and chloride and bromide ions in melts of C a(N 03) 2 • 4 H2O • αCH3CONH2 has been studied at different temperatures between 30 and 70 °C. The formation constants of the complexes PbX+ and PbX2 (X = Cl, Br) were determined from em f measurements by means of Ag/AgX electrodes. The dependence of the formation constants for PbCl+ on the solvent melt composition has been analysed. The thermodynamic parameters ΔH011 and ΔS011 for PbX+ complex formation have been estimated. The parameters are compared with the literature data for the same process in dilute aqueous solutions and in some hydrated and anhydrous salt melts.