Dehydration Of Calcium Oxalate Monohydrate Research Articles

Infrared Spectroscopic Investigations of Calcium Oxalate Monohydrate (Whewellite) Dehydration/Rehydration

Variable-temperature infrared spectroscopy is employed to monitor molecular vibrations during dehydration of calcium oxalate monohydrate (COM) and hydration of anhydrous calcium oxalate (COA). A unique variable-temperature infrared spectroscopy approach combines precise sample temperature control and rapid (2 °C s−1) heating/cooling with high-sensitivity infrared analysis. Infrared spectra are obtained at specific isothermal sample temperatures, while applying linear temperature versus time heating and cooling ramps, and in coordination with programmed temperature steps. Spectrum vibration bands provide information regarding the symmetry and local environments of solid-state water molecules and oxalate anions. When COM is heated, water molecules liberated from different crystallographic sites are selectively detected. Subtle oxalate anion configuration changes associated with water losses are detected based on infrared vibration band shapes and wavenumber trends. Dehydration and rehydration processes occur at lower temperatures and over narrower temperature ranges than conventional thermal analysis methods because samples consist of thin layers of small particles. Reversible and irreversible sample changes are distinguished by using a step heating/cooling temperature program and spectral subtractions. COA temperature-dependent structure variations that occur while heating and cooling samples in an atmosphere with a low water vapor concentration are characterized.

An integral method to determine variation in activation energy with extent of conversion

This paper proposes an integral method that uses local heating rates to evaluate the activation energy dependence on the extent of conversion. The method leads to consistent results with those from a differential isoconversional technique while regular integral isoconversional technique results in systematic errors in the activation energy with the extent of conversion. The method is validated from (1) simulated thermal analysis curves for a single reaction model, (2) simulated thermal analysis curves involving in two parallel reactions, and (3) non-isothermal dehydration of calcium oxalate monohydrate.

Application of iso-temperature method of multiple rate to kinetic analysis

A new method of the multiple rate iso-temperature was used to define the most probable mechanism g(α) of a reaction; the iterative iso-conversional procedure has been employed to estimate apparent activation energy Ea, the pre-exponential factor A was obtained on the basis of Ea and g(α). In this new method, the thermal analysis kinetics triplet of dehydration of calcium oxalate monohydrate is determined, which apparent activation energy Ea is 82.83 kJ mol-1, pre-exponential factor A is 1.142·105-1.235·105 s-1, the most probable mechanism belongs to phase boundary reaction Rn with integral form g(α)=1-(1-α)n and differential form f(α)=n(1-α)1-(1/n), where accommodation factor n=2.40-1.40.

Critical Considerations on the Isoconversional Methods. III. On the evaluation of the activation energy from non-isothermal data

The differential and integral isoconversional methods for evaluation the activation energy, described in the first note of this series, were applied for: a) simulated data for two successive reactions; b) dehydration of calcium oxalate monohydrate. It was shown that for these systems the activation energy depends on the conversion degree as well as on the method of evaluation.

Kinetic concepts of thermally stimulated reactions in solids: A view from a historical perspective

Historical analysis suggests that the basic kinetic concepts of reactions in solids were inherited from homogeneous kinetics. These concepts rest upon the assumption of a single-step reaction that disagrees with the multiple-step nature of solid-state processes. The inadequate concepts inspire such unjustified anticipations of kinetic analysis as evaluating constant activation energy and/or deriving a single-step reaction mechanism for the overall process. A more adequate concept is that of the effective activation energy, which may vary with temperature and extent of conversion. The adequacy of this concept is illustrated by literature data as well as by experimental data on the thermal dehydration of calcium oxalate monohydrate and thermal decomposition of calcium carbonate, ammonium nitrate and 1,3,5,7- tetranitro-1,3,5,7-tetrazocine.

Application of the Šesták-Berggren Equation to Organic and Inorganic Materials of Practical Interest

Reactions that have an initial acceleratory period are common in both organic and inorganic systems. The Sestak-Berggren equation, dx/dt= -kxn(1-x)m[-ln(x)]p, with p set to zero (also called the extended Prout-Tompkins (PT) equation) is an excellent empirical kinetic law for many of these systems. In this work, it is shown to fit both isothermal and constant heating rate pyrolysis data for a well-preserved algal kerogen in a petroleum source rock and two synthetic polymers (polycarbonate and poly-ether-etherketone), dehydration of calcium oxalate monohydrate, decomposition of ammonium percholorate, and diffusive release of gas implanted in materials. Activation energies derived by non-linear regression to multiple experiments are consistent with those derived by simple isoconversional methods. Errors caused by misapplication of first-order kinetics to single-heating-rate data are discussed briefly.

Approximation of the TG curve through a temperature power series in order to evaluate the non-isothermal kinetic parameters of solid-gas decompositions and degradation

A procedure has been developed for evaluating non-isothermal kinetic parameters based on the development of the degree of conversion in a temperature power series. The procedure was applied to the dehydration of calcium oxalate monohydrate and to the thermooxidative degradation of an unsaturated polyester resin.

Non-isothermal kinetics of reactions whose activation energy depends on the degree of conversion

A procedure for evaluating the non-isothermal kinetic parameters of reactions for which the activation energy depends on the degree of conversion is suggested. The procedure has been applied to the dehydration of calcium oxalate monohydrate and to the thermal degradation of poly(vinyl chloride) (PVC) and polychloroprene rubber.

Non-isothermal kinetics of reactions whose activation energy depends on the degree of conversion

A procedure for evaluating the non-isothermal kinetic parameters of reactions for which the activation energy depends on the degree of conversion is suggested. The procedure has been applied to the dehydration of calcium oxalate monohydrate and to the thermal degradation of poly(vinyl chloride) (PVC) and polychloroprene rubber.

Determination of Kinetic Parameters for the Dehydration of Calcium Oxalate Monohydrate by Diffuse Reflectance FT-IR Spectroscopy

Variable-temperature diffuse reflectance infrared spectroscopy was used to study the reaction kinetics of the dehydration of calcium oxalate monohydrate. The kinetic analysis of the dehydration process indicates that there are two reactions taking place, as well as the presence of two different equimole water molecular environments in the crystal structure of calcium oxalate monohydrate. Partial least-squares calibration of the dehydration profiles was carried out to identify the evolution sequence of the two different types of water molecules in the crystal structure of calcium oxalate monohydrate. The results indicate that the water molecules in the crystal structure which are responsible for the stretching absorptions at 3428 and 3336 cm−1 are eliminated first at a higher rate, during the first part of the reaction, and the others responsible for the absorptions at 3486 and 3058 cm−1 are eliminated at a later stage. The unexpected and unidentified band arising from bending modes of one of the types of water molecules is shown to arise from the same water molecules that are responsible for the stretching absorptions at 3486 and 3058 cm−1. Kinetic parameters were determined for the dehydration step, and the overall activation energy calculated for the dehydration step is in agreement with values reported in the literature. However, the clear presence of two distinct reactions invalidates all previous determinations of reaction order.

Determination of kinetic parameters in TG and DSC by means of direct overall evaluation of simple and complex reactions

For kinetic evaluation by TG and DSC of simple and complex reactions we used a direct fit on the basis of TG differential equations and DSC non-linear equations. The advantages of an overall evaluation, i.e. the simultaneous inclusion of all data sets at different heating rates, are described. TG and DSC measurements of the dehydration of calcium oxalate monohydrate and the decomposition of calcium oxalate are used as examples.

Investigation on Dehydration of Calcium Oxalate Monohydrate by Correlative Judging Method

Investigation on Dehydration of Calcium Oxalate Monohydrate by Correlative Judging Method

A computer program for kinetic analysis of non-isothermal thermogravimetric data

A computer program in BASIC which enables regression analysis and determination of kinetic and thermodynamic parameters from experimental TG/DTG non-isothermal thermogravimetric data is presented. This program allows the Freeman-Carroll, Horowitz-Metzger, Coats-Redfern and Doyle (modified by Zsakó treatments) methods to be performed for up to 14 different solid-state rate-controlling reactions, including nth-order, Avrami-Erofeev, phase boundary movement and diffusional models. Data from the literature and the dehydration of calcium oxalate monohydrate were tested as examples of the application of this program.

Fourier Transform Infrared Analysis of Evolved Gases: Determination of Activation Energies

The use of Fourier transform infrared evolved-gas analysis (FT-IR/EGA) to determine activation energies from the heating rate dependency of the maximum evolution of effluent is demonstrated. The mathematical considerations are similar to those normally applied to differential thermal analysis. The determination of the activation energy is substantiated with dehydration of calcium oxalate monohydrate during pyrolysis; the results are compared with thermogravimetry analysis results, and indicate that the activation energy in the dehydration step in the pyrolysis of calcium oxalate monohydrate is temperature dependent.

The deuterium isotope effect on the kinetics of the thermal dehydration of calcium oxalate monohydrate

The deuterium isotope effect in the thermal dehydration of calcium oxalate monohydrate was investigated by means of non-isothermal thermogravimetry. The dehydration consisted of a first order reaction (F 1) and a two-dimensional phase boundary reaction (R 2). The F 1 was the rate determining step at lower temperatures and the R 2 which was the main process of dehydration dominated the dehydration at higher temperatures. The activation energies and pre-exponential factors for these processes were larger for the deuterated specimen than those for the protonated one. The ratio of the rate constants for the R 2 process, k(HR 2)/ k(DR 2), was 1.01, and the kinetic deuterium isotope effect in the R 2 process was not recognized significantly. The ratio for the F 1 process, k(HF 1)/ k(DF 1) decreased with increasing temperature and the mean value was 1.37.

Thermal decomposition kinetics: Part XIV. Kinetics and mechanism of isothermal decomposition of calcium oxalate monohydrate and correlation with sample mass

The mechanisms of the first two stages of the thermal decomposition of calcium oxalate monohydrate have been established from isothermal mass-loss studies. For both stages, the rate-controlling process is phase boundary reaction with cylindrical symmetry. The isothermal kinetic parameters computed from the mechanism-based equation show the same trend as those from the mechanism-non-invoking approach. For the decomposition of calcium oxalate to calcium carbonate and carbon monoxide, the kinetic constants are not appreciably affected by sample mass, while for the dehydration of calcium oxalate monohydrate, they show a systematic decrease with increase in sample mass. The bestfit correlations have been represented as: E (or log A) = C 1 − C 2 m + C 3 m 2

Kinetics of the decomposition of hydrated oxalates of calcium and magnesium in air

Combined thermal analysis was used to establish the dissociation steps for the hydrated oxalates of calcium and magnesium. While dehydration of calcium oxalate monohydrate is governed by three-dimensional phase boundary movement, dehydration of magnesium oxalate is governed by Avrami—Erofeev nuclei growth. Both dehydration reactions demonstrated that the activation energy and the pre-exponential factor vary with temperature over wide ranges. Accordingly, the methods of Reich and Kissinger (based on the maximum rate temperature) or Carroll and Manche are not suitable. Moreover they could not distinguish between the overlapping reactions or mechanisms. They can be only used as a guide in differentiating between the possible mechanisms obtained using the methods of Coats and Redfern or Šatava and Škvàra. The latter method is recommended since it gives a smaller number of possible operating mechanisms. When possible mechanisms have close activation energies both techniques are needed. Both anhydrous oxalates dissociated to the carbonates according to the Avrami—Erofeev nuclei growth, A 2. For the magnesium salt, at low heating rates, this step was accompanied by dissociation of MgCO 3 to MgO. Both carbonates dissociate according to a three-dimensional phase boundary migration mechanism.

A new differential method for determining orders and rates of reactions and its application to solid state reactions. Oart II. some empirical observations on the isothermal dehydration of calcium oxalate monohydrate

The recently described differential method of determining the order and rate of a reaction has been applied to the isothermal dehydration of calcium oxalate monohydrate. Literature data are compared with results obtained in this study, orders of reaction of ∼0.2 and ∼0.5 are found to describe the reaction well. No evidence could be found to support the often quoted orders of 0.666 or 1 for this process.

Kinetics and mechanism of the thermal dehydration of calcium oxalate monohydrate

The kinetics and mechanism of the thermal dehydration of CaC 2O 4 · H 2O were studied by means of thermogravimetry, both at constant temperature and linearly increasing temperature. The dehydration mechanism was determined as a random nucleation and subsequent growth mechanism, an Avrami—Erofeyev law with m ≅ 2. The mechanistic equation could be expressed as [−ln(1 −α)] 11.904 = kt where α is the fraction dehydrated at time t and k is the rate constant. The plausibility of the Avrami—Erofeyev mechanism in the thermal dehydration of CaC 2O 4 · H 2O is discussed.

Thermal reactions of lead(IV) chloride complexes in the solid state. Part IV. Thermolysis of alkali metal hexachloroplumbates

Alkali metal hexachloroplumbates of general formula M 2(I)PbCl 6, with M(I) = K, Rb, Cs, undergo thermal decomposition on heating to 700 K. The thermal processes in the solid state have been studied by differential thermal analysis (DTA) and thermogravimetry (TG) with a derivatograph. The thermolysis occurs in one step M 2(I)PbCl 6→2M(I)Cl (s)+PbCl 2(s)+Cl 2(g) The experimental TG and DTG curves were used to estimate the enthalpy of thermal dissociation. Based on these curves the kinetic parameters of thermolysis have been evaluated. In the linearization procedure of the kinetic equation. ▪ the methods of Freeman and Carroll, Coats and Redfern, Horowitz and Metzger and Gorbachev were applied. Sixteen f(α) functions which describe the primary mechanism of thermal decomposition of solid substances—nucleation, growth of nuclei, phase boundary reactions and diffusion processes—were used in the least squares error analysis. The dehydration of calcium oxalate monohydrate was performed to confirm the validity of the analytical and computational procedures. The differences in values of the kinetic and thermodynamic parameters for hexachloroplumbates thermolyses have been discussed. It was found that K 2PbCl 6 showed atypical thermal properties. This is probably caused by the different crystal structure of K 2PbCl 6 as compared with those of Rb 2PbCl 6 and Cs 2PbCl 6, as well as the abnormal course of its thermolysis. The thermal properties of alkali metal hexachloroplumbates have been compared with those for analogous compounds of the type M 2(I)M(IV)X 6 (M(IV) = Pt, Te, Tc; X = Cl, Br).