Hydrate water thermal analysis

Since the water content of a hydrated mixture in equilibrium conditions is a linear function of the water content of the single hydrated phases, thermal analysis, more specifically TG, provides a good opportunity to resolve hydrated mixtures, provided the single components, the water content of which should be known, display sufficiently distinct dehydration ranges. Unfortunately, this circumstance is very uncommon, since the mixture components normally present wide overlaps in the thermal transformation effects, which prevents an easy resolution of the TG traces. 

Apart from the qualitative observations made previously about suitable solvents for study, the subject of solvates has two important bearings on the topics of thermochemistry which form the main body of this review. The first is that measured solubilities relate to the appropriate hydrate in equilibrium with the saturated solution, rather than to the anhydrous halide. Obviously, therefore, any estimate of enthalpy of solution from temperature dependence of solubility will refer to the appropriate solvate. The second area of relevance is to halide-solvent bonding strengths. These may be gauged to some extent from differential thermal analysis (DTA), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) solvates of "aprotic solvents such as pyridine, tetrahydrofuran, and acetonitrile will give clearer pictures here than solvates of "protic solvents such as water or alcohols. 

Thermogravimetric analysis (TGA) and the differential thermal analysis (DTA) may be used to measure the water of crystallization of a salt and the thermal decomposition of hydrates. 

Nitric acid forms hydrates with water HN03. H20 (m. p. -38°C) and HN03.3H20 (m. p. -18.5°C). The chief evidence for these hydrates is obtained from the thermal analysis of the system nitric acid-water (Fig. 2). Other experimental... 

Thermogravimetric analysis of nickel(II) chloride hexa-hydrate shows that water evolution occurs from ambient temperatures (25°) to 66.6°. The resulting dihydrate is stable up to 133.3°, beyond which temperature further water loss occurs. Differential thermal analysis shows an endotherm at 53.9° related to the first dehydration step, and a second, strong endotherm at 118.9°, not accompanied by any weight loss, indicates the transformation of an octahedrally coordinated to a close-packed cubic structure. 

Thermogravimetric analysis of a sample of the 5 hydrate shows that water evolution occurs between 34.1° and 89.6°, at which latter temperature a dihydrate has formed. This is stable up to 107°, beyond which temperature the remaining two water molecules are slowly lost. Differential thermal analysis shows two dehydration endotherms at 36.4 and 132.8° and a structure transformation (octahedrally coordinated to close-packed hexagonal) endotherm at 151.8°. 

When partially hydrated samples are cooled down to 77 K, no crystallization peak is detected by differential thermal analysis. The x-ray and neutrons show that an amorphous form is obtained and its structure is different from those of low-and high-density amorphous ices already known [5]. Samples with lower levels of hydration corresponding to one monolayer coverage of water molecules are under investigation. This phenomenon looks similar in both hydrophilic and hydrophobic model systems. However, in order to characterize more precisely the nature of the amorphous phase, the site-site partial correlation functions need to be experimentally obtained and compared with those deduced from molecular dynamic simulations. 

Differential thermal analysis and thermogravimetry demonstrate collagen releases its hydrated water at about 40°C. 

Non-isothermal measurements (Chapter 2) have yielded valuable information about reaction temperatures and the successive steps in the removal of water from crystalline hydrates, e g. oxalates [14], sulfates [15-17]. DTA and DSC studies have sometimes provided additional information on the recrystallization of the dehydrated product [18]. The problems of relating kinetic parameters obtained by non-isothermal measurements to those from isothermal experiments are discussed in Chapter 5. The effects of heat transfer and diffusion of water vapour may be of even greater consequence in non-isothermal work. Rouquerol [19,20] has suggested that some of the above problems may be significantly decreased through the use of constant rate thermal analysis. 

As for the compressibility, the thermal expansion is considered to be composed of two main terms, the cavity and the hydration terms. A quantitative analysis is possible when one makes assumptions about the thermal expansion of the hydrational water [64]. 

Calteridol calcium is used as a chelating excipient in a parenteral formulation and was chosen as an example for two reasons. It contains water associated with a metal cation and thus represents a Class 3 hydrate. In addition, water is also contained in channels, as with the second example. What is unique about the calteridol system is that a single crystal structure has been solved for the tetra-decahydrate (reactant) and the tetra-dihydrate (product) using the same crystal. This procedure permits an interesting look not only into the structural differences but also into the energetic differences of the water environment through thermal analysis. A similar study was performed on the dihydrate and trihydrate phases of di-sodium adenosine 5 -triphosphate [25]. 

Similar discontinuities in Arrhenius plots are observed in thermal analysis (TA) as well, in particular, in the dehydration of crystalline hydrates performed in humid air. For illustration. Fig. 3.2 reproduces an Arrhenius plot for the dehydration of calcium oxalate monohydrate in an air flow, carried out under non-isothermal conditions by Dollimore et al. [28]. The equilibrium pressure of water vapour Pgqp measured at temperatures of up to 400 K and comparatively moderate decomposition rates turns out to be lower than its partial pressure in air which implies that the decomposition occurs in the isobaric mode. Above 400 K, the equilibrium pressure of H2O becomes higher than p with the process becoming equimolar. The slope of the plot decreases to one half of its former value in full agreement with theory (see Sect. 3.7). 

Differential thermal analysis under vacuum (DTA heating rate 20 C/min.) as shown in Figure 9 indicated three endotherms with peak temperatures at -98 C, -122 C and -182 C. The first two endotherms correspond to loss of water of hydration and the one at 182 C to melting. 

Studies on these hydrates by differential thermal analysis (DTA), thermogravi-metric analysis (TGA) and differential scanning calorimetry (DSC) showed that the initial decomposition involved elimination of all water molecules via a multi-step process with the formation of the dihydrate and monohydrate intermediates. Further heating led to formation of the stable oxysulfates and, subsequently to the sesqui-oxides (Wendlandt 1958, Wendlandt and George 1961, Niinisto et al. 1982) ... 

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