Thermogravimetric measurement bound water

The structure proposed by Bradley (1940) has three forms of water a zeolitic water, bound water (at the edges of the octahedral sheet), and structural hydroxyls. Using thermogravimetric curves, Caillere and Henin (1961a) attempted to measure the amount of these three types of water for several attapulgites (Table LV). The amount of bound water and hydroxyls differs considerably from that calculated on the basis of the ideal structure (column 5) and suggests there are more structural hydroxyls than proposed for the ideal structure. 

Thermogravimetric (TG) analysis is used to determine the water content of clays. The weight loss at 110°C measures the interlayer water, whereas the bound water is lost between 400°C and 1000°C. The temperature at which bound water is lost is used in the identification of clays. For example, kaolin decomposes at 500-650°C to lose its bound water, whilst illite shows staggered weight loss between 400°C and 1000°C. Identification of clays is also carried out by IR spectroscopy in conjunction with XRD. The crystal structure is usually determined by XRD. Refractive index is also a very useful indicator of clay types. 

Thermogravimetric analysis (TGA) is a widely applied technique in the field of cement science. Measurements of bound water and portlandite content by TGA are often used to follow the reaction of portland cement or to evaluate the reactivity of supplementary cementitious materials (SCMs), such as fly ash and blast furnace slags. TGA is able to identify X-ray amorphous hydrates, such as C-S-H or AHj, and can be used complementarily to other techniques such as X-ray diffraction (XRD). 

TGA, Differential Thermal Analysis (DTA) and Differential Thermogravimetric Analysis (DTG) measurements have been employed to determine the number and nature of water molecules present in the POMs. The results of TGA show the presence of two types of water in POM compounds, namely water of crystalHza-tion and constitutional water molecules. The loss of the former usually occurs at temperatures below 443-473 K [141]. At temperatures exceeding 543 K for H3PM012O40 and 623 K for H3PW12O40, the constitutional water molecules (the acidic protons bound to the oxygens of the polyanion) are lost, according to the literature and for the acid forms [140]. For example the thermolysis of H3PM012O40 proceeds in two steps as schematized below ... 

Simultaneous thermogravimetric and differential thermal analysis (TG/DTA) is a useful technique for the solid-state characterization of pharmaceutical materials. Such characterization includes the determinations of loss on drying, phase transition temperatures, thermal stability, and whether or not water is bound or unbound. TG/DTA combines the measurement of a change in mass of a sample as a function of temperature (TG) with the temperature difference of a sample compared with an inert reference material as a function of temperature (DTA). 

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