Copper sulfate pentahydrate, dehydration

Widjaja E, Chong HH, Tjahjono M (2010) Use of thermo-Raman spectroscopy and chemometric analysis to identify dehydration steps of hydrated inorganic samples -application to copper sulfate pentahydrate. J Raman Spectrosc 41 181-186... 

The DSC, TG curves of solvates and hydrates are related to the phase diagrams between substance and solvent (or water). Eutectic are observed. Fusion or decomposition of the solvate may occur during heating. Therefore, one may observe the melting of the solvate followed by recrystallization into the anhydrous form or the endothermic desolvatation in the solid state. In certain cases both phenomena may over-lapp. Details about experimental factors and examples can be found in Ref. If the anhydrous form is metastable, further phase transitions follow the desolvatation. If several solvates or hydrates exist, the transitions observed depend on the pressure, as demonstrated by Soustelle in the case of copper sulfate pentahydrate. Depending on the pressure, the direct dehydration into the anhydrous or the dehydration via the monohydrate, or the three dehydration steps trihydrate, monohydrate and anhydrous forms may be obtained. Hydrates have been the subject of... 

The first step in any kinetic study is to identify all the products and intermediates of the reaction. Dehydration often involves several distinct steps which may be very dependent upon reaction conditions, e,g, copper sulfate pentahydrate may yield trihydrate and/or monohydrate [29,30], Metastable intermediates may be formed, e,g, amorphous magnesium carbonate produced on dehydration undergoes [18] exothermic recrystallization at higher temperatures,... 

Colvin and Hume [56] showed that, on dehydration, copper sulfate pentahydrate yielded amorphous trihydrate which, in vacuum, reacted further to form the monohydrate, but in water vapour the crystalline trihydrate could be formed (see also Guenot et al. [57]). Cooper et al. [58] attributed an observed decrease in rate during dehydration of the trihydrate to impedance of the escape of water by the residual solid. 

The dehydration of inorganic salt hydrates is usually stepwise but not always predictable. For copper sulfate pentahydrate, three DSC peaks are apparent corresponding to the loss of 2, 2, and 1 waters of hydration. Further, there is an increase in dehydration enthalpy of this salt with increasing numbers of hydration waters removed. This type of study is of importance in the estimation of the purity of the sample since the dehydration enthalpy is directly proportional to the water content of the sample. Similarly, DSC can be used to determine the moisture content of materials generally, since the measured endothermic dehydration DSC peaks are directly proportional to the water content of the material. 

The scope of temperature- and time-resolved SCTGA has been revealed by Parkes, Barnes and Charsley, via studies of the decomposition of inorganic salts. Temperature-resolved SCTA is particularly useful for resolving the dehydration characteristics of copper sulfate pentahydrate. A typical linear heating (LH) TGA profile for this salt is shown in Fig. 10, and is compared with a proportional heating (PH) SCTGA profile over the same temperature range. 

A simple thermogram, for the dehydration of copper sulfate pentahydrate, is shown in Figure 17.1. There are two points of major interest to the analytical chemist. First is the general shape of the thermogram and the particular temperatures at which changes in mass occur. From this information, individual compounds may be identified under given conditions. Unfortunately, the reproducibility of the temperatures at which mass changes occur is severely affected by many experimental conditions, as noted later. For this reason, the obvious qualitative analytical capabilities of TG have yet to be fully realized. 

Figure 12 LH (A) and PH (B) curves for the dehydration of copper sulfate pentahydrate. (Reproduced with permission from Parkes GMB, Barnes PA, and Charsley EL (1999) Analytical Chemistry IV. 2482-2487.)...

Decomposition of model substances method The third method of calibration is by carrying out an experimental run using certain well studied model substances such as copper sulfate pentahydrate, calcium carbonate, calcium oxalate mono hydrate, potassium carbonate, sodium hydroxide, zinc oxalate dihydrate, and benzoic acid. These model substances show well resolved dehydration and decomposition temperatures over a wide temperature range. 

Gopper(II) Sulfates. Copper(II) sulfate pentahydrate [7758-99-8] CuS04-5H20, occurs in nature as the blue triclinic crystalline mineral chalcanthite [13817-21 -5]. It is the most common commercial compound of copper. The pentahydrate slowly effloresces in low humidity or above 30.6°C. Above 88°C dehydration occurs rapidly. 

Copper(II) sulfate monohydrate [10257-54-2] CuS04-H2 0, which is almost white in color, is hygroscopic and packaging must contain moisture barriers. This product is produced by dehydration of the pentahydrate at 120—150°C. Trituration of stoichiometric quantities of copper(II) oxide and sulfuric acid can be used to prepare a material of limited purity. The advantages of the monohydrate as opposed to the pentahydrate are lowered freight cost and quickness of solubilization. However, these advantages are offset by the dustiness of the product and probably less than one percent of copper sulfate is used in the monohydrate form. 

The dehydration of pentahydrated copper sulfate leads to various products, depending on the reaction conditions. This reaction is a transformation only if the final product is known (anhydrous copper sulfate, other hydrated copper sulfates). 

We will illustrate this by an example the dehydration of pentahydrate copper sulfate. 

Figure 3.13. Dehydrations of pentahydrate copper sulfate (a) shows two steps, this means that we obtain one stable phases (b) shows three steps, this means we obtain two stable phases (c) shows four steps, this means we obtain three stable phases...

Anhydrous copper(II) sulfate [7758-98-7] is a gray to white rhombic crystal and occurs in nature as the mineral hydrocyanite. CuSO is hygroscopic. It is produced by careful dehydration of the pentahydrate at 250°C. An impure product can also be produced from copper metal and hot sulfuric acid ... 

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