DEHYDRATION AND THERMAL DECOMPOSITION STUDY OF COPPER SULPHATE PENTAHYDRATE USING THERMOGRAVIMETRY

Introduction:

The mass of crystal water in copper sulphate hydrates can be measured using Thermogravimetry. However, the dehydration of crystal water in hydrated copper sulphate often occurs over several steps at close temperatures and standard test methods often do not completely isolate these multiple dehydration reactions. In the present work, Perkin Elmer’s TGA-DTA-DSC thermal analyzer (STA 6000) has been employed to clearly distinguish these each and every step of the dehydration process. The complete decomposition of the anhydrous salt has also been found to occur very clearly through this course of investigation.

Experimental:

a)      Sample: Copper Sulphate, Pentahydrate (CuSO4.5H2O)

b)      Measuring Cell: STA 6000 (TGA-DTA-DSC)

Sample Type: Copper Sulphate, crystals, 12.675 mg

Copper Sulphate, finely ground, 11.058 mg

c)       Heating Schedule: Room Temperature to 995°C at 10 K/Minute using Pyris Software

d)      Pan: Alumina (70 µl), open

e)      Atmosphere: Nitrogen, 30 cm3/minute

Results:

The following figures show the TGA-DTA-DSC thermogram of the crystal and ground hydrated copper sulphate. When copper sulfate pentahydrate is heated, it loses its water of crystallization. This dehydration process occurs in three steps at different temperatures according to the following equations:

F1

The different dehydration steps that occur during heating of the material are difficult to distinguish in the TGA-signal, but are clearly visible in the DSC-result. The majority of the water is lost below 150°C, but the monohydrate is stable until 241°C. The loss of water from the monohydrate corresponds to a weight loss of approximately 7.0%.If the copper sulfate crystals are ground, the first two dehydration steps can be clearly separated. The same degree of separation cannot be obtained at this heating rate with crystals that have not been ground. The resulting anhydrous copper sulfate then decomposes in 3 further reaction steps:

F2

F3

Table1 shows the respective peak temperatures with their corresponding mass losses in both the cases. The mass losses in both the cases have been compared with that of theoretical stoichiometric mass loss.  The weight decrease rate in each of the dehydration step was almost same as the theoretical value.

F4

Contributed by: Dr.(Mrs) Saswati Ghosh

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