Estimate the purity of materials:

Estimate the purity of materials which melts   during DSC melting endotherm. Describe the procedure.

Answer:

The heat of fusion ?Hf and temperature during melting are measured and a curve is plotted, the slope gives the melting point, depression (?T) due to presence of impurities in sample and Zero intercept represents the melting point of a theoretical sample without impurity (To).

Depression of melting point ( ? T ) can be expressed as

? T = (- RT_o²/? H _f )x_s

where , ?H_y  is the heat of fusion in mol^-1 To is the melting point of the pure solvent system (substances) in Kelvins and, R is the gas constant and xs is the fraction of solute (impurity). This expression can be used to calculate % impurity in the sample.

mole% impurity =100×(? H _f/- RT_o²)× ? T

Area under DSC curve gives heat of fusion for the sample.