Eutectic Purity

Eutectic Purity: What is it?

A eutectic system is a homogeneous mixture of 2 components that melts and solidifies like a pure substance. The presence of an additional component (“impurity”) causes the reduction of the melting point of the main component. This effect can be highlighted with differential scanning calorimetry (DSC).

For materials with a purity higher than 98.5 mol percent, DSC can be used to determine the amount of impurity in the mixture, or rather the purity of the main component. The method is described in ASTM E928, DIN 51004 and DIN 51007. For that, the simplified Van´t Hoff equation is used:

Ts: Melting temperature of the current mixture (sample) [K]
T0: Melting point of the 100% pure substance [K]
R: Gas constant
X2: Mole fraction of the impurity
ΔHf: Heat of fusion [J/mol]
F: Molten fraction

 

This equation can be rewritten as:

or as an equation of a line in a 2-dimensional x-y diagram:

y = m⋅x + b

 

The Ts values plotted as a function of 1 /F should yield a straight line with

• Intercept on y-axis:

 

T0

Slope of the line:

In practice, the result is usually not a straight line because of pre-melting which cannot be sufficiently detected by means of DSC. Linearization is carried out by adding a correction factor c to the total area and to each partial area:

The correction factor c has to be lower than 20% to ensure reliability of the method.

Curve Ts=f(1/F) (non-linearized data) and linearized data by applying a correction factor c

The mole fraction of the impurity (X2) is obtained from the slope of the linearized curve and with knowledge of the heat of fusion of the pure substance.

The substance under investigation should have a defined melting point and a defined molecular weight.

  • This test method is not applicable to materials that decompose during melting.
  • You should clean the sample crucible using an appropriate solvent followed by a pre-heating step.
  • The measurements should be carried out using
    - a small heating rate, e.g., 0.3 to 0.7 K/min (ASTM E928) or 1 K/min (DIN 51 004/DIN 51 007). A larger heating rate can be applied up to 25 K below the melting effect.

    - a small sample mass (about 1 to 3 mg).

  • You should reweigh the sample after completion of the measurement. If the sample lost more than 1 weight %, the data cannot be accepted.

Example: Reduction of the Melting Point due to Impurities in Eutectic Mixtures

  • Main component: Phenacetin;
  • Impurity: p-aminobenzoic acid.

The increase in the p-aminobenzoic acid amount in the mixture leads to a decreasing melting temperature and broadening of the melting peak. The mixture melts as a pure substance.

DSC curves of pure phenacetin (green) and phenacetin with 2 and 5 mol% impurity


Related Methods

DSC