Glossary

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 Temperatures and EnthalpiesThe enthalpy of fusion of a substance, also known as latent heat, is a measure of the energy input, typically heat, which is necessary to convert a substance from solid to liquid state. The melting point of a substance is the temperature at which it changes state from solid (crystalline) to liquid (isotropic melt).melting point of the main component. This effect can be highlighted with differential scanning calorimetry (DSC).

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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:

Graph illustrating the reduction of melting point in eutectic mixtures, showcasing DSC curves of pure phenacetin with impurities.

Ts: Melting Temperatures and EnthalpiesThe enthalpy of fusion of a substance, also known as latent heat, is a measure of the energy input, typically heat, which is necessary to convert a substance from solid to liquid state. The melting point of a substance is the temperature at which it changes state from solid (crystalline) to liquid (isotropic melt).Melting temperature of the current mixture (sample) [K] 

T0: Melting Temperatures and EnthalpiesThe enthalpy of fusion of a substance, also known as latent heat, is a measure of the energy input, typically heat, which is necessary to convert a substance from solid to liquid state. The melting point of a substance is the temperature at which it changes state from solid (crystalline) to liquid (isotropic melt).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:

Graphical representation of DSC curves showing melting points of pure phenacetin and mixtures with impurities, illustrating eutectic purity.

 

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:
Graph illustrating the relationship of melting points in eutectic mixtures, showing effects of impurities on purity measurements.

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

DSC curves illustrating melting points of pure phenacetin and mixtures with impurities, showcasing eutectic purity analysis.

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

Graph depicting the relationship between Ts and 1/F for non-linearized and linearized eutectic purity data, with T0 labeled.
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 Temperatures and EnthalpiesThe enthalpy of fusion of a substance, also known as latent heat, is a measure of the energy input, typically heat, which is necessary to convert a substance from solid to liquid state. The melting point of a substance is the temperature at which it changes state from solid (crystalline) to liquid (isotropic melt).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.

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 Temperatures and EnthalpiesThe enthalpy of fusion of a substance, also known as latent heat, is a measure of the energy input, typically heat, which is necessary to convert a substance from solid to liquid state. The melting point of a substance is the temperature at which it changes state from solid (crystalline) to liquid (isotropic melt).melting temperature and broadening of the melting peak. The mixture melts as a pure substance.

DSC curves depicting pure phenacetin and impurities showing impacts on melting points, highlighting eutectic purity analysis.
DSC curves of pure phenacetin (green) and phenacetin with 2 and 5 mol% impurity
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