Eutectic Composition, Methyl-4-Hydroxybenzoate and 4-Hydroxybenzoic Acid

Sample

Methyl-4-hydroxybenzoate (MpHB), 4-Hydroxybenzoic acid (pHB) as well various mixtures

Application

Preservative

Conditions

Measuring cell: DSC820

Pan: Aluminum 40 µl, hermetically sealed

Sample preparation: Mixtures prepared by grinding in a mortar. The samples were pressed down in the pan.

DSC measurement: Heating from 115 °C to 135 °C at 5 K/min

Atmosphere: Nitrogen, 50 cm³/min

Interpretation

The first two diagrams show the sharp melting peak of the pure MpHB, as well as the eutectic peaks of the different mixtures. Diagram 43a shows the melting curves of the mixtures with 90%, 95% and 98% MpHB, diagram 43b those with 25%, 50% and 75% MpHB. The mixtures with a pHB content of less than 10% show smaller eutectic peaks as well as the depressed melting peak of the excess MpHB. The melting point of pure pHB is about 214 °C and is shown in example 39.

Evaluation

The heats of fusion are calculated from the eutectic peaks for all the mixtures. The mixtures with high pHB content (>10%) are evaluated using the baseline type ‘line’. The other asymmetric peaks are evaluated using the baseline type ‘horizontal left’ and calculating the partial integral. In order to determine the eutectic, a diagram is constructed showing the heats of fusion versus the composition of the mixtures. The point of intersection of the two straight lines, the eutectic point, characterizes the eutectic. The eutectic composition is 14.16 mole % pHB and 85.84 mole % MpHB and the eutectic temperature 117.4 °C.

The eutectic heat of fusion as a function of the composition for mixtures of Methyl-4-hydroxybenzoate and p-Hydroxybenzoic acid.

Conclusion

The evaluation of the heat of fusion of the eutectic peak allows an exact determination of the eutectic composition of this two-component system. The accuracy of the method depends on the number of points measured and the possibility of a precise evaluation of the peaks. It is recommended, that the calculated eutectic composition be checked by measuring a DSC curve of a mixture of the same composition.

_{Eutectic Composition, Methyl-4-Hydroxybenzoate and 4-Hydroxybenzoic Acid | Thermal Analysis Application No. HB808 | Application published in METTLER TOLEDO TA Application Handbook Pharmaceuticals}