Methyl-4-hydroxybenzoate/p-hydroxybenzoic acid
p‑hydroxybenzoates are used as preserving agents and contain minor amounts of free 4-hydroxybenzoic acid as an impurity resulting from the synthesis with which they form a eutectic mixture. The example of the system methyl‑4‑hydroxybenzoate/p‑hydroxybenzoic acid will be used as an example to determine the composition of the eutectic.
1. Methyl-4-hydroxybenzoate
DAB 10 (methyl ester of p-hydroxybenzoic acid, methyl paraben)
Empirical formula: C8H8O3
CAS No.: 99‑76‑3
Molar mass: M: 152.14
Description: white, crystalline, hygroscopic powder, virtually odorless and tasteless
2. p‑hydroxybenzoic acid
(4‑hydroxybenzoic acid, paraben)
Empirical formula: C7H6O3
CAS No.: 69‑72‑7
Molar mass: M: 138.12
Description: colorless crystals
3. Mixtures
Homogeneous mixtures containing 2, 5, 10, 25, 50 and 75 mol% p‑hydroxybenzoic acid in methyl‑4‑hydroxybenzoate were prepared by grinding in an agate mortar. These mixtures as well as the starting substances are the investigation samples.
Instrument: DSC820 with TSW870
The samples were measured in closed standard aluminum crucibles. Initial sample weight: 2 to 4 mg. To assure good contact with the crucible base, the samples were pressed down with a punch.
Temperature program 1: 50 to 250 °C at 10 K/min
Temperature program 2: 115 to 135 °C at 1 K/min
Purge gas: Nitrogen
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The evaluation of the mass-related heat of fusion of the eutectic peak allows an exact determination of the eutectic composition of a pair of substances.
Determination of the Eutectic Composition | Thermal Analysis Application No. UC42 | Application published in METTLER TOLEDO Thermal Analysis UserCom 4