Sensitive apparatus for obtaining freezing curves Purity of 4-aminopyridine

Freezing curves of highly-purified 4-aminopyridine have been obtained with an instrument of new design employing a gold crucible, a platinum resistance thermometer, electrical heating elements on crucible and heat shield, and accessory electrical control devices such that the temperature difference between crucible and shield could be maintained constant within 0.005 degrees while the temperature of the 4-aminopyridine and crucible dropped through the freezing range. No stirring was used. Methods were devised for handling the data as the derivative of temperature with respect to time and of correcting the freezing curve for the heat capacity of the crucible and charge. Although interpretation of the freezing curves obtained on the 4-aminopyridine was confused by attack on the gold by the molten 4-aminopyridine, the initial impurity in the 4-aminopyridine was probably less than the detection limit, 0.001 mole %.