Sublimation and Melting Point Determination of Benzoic Acid

Sublimation and Melting Point Determination of Benzoic Acid

A Formal Written Report by

Escobido, Espino, *Fidel, Folloso, Francisco

ABSTRACT

The study ought to perform a successful sublimation procedure for benzoic acid and determine the difference between melting points of purified, impure and an equal proportion of both of benzoic acid.

Five grams of benzoic acid was sublimated in an evaporating dish where in 2 grams of purified benzoic acid was obtained. The calculated percentage recovery was 40%. Recovered crystals were then collected and triturated into fine powder and were placed inside a capillary tube labelled "purified". Pure benzoic acid was triturated and was directly placed inside a separate capillary tube labelled as "pure". The last capillary tube was filled by a 1:1 ratio of the purified and pure benzoic acid. The first two tubes were attached to a thermometer through a rubber band and were immersed in an oil bath. After melting, the last capillary tube (1:1) was attached to the same thermometer and was then immersed in the previously cooled and reheated oil bath.

The purified, pure and 1:1 ratio capillary showed a melting point reading of 114oC - 118oC, 117oC -122oC and 115oC -120oC, respectively.

INTRODUCTION

Sublimation refers to the process of transition of a substance from the solid phase to the gas phase without passing through an intermediate liquid phase. Sublimation is an endothermic phase transition that occurs at temperatures and pressures below a substance's triple point in its phase diagram. Sublimation is a technique used by chemists to purify compounds.

The melting point of a substance is the temperature at which the solid phase

converts to the liquid phase under 1 atmosphere of pressure. The melting point is one of a number of physical properties of a substance that is useful for characterizing and identifying the substance. To measure the melting point of a substance, it is necessary somehow to gradually heat a small sample of the substance while monitoring its temperature with a thermometer. The temperature at which liquid is first seen is the lower end of the melting point range. The temperature at which the last solid disappears is the upper end of the melting point range.

Many laboratory techniques exist for the determination of melting points. A Kofler bench is a metal strip with a temperature gradient (range from room temperature to 300 oC). Any substance can be placed on a section of the strip revealing its thermal behaviour at the temperature at that point. Differential scanning calorimetry gives information on melting point together with its enthalpy of fusion.

A basic melting point apparatus for the analysis of crystalline solids consists of a oil bath with a transparent window and a simple magnifier. The several grains of a solid are placed in a thin glass tube and partially immersed in the oil bath. The oil bath is heated (and stirred) and with the aid of the magnifier (and external light source) melting of the individual crystals at a certain temperature can be observed. In large/small devices, the sample is placed in a heating block or a capillary tube, and optical detection is automated.

The measurement can also be made continuously with an operating process. For instance, oil refineries measure the freeze point of diesel fuel online, meaning that the sample is taken from the process and measured automatically. This allows for more frequent measurements as the sample does not have to be manually collected and taken to a remote laboratory.

METHODOLOGY

1. 5 grams of benzoic acid was placed in an evaporating dish.

2. The evaporating dish was covered with an inverted watch glass and was placed on a hot plate. A wetted tissue was placed on top of the set up.

3. Set up was heated for 15 minutes until crystals were formed.

4. Crystals were collected and were then triturated into fine powder.

5. Collected powder was placed in a capillary tube packed.

6. Second capillary tube was filled with pure (not purified) benzoic acid.

7. Last