Recrystallization and Melting Point Determination

Solids: Recrystallization and Melting Point Determination

Introduction

During material synthesis, the identity of products can be unknown or unconfirmed as reaction pathways are often unpredictable. One simple method of clarification is melting point determination. Further, the desired product can often be contaminated with unwanted compounds. Therefore, methods such as recrystallization are an important step in ensuring the purity of the product. This experiment will investigate the melting point of known and unknown compounds to verify their identity. In conjunction, calibration of the thermometer will be carried out using samples of ice and boiling water vapor. In the second part, impure samples of benzoic acid and acetanilide will be recrystallized, the latter of which will be carried out using decolorizing carbon, to remove contaminants.

Experimental Method

Part 1: Thermometer Calibration and Melting Point Determination

The observed temperatures were noted for the vapor of boiling water and a well stirred mixture of crushed ice and water.

The melting point of naphthalene was observed using a DigiMelt or MelTemp device until concurrent values agreed to within 1 °C. The melting point range of benzil and cinnamic acid were also recorded. Following this, two melting range measurements were taken for the unknown compound. The identity of this unknown was confirmed using mixed melting point determination.

Part 2: Recrystallization

Distilled water (50 mL) was boiled and a portion (30 mL) was added to a sample of crude benzoic acid (~1 g). This mixture was brought to a boil, gradually adding more water

(<5 mL) until all the benzoic acid had dissolved. Hot gravity filtration was performed, ensuring the original flask was rinsed with boiling water (<2 mL) afterwards. The solution was boiled down until ~20 mL remained, covered loosely with filter paper and allowed to cool to room temperature.

Boiling water (40 mL) was added to an impure sample of...