Inorganic Synthesis

Sodium acetate trihydrate preparation

Theory:

Sodium acetate is a sodium salt of acetic acid. It’s colourless, water soluble deliquescent compound with plethora of uses. It is used as a food additive (E262), as  pH buffer, also as a  pickling agent in chrome tanning and in some other industrial areas.

Very interesting property of sodium acetate is its ability to supersaturate in water solution. That means it is able to form solution that contains more of the dissolved material than could be dissolved by the solvent under normal circumstances. Crystallization of supersaturated sodium acetate can be then caused by sudden mechanical shock in the solution or by inserting a nucleation site into the solution. After that crystallization takes place almost immediately and is completed in few seconds or minutes depending on how much supersaturated the solution is.

Sodium acetate trihydrate can be made by a reaction of acetic acid with sodium bicarbonate (or Na2CO3, NaOH):

CH3COOH + NaHCO3 →CH3COONa + CO2 + H2O

Sodium acetate crystallizes from a solution in two crystal forms – hexagons or as large plates. Hexagons are told to be trihydrate whereas the large plates represent tetrahydrate [1]. The solubility of anhydrous salt and trihydrate was investigated by Green et al. The anhydrous salt solubility curve in a range from 0 °C to 123 °C is shown on the graph below. At temperatures above 123 °C the saturated solution boils [2].

Graph1

Procedure:

50 % Acetic acid (50 mL) was poured into a beaker. Sodium bicarbonate (33.6 g, 0.4 mol) was added slowly in order to avoid overflow. This solution was heated to app. 70°C and let to cool down without any mechanical stress. Glass rod was used to touch the surface of the cold supersaturated solution. The action caused instant crystallization.

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Literature:

[1] Green, W. F. (1907, January). The “Melting-Point” of Hydrated Sodium Acetate: Solubility Curves. J. Phys. Chem., 12(9), 655-660.
[2] Mannan, K. M., & Rahaman, M. H. (1972). Crystallographic data for sodium acetate trihydrate, sodium acetate tetrahydrate, and 2,5-bis(benzylidene)cyclohexanone. Acta Crystallographica Section B, B28, 320.