THE ANALYSIS OF SODIUM SALICYLATE AND SODIUM
The method of analysis of sodium salicylate and sodium benzoate given in the British Pharmacopoeia entails ignition, extraction with water, and titration with acid.
A quicker method of arriving at the same result is as follows :-Direct titration with standard acid, with methyl orange as indicator, is not possible, as the indicator is affected by the liberated acid. In the presence of neutral ether direct titration to a point just short of the true end-point (methyl orange as indicator) can be obtained. By removing the ether when this point is reached, and adding more neutral ether, the titration may be continued to the correct end-point.
A weighed quantity (2 grms.) of sodium salicylate is transferred with water to a cylindrical separator, and methyl orange and about 30 C.C. of neutral ether 150 added. Semi-normal hydrochloric acid is run in with careful shaking (usually the greater part of the standard acid which should be required can be added at once) until the methyl orange turns a distinct red; the colour is best seen while shaking horizontally over a sheet of white paper after each addition of acid. At this stage the greater part of the salicylic acid has been liberated and is in the ethereal layer. The aqueous layer is run off into another separator, and water washings of the ether added; then neutral ether is added, and, on shaking, the indicator will revert to yellow. The titration is now continued (generally only a few more drops are required) until the end-point, which is quite sharp, is reached. The solution can be titrated back if the addition of acid be overdone. A third shaking with neutral ether is not necessary.
The same method is used in the analysis of sodium benzoate, 1 grm. of the sample being used and titrated with 0.5 N hydrochloric acid. The first endpoint is sharp and is within a few drops of the final figure.
The quantities given above for the determinations are those...