Salicylamide (I) reacts with dimethyl-
p-phenylenediamine, and K
3Fe (CN)
6 in an alkaline medium (phosphate buffer : pH 8.0) to give green coloration (indophenol dye). This chromogen produced is extractable with chloroform, the extract showing an absorption maximum at 650mμ.
A new spectrophotometric method, based on this color reaction, has been established to the determination of (I) in pharmaceutical preparations.
Influences of 47 compounds were examined. The interferences from acetaminophene, ethoxybenzamide, salicylate, phenylephrine hydrochloride, sulpyrine, ascorbic acid, etc. are eliminated by the extraction with chloroform in an acidic medium. The recommended procedure is as follows. Sample {containing 100mg of (I)} is taken in a separatory funnel. To it are added 10m
l of water and 10m
l of 0.1
N HCl, and it is extracted 4 times with 30m
l portions of chloroform. The extracts are combined and evaporated on a water bath. The residue is dissolved in 50m
l of ethanol, and is made up exactly to 100m
l with water. A 10m
l aliquot is taken into another 100m
l volumetric flask, and is made up to 100m
l with ethanol. Ten m
l of this solution is taken into another 100m
l volumetric flask and prepared in the similar way.
A 2 m
l aliquot is transfered into a 30m
l glass-stop-pered test tube. Five milliliters of buffer solution (phosphate buffer : pH 8.0), 2m
l of 0.05% dimethyl-
p-phenylenediamine solution and 1 m
l of 1% K
3Fe(CN)
6 solution are added. After 5 min., the solution is shaken with 10m
l of chloroform. The separated chloroform layer is dried with 2 g of Na
2SO
4, and is filtered. The absorbance is measured at 650mμ (
ET) against chloroform. At the same time the absorbance of standard solution (
ES) is determined, and the amount of (I) is calculated by
ET/
ES.
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