A colorimetric method for the determination of phenacetin and paracetamol. Part III. Studies of the indophenol reaction: an alternative manual procedure for the determination of phenacetin and paracetamol and its application to the determination of other pharmaceuticals, including sulphonamides, procaine and related compounds
An alternative manual procedure is described for the colorimetric determination of phenacetin and paracetamol as indophenol dyes. The procedure differs from that described in Part II in that phenacetin and paracetamol are hydrolysed first to p-phenetidine and p-aminophenol, respectively, before reaction with hypochlorite to form p-quinonechlorimide, which then undergoes a reaction with phenol. By using solid p-quinonechlorimide as starting material, the molar absorptivity of the indophenol dye formed, atits wavelength of maximum absorption (625 nm), has been shown to be 2·85 × 104 l mol–1 cm–1. With the introduction of the hydrolysis step, complete reaction of phenacetin and paracetamol is attained, which is expedient in any colorimetric procedure. Furthermore, the pH at which oxidation with hypochlorite is effected is not so critical as in the procedure described in Part II. The main disadvantage of the procedure compared with that in Part II is the increased analysis time. The potentialities of the indophenol reaction as a method of determining other pharmaceutical compounds have been investigated. The identification of the products of reaction of the hydrolysed compounds with hypochlorite has not been attempted but the apparent molar absorptivities (104 l mol–1 cm–1) at 625 nm of the indophenol dye formed in each instance are as follows: p-aminobenzoic acid, procaine and benzocaine, 1·65; aniline and acetanilide, 2·11; sulphanilic acid, 1·01; sulphaguanidine, 1·07; sulphathiazole, 1·00; sulphanilamide, 0·49; folic acid, 0·76; o-aminobenzoic acid, 0·47; and amethocaine, 0·29.