The effect of ethanol on the colorimetric determination of formaldehyde and glycollic acid

Abstract

Contrary to previous reports ethanol has been found to interfere with the colorimetric determination of formaldehyde and glycollic acid with 1,8-dihydroxynaphthalene-3,6-disulphonic acid (chromotropic acid) or 2,7-dihydroxynaphthalene in concentrated sulphuric acid. The effect is greater for glycollic acid than for formaldehyde. The mechanism for these observations is given.DURING the study of the rheological properties of some sodium carboxymethylcelluloses a routine method for the determination of the sodium glycollate impurity was required. The standard procedure, developed by Eastenvood,l involves the preliminary removal of the salt impurities by washing with hot (50" to 60" C), 80 per cent. aqueous ethanol until the wash liquor no longer gives a positive test for chloride ions. The glycollic acid content of these ethanolic wash liquors is then determined by a colorimetric method based on the conversion of glycollic acid into formaldehyde, and its determination with chromotropic acid. Optical density measurements are made at 570nm, and are related to concentration by using a calibration graph prepared for a series of standard aqueous glycollic acid solutions.The use of standard aqueous solutions of glycollic acid is based on previous observations by Bricker and Johnson2 that ethanol and methanol do not interfere with the colour reaction in the determination of formaldehyde. In a later paper, Bricker and Vai13 modified their procedure to remove volatile organic impurities after adding chromotropic acid but before adding concentrated sulphuric acid. Even so, they were still of the opinion that these two specific alcohols did not, in fact, interfere with the colour development of the formaldehyde. This view is also supported by the later work of Ekberg and S i l~e r ,~ and B e r o~a .~ A conductimetric method more suitable for the routine determination of sodium glycollate was developed,6 but when it was applied to the ethanolic wash liquors from sodium carboxymethylcellulose it was found to give consistently higher values for the glycollate content than those obtained by the colorimetric method of Eastenvood. Subsequent investigation revealed the discrepancies to be caused by the interference by ethanol in the colour development with chromotropic acid. This led to a re-appraisal of Easterwood's method and that of the standard colorimetric determination of formaldehyde in ethanolic solution with chromotropic acid.The chemistry of the reaction ofchromotropic acid with form aldehyde is not known with certainty. Feigl' postulates that, as aromatic hydroxy compounds condense with formaldehyde to yield colourless hydroxyphenylmethanes, it is probable that the initial step consists of a condensation of the phenolic chromotropic acid with formaldehyde, followed by oxidation to a 9-quinonoidal compound of the type shown below-SO,H SO,H ' A ' C H J < > / \ H O < d <JPH ' S0,H SOsH / Concentrated sulphuric acid participates in the reaction both as a dehydrant and an oxidising agent. 0 SAC and the authors.

Functional group analysis

“…In colorimetric determinations of formaldehyde and glycolic acid with 1,8-dihydroxy-3,6-naphthalenedisulfonic acid or 2,7-dihydroxynaphthalene, interferences from ethanol have been reported (215).…”