Colorimetric analysis formaldehyde

Free formaldehyde is reacted with acetylacetone in the presence of an excess of an ammonium salt to form the yellow fluorescent compound, 3,5-diacetyl-1,4-dihydrolutidine and subsequently determined spectrophotometrically in methods A-E (14). In these methods, the test sample must be colorless and free from other carbonyl compounds. Some other derivatives have been used to analyze formaldehyde. For example, formaldehyde was reacted with sodium 4,5-dihydroxy-2,7-naphthalene disulfonate in sulfuric acid solution to yield a purple color (580 nm) and then subjected to colorimetric analysis. A purple-colored pararosaniline derivative was used to analyze formaldehyde in air (15). Air sample was passed through an aqueous solution which contained 0.4% of 3-methyl-2-benzothiazolone hydrazone hydrochloride and then a dye produced was determined at 635 or 670 nm (16). Molecular sieve (1.6 mm pettet) was used to trap formaldehyde in air samples. The formaldehyde... 

The progress of the reaction was followed by monitoring the decrease in formaldehyde concentration with time. Previous studies used the hydroxylamine hydrochloride method of analysis (5 -55), but this was avoided in the current study as it requires tedious pH titrations. Instead, a colorimetric method was used that was first developed by Nash (55), involving formation of 3,5-diacetyl-1,4-dihydrolutidine, by reaction of formaldehyde with ammonia and acetyl acetone at neutral pH. The cyclic product absorbs at 412 nm with a molar extinction coefficient of 8,000 (55). Other colorimetric methods cannot be used as they all involve very strongly acidic or basic media (55), which would force the phenol-formaldehyde reaction to completion. 

Chemical methods involving derivative formation have now been largely superseded by physical methods of analysis, particularly gas chromatography. However, under some circumstances it may be impossible to use the latter techniques. Alternative chemical methods of product analysis are described by Melville and Gowenlock ", and are particularly necessary in the field of combustion . For instance, formaldehyde may be estimated by the chromotropic acid method , and glyoxalic acid and glyoxal by colorimetric methods . ... 

Barzana E, Klibanov AM, Karel M. 1989. A colorimetric method for the enzymatic analysis of gases The determination of ethanol and formaldehyde vapors using solid alcohol oxidase. Anal Biochem 182 109-115. 

The need for an automated and reliable system for formaldehyde determination is now clearly recognized. In response to this need, an automated and microprocessor-controlled flow injection analysis (FIA) system was developed in our laboratory. This system is based on the use of the modified pararosaniline colorimetric method (j6). The simplicity, versatility, good precision, high sampling rate, complete automation and relatively low cost of the system make it attractive for the analysis of large numbers of formaldehyde samples. In this chapter, sufficient background in the principle of FIA will be presented to allow the readers to evaluate the technique and its potential application to the routine analysis of formaldehyde will be explored. 

The concentration of carbon dioxide reduction products (formic acid, formaldehyde, and methanol) in the reactor at the end of each run is listed in Table 1. The methanol concentration was determined by gas chromatography using a Porpac T column. Formic acid was determined with an Dionex ion chromatography column, and the formaldehyde was determined with a colorimetric chromatropic acid analysis (J ). We did not examine the solution or gas phase for methane. 

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