Spectrophotometric flow injection analysis of formaldehyde in aqueous solutions using 3-methyl-2-benzthiazolinone hydrazone

Summary A flow injection method for the determination of formaldehyde was developed. It is based on its reaction with 3-methyl-2-benzthiazolinone hydrazone (MBTH) to yield a blue formazan anion. Typically 180 l samples are injected into a buffered carrier stream with a frequency of 30 per hour. This sample line merges with the reagent stream containing MBTH, whereupon an azine is formed. After merging with a second stream containing an acidic solution of FeCl₃ as reagent, MBTH is oxidized to an intermediate that attacks the azine yielding the formazan dye with an _max at 635 nm. The method was applied to formaldehyde concentrations in the range from 0.15 mg/l to 15 mg/l. The relative standard deviation was 2.3% for 3 mg/l formaldehyde, and 5.1% for 0.3 mg/l, respectively. Strong reducing agents like nitrite interfere seriously. Out of the family of carbonyl compounds only acetaldehyde and propionaldehyde cause strong positive interference.