scope:

Introduction

Aldehydes are a class of partially oxidized organic compounds which, owing to the reactive functional group, the carbonyl group, react rapidly at varying rates to form secondary products.

Short-chain aldehydes are formed in the incomplete combustion of hydrocarbons or alcohols, for example in firing plants or gasoline or diesel engines. The reactivity of aldehydes, particularly formaldehyde (methanal), is utilized industrially in the synthesis of some plastics. However, polymerization is not quantitative, so that aldehydes can escape during the curing phase.

Aldehydes are important ozone precursors and intermediates in the formation of "summer smog". In addition to their direct hazard to health [1] they are also hazardous to the environment via their degradation products. Thus, for example, peroxy and hydroperoxy free radicals, which can lead to high concentrations of photooxidants, in particular ozone, in chainreaction systems, are among the secondary products of photochemical aldehyde breakdown [2].

Formaldehyde distinguishes by a pungent aroma; even at very low concentrations it irritates the mucous membranes of the nose, eyes and respiratory pathways. For formaldehyde a limit value for workplace air (TRGS 900) [G1] as well as a MAK value [maximum permissible workplace concentration ¹)] exist and the WHO specifies an air quality guideline value [4]. In 1977 the German Federal Health Office recommended a guideline value for the formaldehyde concentration in indoor air [5; 6]. The emission of formaldehyde is limited under the German Technical Instructions on Air Quality Control - TA Luft [G2].

Part 1 of Guideline VDI 3862 [T1] gives instructions on measuring short-chain aliphatic aldehydes by the MBTH method (3-methyl-2-benzothiazolinone hydrazone). The aldehydes are determined as a total. The sensitivity of response of MBTH to the individual aldehydes is variable. The method is suitable, in particular, for measuring formaldehyde emissions.

In Guideline VDI 3862 Part 2 [T2], a method is presented which enables aliphatic and aromatic aldehydes and ketones to be determined as individual components. The substances are reacted with 2,4- dinitrophenylhydrazine in the absorption solution (acetonitrile) in gas wash bottles to give the corresponding hydrazones and are quantified as individual components. The absorption solution is analyzed using high-performance liquid chromatography (HPLC).

In Guideline VDI 3862 Part 3 [T3], a method is presented which enables the same aldehydes and ketones to be determined individually. The substances are reacted in cartridges with 2,4-dinitrophenylhydrazine (DNPH) on a solid sorption material to give the corresponding hydrazones. After elution with acetonitrile the individual components are analyzed using an HPLC instrument. Owing to the limited sorption capacity for aldehydes and ketones this method is predominantly suitable for measuring diluted exhaust gases or small amounts of samples.

In the present Part 4 a measurement procedure for formaldehyde by the AHMT method is described. The reaction of formaldehyde with AHMT does not take place in an acidic as in other known methods but in an alkaline medium. This has decisive advantages for measurements which show cross sensitivities (e.g. with urotropin) in an acidic medium (see Section 1).

¹) MAK values are published by the "Senatorial commission for the examination of hazardous working materials (Senatskommission zur Prüfung gesundheitsschädlicher Arbeitsstoffe)" of the German Science Foundation (DFG). The list of MAK values is annually checked and enlarged.