L,8-dihydroxynaphthalene-3,6-disulphonic

Chromotropic acid (l,8-dihydroxynaphthalene-3,6-disulphonic acid, formula 53.1) gives water-soluble, brown-red titanium complexes which differ in composition and colour according to the acidity of the medium [22]. The pH at which the colour is developed is therefore critical. The solutions are usually buffered with formate (pH 3-3.5) or acetate (pH 4-5). 

Photometric determination. Formaldehyde yields a red-violet colour with chromotropic acid (l,8-dihydroxynaphthalene-3,6-disulphonic acid) in concentrated sulphuric acid, its intensity being proportional to the concentration of formaldehyde. The measurement is made at 560 nm [23]. 

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