1.8- dihydroxynaphthalene-3,6-disulphonic acid

Alternative names are p-nitrobenzene-azo-1 8-dihydroxynaphthalene-3 6-disulphonic acid and Chromotrope 2B (the latter is the sodium salt). 

Arsenazo I and a number of its analogues are derivatives of chromotropic acid (1,8-dihydroxynaphthalene-3,6-disulphonic acid). Arsenazo I is applied to the determination of rare earths, Th, U, Zr, Ti, Nb, and other metals. 

Glycerol can be oxidized to formaldehyde by periodic acid and the formaldehyde measured spectrophotometrically at 570 nm after reaction with chromotropic acid, or fluorimetrically after the addition of diacetylacetone and ammonia. The chromotropic acid reagent consists of 8-dihydroxynaphthalene-3,6-disulphonic acid dissolved in 50% sulphuric acid. 

Arsenophenylazo)-4,5-dihydroxynaphthalene-2,7-disulphonic acid (trisodium salt) see Arsenazo. 

Amidonaphthol-4 6-disulphonic acid (K acid) yields with mercuric chloride or acetate unstable red compounds, to vrhich no formulae have been assigned the same result is obtained with its mono and dibenzoyl derivatives. 1 8-Amidonaphthol-3 6-disulphonic acid (H acid) and 1 8-amidonaphthol-4-sulphonic acid give,colours and reactions similar to K acid, but these compounds are even more readily decomposed. Constancy of composition has not been obtained in the case of 1 8-dihydroxynaphthalene-3 6-disulphonie acid (chromotrope acid), the derived compounds being orange in colour. ... 

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]. 

S6ls. give dark brown col. with PeQa Boil ag. sol. —> 1 2-dihydroxynaphthalene-3 6-disulphonic acid. Reduces NH3.AgNOs. Does not couple with diazo-comps. Mono-Na salt is the developer Diogen. 

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