Thiobarbituric acid-reacting substances derived from autoxidizing linoleic and linolenic acids

Seven pyridylazo dyes containing the m-tolylenediamine group were synthesized and their analytical potential for the determination of cobalt was studied spectrophotometrically. The molar absorptivities and selectivity of these reagents increased compared with those of PADAB. Cobalt(II) and 3,5-diCl-PADAT (5-[(3,5-dichloro-2-pyridyl)azo]-2,4-diaminotoluene) at pH 3 form a complex which is very stable even in the presence of strong mineral acids. The complex has two absorption maxima at 548 and 590 nm in hydrochloric acid (2.4 M) solution. The color is very stable and the system conforms to Beer's law; the optimal range for measurement in a 1-cm cell is 0.01–0.4 p.p.m. cobalt. In practice, this color reaction is specific. The molar absorptivity is 1.38 ·10⁵ 1 mol^-1 cm^-1 at 590 nm. The sensitivity is 0.00042 μg Co cm^-2 at 590 nm for log I₀/I= 0.001. The method was applied to the determination of cobalt in steel and waspaloy.