Spectrophotometric determination of iron with quinolinic acid

Quinolinic acid forms two complexes with ferrous ion. One is formed at pH 5.9 and has maximum absorption at 420 mμ; the metal to reagent ratio is 1:2 and the instability constant is about 7.10^-9. The other complex is formed with an excess of potassium cyanide and shows maximum absorption at 440 mμ; the metal to reagent ratio is 1:1 the instability constant is only 2.73?10^-5. The latter complex adheres to Beer's law from 1 to 16 p.p.m. of iron, and its optimum concentration range is 4^-16 p.p.m. of iron, where the percent relative error per 1% absolute photometric error is only 2.94.The reagent is highly specific and can be used spectrophotometrically for the determination of very small quantities of iron in presence of many cations.