Flow injection spectrophotometric determination of lactic acid in skimmed milk based on a photochemical reaction

A spectrophotometric method for the determination of lactic acid in milk samples based on the use of a photochemical reaction carried out in a Flow Injection System is proposed. Determination is based on the reaction between lactic acid and Fe(III), which is reduced to Fe(II) in the presence of UV light, being the latter made to react with o-phenanthroline. The complex formed between Fe(II) and o-phenanthroline, Fe(o-phen)32+ (ferroin) is a coloured compound and it can be spectrophotometrically monitored at 512 nm. The method shows a linear range between 0.5 and 50 μg ml−1 with a limit of detection of 0.16 μg ml−1. The precision was ±2.15 expressed as relative standard deviation (n=11) and the sample throughput of 30 samples h−1. Also non-linear adjustments have been made and validated by ANOVA. The proposed method has been applied to the determination of lactic acid in both synthetic and milk samples.