A procedure for assaying commercial samples of eosin.

A simple, rapid and inexpensive assay method for the colored components of commercial samples of eosin is described. Samples are dissolved in dilute aqueous ammonia and absorbance measurements made at wavelengths of 492 nm, 512 nm, and 518 nm. The concentrations of fluorescein, tribomofluorescein and eosin are derived by solution of simulataneous equations in three unknowns. The validity of the method has been confirmed by recovery experiments. Mean recoveries of fluorescein, tribromofluorescein and eosin were 99.1%, 100.2% and 98.8% respectively. Seventeen commercial samples have been assayed by this procedure. Concentrations of fluorescein, tribromofluorescein and eosin ranged from 0.00%-0.82%, 4.34%-29.92% and 44.89%-82.82% respectively. The sodium concentrations of the same samples were measured by atomic absorption spectrophotometry and found to range from 0.10% to 6.13%. The presence of free halide (Br-, Cl-, or both) was demonstrated. These observations, together with color variations in the samples, indicate that in commercial products eosin is in the form of both the free acid and sodium salt. Volatile matter accounted for 1.68%-16.11% of the samples.