The standard calibration curve was prepared for glyoxylic acid concentration according to the optimized procedure conditions. The spectrophotometer was zero sat using the blank sample that comprised water instead of glyoxylic acid in test mixture. Figure shows the relation between absorbance and glyoxylic acid concentration at 560 nm. A linear relation between color intensity and glyoxylic acid concentration was obtained in the range 0 -0.028 M representative for the calibration curve for glyoxylic acid. At the point 0.028 M, it is supposed that glyoxylic acid amount (0.7 x 10 -5 moles) reacts with the corresponding stoichiometric amount of tryptophan (0.48 x 10 -5 moles from Figure ). The further slight increase in absorbance afterwards can be ascribed to interaction of the excess of glyoxylic acid with tryptophan's secondary products with iron II ion.