The change in absorbance at 510 nm vs. time for the
reaction between vanillin and catechin in a solution of 4%
concentrated HC1 in glacial acetic acid is shown in curve
A of Figure 1. When the same reaction was carried out
in methanol (Price et al., 1978), the absorbance after 5 min
was approximately 3-fold lower.
Addition of methanol to the reaction in glacial acetic
acid caused a rapid decrease in absorbance (curves B and
C in Figure 1). The change in ASI0 is not due to a single
second-order reaction for either the initial color formation
or the decrease following the addition of methanol, as
shown by the nonlinearity of the appropriate semilog plots