Dyes: Remazol Red F-3B (C.I. Reactive Red
180) (Fig. 1(a)) was provided by DyStar and Pro-
cion Red MX-5B (C.I. Reactive Red 2) (Fig. 1(b))
was provided by BASF. The structures of both dyes are shown below in their azo tautomeric
form. Although these commercial samples contain
inorganic salts and organic formulating agents the
dyes have been used without further puri®cation.
Spectrophotometric studies: Spectra were
recorded at room temperature on a Philips PU
8720 UV/VIS scanning spectrophotometer using a
5 mm glass cell in the 400±600 nm range. Beer's
law experiments were performed using 2 ml of
stock solution (6.510ÿ4 M Procion Red MX-5B;
3.7510ÿ4 M Remazol Red F-3B), serially dilu-
ted with equal volumes of water. Experiments to
study the eect of the additives were performed at
a given dye concentration (6.510ÿ4 M Procion
Red MX-5B; 3.7510ÿ4 M Remazol Red F-3B)
and varying either the salt or urea concentration
in the range 20±120 g lÿ1. To a 2 ml aliquot of an additive solution was added 2 ml of stock dye
solution and the spectrum recorded.