Besides photolysis, pH can also affect a dye's peak absorption wavelength.

Therefore, RR solutions of 10 mg/L were prepared at different pHs and compared before

and after UV exposure. The change in absorbance (at 538 nm) as a function of pH is

shown in Figure 4-4. At a pH above 8, there was a decrease in absorbance along with a

slight visible change of color from a dark pink to a red. Figure 4-5 displays a section of

the spectrophotometer scan and shows the changes that occur at an alkaline pH. At a pH

of 7, the maximum peak wavelength is 538 nm. At a pH of 9.5, the highest peak occurs at

both 514 nm, as well as 538 nm. After two hours of UV exposure, the max peak returns

back to 538 nm. Also noticeable at pH 9.5 is a slight downshift in the curve after UV

exposure. Therefore, the pH was kept below 8 for comparison studies of gels to ensure

accurate measurements of destruction ability.




 0.3

 0.25

 o 0.2

 S0.15
 0
 0.1
 S0.1 -- Abs. (initial)

 0.05 -- Abs. (final)

 0
 0 2 4 pH 6 8 10 12
 Figure 4-4. Change in absorbance of RR at 538 nm as a function of pH.
Figure 4-4. Change in absorbance of RR at 538 nm as a function ofpH.