Question

an acid base indicator like bromocresol green is blue under basic conditions and yellow under acidic conditions. what would be a good methodology to quantify a substance of this sort which has general equilibrium as followed:
H2O +Hln <--> H3O +In (In is the indicator in equalibrium)
b) explain why a concentration > .01 M is a potential problem in using UV-vis absorption spectroscopy to determine concentration. c) explain why wavelength selection can be critical in the determine concentration using UV-vis spectroscopy

Answer 1

a) A good methodology to study the equlibrium is via UV-Vis absorption spectrophotometry

H2O + HIn (yellow) -------- H3O⁺ + In^-(green)

Ka = [H3O⁺][In^-]/[HIn]

pH = pKa + log[In-]/[HIn]

log[HIn]/[In-] = pH + pKa

y = mx+c

The idea would be to measure the ration of log[HIn]/[In-] at different pH via spectrophotometry. A plot of pH Vs log[HIn]/[In-] would give a straight line with intercept pKa. where Ka = 10^-pKa

3) Colored solution absorb light at a certain wavelength and emit at the complimentary wavelength. Therefore, inorder to measure the concentration of such solutions it important to select the wavelength at which it shows maximum absorbance.