an acid base indicator like bromocresol green is blue under basic conditions and

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

Explanation / Answer

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.

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