an acid base indicator like bromocresol green is blue under basic conditions and
ID: 1050340 • Letter: A
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.