[Wald_ron]

Hello,
I am working on this problem,

HIn  H+ + In-

absorbance data for 5x10^-4 solution of HIn in .1 M NaOH and .1M HCl , measurements at 485 & 625 nm w/ 1cm cells.

.1 M NaOH A(485)= .075 A(625)=.904
.1 M HCl    A(485)= .487 A(625)= .181

I found all off the molar absorptivities (eta)

NaOH E(485) = 150      E(485) = 1808

HCl    E(485) = 974      E(485) = 362

The second part asks us to determine the acid dissociation constant for the indicator if a pH 5 buffer containing a small amount of the indicator exhibits an absorbance of .567 at 485 nm and .395 at 625 nm.

I am utterly clueless as to where to even start with this one ,

any pointers would be appreciated, or similar examples would be equally appreciated.

[Borek]

Your indicator is a weak acid.

You have three data points - high pH, low pH, pH around 5.

Think - what form (HIn or In^-) dominates at low pH? At high pH?

[Wald_ron]

Your indicator is a weak acid.

You have three data points - high pH, low pH, pH around 5.

Think - what form (HIn or In^-) dominates at low pH? At high pH?

Thank you very much, I was able to determine a solution

[Pipette Monkey]

You know extinction coefficients at two wavelengths.  You know the total concentration of the compound.  There is just one species at low pH, one species at high pH, but around the pka, there will be a combination of ionized and neutral forms.

This is a high school algebra problem    Set up two equations with two unknowns.