[anita-573]

I have attached the equation. I have completed a series of UV/Vis absorption titration experiments of a zinc porphyrin and pyridine as a ligand,1:1 but varying the concentration of pyridine whilst keeping the concentration of the porphyrin constant, to allow calculation of the binding constant by plotting the equation. I am having trouble with it because Af is the final absorbance of the fully complexed porphyrin, however, there is a shift in the absorbance when the ligand is bound to the porphyrin from about 419 nm to 428 nm. I was confused as to what wavelength I am interested in, so I'm not sure what absorbances to use. I have attempted but keep on getting mathematical error on my calculator somehow.

Thanks

[Babcock_Hall]

IIRC the logarithm of a negative number is undefined.  Is it possible that this is the problem?  It might be helpful to show your data.

[anita-573]

Thanks for the reply, my issue was getting zero in denominator and I wasn't sure what wavelength I should be taking the absorbances from, since there is a second peak from complexed porphyrins.

[Babcock_Hall]

I don't understand your system as well as I would like.  I would be tempted to treat A_f and K as adjustable parameters in some sort of regression treatment.  That way it should not be equal to A_obs, which sounds like the dependent variable.