Spectrophotometry question - anyone experienced with it?

[omgwtfitsp]

Hey guys, I'm doing an experiment trying to find the equilibrium concentration for the reaction:

Fe 3+ + SCN- = FeSC 2+

So now, I have already graph the calibration curve and found the constant for absorption and concentration.

This next part of the experiment we use a constant volume of Fe 3+ but change the volume of SCN -

So I have to find the equilibrium concentration for the product (FeSCN 2+) for each trial of different volumes.

So would we expect the equilibrium to be the same for all the trials with the different volumes one of the reactant or should they be different?

Because when I calculated the equilibrium constant for each trial, they all roughly came to about the same number, off by a bit here and there. Except for the first test tube that had the lowest volume of one of the manipulated reactant, this trial had a higher constant than the others.

I'm not familiar so much with spectrophotometry, so is there some experimental error that is associated with this device?

 

[symbolipoint]

Not sure exactly what data you have, but absorbance is directly related to concentration of the complexed iron. You can recalculate the absorbance for constant volume. In other words, if you are adding volume increments of your ligand solution then you are increasing the volume of your resulting solution of the the complex. If you were adding only the ligand without its amount of volume of solvent, then your absorbance would be higher. In reality, you will include solvent WITH the added ligand, therefore, you should make a recalculation of absorbance.