Question

Can you explain how a fingerprint left on the spec cell containing one of the test samples would affect the:

a) determination of the equilibrium constant?

b) concentration of Fe3+ remaining at equilibrium for the test sample?

c) absorbance measurement for the test sample?

This is for my determination of an equilibrium constant post-lab, this is what we were working with:

Fe3+ (aq) + SCN-(aq) =/=FeSCN2+(aq)

Answer 1

In a spectrometer, the sample solution is placed inside the cuvet, and a tiny spectrum light beam is shined through it. Some light is absorbed by the sample, and the rest is transmitted and hits the (also tiny) detector on the other side of the cuvet. The "absorbance" is basically the difference between the full spectrum of the beam, i.e., before it passes through the sample, and the detected light, i.e., after it passes through the sample. The cuvet itself isn't supposed to absorb any light, and normally it doesn't but if it's dirty, that's a different situation. The oil or dirt particles will scatter (change the angle of travel of), and possibly absorb, some of the light. This will give a false reading.

(a) Kc is the equilibrium constant and is basically products over reactants. It depends upon how the concentrations of the products and reactants will appear to change based on the effect of fingerprints on absorbance.

(b) Beer's Law states that higher absorbance means higher concentration (A = ECl).

(c) Oil & dirt in the form of fingerprints certainly won't create more light for the detector to see. But it will scatter and absorb some of the beam's light which means that less will reach the detector. Remember that absorbance is the light that doesn't reach the detector.