[aghastamok]

A question on a pre-lab asks,

"Why is it impossible to accurately measure the molar absorptivity constant (?)
for FeSCN2+ by simply adding some Fe3+ and SCN? solutions to a test tube?"

Does this simply refer to the fact that you can't immediately measure the absorptivity, that one must wait for equilibrium to be achieved, or is there some bigger picture I'm missing? Also, from my understanding, don't two compounds in solution have the same absorptivity as their bonded counterpart?
Thanks in advance. No rush on this one.
me.

[Borek]

Fe³⁺ + SCN^- <-> FeSCN²⁺
FeSCN^2- + SCN^- <-> Fe(SCN)₂⁺
Fe(SCN)₂⁺ + SCN^- <-> Fe(SCN)₃
Fe(SCN)₃ + SCN^- <-> Fe(SCN)₄^-
Fe(SCN)₄^- + SCN^- <-> Fe(SCN)₅^2-
Fe(SCN)₅^2- + SCN^- <-> Fe(SCN)₆^3-

[ARGOS++]

Dear aghastamok,

The answers to your Questions are:
           Because ….;  “No”;  No;  YES!;  “Never”;  “Anyway/BUT” .
NO,  - This is NOT a JOKE from my side !!!

You may realise, that mostly for Complexes (?only?) the Rule holds:
           “As more ‘simple’ the question   ?   as more complex the answer. ”

Let’s be guided by the “chain” of answers:
Because:
It is not such simple, Because of AT LEAST four reasons:
    a)   '?' is not THE Constant you, but EVERYBODY else too, likes to believe in !!!!
          (see:  http://www.p-forster.com/english/themes/Spectroscopy/BASICS/Corrected Beer's Law.htm)
    b)   From a): You are toooo imprecise about the “Chemophysical Enviroment”.
          (Not only, but pH, too!).
    c)   The given answer from Mr. pH (“Borek”).
    d)   You have to know ALL the “pK’s” of complexation for each Species.
          (Not all times true !!!).
           But you must use Activities!  It’s only for reliability.
    e)    …….   f)   …..  .)  ….. (for some: see later).

“No”:
You can measure “immediately” an absorbance, whatever “immediately” means, IF you know what you measure, AND how to handle. That’s why -
No:
With simple words: I don’t know any reason, why you should not be able to wait for (all) known equilibrium’s, if it’s under good conditions.

YES! :   -  You are missing some bigger pictures, - but NOT only you.
               Small Hints: Borek’s answer, and “Anyway/BUT”

Never:
If I translate your “counterpart“ into my “bonded Center-atom“ and “bonded Ligands”, then you will know by yourself, that such Changes CAN’T be unrecognised from the chromophore Systems of the “partners”.

“Anyway/BUT” :
As your basic Idea, the one you would like, is correct, you could do it “anyway” this/your “Way”, BUT you have to use the correct Tools, AND the correct “Way”.
(Yes, quite a Lot of “Ways”, isn’t it?)

Recipe:
    A)  Define an Environment where your Experiment is possible AND is making sense.
         It’s not as difficult as you may think!
    B)  Make an EXACT “Dilution-Raw”, and record all DIGITAL Spectra.
         (Analog: Linearity.htm)
    C)  Do some “Reliable Chemometrics” with Spectra and pK’s, OR
         do a Lot of “Reliable Chemometrics” with only enough Spectra and an additional determination.
    D)  Enjoy your results. (But I’m not able to tell you their precision jet!)
         It’s very easy possible, that precision will be better as any Literature.

I, you too?, think, there are a Lot of  facet worth to think/?philosophise? about.

Good Luck!
                    ARGOS⁺⁺