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What is the molar absorptivity of copper sulfate? I am trying to find the molarity of copper sulfate solution by absorption in a spectrophotometer with a cuvette.

However, I can't find $\varepsilon$, the molar absorptivity coefficient of $\ce{CuSO4}$. I searched it up online, and I found experimental data saying that is $\pu{2.81 L mol-1 cm-1}$, but another source says that it is $0.91$.

I have searched on various databases including NIST's, but I couldn't find anything. Is there a definite molar absorptivity for everything, or is it all experimental?

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    $\begingroup$ The value will depend on wavelength so both the values above could be correct; it would be worth checking. Also if you can, measure absorbance vs. concentration. The eqn. is $A=\epsilon [C]l$ where l is the cell path length and $\epsilon $ the extinction coefficient in $\pu{dm^2mol^{-1}cm^{-1}}$ $\endgroup$ – porphyrin May 13 '17 at 12:11
  • $\begingroup$ Right, I searched up 635nm, but I forgot to make sure that it was actually that. Thanks for reminding me $\endgroup$ – abcdefg May 13 '17 at 20:08
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The molar absorptivity of copper sulfate in water at 810 nm (the position of the absorbance maximum) is about 12.3 L/mol/cm

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