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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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These are the absorbance spectra of copper sulfate in water (open markers) and heavy water (filled markers) (1):

Copper sulfate spectrum

The absorbance in water is maximum at 780 nm, as stated in the reference. From the spectrum, the molal absorption coefficient, at 780 nm, appears to be approximately 12.5 $\mathrm{(mol/55.51 mol \ of \ water)^{-1} cm^{-1}}$ with aquamolality equal to 0.5.

Since only ordinary water, i.e., $\ce{H2^16O}$, is of interest in the OP's question, the aquamolality is the same as the molality, so the solution is simply 0.5 molal. The measurements were reported to have been made at room temperature, with no specific temperature given. Hence, the solution molarity is approximately 0.5 M.

Reference

  1. G. Jancso, "Effect of D and $^{18}O$ isotope substitution on the absorption spectra of aqueous copper sulfate solutions", Radiation Physics and Chemistry, 74 (2005) 168-171.
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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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