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?