I did a lab about spectrophotometry and right now I need the molar absorptivity(ε) of thymol blue in acidic solution and also in basic solution. I have tried looking online but I haven't found the data I need.

What is the molar absorptivity of thymol blue in acidic solution and in basic solution? information The specific wavelength is 540.27 nm for the acid and 601.88 nm for the base.

  • $\begingroup$ Are you certain that the purpose of the lab was not to find the molar absorptivity by plotting absorbance as a function of concentration of thymol blue in both acid and base? The slope of that line will be the molar absorptivity times the path length: $$\begin{align} Abs&=\epsilon \ell c\\ y&=mx+b \\ m&=\epsilon \ell\end{align}$$ $\endgroup$
    – Ben Norris
    May 19, 2015 at 1:53
  • $\begingroup$ that is exactly what I did and I have my own results but now I need to calculate the percent error . $\endgroup$
    – ems
    May 20, 2015 at 16:24

1 Answer 1


First of all, molar absorptivity is dependent on wavelength, so you either need to specify a wavelength or you need the entire spectrum.

When searching for absorptivity, it's often effective to search for the older name 'extinction coefficient'. Using that search term in Google, I found an absorption spectrum for thymol blue here (pdf).

  • 1
    $\begingroup$ The wavelength of choice has a maximum absorbance. The molar absorptivity is not usually calculated for other wavelengths. $\endgroup$
    – LDC3
    May 19, 2015 at 1:01
  • $\begingroup$ It's often calculated at the isobestic wavelength. $\endgroup$ May 19, 2015 at 9:39
  • $\begingroup$ If you use the isbestic wavelength, you only find out how much thymol blue is in the solution; you don't find out the pH of the solution, which is the purpose of using the pH indicator. $\endgroup$
    – LDC3
    May 20, 2015 at 1:04
  • $\begingroup$ I added the wavelengths in which the experiments have been done. And also,I was not able to find the information I need in that pdf document. $\endgroup$
    – ems
    May 20, 2015 at 16:28
  • $\begingroup$ Just read them off the graph at Fig. 2. For the acid at 540 nm (are you sure that's the right wavelength? It's a weird wavelength to measure the acid form at) it looks like it's about 0.05 L.(mol.cm)^-1, and for the base at 600 nm it looks like it's about 0.85 L(mol.cm)^-1. You may want to measure that graph more precisely than my eyeball did. $\endgroup$ May 20, 2015 at 18:19

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