# How do you solve for concentration knowing absorbance?

Jun 8, 2016

You use Beer's law:

$\setminus m a t h b f \left(A = \epsilon b c\right) ,$

where:

• $A$ is the absorbance of the solution. This is unitless.
• $\epsilon$ is the extinction coefficient, or molar absorptivity, of the species in solution in "L"/("mol"cdot"cm").
• $b$ is the path length of the cuvet. This tends to be $\text{1 cm}$ for typical UV-Vis spectrometry.
• $c$ is the known concentration of your solution in $\text{M}$ or $\text{mol/L}$.

Hence, all you really need to do is solve for $c$ to get:

$\textcolor{b l u e}{c = \frac{A}{\epsilon b}}$

You already have $A$, you should have been supplied the value of $\epsilon$, and you now know what $b$ is. This is all you need to get $c$.

If you have multiple absorbances, then you have multiple concentrations to solve for.

You don't really need the wavelength to calculate concentration, but you should still report that. I'm guessing it is your ${\lambda}_{\text{max}}$.

Example: That is the wavelength where the light absorption generates the tallest absorbance signal.