Quick question on Lambda Max and Beer Lambert Law

[futureless]

Hey, just wondered if you guys could clear something up for me:D

When using the beer lambert law, should you use the Absorbance value at Lambda max, ie the highest absorbance? (because presumably using different values would lead to different molar absorption coefficients, which isn't good)

[John Cuthber]

Usually, but not always.

At the maximum the change in absorbtivity with wavelength is small so errors in the wavelength don't matter so much.

On the other hand, if there's a "valley" in the spectrum you can use that instead.

Of course, the sensitivity will be lower, but that might be an advantage sometimes. If the solution is very concentrated you might want to use some wavelength where the stuff absorbs less strongly- that way you don't need to dilute the sample to get it within the linear range of the spectrometer.

Also if the solution has some impurity in it that also absorbs at the maximum, but not somewhere else in the spectrum you can sometimes measure the concentration without interference from the impurity.