The measurement must be done when at a maximum of absorption. This is to prevent against any instrumental deviation (where the desired wavelength is not the wavelength used in practice by the spectrometer) or any spectrum modification (of which, strictly speaking, the shape depends of factors such as temperature, ionic force, nature of the solvent, and the nature of other present molecules …), the measurement error of absorbance A is minimised if . Referring back to the studied molecule’s spectrum is necessary in order to determine the wavelength to work with. Many information sources are useful to find out the studied element’s spectrum:
In case of complete absence of information and/or to acquire a spectrum measured in controlled experimental conditions, one needs to establish oneself the spectrum by following the protocol detailed here.