Editing Ultraviolet–visible spectroscopy (section)

====Deviations from the Beer–Lambert law====
At sufficiently high concentrations, the absorption bands will saturate and show absorption flattening. The absorption peak appears to flatten because close to 100% of the light is already being absorbed. The concentration at which this occurs depends on the particular compound being measured. One test that can be used to test for this effect is to vary the path length of the measurement. In the Beer–Lambert law, varying concentration and path length has an equivalent effect—diluting a solution by a factor of 10 has the same effect as shortening the path length by a factor of 10. If cells of different path lengths are available, testing if this relationship holds true is one way to judge if absorption flattening is occurring.

Solutions that are not homogeneous can show deviations from the Beer–Lambert law because of the phenomenon of absorption flattening. This can happen, for instance, where the absorbing substance is located within suspended particles.<ref>{{cite journal |last1=Berberan-Santos |first1=M. N. |title=Beer's law revisited |journal=Journal of Chemical Education |date=September 1990 |volume=67 |issue=9 |pages=757 |doi=10.1021/ed067p757 |bibcode=1990JChEd..67..757B }}</ref><ref>{{cite journal |last1=Wittung |first1=Pernilla |last2=Kajanus |first2=Johan |last3=Kubista |first3=Mikael |last4=Malmström |first4=Bo G. |title=Absorption flattening in the optical spectra of liposome-entrapped substances |journal=FEBS Letters |date=19 September 1994 |volume=352 |issue=1 |pages=37–40 |doi=10.1016/0014-5793(94)00912-0 |pmid=7925937 |s2cid=11419856 |doi-access= }}</ref> The deviations will be most noticeable under conditions of low concentration and high absorbance. The last reference describes a way to correct for this deviation.

Some solutions, like copper(II) chloride in water, change visually at a certain concentration because of changed conditions around the coloured ion (the divalent copper ion). For copper(II) chloride it means a shift from blue to green,<ref>{{cite journal |last1=Ansell |first1=S |last2=Tromp |first2=R H |last3=Neilson |first3=G W |title=The solute and aquaion structure in a concentrated aqueous solution of copper(II) chloride |journal=Journal of Physics: Condensed Matter |date=20 February 1995 |volume=7 |issue=8 |pages=1513–1524 |doi=10.1088/0953-8984/7/8/002 |bibcode=1995JPCM....7.1513A |s2cid=250898349 }}</ref> which would mean that monochromatic measurements would deviate from the Beer–Lambert law.