Editing Absorbance (section)

==Measurements==

===Logarithmic vs. directly proportional measurements===
The amount of light transmitted through a material diminishes [[Exponential function|exponentially]] as it travels through the material, according to the Beer–Lambert law ({{math|1=''A'' = (''ε'')(''l'')}}). Since the absorbance of a sample is measured as a logarithm, it is directly proportional to the thickness of the sample and to the concentration of the absorbing material in the sample. Some other measures related to absorption, such as transmittance, are measured as a simple ratio so they vary exponentially with the thickness and concentration of the material.

{| class="wikitable sortable" style="text-align: center;"
|+ Absorbances and equivalent transmittances
|-
! scope="col" | Absorbance: <math display="inline">-\log_{10}\left(\Phi_\mathrm{e}^\mathrm{t}/\Phi_\mathrm{e}^\mathrm{i}\right)</math>
! scope="col" | Transmittance: <math display="inline">\Phi_\mathrm{e}^\mathrm{t}/\Phi_\mathrm{e}^\mathrm{i}</math>
|-
| 0
| 1
|-
| 0.1
| 0.79
|-
| 0.25
| 0.56
|-
| 0.5
| 0.32
|-
| 0.75
| 0.18
|-
| 0.9
| 0.13
|-
| 1
| 0.1
|-
| 2
| 0.01
|-
| 3
| 0.001
|}

===Instrument measurement range===
Any real measuring instrument has a limited range over which it can accurately measure absorbance. An instrument must be calibrated and checked against known standards if the readings are to be trusted. Many instruments will become non-linear (fail to follow the Beer–Lambert law) starting at approximately 2 AU (~1% transmission). It is also difficult to accurately measure very small absorbance values (below {{val|e=-4}}) with commercially available instruments for chemical analysis. In such cases, [[Laser absorption spectrometry|laser-based absorption techniques]] can be used, since they have demonstrated detection limits that supersede those obtained by conventional non-laser-based instruments by many orders of magnitude (detection has been demonstrated all the way down to {{val|5e-13}}). The theoretical best accuracy for most commercially available non-laser-based instruments is attained in the range near 1 AU. The path length or concentration should then, when possible, be adjusted to achieve readings near this range.

===Method of measurement===
Typically, absorbance of a dissolved substance is measured using [[absorption spectroscopy]]. This involves shining a light through a solution and recording how much light and what wavelengths were transmitted onto a detector. Using this information, the wavelengths that were absorbed can be determined.<ref>{{cite web|last1=Reusch|first1=William|title=Visible and Ultraviolet Spectroscopy|url=https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/Spectrpy/UV-Vis/spectrum.htm|access-date=2014-10-29}}</ref> First, measurements on a "blank" are taken using just the solvent for reference purposes. This is so that the absorbance of the solvent is known, and then any change in absorbance when measuring the whole solution is made by just the solute of interest. Then measurements of the solution are taken. The transmitted spectral radiant flux that makes it through the solution sample is measured and compared to the incident spectral radiant flux. As stated above, the spectral absorbance at a given wavelength is

<math display="block">A_\lambda = \log_{10}\!\left(\frac{\Phi_{\mathrm{e},\lambda}^\mathrm{i}}{\Phi_{\mathrm{e},\lambda}^\mathrm{t}}\right)\!.</math>

The absorbance spectrum is plotted on a graph of absorbance vs. wavelength.<ref>{{cite web|last1=Reusch|first1=William|title=Empirical Rules for Absorption Wavelengths of Conjugated Systems|url=https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/Spectrpy/UV-Vis/uvspec.htm#uv1|access-date=2014-10-29}}</ref>

An [[Ultraviolet-visible spectroscopy#Ultraviolet–visible spectrophotometer]] will do all this automatically. To use this machine, solutions are placed in a small [[cuvette]] and inserted into the holder. The machine is controlled through a computer and, once it has been "blanked", automatically displays the absorbance plotted against wavelength. Getting the absorbance spectrum of a solution is useful for determining the concentration of that solution using the Beer–Lambert law and is used in [[HPLC]].