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Module 1: Técnicas espectroscópicas en análisis químico

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XSIQ
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Chemistry - Colorimetry

Colorimetry

One of the simplest forms of instrumental analysis is _colorimetry._ This
is based on the fact that the amount of light absorbed by a coloured
solution is closely related to the concentration of the light-absorbing
species in the solution. You should be well aware that an aqueous solution
of copper II sulfate CuSO4(aq) is blue, due to the presence of Cu(aq) ions.
The higher the _c_[Cu(aq)] the deeper will be the blue colour. The blue
colour is the result of the absorption of the orange-red wavelengths of the
visible light interacting with the solution.

The relationship between amount of light absorbed and c(Cu), coupled with
our capacity to readily distinguish different shades of blue, enables us to
make simple estimates of the c(Cu) in aqueous solutions - as long as we
have some reference solutions of known concentration.

If you make up a set of aqueous solutions of copper sulfate of known
concentrations - standards - it is then possible to estimate the _c_(CuSO4)
in a solution of unknown concentration by simply comparing the colours of
the solutions.

Consider the situation of quickly estimating the concentration of Cu(aq)
in a solution of CuSO4(aq) which has lost its label.

* Make up standard solutions of known concentration of the species being
tested for - i.e. Cu

* Compare the colour of the solution of unknown concentration with the
colours of the standard solutions of known concentration.

Visually it appears that the unknown is approximately 0.35 M Cu(aq).

A more sophisticated approach to colorimetry involves the use of an
instrument known as a colorimeter. This instrument compares the intensity
of a particular wavelength of light as it enters a cell containing the
solution of the species being analysed with its intensity as it leaves the
solution.

The key components of a colorimeter are represented in the diagram below.

The key to this device is using a wavelength of light that is strongly,
but not totally, absorbed by the species being analysed.

So if analysing for Cu(aq), wavelengths in the red section of the spectrum
would be appropriate.

Ideally the intensity of the light leaving the solution will be lower than
the intensity of the light entering because some of it has been absorbed.

The amount of light absorbed by the solution is called the _absorbance_.

A colorimeter is calibrated by measuring the absorbance of light of a
particular wavelength by solutions of known concentration (of the species
being analysed for, e.g. Cu). A graph of absorbance v. concentration is
then established. Calibration establishes the relationship that exists
between absorbance and the concentration of the species being analysed.

The absorbance of a solution of unknown concentration is then measured
using the calorimeter and the colibration curve used to determine the
required concentration.

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