There as real deviations that cause a

There are five main limitations to the beer
lambert law, they can be within three forms. Some are known as real deviations
that cause a real limitation to the beer lambert law. Whilst others are either
chemical or instrumental deviations whereby there is a chemical change in the
solution or deviation is caused by the method used. All three of these can lead
to the law not being obeyed and a linear function not being produced.

A real
deviation is the concentration of the solution, this is because beer lambert
law only applied in dilute concentrations. According to Skoog
et al (2014) this is because as the concentration
increased above 0.01 M the distance between the molecules that are responsible
for absorbing the incident light rays are greatly reduced, this led to the
distribution of charge altering and hence the absorption of neighbouring
molecules may become stronger or weaker. This ultimately leads to a deviation
in the linear relationship that would be expected from absorbance against
concentration.

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                In
terms of chemical deviations that limit the beer lambert law this is caused by
the stability/solvent. The analyte can produce a different characteristic for
the absorbance spectrum due to dissociating, associating or interacting with
the solvent. This affects electrons in the sample which them leads to the
change in absorption spectrum. Also, the solvent can affect the
spectrophotometry as when light is absorbed by the solution the solvent can
transfer some of the light energy to itself. This light energy is then spread
around the maximum wavelength (Mehta, 2012).

Finally,
the other source of limitation is from instrumental deviation, three of which
are stray/scattered light, monochromatic light and Fluorescence. This is
important as stray light rays outside of the selected wavelength reach the
solution leading to a decreased absorbance reading as more transmitted light
passes through the sample (Agilent Technologies Inc, 2011). Due to this the results are not a true
reflection of solely monochromatic light ray’s (which the beer lambert law is
based on) and so transmittance by the sample is caused by the stray light also.
Similarly, to this Mehta (2012) also states scattered light radiation also can
occur due to the rays being scattered by surfaces and the filters leading to
the same effect on the results obtained. Another instrumental deviation is that
the sample should not fluoresce as the rays of light emitted by the sample can affect
the reading for absorbance/transmittance.

In
terms of the results and graphs collected, Table 1 highlighted the specific
absorbance values for potassium permanganate, It would be expected that the
highest region would be around the lambda max value. This was achieved with the
highest recordings being around 490 – 550 nm which was around the 530 maxima.
Table 2 showed the recordings for transmittance and absorbance, which when the
transmittance was calculated into Log (100/T) the values are similar to
absorbance suggesting that the relationship of A and T stated by the beer lambert
law is correct.

Looking
at figure 2 it showed the relationship between wavelength of light and
absorbance. The highest peak on the graph was at 530 nm meaning that this was
the specific wavelength in which maximum absorbance of the potassium permanganate
was achieved. This value is characteristic of potassium permanganate as well as
being the wavelength where maximum sensitivity was present. This means changes
in intensity are best detected and risk of deviation from the beer lambert law were
minimised.

                Figure
3 and 4 showed the relationship between concentration vs percentage
transmittance and concentration vs 100/T respectively. The results obtained led
to a non-linear function being produced for both graphs with figure 3 being a
negative correlation and figure 4 being positive. Figure 3 showed that as
concentrations increases, transmittance decreases exponentially. On the other
hand, figure 4 showed that as concentration increases, 100/T increases
exponentially. Both graphs would not be appropriate to use for the beer lambert
law as they are not suitable to do a calibration with.

The
general shape of figure 5 showed that the relationship between concentration
and absorbance is true and that beer lambert law had been obeyed. The graph
showed a linear function meaning that as concentration increased so did the
absorbance of the solutions, giving a straight line on the graph. The figure
suggested that for the limited range of concentrations of potassium
permanganate the beer lambert law was obeyed. This was because of the linear
relationship present as well as the values obtained for Log (100/T) were within
a very close range to the values for absorbance which also proved beer lamberts
law to be true.

 

Conclusion:

The aim of the practical was to investigate
the beer lambert law with a range of potassium permanganate solutions. The
results and graphs support that the law had been obeyed under these
experimental conditions. This is because figure 5 produced a graph that had a
linear relationship between both concentration and Log (100/T), the
relationship between T and A was established and the values of Log (100/T) were
similar to Absorbance, all of these features would be expected of the beer
lambert law.