Experiment 4C- Page 1
Analysis of Iron in an Iron(III) Oxalate Complex
In the previous experiment, an iron(III) oxalate complex was analyzed for its oxalate content.
In this experiment,
the iron content in this complex will be determined using spectrophotometry, one of the most widely used analytical
methods of analysis today.
The use of this technique, particularly in the visible region, is routine in clinical,
biochemical, and environmental laboratories.
Multiple samples can be quickly analyzed.
In a spectrophotometric method of analysis, the sample solution absorbs electromagnetic radiation from an
appropriate light source, and the amount of radiation absorbed can be related to the concentration of material of interest
(analyte) in the solution.
The energy of radiation absorbed, although often in the visible region, may be in any of the
regions of the electromagnetic spectrum.
If visible light is absorbed, we can see the results.
A copper(II) solution
appears blue to our eyes because it absorbs the complementary color yellow from white light and transmits the
remaining blue light to our eyes.
Spectrophotometric methods can be easily quantitated.
The amount of light absorbed is directly related to the
amount of analyte through which the light is passed.
This is determined by both the concentration of the analyte (c)
and the path length of the sample (b).
The ratio of the intensity of light entering the cell, I
, and that leaving the cell, I,
is related to these two factors by Beer's Law.
The transmittance (T) or fraction of radiant energy that reaches the other
side of the cell is given by I/I
Beer's Law states that:
k b c
The constant k often includes the conversion from natural logarithms to base ten logs and is called the molar
, with units of M
The term log I
/I is renamed absorbance (A), and A =
b c, where A = log 1/T.
Note that absorbance is directly
proportional to concentration.
Instruments used for spectrophotometric measurements often have scales that indicate both absorbance and percent
transmittance (%T = 100T).
The conversion between these is:
Since the absorbance unit is linearly related to concentration, it is the unit of preference.
However, the scale on the
instrument is linear in %T.
Hence, the %T scale is usually recorded for ease and accuracy of readings and the values
are mathematically converted to absorbance values (See examples in Table I).