Spectrophotometric Determination of Iron

Chem 128
September 27, 2002
web copy: edited september 25, 2002


To determine the iron content of a sample


Spectrophotometry (absorption of light)

General Statement:

Spectrophotometry is an extremely rapid way to measure the concentration of solutions. It is one of the most widely used methods of quantitative analysis. It can be applied to a wide range of chemical species.
  • If the species is not strongly colored

    Background--Absorption of Light and Absorbance

    Beer's Law states


    The Chemistry Involved

    We want to determine the concentration of iron (Fe2+ or Fe3+) in a sample. Iron compounds are generally not strongly colored, so direct spectrophotometry is not practical. However, we can find a reagent that reacts with Fe2+ to produce a strongly colored molecular complex. Measuring the concentration of this species is equivalent to measuring the concentration of iron.

    The reagent of choice is a colorless compound called 1,10 phenanthroline.

    This species has two nitrogen atoms with available electron pairs. Each of the nitrogens can bind to an Fe2+ ion. In the language of week two, this is a bidentate ligand. If there is an excess of the reagent, we will end up with three phenanthroline molecules bound to each iron ion.

    This complex is an intense red color and it easy to measure with a spectrophotometer. It is easy to measure the iron in very dilute solutions-- concentrations of 1 mg/liter (1 part per million) are easily measured. That's around 2 x 10-5 Molar.

    We have two complications when we analyze our K3Fe(C2O4)3 :

    Experimental Procedure

    Chemicals Required:

    Special Equipment

    Standard Solutions and the Calibration Plot

    Solution of the Iron Salt

    Notice that we only use 1% of the sample we weighed out. If we had tried to weigh this directly it would have weighed 3 mg but the balances can only weigh to +1 mg.
    The Spectronic 20 Spectrophotometer

    The basic design of the instrument is shown in the cartoon

    An external diagram of a Spectronic 20


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