The purpose to this experiment was to determine
the concentration of both Cr3+ and Mn2+ using
spectrophotometry. To do this I dissolve the steel sample in sulphuric and
nitric acids, which oxidizes the chromium and manganese into dichromate and
permanganate when introduced to a small amount of silver ions. 

We are able to then determine the
concentrations of dichromate as well as the permanganate from their absorption
at the 440 and 545nm wavelengths. In order to make sure that the manganese ion
is completely oxidized, I used potassium periodate if it is in slight excess as
shown here:

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 During the reaction, phosphoric acid was
added to create a complex with the iron ion into a colourless phosphate
complex, and by doing this, it reduces the interference from Fe3+ ions.

 

Utilizing the formula given by Beer’s
Law:

We can determine the concentrations of Cr2O72-
and MnO4- by using the measurements taken for the
absorbance at the 440 and 545nm wavelengths. I will be preparing a calibration
curve to determine the molar absorptivity’s of the ions at the two wavelengths
and use these values to then determine the amount of Mn and Cr in my sample.

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