The principles of colorimetry - testing the composition of a paperclip

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Shaima khelfa

                                       Principles of colorimetry

Colorimetry is used to measure the intensity of absorption of coloured compounds over a narrow range of frequencies. It is a useful way of determining the concentration of a chemical in a coloured solution.

Beers law states that the amount of light absorbed by a solution is directly proportional to path length and concentration.

                                                      A = abc

This graph shows the relationship between absorbance and concentration that they are proportional to each other.


In a colorimeter a narrow beam of light passed through a filter towards a tube containing the coloured solution. Any light that passes through the solution is detected by a photocell and a reading of absorbance is displayed on the meter.

Before you start to measure the absorbance of any sample you firstly have to calibrate or reference the colorimeter with distilled water so that the baseline will be placed at 0.00Abs or 100% T. Also appropriate wavelengths have to be selected before starting to measure the absorbance of your sample.

How much manganese is there in a paperclip?


  1. Cut up a paper clip in to small pieces and place in a weighing boat to weigh accurately about 0.2g using a weighing balance.
  2. Measure approximately 70cm3 nitric acid using a measuring cylinder and pour it into a beaker. Then add the small pieces of paperclip to it.
  3. Place the beaker with the nitric acid and paperclip under a Bunsen burner to warm up but make sure to not let it boil. The nitric acid oxidises the Manganese to Mn 2+ (aq) ions  
  4. Add about 10 cm3 of phosphoric(v) acid to the beaker followed by 10 cm3 of potassium iodate (VII) solution. Boil the solution carefully for 10 minutes and then allow to cool. (The phosphoric(v) acid prevents the precipitating of insoluble iron (III) salts).
  5. Pour the solution the solution into 100cm3 volumetric flask using a small funnel. Rinse the remaining solution from the beaker and funnel into the flask with distilled water and add further more distilled water to the flask till the graduation mark.
  6. Stopper the flask and shake it to ensure that the solution is uniform and has been mixed thoroughly. All the Manganese in the 0.2g of paperclip is now in the purple solution as the Manganate.
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Using colorimetry to find the % in a paperclip

  1. Pipette 10cm3 of the prepared potassium Manganate (VII) solution into a 100cm3 volumetric flask and make up to the graduation mark with distilled water. This corresponds to the solution which would be obtained from the paperclip containing 2.0% Mn (2.0% Mn solution). Set aside a sample of this solution to use in the colorimeter.
  2. Take two burettes and fill one with water and the other with 2.0% Mn solution. Make up 20cm3 samples of solutions to correspond to 1%, 0.8%, 0.6%, 0.4% and 0.2% solutions ...

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