Understanding colorimetric analysis

In colorimetry, light of a specific wavelength is absorbed by a coloured solution. The concentration of this solution is determined by the amount of light absorbed.

Remember that a blue solution reflects blue light and absorbs the other colours.

In colorimetric analysis the species being analysed is converted into a coloured solution of known volume.

Standard solutions, of known concentration, are also prepared in the same way.

The absorbance of the standard solutions is measured and plotted on a graph, then the absorbance of the unknown solution is measured and its concentration determined using the graph.

In this example the unknown nickel solution has a concentration of 0.06 mol L–1.

Manganese in steel wool

Weigh accurately about 0.2 g of steel wool.

Place the steel wool in a conical flask. In a fume cupboard, add 20 mL of conc nitric acid.

The steel wool is oxidised by the nitric acid, releasing NO2 gas.

Allow this gas to disperse, then heat the solution for 2 minutes.

Add about 80 mL distilled water…

followed by 5 mL of conc phosphoric acid.

The phosphoric acid forms a colourless complex with Fe3+.

Weigh about 0.5 g of potassium periodate powder.

Add the potassium periodate to the flask.

Heat the solution for about 10 minutes.

A pink colour should slowly develop.

Allow the solution to cool, then transfer it to a 200.0 mL volumetric flask.

Rinse the flask and funnel with distilled water.

Finally make the solution up to volume with distilled water.

Remember that the liquid must be at 20 °C before the flask is topped up.

Take a standard solution of potassium permanganate and perform a series of dilutions to estimate the concentration of the sample solution.

The steel wool solution is closest in colour to the 1/20 dilution.

Make up a series of solutions around this concentration. Take 25.0 mL of 0.02 mol L–1 KMnO4 and dilute it to 100.0 mL in a volumetric flask.

Take 15.0 mL, 10.0 mL, 5.0 mL, 2.5 mL and 1.0 mL of this solution and make up each one to 100.0 mL.

14

4 1

4

4 1 1

1

25.0 mL 15.0 mL(MnO ) 0.02027 mol L

100.0 mL 100.0 mL7.601 10 mol L

(Mn) (MnO ) (Mn)

7.601 10 mol L 54.94 g mol

0.04176 g L

c

m c M

Calculate the exact concentration of each of the standard solutions.

Volume taken (mL)

15.0 10.0 5.0 2.5 1.0

Conc(× 10-4

mol L-1)7.60 5.07 2.53 1.27 0.507

Using the colorimeter

•Unless you have special (very expensive) colorimetry cells, make sure you use the same test tube for all your solutions – the slight variations in glass between test tubes will alter your readings. Make sure the test tube is facing the same way for all readings too.

•Exclude all light from the colorimeter before taking your reading. More expensive colorimeters have light-blocking lids. If you are using a ‘home made’ LED colorimeter you will need to block out the light another way.

•You will need to wait several minutes (perhaps longer) for the colorimeter to warm up and for its empty reading to stabilise. You may find you need to use a good quality power supply which can supply a constant voltage.

Place the sample solution in the colorimetry test tube and cover to exclude the light.

Record the absorbance of the solution.

Rinse the test tube and replace with the most dilute standard solution. Be careful to use the same test tube and have it facing the same way.

Record the absorbance of this solution.

Repeat for each of the standard solutions, being careful to face the test tube the same way for each solution.

Finally, re-read the absorbance of the sample solution.

Conc(× 10-4

mol L-1)7.60 5.07 2.53 1.27 0.507

Absorbance

27.92 24.86 23.04 21.18 19.86

Absorbance of sample: 25.04 and 24.89

In an ideal world the absorbance of the sample would be the same both times, but you are unlikely to get perfect results with a simple LED colorimeter.

Plot a graph of your standard solutions and use it to determine the concentration of the sample solution.

The sample solution has a concentration of MnO4-

4.2 × 10-4 mol L-1.

Convert this concentration into g L-1 of Mn, and hence calculate the % of Mn in the steel wool.

Molar mass of Mn

We made up the sample solution in a 200.0 mL flask.

We started with 0.214 g of steel wool.

Important

Many of the coloured solutions made during colorimetric analysis change in intensity with time, and the equipment itself will give different readings when hot or cold. Make sure you take all measurements at the same time (ie during the same 20 minute session), and measure the absorbance of all solutions soon after they are made. If you are analysing a large number of samples over several days or weeks, you will need to make a new calibration curve for each day of analysis.