A-level Applied Science/Finding out about substances/Colorimetry

Uses of colorimetry

Colorimetry is used in chemistry and in other sorts of places such as in industries, color printing, textile manufacturing, paint manufacturing and in food industries (including the chocolate industry). Colorimetry is also used in aspirin.

Colorimetry can detect the smallest colour difference that the human eye cannot pick up. Under the action of chemical agents, samples develop a specific colour that shows the concentration of the substance being tested.

Colorimetry is just one of the types of photometric analysis techniques i.e. it is a way of measuring light.

Below are most possible colorimetric tests:

Colorimetry can be used to find out the concentration of any coloured substance.

Applications of colorimetry

Food & Beverage Quality Control

1. Alpha Amylase Activity
2. Milk Quality
3. Miscellaneous Quality Tests

Mineral Oils & Fuels

1. Anti-Icing Additive in Aviation Fuels
2. Marked Oils
3. Carbonisable Substances
4. Lead content

Medical & Clinical Tests

1. Alpha Amylase in Blood or Urine
2. Bilirubin
3. Cholesterol
4. Cholinesterase, Activity in Blood
5. Haemoglobin Content of Blood cells
6. Iron in Serum
7. Lactate Dehydrogenase
8. Lead in Urine/Feaces
9. pH Value of Blood/Urine
10. Phenolsulphophhtalein (Phenol Red) Excretion Test
11. Phenylpyruvic Acid
12. Phosphatase in Blood
13. Phosphorus, Inorganic, in Blood
14. Proteins
15. Salicylate In Blood
16. Sugar/Glucose in Blood
17. Sulphobromophthalein Retention
18. Sulphonamides in Blood/Urine
19. Trichloroaccetic Acid
20. Urea in Blood
21. Uric Acid in Serum

Air Monitoring Tests

1. Chromium
2. Lead
3. Pipino

Method

Colorimetry measurements are made by using a light which passes through a colour filter. The light then passes through a little box (cuvette) with the actual chemical substance. The light leaving the actual sample should be less than the light that actually entered the compound. The loss of light always reflects the concentration of the compound.

Colorimetry can only be done to measurements which are within the visible region of the electromagnetic spectrum, which is 380 – 780 nm. The main general factors which affect the amount of light absorbed by the sample are the wavelength of the light and the colour of the solution.

When manufactures are using a colorimeter, they have a choice of which wavelength they use. The options are:

1. Gelatin filters
2. Interference filters
3. Grating Monochromators
4. Prisms

Sample preparation

Weighed into the crucible, the cotton thread is attached to the wire, and the crucible is put into the vessel. This applies to powdery substances

Colorimetric Measurement of Iron Concentrations

By making several solutions of iron (II) sulfate with known concentrations, the absorbance of each can be determined and a calibration curve can be constructed. Given a solution of unknown concentration, its concentration can be determined from its absorbance.

You will be given iron (II) sulfate solution of known concentration.

Equipment

• Volumetric flasks, 100cm³
• Colorimeter
• graduated Pipette
• Pipette filler
• cuvette

Reagents

• Iron (II) sulfate (0.100 mol dm^-3)

Precautions

Wear eye protection, protective gloves and laboratory coat at all times.
Make sure you wear shoes.

Procedure

1. Use the colorimeter to measure the absorbance of the water sample and the standard solutions across a range of wavelengths. Choose a suitable wavelength for the rest of the experiments.
2. As a group, decide on how you will need to dilute the standard solutions. You will need to make a series of dilutions which will have absorbances similar to the water sample.
3. Use a colorimeter to determine the absorbance, at the appropriate wavelength of every diluted standard solution made.

Results

• Plot a graph of absorbance against wavelength for the unknown sample and the standard solutions.
• Explain your choice of wavelength.
• Plot a graph of absorbance against concentration for the diluted standard solutions.
• Calculate the concentration of the unknown solution.
• Comment on the shape on the graph: According to the Beer Lambert Law it should be a straight line passing through the origin (i.e. directly proportional).

Risk assessment

• Experiment or Activity: Colorimetry
• Material used or Procedure:Hazard
• (the harm it can cause):Risk
• (the chance of harm):Reducing the risk
• (What can be done to make it safer):Emergency Procedures (What to do if something goes wrong, e.g. First Aid)
• Nitric acid
• Phosphoric acid
• Potassium iodate
• Potassium mangonate

All of these are harmful to skin. If in contact with skin they are an irritant. Also can cause burns to skin if left in contact too long. Also can get in eyes and cause blindness

Wear protective gloves when handling acid. Wear protective goggles at all times when conducting experiment.

If in contact with skin run under cold water immediately. If acid gets in eyes rinse and wash out eyes.

Observations and measurements

once you have carried out they experiment your results should show a clear pattern. As the concentration of the solution increases so should the absorbance recorded, meaning they are proportional to one another.

Results, calculation and evaluation

Percentage %Unknown (A)0.1 M (A) 100% 0.13 0.29 75% 0.10 0.24 50% 0.08 0.15 25% 0.06 0.11

Evaluation of our colorimetry experiment: The good thing about our experiment is that we got good results and it compared well against the rest of the class because there percentages as seen of the graph were around the same mark as ours was. A bad thing about our colorimetry experiment is that the colorimeter was playing up and sometimes gave us the wrong readings and this set us back a bit because we had to change the colorimeter meaning again we could not repeat.

Scientific principles behind colorimetry

Colorimetry is the science that describes colours in numbers. It is used in chemistry, and in industries such as colour printing, textile manufacturing, and paint manufacturing.

A colorimeter measures the intensity of light shining through a coloured solution compared to the intensity of light passing into the solution. A detector measures the transmittance (T) (% of light passing through) of the solution. This is mathematically converted to absorbance (A = #log₁₀T). The absorbance is directly proportional to the concentration (Beer#Lambert law). The colorimetry of sulfuric acid is a blue colour and will give an absorption of 0.42 when doing serial dilution.