A-level Applied Science/Finding out about substances/Colorimetry
- 1 Uses of colorimetry
- 2 Sample preparation
- 3 Standard procedures and equipment
- 4 Risk assessment
- 5 Observations and measurements
- 6 Results, calculation and evaluation
- 7 Scientific principles behind colorimetry
Uses of colorimetry
Colorimetry is used in chemistry and in other sorts of places such as in industries colour printing, textile manufacturing, paint manufacturing and in food industries (including the chocolate industry). Colorimetry is also used in asprin.
Colorimetery can detect the smallest colour difference that the human eye can not 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 subsistent.
Food & Beverage Quality Control - Alpha Amylase Activity - Milk Quality - Miscellaneous Quality Tests
Chemicals & Petrochemicals, Miscellaneous Quality Tests
Mineral Oils & Fuels - Anti-Icing Additive in Aviation Fuels - Marked Oils - Carbonisable Substances - Lead Content Aggregates, Miscellaneous Quality Tests Medical & Clinical Tests - Alpha Amylase in Blood or Urine - Bilirubin - Cholesterol - Cholinesterase, Activity in Blood - Haemoglobin Content of Blood - Iron in Serum - Lactate Dehydrogenase - Lead in Urine/Faeces - pH Value of Blood/Urine - Phenolsulphophthalein (Phenol Red) Excretion Test - Phenylpyruvic Acid - Phosphatase in Blood - Phosphorus, Inorganic, in Blood - Proteins - Salicylate In Blood - Sugar/Glucose in Blood - Sulphobromophthalein Retention - Sulphonamides in Blood/Urine - Trichloro-Acetic Acid - Urea in Blood - Uric Acid in Serum
Air Monitoring Tests - Chromium - Lead
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
Gelatin filters Interference filters Grating Monochromators Prisms
Place chemical sample in cuvette while avoiding to cause any marks (such as finger smudges) on the clear sides of the cuvette.
Standard procedures and equipment
Colorimetric Measurement of Iron Concentrations
By making several solutions of iron (II) sulphate 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) sulphate solution of known concentration.
- Volumetric flasks, 1000cm³
- Burettes, 50 cm³
- Pippette filler
- Iron (II) sulphate (0.100 mol dm-3)
Wear eye protection, protective gloves and laboratory coat at all times.
Make sure you wear shoes
- 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.
- 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.
- Use a colorimeter to determine the absorbance, at the appropriate wavelength of every diluted standard solution made.
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 of the graph: According to the Beer Lambert Law it should be a straight line passing through the origin (i.e. directly proportional).
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)
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 acids 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
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 0.41 when doing serial dilution.