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

From Wikibooks, open books for an open world
Jump to: navigation, search

Uses of colorimetry[edit]

Colorimetry is used in chemistry and in other sorts of places such as in industries like colour printing, textile manufacturing, paint manufacturing and in food industries. Colorimetry is also used in asprin.

Colorimetery can detect the smallest colour diffrence 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

Sample preparation[edit]

uses of cuvette

Standard procedures and equipment[edit]

Colorimetric Measurement of Iron Concentrations[edit]

By making several solutions of iron (III) 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 (III) sulphate solution of known concentration.

Apparatus[edit]

  • Volumetric flasks, 100cm³
  • Burettes, 50 cm³
  • Colorimeter
  • Pippette
  • Pippette filler

Reagents[edit]

  • Iron (III) sulphate (0.100 mol dm-3)

Precautions[edit]

Wear eye protection, protective gloves and laboratory coat at all times.

Procedure[edit]

  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[edit]

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).

Risk assessment[edit]

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


Medium     

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[edit]

Results, calculation and evaluation[edit]

Scientific principles behind colorimetry[edit]

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).