Colourimetric determination of copper ore

An ore is any rock from which a metal may be extracted. Ores usually contain a compound of the metal, a mineral, together with waste material. To decide whether an ore is worth mining it is necessary to find out how much of the useful mineral it contains, and how much is waste. This experiment illustrates one way in which this might be done using a form of colourimetry in which comparisons of depth of colour are made by eye without necessarily using a colourimeter.

The experiment depends on the making and use of a set of comparison solutions of known copper concentration of the kind that might be used with a colourimeter. Here the comparison is made by eye but, as an extension, the solutions could be used in a colourimeter and a proper calibration curve drawn and used.

Equipment

Apparatus

• Eye protection (goggles)
• Beaker, 250 cm³
• Beaker, 100 cm³
• Volumetric flask, 100 cm³
• Small filter funnel and filter paper, to fit volumetric flask
• Test tubes, x6 (must have a capacity of at least 10 cm³)
• Test tube rack
• Plastic weighing dish (boat)
• Measuring cylinder, 50 cm³
• Measuring cylinder, 10 cm³
• Access to a balance, weighing to the nearest 0.1 g

Chemicals

• Purified (deionised or distilled) water
• Dilute sulfuric acid, 2 M (CORROSIVE), 40 cm³
• Sample of powdered ore (see technical notes below) (HARMFUL, DANGEROUS FOR THE ENVIRONMENT), 10 g
• Copper(II) sulfate solution, 1 M, 25 cm³ (HARMFUL, DANGEROUS FOR THE ENVIRONMENT)

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection (goggles) throughout.
• Dilute sulfuric acid, 2M H₂ SO₄ (aq), (CORROSIVE) – see CLEAPSS Hazcard HC098a and CLEAPSS Recipe Book RB098.
• Sample of powdered ore – a simulated copper ore made up with a minimum of 30% by mass of copper(II) carbonate, CuCO₃ (s), (HARMFUL – see CLEAPSS Hazcard HC026) thoroughly mixed with dry ‘silver‘ sand or washed and dried building sand.
• Copper(II) sulfate solution, CuSO₄ (aq), (HARMFUL, DANGEROUS FOR THE ENIVORNMENT) – see CLEAPSS Hazcard HC027c and CLEAPSS Recipe Book RB031.

Procedure

1. Weigh out as exactly as possible 10 g of the ground ore and transfer it into a 250 cm³ beaker.
2. Add 40 cm³ of the dilute sulfuric acid a little at a time, allowing the effervescence to subside between additions. (There is some risk of spray.)
3. When the reaction has finished filter the mixture into the volumetric flask.
4. Add purified water until the total volume of liquid in the flask is exactly 100 cm³.
5. Using the copper(II) sulfate solution provided, prepare six tubes of diluted copper(II) sulfate, according to the following table. Ensure the solutions are well mixed.

6. Pour a 10 cm³ sample of the copper solution from your volumetric flask into another test tube. 
7. Compare the colour of your tube from step 6 with those from step 5. Which one matches the colour best?
8. Estimate the mass of copper mineral in 10 g of the ore using the following table:

Teaching notes

It is a good idea to set up the standard colour test tubes in a rack, put white paper under the tubes and observe by looking down through the solutions. 

When students have completed this experiment they are probably going to ask two things:

1. What is the correct answer?
2. How does the arithmetic work?

For the answer to the first question, consult the person who made up the ‘ore’ mixture – it is best to ‘come clean’ and confess that the ore is not a real one. Samples of copper ore, such as malachite, could be shown, if available.

For the second question, work out the concentration of copper in, say, test tube 3:

Concentration Cu (as Cu²⁺) = 4/10 x 1 M = 0.4 M

Work out the concentration of copper ions when 5 g of copper carbonate is dissolved and made up to 100 cm³ of solution (formula mass of CuCO₃ = 124):

Concentration Cu = (5/124) x (1000/100) = 0.4 M

The two concentrations should be the same. However, this calculation works only approximately because ‘basic’ copper carbonate also contains an equimolar amount of copper hydroxide and some water.

It should be stressed that copper ores are seldom as concentrated as this.