Colorimetric determination of a copper ore

Class practical

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 colorimetry in which comparisons of depth of colour are made by eye without necessarily using a colorimeter.
 

Lesson organisation

This 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 colorimeter. Here the comparison is made by eye but, as an extension, the solutions could be used in a colorimeter and a proper calibration curve drawn and used.

Health & Safety and Technical notes

Read our standard health & safety guidance

Wear eye protection throughout. 

Dilute sulfuric acid, H₂SO₄(aq), (CORROSIVE) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.

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) 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 and CLEAPSS Recipe Book. 

1 Test-tubes must have a capacity of at least 10 cm³.

Procedure

a Weigh out as exactly as possible 10 g of the ground ore and transfer it into a 250 cm³ beaker.

b Add 40 cm³ of the dilute sulfuric acid a little at a time, allowing the effervescence to subside between additions.

c When the reaction has finished filter the mixture into the volumetric flask.

d Add purified water until the total volume of liquid in the flask is exactly 100 cm³.

e 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.

f Pour a 10 cm³ sample of the copper solution from your volumetric flask into another test-tube. 

g Compare the colour of your tube from part f with those from part e. Which one matches the colour best?

h 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.

Health & Safety checked March 2009

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