Volumetric Analysis (6) Estimating the % of copper in brass–a redox titration.

In this extensive blog about this titration I’m going to discuss the theory behind what happens then describe a simple procedure and work you though the calculation that you will need to do.

Theory

Brass is an alloy of copper and zinc.

I love this titration because it involves some superb colours and a lovely toxic and corrosive concentrated acid–nitric acid.

The reaction involves several redox reactions:

First, the reaction between copper and concentrated nitric acid

Cu + 4H⁺ + 2NO₃^–  =   Cu²⁺ + 2NO₂ + 2H₂O

Or more simply

Cu (s)     =    Cu ²⁺ (aq)  +   2e^–

Second, the reaction between copper ions and iodide ions to release iodine

2Cu ²⁺ (aq)    +    4I^– (aq)   =   2CuI(s)   +   I₂ (aq)

Third, the reaction between iodine and sodium thiosulphate solution

2Na₂S₂O₃ (aq)    +   I₂ (aq)   =    2NaI (aq)      +    Na₂S₄O₆ (aq)

The basic procedure goes like this:

We measure out a given mass of brass and dissolve it in an excess of concentrated nitric acid. 

You can of course break British law and use a copper coin instead of brass and so find out the % of copper in coinage.

You could work on coins from different times to see if the Government has gone cheap in its production of coinage.

I wouldn’t be surprised to see that the % of copper has fallen over the years. 

You will need to wait longer than you think until all the copper dissolves to form a green solution; it is a reaction best done in a fume cupboard since the brown fumes evolved are toxic nitrogen dioxide (NO₂).

The acidic green solution consists of copper complexes with nitrogen oxides in the solution.

The acidic solution contains a high concentration of nitrate ions (NO₃^– ) and under these conditions these ions are oxidising.

If nitrate ions and iodide ions are in the same solution then a redox reaction occurs because iodide ions are reducing agents like so:

2NO₃^- + 4H⁺ + 2I^-   =  I₂ + 2NO₂ + 2H₂O.

If left the nitrate ions will affect the end point of the titration as they will remove iodide ions from the solution. 

So we need to neutralise the solution with sodium carbonate, a non oxidising compound.

You will see a precipitate for when you add the sodium carbonate this precipitate is copper hydroxide. 

Adding a weak acid like a dilute solution of ethanoic acid will redissolve this precipitate.

Now if you add potassium iodide to the reaction mixture the iodide ions will only react with copper(II) ions and not the nitrate ions present. 

Iodine is liberated together with copper(I) iodide which is a white precipitate.

Your solution should go a thick creamy colour looking like toffee yogurt (ugh!).

As you titrate this stuff with sodium thiosulphate the mixture goes pale then you can add the starch indicator to give the usual purple colour. 

The end point occurs when this purple colour disappears with the addition of one drop of sodium thiosulphate. 

Here you will find a youtube video of the titration procedure

Here are some instructions you could follow to carry out this titrimetric determination.

Procedure

First, weigh accurately a piece of brass (or your copper coin) it should be about 2.5g.

Second, add the brass to about 20ml of concentrated nitric acid in a 250ml beaker., do this in the fume cupboard.

Third, let the beaker and contents stand in a fume cupboard until the copper has all dissolved. (this take ages!!)

Fourth, very carefully transfer all this mixture to a 250ml volumetric flask with all the washings—the solution should look green, but do not make up to the mark. 

Fifth, now add 2M sodium carbonate solution drop by drop until a faint precipitate appears; follow the sodium carbonate with 2M ethanoic acid drop by drop until the precipitate just dissolves. 

Sixth, now you can make up to the mark with distilled water!!

Seventh, pipette 25ml of your green copper(II) solution into a 250ml conical flask, add about 10ml of 1M potassium iodide solution (measuring cylinder will do) to liberate the iodine and precipitate copper(I) iodide and then titrate with standardised 0.100M sodium thiosulphate solution.      

Eighth, when the iodine colour in the flask is pale brown (remember this looks like toffee yoghurt!) add 1ml starch solution to turn it purple.

Ninth, continue titrating with 0.100Msodium thiosulphate solution until one drop removes the purple starch/iodine colour and leaves a creamy-white colour in the flask.

You will concordant results i.e. three titres to with 0.1ml.

Here is a typical set of results

Mass of brass sample 2.67g

Titration results:

Pipette solution:    copper(II)                                       x  mol/dm³

Burette solution:    Sodium thiosulphate                    0.100mol/dm³

Indicator:                 starch solution

Burette readings:               Rangefinder            1                     2                     3

Final reading (ml):           25.30                        25.10              25.20           25.20

First reading (ml):              0.00                         0.00               0.00               0.10

Volume NaOH (ml):           25.30                        25.10             25.20           25.10

Average titre (ml):                        25.1(3)

1.    Work out amount of sodium thiosulphate used.

Amount thiosulphate   =    25.1  ×  0.100   =   2.51mmol.

2.    This amount of thiosulphate reacts with a tenth of the mixture in the flask so amount thiosulphate equivalent to all the liberated iodine is 25.1mmol.

3.    Use the reaction equations to determine the amount of copper in the sample of brass

2Na₂S₂O₃ (aq)    +   I₂ (aq)   =    2NaI (aq)      +    Na₂S₄O₆ (aq)

25.1mmol          12.55mmol

2Cu ²⁺ (aq)    +    4I^– (aq)   =   2CuI(s)   +             I₂ (aq)

25.1mmol                                                          12.55mmol

So the amount copper in the brass sample was 25.1mmol.

4.    Work out the mass of copper in this amount.

Mr (Cu)  =  63.5 g/mol  therefore mass of copper is

63.4  ×  25.1/1000  =   1.59g  copper.

5.    Work out percentage of copper in the brass sample as

Mass of copper ×  100/mass of sample   =   % copper

1.59  ×   100/2.67    =   59.6%

Extension questions:

1.    You could analyse the redox equations and work out the oxidation number changes

2.    You could estimate the percentage error in the final result and work out where the greatest error is likely to be. 

3.    You could work out the kind of complex ions copper makes with nitrogen compounds not just nitric acid.

4.    You could look up the way copper reacts with nitric acid under different conditions.