Colourful Solutions > Practical Scheme of Work > E102 - Back-titration - analysis of an insoluble carbonate

All carbonates except group 1 and ammonium carbonate are insoluble. This means that they cannot be determined directly by titration. However, all carbonates are salts of a very weak acid (carbonic acid) and react with stronger acids forming salts and water and carbon dioxide.

Note: Titrations work better with solution concentrations of between 0.01 and 0.2 mol dm-3.

Background

Metal carbonates react with acids in a neutralisation reaction forming a salt, water and carbon dioxide. The general equation is shown below

Neutralisation of a metal carbonate

MCO3(s) + 2HCl(aq) → MCl2(aq) + CO2(g) + H2O(l)

This allows us to react the metal carbonate with a known amount of excess acid and then determine the excess.

We can then find the moles of carbonate reacted and hence, the relative mass of both the carbonate and the identity of the metal ion. This is known as back-titration.

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Requirements

Chemicals

Apparatus

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Procedure

Measure out 50ml of 2mol HCl into a 250ml conical flask using a pipette and pump.

Weigh out accurately approximately 4g of the unknown carbonate, XCO3.

Carefully add the unknown carbonate to the acid in a 250ml conical flask, rinsing with water to ensure that all the carbonate enters the flask.

When the reaction has completely subsided, transfer the contents of the conical flask to a 250ml volumetric flask and make up to the mark with water.

Titrate 25 ml aliquots of this solution against 0.1 mol dm-3 NaOH or KOH solution (you need to prepare this from solid) until concordant results have been obtained.

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Data analysis

Titres 2 & 3 are concordant and were used to average the titration volume = (24.80 + 24.75)/2= 24.775

This is rounded to three sigificant figures as the inaccuracy is ± 0.1

Hence titre = 24.8 ml ± 0.1

The moles of potassium hydrogenphthalate used = mass/Mr = 5.22/204.23 = 0.00257

Molarity of the potassium hydrogenphthalate solution used = 0.00256/0.25 = 0.102 mol dm-3

Moles of potassium hydrogenphthalate in 25 ml aliquot = 0.102 x 0.025 = 2.56 x 10-3

Equation for the reaction between sodium hydroxide and potassium hydrogenphthalate (abbreviated to KHPh):

NaOH + KHPh KNaPh + H2O

Hence moles of sodium hydroxide also = 2.56 x 10-3

Volume of sodium hydroxide from the titration = 24.8 ml

Hence molarity of sodium hydroxide = 0.103

The was diluted in a ratio of 1:10 from the original stock bottle

Hence concentration of sodium hydroxide in the original stock bottle = 1.03 mol dm-3

Treatment of errors and inaccuracies

These should be recorded as percentage inaccuracy and then propagated through a typical series of steps:

1 Making the potassium hydrogenphthalate solution:

Mass of potassium hydrogenphthalate = 5.22g ± 0.01 = 0.19% (inaccuracy)

Volumetric flask 250 ml ± 0.23 = 0.092%

2 Transfering 25 ml aliquot (pipette)

25 ml ± 0.06 = 0.24%

3 Titration inaccuracy

24.8 ml ± 0.1 = 0.40%

Total inaccuracy = 0.19 + 0.09 + 0.24 + 0.40 = 0.92%

Applying this inaccuracy to the sodium hydroxide give molarity = 1.03 ± 0.01

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Titrations