IB Chemistry home > Syllabus 2016 > Practical Scheme of Work > Determining solutions

Titration - determining the strength of an unknown base

The determination of a base is carried out by titration of the unknown solution using a standard acid solution.

The bench sodium hydroxide is a solution of approximately 1 mol dm-3. The purpose of this experiment is to determine its exact molarity. It is to be determined by reaction with a potassium hydrogenphthalate primary standard solution.

Note: Titrations work better with solution concentrations of between 0.01 and 0.2 mol dm-3. The bench sodium hydroxide cannot be titrated directly.

Standard solutions

Potassium hydrogenphthalate is a primary standard, having the following characteristics

These characteristics allow the preparation of a standard solution to a good degree of accuracy.


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Procedure

Part 1 - preparing the standard solution

Accurately weigh out approximately 5.2 g of potassium hydrogenphthalate (Mr = 203.23)

Transfer to a volumetric flask and dissolve in about 100 ml of distilled (deionised) water. Then make up to the mark with distilled (deionised) water.

Invert several times to ensure complete mixing.

Part 2 - preparing the sodium hydroxide solution

Transfer a 25 ml aliquot of the 'bench' sodium hydroxide solution into a 250 ml volumetric flask using a pipette and filler.

Add distilled (deionised) water to the mark.

Invert several times to ensure complete mixing.

Part 3 - the titration

Transfer a 25ml aliquot of the potassium hydrogenphthalate solution into a conical flask using a pipette and filler.

Add 5 drops of phenolphthalein indicator solution

Fill a 50 ml burette to the 0.00 mark with the diluted sodium hydroxide solution.

Add the sodium hydroxide slowly to the acid/indicator solution in the conical flask swirling constantly until the first permanent hint of pink appears (the end-point).

Repeat the steps above, adding the sodium hydroxide dropwise near the end-point.

Repeat the titration as many times as needed to obtain concordant results (results within 0.1 ml of each other)


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Recording the data

Ensure that all of the data is recorded with suitable units and inaccuracies.

The following shows example raw data and analysis.

Raw data

Mass of potassium hydrogenphthalate = 5.22g ± 0.01

The initial solutions are all colourless.

Titre Initial burette reading /ml ± 0.05 Final burette reading /ml ± 0.05 Volume added /ml ± 0.1 Observations
1 0.00 25.25 25.25 red end
2 0.05 24.85 24.80 pink end
3 0.00 24.75 24.75 pale pink

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 = 5.22/203.23 = 0.00257

Molarity of the potassium hydrogenphthalate solution used = 0.00257/0.25 = 0.103 mol dm-3

Moles of potassium hydrogenphthalate in 25 ml aliquot = 0.103 x 0.025 = 2.57 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 = 2.57 x 10-3

Volume of sodium hydroxide from the titration = 24.8 ml

Hence molarity of sodium hydroxide = 0.104

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

Hence concentration of sodium hydroxide in the original stock bottle = 1.04 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 = 1.04 ± 0.01


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