Stoichiometry: 3.54

Titration is an experimental technique used to determine the concentration of an unknown solution. It usually involves acids and bases, but may be extended to any addition of one solution to another to find out the amounts required for reaction.

Syllabus reference

Reactivity 2.1.2 - The mole ratio of an equation can be used to determine:

• the masses and/or volumes of reactants and products
• the concentrations of reactants and products for reactions occurring in solution.
Calculate reacting masses and/or volumes and concentrations of reactants and products.

Guidance

Avogadro’s law and definitions of molar concentration are covered in Structure 1.4.
The values for Ar given in the data booklet to two decimal places should be used in calculations.

Tools and links

Structure 1.5 - How does the molar volume of a gas vary with changes in temperature and pressure?
Nature of science, Structure 1.4 - In what ways does Avogadro’s law help us to describe, but not explain, the behaviour of gases?

General experimental procedure
Calculations
Calculation schematic

General experimental procedure

The procedure involves adding one solution to another solution, in the presence of an indicator that shows when the two solutions have completely reacted. If the concentration and volume of one of the solutions is known and the volume of the other solution is known, then the unknown concentration can be calculated providing the stoichiometry of the reaction is known.

The experimental procedure is very accurate and liquids are carefully measured, using specially designed glass apparatus with low margins of error. See errors and inaccuracies.

Titrations are intended for analytical and determination/estimation purposes. If the compound being used for the titration cannot be obtained in a very pure state then it first must be standardised against a solution whose concentration can be determined to a high degree of accuracy. Such substances are called primary standards.

In summary:

Measure 25cm³ of one solution into a conical flask
Add a few drops of indicator solution
Add a second solution very slowly from a burette until the indicator just changes colour
Repeat the process until concordant results are obtained (results within 0.1 cm³ of one another)

Click on the 'titrate' button to see the general experimental procedure.

Titration is carried out several times until results within 0.1cm³ of one another are obtained. These are then said to be 'concordant results'

Readings on the burette should be taken at eye level; the bottom of the meniscus at the surface of the solution is usually read.

Once concordant results have been obtained an average of these is used for calculations.

	Measure 25cm³ of one solution into a conical flask. Add a few drops of indicator solution. The reading on the burette is recorded taking into account the tolerance or error limits (e.g. ±0.05 cm³).		Close to the 'end-point', i.e. the point at which the solute in the flask has been completely reacted, the solution is added from the burette 1 drop at a time, swirling the solution in the flask gently between each addition, until the colour just reaches the 'end-point'.
	The solution is added slowly from the burette into the reagent solution in the flask until the colour of the indicator starts to change.		The final reading on the burette is recorded, once again taking into account the error limits.

NOTE The solution in the conical flask does not have to be the unknown, the choice is usually dependent on the ease of observation of the indicator colour change. For example, it is easier to see the first hint of pink appearing from a colourless solution in a flask on a white tile, than to carry out the titration the other way round.

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Calculations

The calculation is carried out as follows:

The number of moles of solute in the known solution is calculated using the relationship:

moles = molarity x volume

Then the stoichiometric relationship between this substance and the other reacting solution is used to determine the number of moles of solute in the unknown solution.

Example of equation stoichoimetry:

Sodium hydroxide	+	nitric acid	sodium nitrate	+ water
NaOH	+	HNO₃	NaNO₃	+ H₂O
1 mole	≡	1 mole

Here the stoichiometric relationship is:

moles of base = moles of acid

The molarity of the unknown solution can now be calculated from the number of moles present and the volume used:

molarity = moles/volume(litres)

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Calculation schematic

The calculation may be represented by a flow scheme showing the order of the operations to be carried out.

Example - Determination of the concentration of an unknown sodium hydroxide solution.

Procedure

25cm³ of the unknown base is measured into a conical flask using a pipette.
5 drops of phenolphthalein indicator is added
A standard solution of potassium hydrogen phthalate (an acid) is added from a burette until the red colour of the inicator has completely disappeared. The burette reading is noted.
The process is repeated several times until concordant results are obtained (results within 0.1 cm³ of one another)

The potassium hydrogen phthalate reacts in a 1:1 ratio with sodium hydroxide

Therefore at the end-point (equivalence point)

Moles of potassium hydrogen phthalate = moles of sodium hydroxide

molarity(acid) x volume(acid) = molarity(base) x volume(base)

The molarity of the unknown sodium hydroxide solution may now be calculated.

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