Titrations

The pH scale for acids and alkalis goes from 0 (strongly acidic) to 14 (strongly alkaline). Litmus and phenolphthalein are single indicators used to show whether a solution is acidic or alkaline, while universal indicator is a mixed indicator that can give an estimate of the pH. A pH curve shows how the pH changes when an acid and alkali are mixed together. Titrations are used to determine the concentration of an acid or alkali.

pH and indicators

The pH scale

The pH scale gives a measure of how acidic or alkaline a solution is. It normally runs from 0 (most acidic) to 14 (most alkaline). A neutral substance is pH 7 at 25°C.

Indicators

An indicator changes colour depending on the properties of the substance it is added to. Three indicators are commonly used to show whether a solution is acidic or alkaline:

• litmus
• phenolphthalein
• universal indicator

Litmus and phenolphthalein are single indicators (they only contain one colour-changing substance) whereas universal indicator is a mixed indicator (it contains several different colour-changing substances).

Litmus

Litmus paper can be red or blue. The table shows its colours in acidic, neutral and alkaline solutions.

	Acidic	Neutral	Alkaline
Red litmus	Stays red	Stays red	Turns blue
Blue litmus	Turns red	Stays blue	Stays blue

A litmus solution shows the same changes for acids and alkalis, but turns purple in neutral solutions.

Phenolphthalein

Phenolphthalein changes colour sharply at about pH 8. For most purposes, this means that it is pink in alkaline solutions and colourless in acidic solutions.

Note that it would be incorrect to say that it is ‘clear’ in acidic solutions. A coloured solution can be described as clear if you can see through it, so you should use the word ‘colourless’ when the solution has no colour.

Universal indicator

Universal indicator shows a range of colours depending on the pH of the solution (see below).

pH scale and universal indicator colours                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             

pH curves

The change in pH can be estimated using universal indicator, or measured more accurately using a pH meter, when an acid and an alkali are mixed.

A graph of pH (on the vertical axis) against total volume of acid or alkali added (on the horizontal axis) is called a pH curve. You need to able to interpret a simple pH curve. If you are taking the Higher Tier paper you will also have to be able to sketch a pH curve.

Adding acid to alkali

The pH curve below shows what happens to the pH when a strong acid (such as hydrochloric acid) is added to 25 cm³ of a strong alkali (such as sodium hydroxide). The acid and the alkali started off at the same concentration.

pH curve adding acid to alkali

Note that the pH falls:

• slowly at first as acid is added to the alkali
• rapidly at the end-point (the point where the alkali is completely neutralised)
• slowly again once excess acid is being added

In this example, 25 cm³ of acid was needed to neutralise the alkali. If the acid had been more concentrated than the alkali, the volume needed would have been less than 25 cm³. The mixture was pH 12 at 20 cm³ and at pH 2 at 30 cm³. It was pH 7 at the end-point.

Adding alkali to acid

The pH curve below shows what happens to the pH when a strong alkali is added to 25 cm³ of a strong acid. As before, they both started off at the same concentration.

The pH curve for an alkali being added to an acid

Note that the pH rises:

• slowly at first as alkali is added to the acid
• rapidly at the end-point (the point where the acid is completely neutralised)
• slowly again once excess alkali is being added

In this example, 25 cm³ of alkali was needed to neutralise the acid. If the acid had been more concentrated than the alkali, the volume needed would have been more than 25 cm³.                                                                                                                                                                                                                                                                                                                                                                                                      

Carrying out a titration

The concentration of an acid or alkali can be calculated by carrying out an experiment called a titration. You should be able to identify the apparatus needed to carry out a simple acid-alkali titration, and to describe how it is done.

Materials

The apparatus needed includes a:

• pipette to accurately measure a certain volume of acid or alkali
• pipette filler to use the pipette safely
• conical flask to contain the liquid from the pipette
• burette to add small, measured volumes of one reactant to the other reactant in the conical flask

Apparatus needed to carry out a titration

Method

1. Use the pipette and pipette filler to add 25 cm³ of alkali to a clean conical flask.
2. Add a few drops of indicator and put the conical flask on a white tile (so you can see the colour of the indicator more easily).
3. Fill the burette with acid and note the starting volume.
4. Slowly add the acid from the burette to the alkali in the conical flask, swirling to mix.
5. Stop adding the acid when the end-point is reached (the appropriate colour change in the indicator happens). Note the final volume reading.
6. Repeat steps 1 to 5 until you get consistent readings

The same method works for adding an alkali to an acid - just swap around the liquids that go into the conical flask and burette.

The titre

The difference between the reading at the start and the final reading gives the volume of acid (or alkali) added. This volume is called the titre.

For example, if the reading at the start is 1.0 cm³ and the final reading is 26.5 cm³, then the titre is 25.5 cm³ (26.5 – 1.0). Note that the titre will depend upon the volume of liquid in the conical flask, and the concentrations of the acid and alkali used.

It is important to repeat the titration several times to check that your titre value is consistent so that your calculations are reliable.

If universal indicator is used, the colour changes gradually through a range of colours. On the other hand, a single indicator like litmus or phenolphthaleingives a sharp end-point where the colour changes suddenly.

Read on if you're taking the higher paper                                                                                                                                                                                                                                                                                                                         

Titration calculations – Higher tier

You should be able to use titration results to calculate the concentration of anacid or alkali. If several runs have been carried out, any irregular titres should be ignored before calculating the mean titre.

Example

27.5 cm³ of 0.2 mol/dm³ hydrochloric acid is needed to titrate 25.0 cm³ of sodium hydroxide solution. What is the concentration of the sodium hydroxide solution?

Step 1: Convert all volumes to dm³

27.5 cm³ = 27.5 ÷ 1000 = 0.0275 dm³

25.0 cm³ = 25.0 ÷ 1000 = 0.025 dm³

Step 2: Calculate the number of moles of the substance where the volume and concentration are known

number of moles = concentration × volume

number of moles of hydrochloric acid = 0.2 × 0.0275 = 0.0055 mol (5.5 × 10^–3mol)

Step 3: Calculate the unknown concentration

We can say that 0.0055 mol of acid will react with 0.0055 mol of alkali

concentration of alkali = moles ÷ volume = 0.0055 ÷ 0.025 = 0.22 mol/dm³

A quick check

You can check your answer using this quick method (but which misses out the chemical understanding that may attract full marks).

unknown concentration = known concentration × ^{volume of known} / _{volume of unknown}

In the example above, this would be:

unknown concentration = 0.2 × ^27.5 / _25.0 = 0.22 mol/dm³