9.3 - The pH scale

9.3.1: Distinguish between aqueous solutions that are acidic, neutral or basic using the pH scale.

Using the pH scale

The concept of pH comes from a consideration that acidity is due to the presence of H⁺ ions in the water. These H⁺ ions come from the reversible breakdown of the water H₂O molecules.

H₂O H⁺ + OH^-

It may be seen that the breakdown of one molecule of water will produce 1 of each of the resulting ions.

We could therefore measure acidity by simply measuring the concentration of hydrogen ions in the sample. This would, however, lead to very small and inconvenient numbers as the concentration in pure water at 25ºC is actually 1 x 10^-7 moles dm^-3

An attempt is made to produce numbers that are more easily handled using logarithms (i.e. expressing the number as the power to which 10 has to be raised to achieve the number - this is a common mathematical artifice)

log 1 x 10^-7 = -7

This gives very small numbers a negative value and so a simple change of sign is performed making it positive. Thus the expression for potential hydrogen (pH) is:

pH = - log [H⁺]

Low pH value are acidic, higher values are basic.

pH 1 represents strong acid, pH 7 is neutral, pH 14 represents strong base

pH scale

Example:

Calculate the pH of a 0,01 moles dm^-3 solution of a strong acid HCl.

As the acid only contains one hydrogen atom it will only produce one hydrogen ion per molecule when dissolved in water.

Therefore the H⁺ concentration is = 0,01 moles dm^-3

log 0,01 = -2

pH = 2

The pH range

At 25ºC the equilibrium H2O H⁺ + OH^- gives a value for both the H⁺ and OH^- ions concentrations as 1 x 10^-7

According to the equilibrium law, Kc (the equilibrium constant) must remain the same providing the conditions remain the constant.

therefore:

As the water concentration is very large in comparison to the other concentrations, slight changes in H⁺ and OH^- do not affect it and so we may remove it from the equation, defining a new constant Kw (called the ionic product of water).

Kw will always be equal to 1 x 10^-14 at 25ºC .... consequently any increase in [H⁺] must be accompanied by a corresponding decrease in [OH^-] to maintain the constant value of Kw.

The other consequence of this is that the pH range becomes 0 to 14 under the usual concentration conditions of the laboratory.

pH 1 represents strong acid, pH 14 represents strong base

Example 2:

Calculate the pH of a 0,01 moles dm^-3 solution of a strong base NaOH

As the base only contains one OH group it will only produce one hydroxide ion per molecule when dissolved in water.

Therefore the OH^- concentration is = 0,01 moles dm^-3

As the ionic product of water at 25ºC [H⁺] x [OH^-] = 1 x 10^-14

Then [H⁺] = 1 x 10^-12

log 1 x 10^-12 = -12

pH = 12

9.3.2: Identify which of two or more aqueous solutions is more acidic or basic, using pH values. Measure pH using a pH meter or pH paper. Students should know that pH paper contains a mixture of indicators. The theory of pH meters is not required.

pH measurement

pH can be measured with:

• A pH meter
• Indicator solution
• Indicator paper

A pH meter uses electrochemical factors to calculate the hydrogen ion concentration in a sample solution and converts it to pH on the instrument readout.

Indicators are many and varied both in chemical constitution and their colour responses to acids and bases. The indicators most commonly used are universal indicator, litmus, phenolphthalein and methyl orange.

Indicator paper is simply absorbant paper containing either universal indicator (a mixture of pH sensitive dyes) or litmus (in the red or the blue form)

Realistically the only indicator that gives an approximate value for pH is universal, the others are useful for titrations or for performing spot tests to differentiate between acids and bases.

9.3.3 - State that each change of one pH unit represents a tenfold change in the hydrogen ion concentration [H+(aq)]. Relate integral values of pH to [H+(aq)] expressed as powers of ten. Calculation of pH from [H+(aq)] is not required.

pH scale calculations

Remember that a logarithm is the power to which 10 must be raised to get the given number:

Thus a change of 1 in the pH scale represents a change by a factor of 10 in the acidity or basicity of the solution.

Example:

Hydrochloric acid with a measured pH of 2,3 is ten times as 'strong' as hydrochloric acid with a measured pH of 3,3. The term 'strong' means that there is a concentration of ten times as many hydrogen ions in the solution with a lower pH.

Powerpoint: The pH scale