Core Practical: Investigating pH (Edexcel GCSE Chemistry: Combined Science)

Aim: 

• To investigate the changes in pH of a fixed volume of dilute HCl on addition of varying amounts of a solid base

• Dilute HCl (0.5M or 1M), solid base such as CaO or Ca(OH)₂
• Conical flask, 25 cm³ or 50 cm³ volumetric pipette, glass rod
• Spatula and weighing boat
• pH probe or Universal Indicator paper

Method:

• Use a pipette to measure a fixed volume of dilute HCl into a conical flask
• Add one spatula of calcium oxide or calcium hydroxide to the flask and swirl
• When all the base has reacted record the pH of the solution
• If using U.I. paper use the glass rod to extract a sample from the flask
• Repeat for different numbers of spatula (1-10) of solid but the same volume of HCl
• Record your results neatly in table format

• Plot a graph of the amount of the base on the X-axis against the pH recorded on the Y-axis
• The resulting graph should look something like the one below

Investigating the change in pH during neutralisation of an acid

Conclusion:

• The graph indicates a sudden change in pH which corresponds to the vertical section of the graph
• This indicates that the more solid base is added the higher the pH, therefore the base is neutralising the acid
• From the sample graph it can be seen that 6 spatulas of the base are required to completely neutralise the acid

Hazards, risks and precautions

Hazard symbols to show substances that are corrosive, harmful to health and hazardous to the environment

• Copper(II) oxide can cause serious eye irritation and is a skin irritant. It is harmful if swallowed or inhaled and is toxic to aquatic life
• Dilute hydrochloric acid is not classified as hazardous at the concentrations typically used in this practical, however it may still cause harm to the eyes or the skin
• For both substances, avoid contact with the skin and use safety goggles
• For copper(II) oxide, care should be taken not to inhale the powder