Three colourless solutions sit on the bench. Test them with Universal Indicator or litmus paper, record what you see, then mix two of them and follow the pH.
A, B and C are all colourless
Each tube holds 10.0 cm3
A and B are both 0.100 mol/dm3, reacting 1 : 1
Click any tube to test just that one. Rinsing clears every tube so you can try a different test.
Reading the bench. The beaker starts with 25.0 cm³ of solution A and a few drops of Universal Indicator, so the liquid colour always matches the pH meter. The graph traces the pH as far as you have added, so drag slowly through the middle — the colour changes over a very small volume there. Use the 0.1 cm³ buttons to pin down the exact point.