pH – Problem Solving (OCR A Level Chemistry A): Revision Note

PAG 11.1: pH – Problem Solving

Testing the pH of different solutions

• According to the Practical Activities Support Guide published by OCR:

  • In PAG 11.1, students will be given a number of colourless solutions that could be acids and / or alkalis

  • Students are to plan and implement a practical procedure to work out the identity of each solution

• However, the key skill / technique for Practical Activity Group 11 is the measurement of pH using charts, meters and probes attached to data loggers

• Therefore, the aim of this practical will be to test a range of solutions using a combined sequence of tests

• The solutions given could be the following:

Method

Test 1 - Addition of universal indicator 

• Add 5 cm³ of each solution to separate test tubes

• Add universal indicator solution to each test tube

• Record the pH’s of each solution in a results table

  • Use the colour chart to identify the correct pH

Universal indicator pH chart 

Test 2 - Using a pH probe 

• Before completing this test, the pH probe will need calibrating

  • After storage, a pH meter does not give accurate readings because the glass electrode in the pH meter does not give a reproducible emf over longer periods of time

• Calibration of a pH probe 

  • Rinse the pH probe with distilled water and shaken to remove excess water

  • Place the tip of the pH probe in pH 4 buffer solution, ensuring the bulb is fully immersed 

  • Allow to sit until the pH stabilises 

  • Adjust reading to the pH of the buffer

  • Repeat this process, including the washing with distilled water with a pH 9 buffer solution

• Once calibrated, the tip of the pH probe is rinsed with distilled water and shaken to remove excess water

• The tip is placed in the unknown solution, again, ensuring the bulb is fully immersed 

• The pH is allowed to stabilise and then the reading recorded in a results table

  • In practice, recalibration may sometimes be required during testing

• This process can then be repeated for each unknown solution

Test 3 - Using phenolphthalein indicator 

• This can be done with a spotting tile

• Add a few drops of each solution to a labelled spotting tile (so each solution can be identified)

• Add 1 drop of phenolphthalein in each well

• Record observations in a results table 

Specimen results