: pH – Problem Solving (OCR A Level Chemistry)

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:

Solution	Approximate pH
Ammonia	11.00
Ethanoic acid	2.90
Hydrochloric acid	1.10
Sulfuric acid	1.00
Methanoic acid	2.40
Sodium hydroxide	13.00
Ethanoate buffer solution	4.76

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 

Solution	Test 1 - Using UI	Test 2 - pH probe	Test 3 - Phenolphthalein
Ammonia	Pale blue = pH 10	10.39	Turns pink
Ethanoic acid	Orange = pH 3	2.89	No change
Hydrochloric acid	Red = pH 1	1.20	No change
Sulfuric acid	Red = pH 1	1.46	No change
Methanoic acid	Pink = pH 2	2.30	No change
Sodium hydroxide	Purple = pH 14	13.60	Turns pink
Ethanoate buffer	Orange = pH 3	2.96	No change