Module 6 – Acid/Base Reactions
Measuring pH
By measuring the pH of various substances, you can determine how acidic or basic they are.
In this investigation you will be able to determine the actual pH of these substances
universal indicator and a pH meter. An advantage of using a pH meter is that it does not
involve adding any additional substances to the solution and the pH can usually be found
with a greater degree of accuracy, provided the meter is calibrated properly
Aimq
To measure and compare the pH of various substances
Materials
 Various household substances (for example, vinegar, lemon juice, dishwasher
detergent, shampoo, antacid tablet, baking soda, vitamin C, bleach, washing
detergent, fabric conditioner, soda water, salt, sugar)
 Soil samples (including potting mix)
 Samples of waster from swimming pools and local waterways
 1 mol L-1 and 0.0001 mol L-1 hydrochloric acid (HCl)
 1 mol L-1 and 0.0001 mol L-1 sodium hydroxide (NaOH)
 Barium sulfate (BaSO4) powder
 Universal Indicator solution and pH colour chart
 Distilled water
 Petri dishes (one for each soil sample)
 Test tubes and test-tube rack
 pH meter or pH probe
 Spatula
 Filter funnel and paper
 Gloves
 Safety glasses
Risk Assessment
What are the risks in doing this
investigation?
Household chemicals can burn the skin
Sodium hydroxide is caustic and
hydrochloric acid is corrosive
How can you manage these risks to stay
safe?
Food products can be contaminated in the
laboratory
Soil samples may contain harmful microorganisms
What other risks are associated with your investigation? How can you manage these?
Method
PART A: TESTING HOUSEHOLD SUBSTANCES, HYDROCHLORIC ACID AND SODIUM
HYDROXIDE SOLUTIONS
1. Set up test tubes in the rack. Place a few millilitres (enough to allow measurement
using the pH meter) of the substance to be tested in a test tube. (You may need to
add distilled water if the substance is a solid.)
2. Test the pH with the pH meter.
3. Record the pH.
4. Rinse the pH meter with distilled water.
5. Add one drop of universal indicator solution.
6. Record the colour and pH.
7. Repeat with all other household substances, hydrochloric acid and sodium hydroxide
solutions.
PART B: TESTING WATER SAMPLES
1. If the samples contain a lot of suspended sediment, filter or allow them to settle
before testing.
2. Repeat the method used in part A to test the samples.
PART C: TESTING SOIL SAMPLES
1. Place approximately a spatula of soil in a Petri dish.
2. Add a few drops of indicator to make a paste and stir it.
3. Lightly sprinkle a thin layer of barium sulfate powder over the soil paste. Do not stir.
4. Allow a few minutes for the powder to absorb the indicator. Record the colour and
pH values.
5. Repeat steps 1-4 with other soils samples.
Results
Record your results in a table. Include an additional column for hydrogen ion
concentration.
Analysis of Results
1. List the substances from the most acidic to the most basic.
2. Record which substances, if any, were neutral.
Discussion
1. Identify where there were significant differences between the pH values obtained
using universal indicator and the pH meter. Suggest possible reasons for these
differences.
2. Suggest a procedure for measuring the pH of the soil samples using a pH meter.
3. Identify any noticeable difference in the pH of the water samples and suggest a
reason for this difference.
4. Compare the hydrogen ion concentration of the samples of HCl used with the pH
reading obtained from the meter. Suggest what the difference in these readings
could imply.
5. Use the table below to determine the hydroxide ion concentration of the sodium
hydroxide samples from the pH measured. Compare the value obtained from the pH
meter with that of the solutions. Suggest possible reasons for any differences.
6. Reflect on possible errors that could have occurred and suggest what effect these
may have had on results.
Conclusion
Identify any generalizations related to the acidity and alkalinity of the substances tested.
(Nelson Chemistry 12)