Investigating Science 11–12 · Year 11

Acids and bases: indicator colour against pH readings in a dilution series

Module 1: Cause and Effect – Observing

Practical, model not builtLow risk

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The idea

Indicator colour is a qualitative observation and a pH meter reading is a quantitative one; a tenfold dilution raises the pH of a strong acid by one unit but of a weak acid by only about half a unit.

Safety card

Low riskAn adult supervises

Hazards

dilute acids and sodium hydroxide irritate eyes and skin
glass breakage

Controls

safety glasses
wipe spills with plenty of water
dispose of solutions as directed in the school’s CSIS procedures

Note

Chemicals are used: follow the NSW Department of Education Chemical Safety in Schools (CSIS) package, Section 1 (general information for all staff, including the CSIS 1.7 risk assessment requirement), read the Safety Data Sheet for each chemical and complete a RiskAssess risk assessment before the lesson.

What you need

0.10 mol/L hydrochloric acid, 50 mL; 0.10 mol/L ethanoic (acetic) acid, 50 mL; 0.10 mol/L sodium hydroxide, 20 mL
distilled water, universal indicator with colour chart, pH meter calibrated with pH 4.00, 7.00 and 10.00 buffers (readings below 4 and above 10 lie outside the span the buffers cover)
10 mL pipette or measuring cylinder, 100 mL beakers, wash bottle, labels

How to do it

Make a tenfold dilution series of each acid: 10 mL of solution made up to 100 mL, three times, giving 0.10, 0.010, 0.0010 and 0.00010 mol/L.
Test a sample of each with universal indicator and record the colour and the chart value.
Measure each with the pH meter, rinsing the probe with distilled water between readings, from the most dilute to the most concentrated.
Measure the sodium hydroxide solution the same way.
Plot pH against log₁₀(concentration) for both acids and compare the gradients.
List the benefits and drawbacks of the indicator and the meter as observation tools.

What you should see

Calculated from concentration, hydrochloric acid is pH 1.00, 2.00, 3.00 and 4.00, a gradient of −1 on the pH against log₁₀ c graph. Ethanoic acid (pK_a 4.76 at 25 °C, PubChem) gives pH 2.88 at 0.10 mol/L, 3.39 at 0.010 and 3.91 at 0.0010 mol/L, a gradient near −0.5, because only 1.3 % of the molecules ionise at 0.10 mol/L and 12 % at 0.0010 mol/L. Sodium hydroxide at 0.10 mol/L is pH 13.0 by the same calculation. A meter responds to hydrogen-ion activity, not concentration: the Davies equation gives an activity coefficient of about 0.78 at an ionic strength of 0.10 (0.782 with A = 0.5092, 0.785 with A = 0.50), so 0.10 mol/L hydrochloric acid reads about 1.11 and 0.10 mol/L sodium hydroxide about 12.89, and both readings also lie outside the 4 to 10 calibrated span. The indicator separates acidic, neutral and basic bands but cannot resolve 0.1 pH unit; the meter readings are compared with both the calculated and the activity-corrected values.

What changes

What you change: acid concentration and acid type
What you measure: pH
What you keep the same: temperature near 25 °C
meter calibrated before use
probe rinsed between samples
same dilution technique

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

A weak acid is the same as a dilute acid.
pH changes by the same amount for every dilution whatever the acid.
An indicator gives an exact pH.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Investigating Science Stage 6 Syllabus (2017), NESA; currentINS11-8 INS11/12-1 INS11/12-3 INS11/12-4
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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