Chemistry 11–12 · Year 12

Titration curves with a pH probe and a conductivity probe: strong and weak acids against a strong base

Module 6: Acid/Base Reactions

Practical, model not builtLow risk

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

Logging pH and conductivity as base is added reveals the shape of the curve, the equivalence point, the buffer region of a weak acid and the pH at which an indicator must change.

What you need

pH probe and conductivity probe with data logger and drop counter or a burette
0.100 mol/L hydrochloric acid, ethanoic acid and ammonia, 100 mL each; 0.100 mol/L sodium hydroxide, 150 mL (the two runs to 50 mL take 100 mL, and the rest rinses and fills the burette)
Magnetic stirrer and stirrer bar, 100 mL tall-form beaker, 25.00 mL pipette, burette
Phenolphthalein and methyl orange for comparison

How to do it

Pipette 25.00 mL of hydrochloric acid into the tall-form beaker so the probe tips are covered (if water must be added to cover them, measure it and enter it in the simulation, because it changes every pH value before the end point); start the stirrer and the logger.
Add sodium hydroxide from the burette in 1.0 mL steps, recording pH and conductivity after each addition; reduce the step to 0.2 mL within 3 mL of the expected equivalence and continue to 50 mL.
Repeat with ethanoic acid against sodium hydroxide, then hydrochloric acid against ammonia.
Plot pH against volume for each and mark the equivalence point (steepest slope) and, for the weak acid, the half-equivalence pH; plot conductivity against volume and mark the equivalence point where the conductivity line changes slope (a minimum only for hydrochloric acid with sodium hydroxide).
Compare the equivalence pH with the ranges of phenolphthalein and methyl orange and state which indicator suits each pair.
Overlay the logged curves on the simulation's computed curves and account for any offset.

What you should see

Strong acid with strong base: pH 1.0 at the start, 1.48 at half-way, jumping from 3.7 at 24.9 mL through 7.0 at 25.0 mL to 10.3 at 25.1 mL, then 12.52 at 50 mL; conductivity falls to a minimum at equivalence and rises again. Weak acid with strong base: starts at 2.88, sits in a buffer region with pH 4.76 at half-equivalence (equal to pKa), reaches 8.73 at equivalence (alkaline, so phenolphthalein suits it and methyl orange does not) and 12.52 at 50 mL; its conductivity dips slightly over the first millilitre, then rises steadily to equivalence and more steeply after it, so equivalence shows as a change of slope, not a minimum. Strong acid with weak base gives an acidic equivalence near pH 5.3, suiting methyl orange; its conductivity falls to equivalence and then stays nearly level, because excess ammonia is a weak electrolyte.

What changes

What you change: volume of titrant added, and the acid-base pair
What you measure: pH and conductivity at each volume
What you keep the same: concentrations at 0.100 mol/L
aliquot volume
stirring
temperature
probe calibration

Common misconceptions

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

The equivalence point of any titration is pH 7 (only for strong acid with strong base; a weak acid gives an alkaline equivalence).
Conductivity always falls to a minimum at equivalence (only for a strong acid with a strong base; with ethanoic acid it rises after the first millilitre and changes slope at equivalence, and with ammonia as the titrant it levels off after equivalence because excess ammonia is a weak electrolyte).
A flat region on the curve means no reaction is happening (the buffer region is where the reaction is running with the least pH change).

Safety card

Low riskLearners carry it out

Hazards

0.1 mol/L sodium hydroxide irritant
ammonia vapour
glass probes

Controls

eye protection
ammonia burette capped between additions
probes rinsed and stored correctly

Note

NSW Department of Education Chemical Safety in Schools (CSIS) package, 2021 Technical Update: Section 1.7 (risk assessment) and Volume 2 Appendix D (generic assessment advice and DoE chemical categories); record a RiskAssess (riskassess.com.au) risk assessment before the lesson and check the school's hazardous chemical register (CSIS Section 1.9) for local restrictions.

Curriculum references

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

Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document)CH12-13 CH11/12-2 CH11/12-5
Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document); a Working Scientifically outcome not among those the syllabus targets in Module 6, which it allows in any moduleCH11/12-4
Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taughtCH-12-02 CH-12WS-02 CH-12WS-04 CH-12WS-05
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities