Chemistry • Year 12 • Module 6 • Lesson 19
Acid/Base Analysis Techniques: Industrial & Digital
Lock in the vocabulary, the glass electrode mechanism, two-point calibration, and the four main analytical methods before you attempt data tasks.
1. Label the glass electrode pH probe
The diagram below shows a cross-section of a glass electrode pH probe in a test solution. Write the missing labels into the answer table below — each label must come from the lesson's Key Terms or Cards 1–2. 8 marks
| Box | Your label |
|---|---|
| A | |
| B | |
| C | |
| D | |
| E | |
| F | |
| G | |
| H |
2. Term–definition match
Match each term from the word bank to the correct definition. Write the term in the right-hand column. 10 marks
Word bank: pH meter · calibration · glass membrane · Nernst equation · two-point calibration · automated titration · conductometric titration · back titration · equivalence point · buffer solution
| # | Definition | Matching term |
|---|---|---|
| 2.1 | An electrochemical sensor that converts [H⁺] into a measurable voltage and displays a pH reading. | |
| 2.2 | A solution of precisely known pH used to establish the slope and intercept of a pH probe's voltage–pH relationship. | |
| 2.3 | The thin electrode component that is selectively permeable to H⁺ ions; a potential difference develops across it. | |
| 2.4 | The mathematical relationship E ≈ E° − 0.0592 × pH that connects the measured voltage to the pH of a solution at 25 °C. | |
| 2.5 | A calibration procedure using two buffer solutions of different pH values to set both the offset and the sensitivity of the probe. | |
| 2.6 | A titration system in which a pump-controlled burette and a pH sensor detect the equivalence point without human intervention. | |
| 2.7 | A titration technique that detects the equivalence point using changes in electrical conductivity rather than a colour indicator. | |
| 2.8 | An analytical technique in which an excess of a known reagent is added to the analyte, and the unreacted excess is then titrated; used for insoluble analytes. | |
| 2.9 | The point in a titration at which stoichiometrically equivalent amounts of acid and base have reacted. | |
| 2.10 | A solution that resists large changes in pH when small amounts of acid or base are added; used as a calibration standard. |
3. True or false — with correction
Circle T or F. If false, write the corrected version. 8 marks (1 T/F + 1 correction each)
3.1 A glass electrode pH probe measures the concentration of H⁺ ions directly without any intermediate conversion step. T / F
3.2 Two-point calibration of a pH probe sets both the intercept and the slope of the voltage–pH relationship. T / F
3.3 A conductometric titration is the best choice for analysing a dark red wine sample because it uses electrical measurement to detect the equivalence point without relying on a colour indicator. T / F
3.4 A direct pH probe reading gives concentration as precisely as a volumetric titration when the Ka of the acid is known. T / F
4. Function recall
Answer each in 1–2 sentences using precise terms from the lesson. 10 marks (2 each)
4.1 What is the function of the glass membrane in a pH electrode?
4.2 Why must a pH probe be calibrated before every use, not just when it is new?
4.3 What is the function of the reference electrode in a combination pH probe?
4.4 Why is a back titration used to determine the CaCO₃ content of an antacid tablet rather than a direct titration?
4.5 What is the function of continuous pH monitoring in an industrial wastewater treatment plant?
5. Connect the key concepts
Draw labelled arrows between the six terms below. Each arrow must carry a linking phrase (e.g. "measures", "requires", "converts to", "detects"). Aim for at least 6 labelled arrows. 6 marks
Supplied terms: glass membrane · H⁺ concentration · voltage (potential difference) · Nernst equation · calibration · pH reading
6. Fill in the blanks
Complete the paragraph using words from the word bank. Each word is used once. 8 marks
Word bank: slope · intercept · voltage · pH 4.00 · glass membrane · Nernst · aging · temperature
A pH probe must be calibrated before each use because the properties of the (1) change over time due to electrode (2) and because (3) affects the (4) of the (5) equation. In two-point calibration, the first buffer (for example (6)) sets the (7) of the (8)–pH linear relationship, and the second buffer sets the slope (sensitivity) of that relationship.
Q1 — Labelled glass electrode diagram
A: Glass membrane (thin SiO₂-rich membrane selectively permeable to H⁺ ions). B: Hydrated gel layer (where H⁺ ions partition from the test solution). C: Internal reference solution (0.1 mol/L HCl of fixed, known pH). D: Reference electrode (Ag/AgCl — provides stable, constant potential). E: Test solution (the solution being measured). F: Voltmeter / pH meter electronics (measures the potential difference and converts to a pH value using the calibrated Nernst relationship). G: Potential difference / voltage across the membrane (proportional to the difference in [H⁺] across the glass membrane). H: H⁺ ion exchange at the outer gel layer surface (H⁺ ions partition into the outer gel layer, creating the potential difference).
Q2 — Term–definition matches
2.1 pH meter · 2.2 calibration · 2.3 glass membrane · 2.4 Nernst equation · 2.5 two-point calibration · 2.6 automated titration · 2.7 conductometric titration · 2.8 back titration · 2.9 equivalence point · 2.10 buffer solution
Q3 — True / false with correction
3.1 False. A glass electrode measures a voltage (electrical potential difference) across the glass membrane; the meter electronics then convert this voltage to a pH value using the Nernst relationship. It does not measure concentration directly.
3.2 True.
3.3 True.
3.4 False. A direct pH probe reading propagated through a reverse Ka calculation gives concentration with ~5–10% uncertainty, compared with ~0.1–0.3% for a volumetric titration with concordant titres. The pH probe method is far less precise for determining concentration.
Q4.1 — Function of the glass membrane
The glass membrane is selectively permeable to H⁺ ions. H⁺ ions from the test solution exchange into the outer hydrated gel layer of the membrane, while the inner gel layer is in contact with the fixed internal reference solution. Because [H⁺] differs on the two sides, a potential difference (voltage) develops across the membrane that is proportional to pH.
Q4.2 — Why calibrate before every use
The voltage–pH relationship drifts with time because the hydrated gel layer properties change (electrode aging), and the Nernst slope changes with temperature. Calibration with fresh buffer solutions corrects for both types of drift simultaneously, re-establishing the accurate slope and intercept for current conditions.
Q4.3 — Function of the reference electrode
The reference electrode (typically Ag/AgCl or calomel) provides a stable, constant electrical potential against which the glass electrode potential is measured. Without a stable reference, the measured voltage would be meaningless — there would be no fixed baseline from which to calculate the potential difference across the glass membrane.
Q4.4 — Why back titration for CaCO₃
CaCO₃ is insoluble and reacts slowly with HCl. In a direct titration it is impossible to detect a clean equivalence point because the solid is not fully dissolved. In a back titration, a known excess of HCl is added to dissolve the tablet completely (and CO₂ is driven off), then the unreacted HCl excess is titrated with standard NaOH — giving a precise result even for an insoluble analyte.
Q4.5 — Function of continuous pH monitoring in wastewater
Continuous pH monitoring ensures that the pH of the effluent discharged from an industrial plant remains within the legal discharge limit (pH 6.5–8.5 under NSW EPA regulations). If the pH drifts outside limits, an automated dosing system adds neutralising agent immediately, preventing regulatory violations and environmental damage.
Q5 — Sample concept map arrows
Acceptable arrows include: H⁺ concentration → partitions into → glass membrane → creates → voltage (potential difference) → related to pH by → Nernst equation → used during → calibration → converts voltage to → pH reading. Also: calibration → corrects slope and intercept of → Nernst equation. Award 1 mark per correctly labelled directional arrow (minimum 6).
Q6 — Cloze answers (in order)
(1) glass membrane (2) aging (3) temperature (4) slope (5) Nernst (6) pH 4.00 (7) intercept (8) voltage