pH and pKa
Apply dominant-form reasoning to indicator choice in the steep titration region.
AP Chemistry Topic 8.7 · CED printed p. 155 / PDF p. 162. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- Conjugate-pair equilibrium
- Logarithmic ratios
- Titration curves
pH tells you the proton environment; pKa tells you how readily a particular acid gives up its proton. Compare the two. If pH is below pKa, HA predominates over A−. If pH is above pKa, A− predominates. At equality they have equal concentrations in the ideal model. Quantitatively, [A−]/[HA] = 10^(pH − pKa), so a difference of one pH unit gives a tenfold ratio. For a base B, use the pKa of its conjugate acid HB+ when comparing with pH: below that pKa favors protonated HB+, above it favors B. Alternatively compare pOH with pKb consistently; do not compare pH directly to an unrelated pKb.
An acid–base indicator is itself a conjugate pair whose forms have different observable properties, often different colors. Its color follows the mixture of protonation states, so it responds over a transition interval rather than at one mathematically exact pH. Select an indicator with pKa near the titration’s equivalence pH and a transition lying in the steep portion of the curve. Then a small added volume crosses the color interval near the correct stoichiometric point. A mismatched indicator may change long before or after equivalence and bias the calculated analyte concentration. Use only a small indicator amount in the conceptual method: the indicator is another acid/base and should not consume a significant analyte fraction.
Let's work through it
- A drug-like model base B has conjugate-acid pKa = 9.00. At pH 7.00, [B]/[HB+] = 10^(7.00 − 9.00) = 0.0100.
- HB+ is about 100 times as abundant as B, so the protonated form predominates.
- At pH 10.00 the ratio is 10:1 and B predominates. Raising pH reduces protonation; it does not change the pKa at a fixed temperature and specified medium.
- An indicator pair with the same pKa would transition near pH 9. Its suitability still depends on the measured titration curve’s steep region.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
If pH is two units below pKa, which form dominates?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
If pH is two units below pKa, which form dominates?
Worked answer and reason
HA is about 100 times as abundant as A− in the ideal model.
The logarithmic difference corresponds to a concentration ratio.
Retrieval 2
Why select pKa near equivalence pH?
Worked answer and reason
The indicator transition then occurs near the steep stoichiometric region.
The color signal can closely approximate the equivalence volume.
Catch the tempting mistake
- pKa is the pH of every solution of that acid.
- pKa is a strength parameter; pH also depends on concentration and composition.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 8 problem 11 · ap application · multiple
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For the teacher
- Give students a colored indicator pair and a curve; assess both dominant-form reasoning and endpoint suitability.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 8.7.A
- Explain the relationship between the predominant form of a weak acid or base in solution at a given pH and the pK_a of the conjugate acid or the pK_b of the conjugate base.
- 8.7.A.1
- The protonation state of an acid or base (i.e., the relative concentrations of HA and A−) can be predicted by comparing the pH of a solution to the pK_a of the acid in that solution. When solution pH < acid pK_a, the acid form has a higher concentration than the base form. When solution pH > acid pK_a, the base form has a higher concentration than the acid form.
- 8.7.A.2
- Acid-base indicators are substances that exhibit different properties (such as color) in their protonated versus deprotonated state, making that property respond to the pH of a solution.
- 8.7.A.3
- To ensure accurate results in a titration experiment, acid-base indicators should be selected that have a pK_a close to the pH at the equivalence point.
Science references for this teaching
- AP Chemistry Course and Exam Description, effective Fall 2024 — Required topic-numbered learning objectives and essential knowledge, all continuations and AP exclusions; supplied PDF SHA256 b5dfe8677ef3d88c613865d2e2a3e8d6125d652e2b24c71ef1e8ce4e011094f0.
- OpenStax Chemistry 2e §14.7, Acid–Base Titrations — Titration regions, equivalence species, and indicator behavior.
Glossary and deeper reading · 5 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Titration
Titration measures an analyte by its reaction with a measured amount of another reagent. The balanced reaction and the observed endpoint must be connected to the equivalence condition. For example, an acid's amount can be inferred from a measured volume of a base with known concentration.
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pH
pH is the negative base-ten logarithm of hydronium activity. In suitable dilute models, activity is approximated by molar concentration relative to the standard concentration. For example, lowering hydronium concentration by a factor of ten raises pH by one unit.
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pKa
pKa is the negative base-ten logarithm of an acid's dissociation constant. A smaller pKa means a stronger acid under comparable conditions. For example, when an acid and its conjugate base have equal activities, their buffer's pH equals that acid's pKa.
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Acid–base indicator
An acid–base indicator has differently colored acid and conjugate-base forms. Its color change occurs over a pH range, so selection must fit the titration's equivalence region. For example, an indicator changing in a basic range can be suitable for some weak-acid–strong-base titrations.
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Conjugate acid–base pair
A conjugate acid–base pair consists of species differing by one proton. Losing a proton forms an acid's conjugate base, while gaining one forms a base's conjugate acid. For example, ammonium and ammonia form a conjugate acid–base pair.