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Phenolphthalein is used as an indicator for the titration of sodium hydroxide solution against a standard solution of oxalic acid. The colour change observed at an alkaline pH close to the equivalence point is :
Options
1
Pinkish red to yellow
2
Yellow to pinkish red
3
Colourless to pink
4
Pink to colourless
Correct Answer
Option 3 : Colourless to Pink
Step-by-Step Solution
1

Setup of the titration:

Burette: NaOH (strong base) — the titrant being added

Conical flask: Oxalic acid (standard, weak diprotic acid) — the analyte

Indicator: Phenolphthalein

2

Before equivalence point:

Flask contains excess oxalic acid → solution is acidic

Phenolphthalein in acid: colourless

3

At/after equivalence point:

All acid neutralised → solution becomes alkaline (Na₂C₂O₄ is basic — salt of strong base + weak acid)

Phenolphthalein in alkaline: pink

Colour change: Colourless → Pink

Acid (colourless) + NaOH → Alkaline (pink)

Phenolphthalein: colourless in acid, pink in base

Colour change at equivalence: colourless → pink ✅

Theory: Indicators & Acid-Base Titrations
1. Phenolphthalein — How It Works

Phenolphthalein (PP) is a weak acid indicator (HIn). In acidic solution: HIn form (colourless) predominates — the molecule exists in a closed lactone ring form which absorbs no visible light. In basic solution (pH > 8·2): In⁻ form (pink/magenta) predominates — the ring opens, forming a quinoid structure that absorbs visible light and appears pink/red. Colour change range: pH 8·2 (colourless) to 10·0 (pink). At pH > 12: decolourises again (In²⁻, colourless alkaline form). PP is suitable for titrations where equivalence point pH is in the range 8·2–10·0.

2. Why PP is Correct for NaOH vs Oxalic Acid

📌 Oxalic acid (H₂C₂O₄) = weak diprotic acid, Ka₁ = 5·9×10⁻², Ka₂ = 6·4×10⁻⁵

📌 NaOH = strong base

📌 At equivalence point: Na₂C₂O₄ formed (salt of strong base + weak acid) → solution is basic (pH > 7)

📌 Equivalence point pH ≈ 8-9 → falls in PP colour change range (8·2-10) ✅

📌 Hence PP is the correct indicator — changes colour precisely at equivalence

📌 Methyl orange (changes at pH 3·1-4·4) would be WRONG for this titration

3. Common Indicators and Their pH Ranges

📌 Methyl orange: pH 3·1-4·4, red (acid) → yellow (base) — strong acid vs strong base, or strong acid vs weak base

📌 Methyl red: pH 4·4-6·2, red → yellow — stronger acids

📌 Litmus: pH 6·0-7·6, red → blue — not sharp, not used in volumetric analysis

📌 Phenol red: pH 6·8-8·4, yellow → red

📌 Phenolphthalein: pH 8·2-10·0, colourless → pink — weak acid vs strong base

📌 Thymolphthalein: pH 9·3-10·5, colourless → blue

📌 Thymol blue (alkaline): pH 8·0-9·6, yellow → blue

4. Selecting the Right Indicator for a Titration

The indicator must change colour at the equivalence point pH. Rule: the indicator's colour change pH range must include the equivalence point pH. Strong acid + Strong base: equivalence pH = 7; use litmus, phenol red, or any indicator near pH 7. Strong acid + Weak base: equivalence pH < 7; use methyl orange or methyl red. Weak acid + Strong base: equivalence pH > 7; use phenolphthalein. Weak acid + Weak base: no sharp change — no suitable indicator. Titration curve shape determines the feasibility: steep endpoint only for strong acid/strong base or one strong component.

5. Why is Option 4 (Pink to Colourless) Wrong?

Option 4 (pink → colourless) would describe the reverse situation: if the flask contained NaOH (pink with PP) and acid was being added from the burette. As acid neutralises the base, the solution turns acidic near the equivalence point → PP becomes colourless (pink → colourless). But in THIS question: NaOH is in the burette (titrant), oxalic acid is in the flask. Flask starts acidic → PP colourless. As NaOH is added → solution becomes alkaline → PP turns pink. Change = colourless → pink.

6. Oxalic Acid as a Primary Standard

Oxalic acid (H₂C₂O₄·2H₂O) is a primary standard for standardising NaOH solutions. Primary standard requirements: (1) High purity (≥99·9%). (2) Stable in air (doesn't absorb moisture or CO₂). (3) High molar mass (to reduce weighing errors). (4) Reacts completely and rapidly with the titrant. (5) Has a definite composition. Oxalic acid meets all criteria. Other primary standards: K₂Cr₂O₇ (for KMnO₄), Na₂CO₃ (for HCl), potassium hydrogen phthalate (KHP) for NaOH. NaOH itself is NOT a primary standard (absorbs CO₂ and moisture from air).

7. Titration Curve — Weak Acid vs Strong Base

For oxalic acid + NaOH: initially pH is low (acidic). As NaOH is added: first, the steep region is less steep than strong acid because Ka of oxalic acid is not too small. Buffer region appears (mixture of H₂C₂O₄ and HC₂O₄⁻, then HC₂O₄⁻ and C₂O₄²⁻). At half-equivalence point: pH = pKa. At equivalence point: solution is basic (Na₂C₂O₄). After equivalence: pH rises steeply with excess NaOH. The steep jump at equivalence is less sharp than strong acid titration — but still detectable with PP. The equivalence point pH (≈ 8-9) falls perfectly in PP's range.

8. Back Titration and Double Indicator Method

Double indicator method: uses two indicators for a mixture of Na₂CO₃ and NaOH, or Na₂CO₃ and NaHCO₃. First endpoint: PP (pH ≈ 8·3) — all NaOH neutralised + Na₂CO₃ → NaHCO₃. Second endpoint: methyl orange (pH ≈ 4) — all NaHCO₃ neutralised. From two burette readings, amounts of each component are calculated. This is a common NEET topic. Back titration: when direct titration is difficult (e.g., solid doesn't dissolve quickly) — add excess standard acid, let react, then titrate the excess acid with NaOH. Useful for insoluble carbonates like CaCO₃.

Frequently Asked Questions
1. Why does phenolphthalein turn pink in base but not in acid? ⌄
Phenolphthalein (HIn) is itself a weak acid. In acid: HIn form has a closed lactone ring structure — no extended conjugation — absorbs only UV → appears colourless. In base: OH⁻ + HIn → In⁻ + H₂O. The In⁻ form has an open quinoid structure with extended conjugation — the chromophore absorbs visible light (550-570 nm, green region) → appears pink/magenta (complementary colour). At very high pH (>12): In²⁻ forms (a third colourless form). This is why very concentrated NaOH causes PP to become colourless again.
2. What is the equivalence point and endpoint? ⌄
Equivalence point: the theoretical point where stoichiometrically equivalent amounts of acid and base have reacted — moles of acid = moles of base (considering basicity/acidity). For oxalic acid + NaOH: moles of H₂C₂O₄ × 2 = moles of NaOH (diprotic acid). Endpoint: the point where the indicator changes colour — observable in practice. The endpoint should be as close to the equivalence point as possible (indicator error). Phenolphthalein endpoint ≈ pH 8·5 for this titration; equivalence point ≈ pH 8-9. Small error, acceptable for practical purposes.
3. Why is NaOH not a primary standard? ⌄
NaOH fails primary standard criteria because: (1) Absorbs CO₂ from air: 2NaOH + CO₂ → Na₂CO₃ + H₂O — this changes its concentration over time. (2) Absorbs moisture — NaOH is hygroscopic (absorbs water), which changes its effective concentration. (3) It's a strong base — difficult to handle precisely. Therefore, NaOH solutions must be standardised against a primary standard (like oxalic acid or potassium hydrogen phthalate KHP). The standardisation gives the exact molarity of the NaOH solution before it's used for other titrations.
4. What happens if you use methyl orange for weak acid + strong base titration? ⌄
Methyl orange changes colour at pH 3·1-4·4. For weak acid (oxalic acid) + NaOH: the equivalence point is at pH ≈ 8-9. Methyl orange changes colour long BEFORE the equivalence point — in the buffer region, not at equivalence. The titration would be stopped too early → inaccurate result. The measured volume of NaOH would be less than the actual equivalence volume → error in result. This is why indicator selection based on equivalence point pH is critical.
5. What colour change occurs when titrating strong acid with strong base? ⌄
Strong acid (e.g., HCl in flask) + strong base (NaOH in burette): equivalence pH = 7. Any indicator changing near pH 7 works: litmus (6-7·6, red→blue), phenol red (6·8-8·4), bromothymol blue (6·0-7·6). Methyl orange also works (changes before pH 7 but the titration curve is so steep near pH 7 that even a slight excess of base causes a massive pH jump through the methyl orange range). Phenolphthalein also works (the steep jump at pH 7 passes through pH 8·2-10·0). For strong + strong titrations, most indicators work due to the very steep pH jump at equivalence.
6. What is the double indicator method for Na₂CO₃ + NaOH mixture? ⌄
Mixture of Na₂CO₃ + NaOH titrated with HCl. Two indicators: (1) Phenolphthalein first — endpoint when PP just becomes colourless. Volume V₁ used: all NaOH neutralised (NaOH + HCl → NaCl + H₂O) AND half of Na₂CO₃ neutralised (Na₂CO₃ + HCl → NaHCO₃ + NaCl). (2) Add methyl orange — continue titrating. Volume V₂ used: remaining NaHCO₃ neutralised (NaHCO₃ + HCl → NaCl + H₂O + CO₂). Calculations: moles Na₂CO₃ = moles HCl × V₂; moles NaOH = moles HCl × (V₁ − V₂). If V₁ < V₂: mixture is Na₂CO₃ + NaHCO₃.
7. What is the role of oxalic acid in permanganate titrations? ⌄
Oxalic acid (H₂C₂O₄) is also used to standardise KMnO₄: 2KMnO₄ + 5H₂C₂O₄ + 3H₂SO₄ → K₂SO₄ + 2MnSO₄ + 10CO₂ + 8H₂O (in acidic medium). KMnO₄ is self-indicating (purple → colourless as Mn⁷⁺ → Mn²⁺). The oxalic acid + warm H₂SO₄ + KMnO₄ reaction is slow initially (autocatalytic — MnSO₄ formed catalyses the reaction). Temperature must be maintained at 60-70°C for good reaction rate. KMnO₄ is NOT a primary standard (contains impurities MnO₂), so it must be standardised against oxalic acid or sodium oxalate.
8. Why is the endpoint of PP visible even though the change is at pH 8·2? ⌄
The sharp steep portion of the titration curve (where pH changes rapidly for tiny additions of NaOH) passes through pH 8·2-10·0 for weak acid + strong base titrations. Near the equivalence point, adding just one drop of NaOH (≈0·05 mL) can change pH by 2-3 units — this rapid change causes the indicator to change colour sharply. The human eye can detect a faint pink colour (endpoint) at concentrations as low as 10⁻⁵ mol/L of the pink form. This sensitivity, combined with the steep pH jump, makes the PP endpoint clearly visible and reproducible.
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