Indicator colours at the end-point
Methyl orange is red in acid and yellow in alkali, passing through orange at the neutral end-point; a permanent pink means excess acid. Litmus is red in acid and blue in alkali. Phenolphthalein (Extended) is colourless in acid and pink in alkali. Indicators are weak acids or bases whose ionised and un-ionised forms differ in colour, so adding H⁺ or OH⁻ shifts the equilibrium between the two forms and the colour flips at the end-point.
Standard titration procedure and apparatus
Rinse the burette with the titrant, fill it, clear the jet bubble and record the initial reading. Rinse the pipette with the analyte and pipette 25.0 cm³ into a conical flask; add indicator. Titrate while swirling, dropwise near the end-point, and record the final reading at the first permanent colour change. Titre = final − initial. Repeat for concordant titres (within 0.10 cm³) and mean them. Apparatus: burette (titrant), pipette (fixed analyte), conical flask, white tile.
(Extended) Titration calculations with n = c × V
(Extended only.) Use n = c × V with V in dm³ (divide cm³ by 1000). Method: write the balanced equation; find moles of the known solution (n = c × V); apply the mole ratio; then divide the unknown's moles by its volume in dm³ for concentration in mol/dm³. Example: 20.0 cm³ of 0.100 mol/dm³ HCl neutralises 25.0 cm³ of NaOH (HCl + NaOH → NaCl + H₂O). n(HCl) = 0.100 × 0.0200 = 2.00 × 10⁻³ mol; ratio 1:1, so n(NaOH) = 2.00 × 10⁻³ mol; c = 0.00200 ÷ 0.0250 = 0.0800 mol/dm³.
Drawn from real examiner reports.
Methyl orange is yellow in alkali, not pink
In alkali methyl orange is yellow, not pink — this is the single most common wrong answer. In acid it is red, and the neutral end-point is orange. Pink belongs to phenolphthalein in alkali, and students routinely swap the two indicators. Use the memory aid R-A-Y: Red in Acid, Yellow in alkali. A permanent pink signals excess acid.
Section 10 — methyl orange colour in alkali poorly recalled, "pink" the common error: W22 P42 Q2(c)(iii); S22 P31 Q4(a).
(Extended) Forgetting to convert cm³ to dm³
n = c × V needs V in dm³. Substituting cm³ directly (n = 0.100 × 25.0 = 2.5 mol) makes the answer 1000× too large; the correct value is n = 0.100 × 0.0250 = 0.00250 mol. Always divide the volume in cm³ by 1000 first. A linked slip is leaving the final answer as a bare sum with no unit — that scores zero even when the method is right.
Section 4 — units omitted on final answers, answers left as unfinished sums: W22 P42 Q2(c)(iv).
Describing the end-point too vaguely
"When the indicator changes colour" is too vague for full marks. Name the indicator, state that it is the first permanent colour change, and give both colours and the direction, e.g. yellow to orange for methyl orange. A brief flash of colour that swirls away is not the end-point — only a change that persists on swirling counts.
Section 2 — observations described as product names rather than the observable change: W22 P31 Q6(a); S23 P31 Q4(d)(ii).
Confusing the burette and the pipette
The burette, with its tap and scale, holds the titrant that is added a little at a time; the pipette measures one fixed volume of the analyte into the conical flask. Never write "add the alkali using the pipette from the burette". Read the question to see which solution it places in the burette and which in the flask, because either can be the acid or the alkali.
(Extended) Assuming a 1:1 mole ratio
After finding moles of the known solution, apply the ratio from the balanced equation rather than assuming 1:1. For H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O, moles of acid = moles of NaOH ÷ 2. Treating this as 1:1 doubles the calculated acid concentration. Identify the coefficients of the two species that actually react and use only those.
Section 4 — correct moles found but the wrong stoichiometric ratio applied: W22 P21 Q9.
Averaging the rough titre with the good ones
Concordant titres agree within 0.10 cm³ of each other. Average only the concordant results and leave the rough (trial) run out of the mean. Burette readings are taken to the nearest 0.05 cm³ from the bottom of the meniscus, both recorded to 2 decimal places, so the titre is a difference of two such readings.
(Extended) Five-step calculation scaffold
For any titration sum work through: (1) write the balanced equation; (2) convert both volumes from cm³ to dm³; (3) moles of the known = c × V; (4) apply the mole ratio; (5) unknown concentration = moles ÷ volume in dm³. Check the final answer carries the unit mol/dm³.
Structuring a "describe the titration" answer
Score the method marks in order: rinse and fill the burette → pipette the analyte into the conical flask → add indicator → titrate while swirling → dropwise near the end-point → record the first permanent colour change → repeat for concordant titres and mean them.
(Extended) Show working and state units
Write n = c × V and c = n ÷ V explicitly and put a unit on every final answer. Method marks are awarded for the correct expression even if the arithmetic slips, and an answer left as an unfinished sum or with no unit is thrown away.
Match the colour change to the direction
Before quoting a colour change, check whether the acid or the alkali is in the burette. Adding acid to alkali with methyl orange gives yellow to orange; adding alkali to acid gives red to orange. The end-point colour you state must match the direction the question describes.
Titration is a quantitative technique for finding the volume of one solution that exactly reacts with a known volume (and concentration) of another solution. For acid–base titrations, an indicator signals the end-point — the volume at which the acid and alkali have exactly neutralised each other.
Indicator: a substance that is a different colour in acidic and alkaline solutions, used to show when the end-point of a titration has been reached.
End-point: the point in a titration at which the indicator shows the first permanent colour change, indicating that the exact volume of titrant needed to neutralise the analyte has been added.
(Extended) Write the ionic equation (with state symbols) for the neutralisation of any strong acid by any strong alkali, and say what it tells you.
(Extended) In a titration, of potassium hydroxide () solution is placed in a conical flask. The mean titre of hydrochloric acid () needed to reach the end-point is .
(a) Calculate the number of moles of HCl used in the titration. [2]
(b) Determine the number of moles of KOH in the conical flask. [1]
(c) Calculate the concentration of the KOH solution in mol/dm³. [2]
