Titration: The Idea

Introduction

Suppose you have a bottle of hydrochloric acid and you want to know exactly how much sodium hydroxide solution is needed to neutralise 25.0 cm³ of it — not roughly, but to the nearest tenth of a cubic centimetre. The pH curve shows that neutralisation happens in a single drop, so the question is how to catch that drop. Titration is the chemist's answer: a careful, repeatable method for finding exact reacting volumes, which can then be used to work out unknown concentrations.

Core explanation

The purpose. A titration measures the volume of one solution needed to react exactly with a known volume of another. If the concentration of one solution is known, the concentration of the other can be calculated from the reacting volumes and the balanced equation.

The apparatus.

- A pipette (with a pipette filler for safety) measures a fixed volume, commonly 25.0 cm³, of one solution very accurately. This is transferred to a conical flask. - A burette holds the other solution. It is graduated in 0.1 cm³ divisions, and its tap allows the solution to be added quickly at first and then drop by drop. - A conical flask holds the pipetted solution. Its shape lets it be swirled without splashing. - A white tile under the flask makes the colour change of the indicator easy to see. - An indicator , a few drops of which are added to the flask, changes colour at the end point.

The general method. The burette is rinsed with the solution it will contain, filled, and the starting reading recorded. The pipetted solution and indicator go into the flask. Solution is run in from the burette while the flask is swirled. Near the end point, the colour change starts to linger where each drop lands, so the solution is then added one drop at a time. As soon as a single drop causes a permanent colour change, the tap is closed and the final reading taken. The titre is final reading minus initial reading.

Rough and accurate runs. The first titration is a quick "rough" run to find the approximate end point. Later runs are done slowly near that volume. Titrations are repeated until at least two titres are concordant , agreeing within 0.10 cm³. The mean of the concordant titres is used in calculations; the rough titre and any outliers are left out.

Reading the burette. Readings are taken at eye level from the bottom of the meniscus and recorded to two decimal places, with the second place being 0 or 5 (for example 23.45 cm³), because a reading can be estimated to half a division.

Safe practice. Titrations in school use dilute solutions, but acids and alkalis can still irritate skin and damage eyes, so eye protection is worn and spills are wiped up promptly. A pipette filler is always used — never the mouth.

Step-by-step reasoning

To decide which titres to average:

1. Ignore the rough titre. 2. Look for results within 0.10 cm³ of each other. 3. Discard any result that differs by more than this. 4. Calculate the mean of the concordant titres.

Visual explanation

Imagine a tall burette clamped above a conical flask on a white tile. A stream of alkali runs into colourless acid containing phenolphthalein. Pink flashes appear and vanish as the flask is swirled, lasting longer each time, until one final drop turns the whole solution a pale, permanent pink. The titration simulation lets you practise stopping at that drop.

Real-world analogy

Filling a glass to the brim without spilling works the same way: pour quickly at first, then slowly, then drop by drop as you near the top. A titration is controlled pouring, with the indicator telling you exactly when the "glass" is full.

Real-world example

Quality-control laboratories use titration to check the acidity of vinegar, the vitamin C content of fruit juice and the concentration of cleaning products. Winemakers titrate wine samples with alkali to measure total acidity, which strongly affects taste.

Why?

Why is the solution added drop by drop near the end? The pH curve shows that one drop can change the pH by several units. Adding liquid quickly would overshoot the end point, making the titre too large.

Common misconception

"The end point is when the solution first turns colour." A brief flash of colour where the drop lands does not count; it disappears on swirling. The end point is the first permanent colour change throughout the whole solution.

Worked example

Question: A student records titres of 24.60 (rough), 24.15, 24.25 and 24.20 cm³. Calculate the mean titre.

Reasoning: Ignore the rough titre. The other three all lie within 0.10 cm³ of each other, so all are concordant.

Answer: (24.15 + 24.25 + 24.20) ÷ 3 = 24.20 cm³.

Quick check

1. Why is a white tile placed under the conical flask during a titration? Answer: It makes the colour change of the indicator easier to see.

Exam focus

Name the apparatus and state what each piece is for; describe the method in a logical order; explain concordant results; and record burette readings to two decimal places. Examiners often ask why the flask is swirled (to mix the solutions so the reaction is complete).

Advanced insight

Rinsing apparatus matters. The burette and pipette are rinsed with the solutions they will contain, because leftover water would dilute them. The conical flask is rinsed with distilled water only: extra water changes the volume but not the moles of reactant pipetted into it, so it does not affect the titre.

Summary

Titration finds the exact volume of one solution that reacts with a known volume of another. A pipette delivers a fixed volume into a flask with indicator, and a burette adds the other solution until one drop gives a permanent colour change. Titres are repeated until concordant, within 0.10 cm³, and averaged.

Practice questions

1. State the function of the pipette and of the burette in a titration. Answer: The pipette measures an accurate fixed volume into the flask; the burette adds the other solution gradually and measures the volume added. 2. A burette reads 1.20 cm³ at the start and 25.65 cm³ at the end. Calculate the titre. Answer: 25.65 − 1.20 = 24.45 cm³. 3. Explain what is meant by concordant results. Answer: Titres that agree closely with each other, usually within 0.10 cm³. 4. Why is the conical flask swirled throughout the titration? Answer: To mix the solutions thoroughly so that the added solution reacts completely and the colour change is seen accurately.