Watching Neutralisation with an Indicator

Introduction

Neutralisation happens between colourless solutions, so without help you cannot see it taking place. Hydrochloric acid and sodium hydroxide solution both look like water, and so does the salt solution they form. An indicator solves the problem by turning the invisible change in pH into a visible change in colour. By watching the colours carefully, you can tell how far a neutralisation has gone and spot the moment it becomes neutral.

Core explanation

Using universal indicator. Suppose a few drops of universal indicator are added to some dilute hydrochloric acid in a conical flask. The solution turns red , showing a pH of about 1–2. Dilute sodium hydroxide solution is then added a little at a time, with swirling. The colour changes in a clear sequence:

Stage Colour Approximate pH --- --- --- Start: acid only red 1–2 Some alkali added orange 3–4 More alkali added yellow 5–6 Neutral point green 7 Slight excess of alkali blue 8–10 Large excess of alkali purple 11–14

The green colour marks the neutral point, where the alkali has exactly neutralised the acid.

Why add alkali slowly? For much of the process, each portion of alkali changes the pH only a little, so the colour stays red or orange. Close to neutral, the pH changes very rapidly — a single drop can move it from yellow straight past green to blue. To catch the neutral point, the alkali is added in larger portions at first and then dropwise as the colour begins to change. Swirling after each addition mixes the solution so the colour is the same throughout; a flash of green or blue that fades on swirling shows you are getting close.

Why a conical flask? Its sloping sides allow swirling without splashing, and a white tile underneath makes colour changes easier to see.

Using phenolphthalein instead. Phenolphthalein is colourless in acidic and neutral solutions and turns pink at about pH 8–10. When alkali is added to acid containing phenolphthalein, nothing appears to happen until, suddenly, one drop produces a permanent pale pink colour. This is the end point . Because the pH jump near neutral is so steep, that first permanent pink shows that the acid has been neutralised, with at most a drop of extra alkali. Single indicators like phenolphthalein give one sharp change, which makes the end point easier to judge than the gradual shades of universal indicator.

Going the other way. If acid is added to alkali, the sequence reverses: purple, blue, green, yellow, orange, red with universal indicator, or pink fading to colourless with phenolphthalein.

Step-by-step reasoning

To follow a neutralisation with an indicator:

1. Add a few drops of indicator to the acid and note the starting colour. 2. Add the alkali in portions, swirling after each. 3. Watch for the first signs of colour change, then add dropwise. 4. Stop when the indicator shows the neutral or end-point colour throughout. 5. Record the volume of alkali used.

Visual explanation

Picture a row of six conical flasks on a white tile, each taken at a later stage of the same neutralisation. The first is red, then orange, yellow, green, blue and finally purple. It looks like a rainbow read from left to right, with green — the neutral flask — in the middle.

Real-world analogy

Adding alkali dropwise near the neutral point is like filling a glass to the brim. You pour quickly at first, then slow to a trickle as the water nears the top, because the final few drops decide whether it is full or overflowing.

Real-world example

Pool maintenance kits use indicator tablets or drops to check swimming-pool water. The water sample changes colour, and the colour is compared with a chart. If it is too acidic or too alkaline, a small amount of a pool chemical is added and the water is tested again, following the colour back towards the ideal range.

Why?

Why does one drop cause a big colour change near neutral? The pH scale is based on tenfold steps. Near neutral, very few hydrogen ions remain, so one drop of alkali can remove almost all of them and then supply an excess of hydroxide ions, shifting the pH by several units at once.

Common misconception

"When the indicator changes colour, the reaction stops." The reaction happens every time acid and alkali meet. The colour change only shows that the acid has been used up; adding more alkali simply makes the solution alkaline.

Worked example

Question: A student adds sodium hydroxide solution to hydrochloric acid containing universal indicator. After swirling, the solution is blue. What does this show, and what should the student do next time?

Reasoning: Blue means pH about 8–10, so the solution is slightly alkaline. The student has added slightly more alkali than needed.

Answer: The alkali is in slight excess; next time the student should add the alkali dropwise near the end and stop at green.

Quick check

1. What colour does universal indicator show at the neutral point? Answer: Green.

Exam focus

Be ready to give the universal indicator colour sequence during neutralisation, to state that green means pH 7, and to explain why alkali is added dropwise with swirling near the end. Know that phenolphthalein goes from colourless to pink when alkali is added to acid.

Advanced insight

Chemists use this method, with a burette and a single indicator, to find exact volumes in a titration. The volume of alkali needed at the end point allows the concentration of the acid to be calculated. Choosing the right indicator matters: its colour change must fall within the steep jump in pH for that particular acid and base.

Summary

Indicators make neutralisation visible. With universal indicator, an acid changes from red through orange and yellow to green at the neutral point, then blue and purple as alkali is added in excess. Phenolphthalein turns from colourless to pale pink at the end point. Alkali is added dropwise with swirling near the end because the pH changes sharply close to neutral.

Practice questions

1. List the universal indicator colours seen as sodium hydroxide is added to hydrochloric acid until the alkali is in large excess. Answer: Red, orange, yellow, green, blue, purple. 2. Why is the alkali added dropwise near the neutral point? Answer: Because the pH changes very rapidly near neutral, so a single drop can overshoot the neutral point. 3. Why is a white tile placed under the flask? Answer: It makes the colour change easier to see. 4. Describe the colour change with phenolphthalein when acid is added to alkali until the acid is in excess. Answer: Pink to colourless.