Universal Indicator and Its Colour Chart

Introduction

Litmus can tell you whether a solution is acidic or alkaline, but not how acidic or alkaline it is. Lemon juice and battery acid both turn blue litmus red. To go further, chemists invented universal indicator , a carefully designed mixture of several indicators. It shows a different colour for each level of acidity or alkalinity, and a colour chart translates each colour into a number. It is the bridge between simple indicators and the pH scale.

Core explanation

A mixture of indicators. Each single indicator changes colour at its own level of acidity: methyl orange around pH 3 to 4, methyl red around pH 4 to 6, bromothymol blue around pH 6 to 7.6, and phenolphthalein around pH 8 to 10. Universal indicator mixes several such indicators together. As a solution becomes gradually less acidic, first one component changes, then the next, and so on. The combined colours form a smooth sequence from red through orange, yellow, green and blue to purple.

The colour chart. Every bottle or pad of universal indicator comes with a colour chart printed by the manufacturer. A typical chart looks like this:

pH Colour Description Example --- --- --- --- 0–2 red strongly acidic hydrochloric acid, stomach acid 3–4 orange acidic vinegar, lemon juice 5–6 yellow weakly acidic rainwater, black coffee 7 green neutral pure water, salt solution 8–9 blue-green weakly alkaline seawater, baking soda solution 10–11 blue alkaline milk of magnesia, soap solution 12–14 violet or purple strongly alkaline sodium hydroxide solution, oven cleaner

The pH number describes acidity: 7 is neutral, numbers below 7 are acidic and numbers above 7 are alkaline. The next page explores the pH scale in detail.

Two forms. Universal indicator is sold as a solution , added a few drops at a time, and as universal indicator paper , which is quick and convenient. A drop of the test liquid is placed on the paper with a clean glass rod, and the colour is compared with the chart straight away, before it dries or changes.

How precise is it? Universal indicator estimates pH to within about one unit. Colours between two shades must be judged by eye, and different brands use slightly different mixtures and charts. For more accurate work, an electronic pH meter is used.

Limitations. Coloured or cloudy test solutions, such as fruit juice or muddy water, can hide the indicator colour. Colour-blind users may also find some shades hard to distinguish.

Step-by-step reasoning

To estimate the pH of a liquid with universal indicator paper:

1. Tear off a small strip of paper and place it on a white tile. 2. Dip a clean glass rod in the liquid and touch it on the paper. 3. Compare the colour with the chart immediately. 4. Read off the pH of the closest matching colour and state whether the liquid is acidic, neutral or alkaline.

Visual explanation

Picture a row of fifteen test tubes, labelled pH 0 to 14, each holding a few drops of universal indicator. From left to right the colours run deep red, orange, yellow, green, blue and finally violet, like a rainbow bent across the rack. The green tube sits exactly in the middle at pH 7.

Real-world analogy

A single indicator is like a light switch: on or off. Universal indicator is like a dimmer switch with a dial marked in numbers, showing not only whether the light is on but how bright it is.

Real-world example

Pool and aquarium owners use test kits based on indicator mixtures. A few drops of indicator are added to a water sample and the colour is matched to a chart. If the water is too acidic or too alkaline, chemicals are added to adjust it, protecting swimmers' eyes or the health of fish.

Why?

Why does mixing indicators give more information than one indicator alone? Each component changes colour at a different level of acidity, so at every pH at least one of them is changing. Together they give a different overall colour at almost every pH value.

Common misconception

"Green universal indicator means a solution contains no acid or alkali." Green shows the solution is neutral, around pH 7. A salt solution made by neutralisation can be green even though it was made from an acid and an alkali.

Worked example

Question: Universal indicator turns orange in liquid A, green in liquid B and blue in liquid C. Estimate the pH of each and classify them.

Reasoning: Using the chart, orange corresponds to about pH 3 to 4, green to pH 7 and blue to about pH 10 to 11.

Answer: A is about pH 3–4 and acidic; B is about pH 7 and neutral; C is about pH 10–11 and alkaline.

Quick check

1. What colour is universal indicator in a neutral solution? Answer: Green.

Exam focus

Learn the order of colours from red at low pH through orange, yellow, green and blue to purple at high pH. Explain that universal indicator is a mixture, and that it shows how strongly acidic or alkaline a solution is, which litmus cannot.

Advanced insight

Manufacturers design universal indicator so that the individual colour changes overlap evenly across the scale. Narrow-range indicator papers use the same idea over a small span, for example pH 4 to 7, giving finer steps of about half a unit where extra precision is needed.

Summary

Universal indicator is a mixture of several indicators, each changing colour at a different acidity. It shows a sequence of colours, red, orange, yellow, green, blue and purple, across pH 0 to 14. A colour chart converts each colour into a pH number, estimating acidity to about one unit, which gives far more information than litmus.

Practice questions

1. Why is universal indicator called "universal"? Answer: It changes colour across the whole range of acidity and alkalinity, rather than at a single point like litmus. 2. Explain why universal indicator gives more information than litmus. Answer: It shows many colours matched to pH numbers, so it tells how strongly acidic or alkaline a solution is, not only which type it is. 3. A liquid turns universal indicator violet. What is its approximate pH and what type of solution is it? Answer: About pH 12 to 14; it is strongly alkaline. 4. Give one limitation of universal indicator. Answer: It estimates pH only to about one unit, and it is hard to use with coloured or cloudy solutions.