Synthetic Indicators: Phenolphthalein

Introduction

Natural indicators from plants are cheap and fun, but their colours vary and they spoil quickly. Chemists therefore use synthetic indicators , made in factories with a fixed, pure composition. One of the best known is phenolphthalein (said "fee-nol-thay-leen"). It is colourless in acids and in water, but turns a striking pink or magenta in alkalis. That sudden appearance of colour makes it a favourite for detecting alkalis and for titrations.

Core explanation

What it is. Phenolphthalein is an organic compound, first made in 1871 by the German chemist Adolf von Baeyer. It is a white or pale yellow solid that does not dissolve well in water, so in the laboratory it is usually supplied as a solution in ethanol mixed with water.

Its colours.

Solution Example Colour of phenolphthalein --- --- --- Acidic hydrochloric acid, vinegar colourless Neutral pure water, salt solution colourless Weakly alkaline (just above neutral) very dilute baking soda solution colourless or faintly pink Alkaline sodium hydroxide solution, limewater pink to magenta

Phenolphthalein starts to turn pink at around pH 8 and is fully pink by about pH 10 . (The pH scale, a number scale for acidity, is introduced shortly; for now, pH 7 is neutral, lower numbers are acidic and higher numbers are alkaline.)

A one-sided test. Because phenolphthalein is colourless in both acids and neutral solutions, it cannot tell them apart. It is really a test for alkalis. To distinguish an acid from water, another indicator such as blue litmus or methyl orange is needed.

How the colour appears. In acidic and neutral solutions, the phenolphthalein molecule has a structure that absorbs only ultraviolet light, which our eyes cannot see, so it looks colourless. In alkaline solution, it loses hydrogen ions and its structure opens out into a charged form that absorbs green light, so we see pink-magenta.

Use in titrations. In a titration, an alkali is added gradually to an acid (or the reverse) until they have exactly reacted. Phenolphthalein is ideal when an alkali such as sodium hydroxide is being added: the solution stays colourless until the acid has been used up, and then a single extra drop of alkali turns it permanently pink. This sharp end point makes measurements precise.

Safety. Phenolphthalein was once used as a laxative, but it was withdrawn from medicines in many countries because of health concerns. Laboratory solutions are also flammable because they contain ethanol. It is handled with gloves and eye protection like other laboratory chemicals and never tasted.

Step-by-step reasoning

To test whether a colourless liquid is alkaline with phenolphthalein:

1. Place a small sample in a clean test tube. 2. Add two or three drops of phenolphthalein. 3. If the liquid turns pink, it is alkaline. 4. If it stays colourless, it is acidic or neutral, and a second indicator is needed to decide.

Visual explanation

Imagine a beaker of clear liquid on a white tile. Drops of alkali are added from above; at first each drop makes a pink swirl that vanishes as it is stirred. Then one final drop leaves the whole beaker a steady pale pink. That moment is the end point.

Real-world analogy

Phenolphthalein is like invisible ink that appears only when a special developer is brushed over the page. Until then the page looks blank, whether the paper is new or old. Likewise, phenolphthalein shows no colour in acids or water, and its pink colour appears only when enough alkali is present to act as the developer.

Real-world example

Phenolphthalein is used to test concrete. Fresh concrete is strongly alkaline, which protects the steel bars inside from rusting. Engineers spray phenolphthalein on a freshly broken surface: pink areas are still alkaline, while colourless areas near the surface show where carbon dioxide from the air has reduced the alkalinity and the steel may be at risk.

Why?

Why do titrations with sodium hydroxide often use phenolphthalein instead of litmus? Litmus changes gradually through a muddy purple, making the exact end point hard to judge. Phenolphthalein changes from completely colourless to clearly pink, which is much easier to see.

Common misconception

"If phenolphthalein stays colourless, the solution must be acidic." It may equally be neutral or only very slightly alkaline. Colourless simply means "not alkaline enough to turn it pink".

Worked example

Question: Three drops of phenolphthalein are added to solutions A, B and C. A turns pink, B and C stay colourless. Blue litmus turns red in B but stays blue in C. Identify each.

Reasoning: Pink means A is alkaline. B turns blue litmus red, so it is acidic. C does not affect phenolphthalein or blue litmus, so it is neutral.

Answer: A is alkaline, B is acidic and C is neutral.

Quick check

1. What colour is phenolphthalein in sodium hydroxide solution? Answer: Pink (magenta).

Exam focus

Memorise: phenolphthalein is colourless in acid and pink in alkali. Many exam answers lose marks by writing "clear" instead of "colourless"; clear means transparent, and a pink solution can still be clear.

Advanced insight

In very strongly alkaline solutions, above about pH 13, the pink colour of phenolphthalein slowly fades again, because the molecule reacts further to form a second colourless form. Chemists also avoid it for titrations of a strong acid with a weak base, where the end point lies below the pH at which it changes colour.

Summary

Phenolphthalein is a synthetic indicator that is colourless in acidic and neutral solutions and pink in alkaline solutions, changing between about pH 8 and pH 10. It detects alkalis but cannot distinguish acids from neutral substances. Its sharp change from colourless to pink makes it excellent for titrations using alkalis.

Practice questions

1. State the colour of phenolphthalein in lemon juice and in limewater. Answer: Colourless in lemon juice (acidic) and pink in limewater (alkaline). 2. Why can phenolphthalein not be used on its own to show that a solution is acidic? Answer: It is colourless in both acidic and neutral solutions, so the result is the same for both. 3. What is meant by the end point of a titration? Answer: The moment when the indicator changes colour, showing that the acid and alkali have just reacted completely. 4. Why is "clear" not the correct word for phenolphthalein in acid? Answer: Clear means see-through, which a coloured solution can also be; the correct word is colourless, meaning no colour at all.