pH and Indicators

pH Scale

The pH scale measures how acidic or alkaline a substance is. It ranges from 0 to 14:

• Acidic solutions have a pH less than 7.
• Neutral solutions have a pH exactly 7 (pure water is neutral).
• Alkaline (basic) solutions have a pH greater than 7.

The scale is logarithmic, meaning each whole number change represents a tenfold change in hydrogen ion concentration. For example, a solution with pH 4 is ten times more acidic than one with pH 5.

For instance, lemon juice typically has a pH around 2 (acidic), pure water is 7 (neutral), and soapy water might be around pH 12 (alkaline).

Indicators

Indicators are substances that change colour depending on the pH of the solution they are in. They help us identify whether a solution is acidic, neutral, or alkaline.

Indicators are usually weak acids or bases that change their molecular structure and colour depending on the pH of the solution.

Common indicators include:

• Litmus paper: turns red in acid and blue in alkali.
• Methyl orange: turns red in acid and yellow in alkali.
• Phenolphthalein: colourless in acid and pink in alkali.

Each indicator changes colour over a specific pH range, so the choice of indicator depends on the expected pH of the solution.

For example, if you test vinegar (an acid) with litmus paper, it will turn red, showing it is acidic. If you test baking soda solution (alkaline) with phenolphthalein, it will turn pink.

Measuring pH

There are two common methods to measure pH:

• Universal indicator: A mixture of dyes that shows a range of colours across the pH scale. It gives an approximate pH value based on the colour shown.
• pH meter: An electronic device with a probe that measures the exact pH value of a solution.

Universal indicator is useful for quick, visual checks, while a pH meter provides precise numerical readings.

Interpreting pH values:

• pH 0–6.9: acidic
• pH 7: neutral
• pH 7.1–14: alkaline

For example, if a universal indicator turns green, the pH is around 7, indicating a neutral solution.

Example: If you test a solution with a pH meter and get a reading of 9.5, this means the solution is alkaline.

Applications of pH and Indicators

Indicators and pH measurements are used in many practical situations:

• Testing acidity of substances: Food products like fruit juices or soft drinks can be tested to check their acidity.
• Monitoring neutralisation reactions: When acids and alkalis react, indicators show when the solution becomes neutral.
• Making salts: Indicators help find the exact point when an acid is neutralised by an alkali during salt preparation (see Making Salts topic for details).

For example, in making sodium chloride salt, methyl orange can be used to show when the acid (hydrochloric acid) has been completely neutralised by sodium hydroxide.

Example: Using universal indicator to test the acidity of lemon juice

Dip a strip of universal indicator paper into lemon juice. The paper turns red, indicating a pH around 2, confirming the lemon juice is acidic.