What is the pH Scale?
The pH scale ranges from 0 to 14 and measures how acidic or alkaline a solution is.
Acids have pH values less than 7. The smaller the pH value, the more acidic the substance is. Concentration of hydrogen ions increases as the pH of the substance decreases.
Alkalis have pH values more than 7. The bigger the value, the more alkaline the substance is. Concentration of the hydroxide ions increases as the pH of the substance increases.
Neutral substances have pH of 7.
- Acids: pH < 7 (lower pH = stronger acid)
- Alkalis: pH > 7 (higher pH = stronger alkali)
- Neutral: pH = 7
Measurement of pH
The pH of a solution can be measured with Universal Indicator or with a pH meter.
1. Universal Indicator
pH of a substance can be approximately measured using a Universal Indicator. A Universal Indicator comes in the form of a solution or pH paper.
In the form of solution
Drip a few drops of Universal Indicator into the solution. Compare the colour with the pH chart (as shown in the featured picture).
In the form of pH paper (using for soil testing)
Dip the pH paper into the solution. Compare the result with the pH chart.
Colour of Universal Indicator
- Strong acids → Red
- Weak acids → Yellow / Orange
- Neutral → Green
- Weak alkalis → Blue
- Strong alkalis → Purple
2. pH Meter
pH can be measured accurately using a pH meter. It is an electrical method of measuring the pH of a solution. It consists of a pH probe connected to a meter. pH of the substance is measured by dipping the probe into the solution. The meter shows the pH either on a scale or digitally.
Common pH Indicators Students Must Know
| Indicator | Colour in Acid | Colour in Neutral | Colour in Alkali | pH Transition Range |
|---|---|---|---|---|
| Universal Indicator | Red / Orange | Green | Blue / Purple | Whole scale (0–14) |
| Litmus | Red | Purple | Blue | 5 – 8 |
| Methyl Orange | Red | Orange | Yellow | 3.1 – 4.4 |
| Phenolphthalein | Colourless | Colourless | Pink | 8.2 – 10.0 |
| Bromothymol Blue | Yellow | Green | Blue | 6.0 – 7.6 |
Key Points
- Litmus is commonly used to test for acids and alkalis.
- Phenolphthalein is often used in acid-base titrations (colourless in acid, pink in alkali).
- Methyl Orange is useful for strong acid – strong base titrations.
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