About Universal Indicator
The common indicators (like litmus) can tell us whether the given substance is an acid or a base. They cannot tell us whether the given substance is a strong acid, a weak acid, a strong base or a weak base. In other words, the common indicators cannot tell us the relative strength of acids or bases. For example, litmus can tell us that sulphuric acid vinegar are both acidic but it cannot tell us that sulphuric acid is a stronger acid (more acidic) than vinegar solution. We can, however, measure the strength of an acid solution or a base solution by using a special type of indicator called ‘universal indicator’ (which works by measuring the pH of a solution). Thus, to obtain an idea of how acidic basic a substance is, universal indicator is used. This will become clear from the wing discussion.
A common method of measuring the pH of a solution in the school laboratory is to universal indicator. Universal indicator is a mixture of many different indicators (or- dyes) which gives different colours at different pH values of the entire pH scale. Since the pH of a solution depends on the hydrogen ion concentration, so we can also say that the universal indicator shows different colours at different concentrations of hydrogen ions in the solution. When an acid or base solution is added to the universal indicator, the indicator produces a new colour. The colour produced by universal indicator is used to
find the pH value of the acid or base solution by matching the colour with the colours on pH colour chart. And knowing the pH value, we can make out whether the given solution is a strong acid, weak acid, strong base or a weak base. The colours produced by universal indicator at various pH values are given below:
Use of Universal Indicator
The makers of universal indicator provide a pH colour chart to compare the colour produced on the indicator paper (or solution) on adding acid or base solution and find out its pH value. Just like litmus, universal indicator can be used either in the form of a solution or in the form of universal indicator paper We will now de scribe how a universal indicator paper is used to measure the pH value of a solution.
A drop of the solution to be tested is put on a strip of universal indicator paper. The indicator paper will undergo a change in colour (where the solution has been added). The colour produced on indicator paper is then matched with a colour on the standard pH colour chart (available in the laboratory). We can read the pH value corresponding to this colour from the pH colour chart. And knowing the pH value, we can tell whether the given solution is a strong acid, a weak acid, a strong base or a weak base.
For example, if on putting the drop of a solution on the universal indicator paper, the paper turns dark red, then its pH will be around 0 (zero) and hence it will be a strong acid. On the other hand, if an orange colour is produced, then the pH will be about 4 and it will be a weak acid. There are also different colours produced with different basic solutions (or alkaline solutions). The weakly basic solutions produce blue colour whereas highly basic solutions give violet colour with universal indicator. Please note that if a solution turns universal indicator green, then it will be a neutral solution (which is neither acidic nor basic). Thus, water will produce a green colour with universal indicator. Check out Chemistry Formulas and NCERT Solutions for class 11 Chemistry prepared by Physics Wallah.
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