Acid-Base Indicators

An acid-base indicator is either a weak acid or weak base that exhibits a color change as the concentration of hydrogen (H+) or hydroxide (OH-) ions changes in an aqueous solution. Acid-base indicators are most often used in a titration to identify the endpoint of an acid-base reaction. They are also used to gauge pH values and for interesting color-change science demonstrations.

pH Scale

The pH scale is used to indicate how acidic or basic a given solution is. The scale generally consists of a series of numbers from 0 to 14. Each number corresponds to a colour.

According to this scale the pH value of neutral substances such as water is 7. The pH value of acidic solutions is less than 7 whereas the pH value of basic solutions is greater than 7. Acidity decreases from 0 to 6 while the basicity increases from 8 to 14.

pH papers are available in the form of books or rolls in the laboratory. These have been prepared by mixing several indicators. The pH value can be found by dipping a pH paper in the relevant solution and comparing the colour of the paper with the colour code. Accordingly, the acidity, basicity or the neutrality of the solution can be identified. Further, it gives an idea about the strength of the acid or the base.

Acid–base indicators are substances that change color as a function of pH.Each acid–base indicator has a specific color change pH intervals. Within the color change interval, indicators are often a mixture of colors between the low and high pH colors. Generally, the color change starts and ends approximately one pH unit. Given below the color change pH interval, some indicators are better for measuring the pH of acidic solutions, whereas others are more suitable for measuring the pH of basic solutions.

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