2.5 Acids, Bases and Salts
chemistry
pH Indicators
The pH of acids and bases cannot be determined by sight.
- Instead, pH is measured by other chemicals called indicators, or by a pH meter that measures the electrical conductivity of the solution.
pH indicators change colour based on the solution they are placed in.
- Litmus is the most common indicator, used on litmus paper.
- Two colours of litmus paper: Blue = basic and Red = acidic.
- Blue = pH above 7, Red = pH below 7
- Universal indicator contains many indicators that turn different colours at different pH values (can be in liquid form, or on paper strips like litmus)
- A pH meter uses electrical probes to measure how solutions conduct electricity
- Indicators change colour at different pH values, so different indicators are used to identify different pH values
- Bromothymol blue for pH 6 - 7.6, phenolphthalein for pH 8.2 - 10
- Many natural sources, such as beets and cabbage, are also indicators