pH indicator definition and examples are listed here. A pH indicator or acid-base indicator is a compound that changes colour in the solution over a narrow range of pH values . Only a small amount of indicator compound is required to produce a visible color change. When used as a dilute solution, a pH indicator has no significant impact on the acidity or alkalinity of a chemical solution.
The principle behind the function of an indicator is that it reacts with water to form the hydrogen cation H + or the hydronium ion H 3 O + .
The reaction changes the color of the indicator molecule. Some indicators change from one color to another, while others change between colored and colorless states. The pH indicators are generally weak acids or weak bases . Many of these molecules occur naturally. For example, the anthocyanins found in flowers, fruits and vegetables are indicators of pH. Plants containing these molecules include red cabbage leaves, rose petal flowers, blueberries, rhubarb stalks, hydrangea flowers, and poppy flowers. Sunflower is a natural pH indicator derived from a mixture of lichens.
For a weak acid of formula HIn, the chemical equation of equilibrium would be:
Hin (aq) + H 2 O (l) ⇆ H 3 O + (aq) + In – (aq)
At low pH, the concentration of hydronium ion is high and the equilibrium position is on the left. The solution has the color of the HIn indicator. At a high pH, the hydronium concentration is low, the equilibrium is on the right and the solution has the color of the conjugate base In – .
In addition to pH indicators, there are two other types of indicators used in chemistry. Redox indicators are used in titrations involving oxidation and reduction reactions. Complexometric indicators are used to quantify metal cations.
Examples of pH indicators
- Methyl red is a pH indicator used to identify pH values between 4.4 and 6.2. At low pH (4.4 and lower), the indicator solution is red. At high pH (6.2 and above), the color is yellow. Between pH 4.4 and 6.2, the indicator solution is orange.
- Bromocresol green is a pH indicator used to identify pH values between 3.8 and 5.4. Below pH 3.8, the indicator solution is yellow. Above pH 5.4, the solution is blue. Between the pH values of 3.8 and 5.4, the indicator solution is green.
Because the indicators change color over different pH ranges, they can sometimes be combined to provide color changes over a wider pH range. For example, ” universal indicator ” contains thymol blue, methyl red, bromothymol blue, thymol blue and phenolphthalein. It covers a pH ranging from less than 3 (red) to more than 11 (purple). Intermediate colors include orange / yellow (pH 3 to 6), green (pH 7 or neutral) and blue (pH 8 to 11).
Uses of pH Indicators
PH indicators are used to give an approximate value of the pH of a chemical solution. For precise measurements, a pH meter is used. Alternatively, absorbance spectroscopy can be used with a pH indicator to calculate the pH using Beer’s law. Spectroscopic pH measurements using a single acid-base indicator are accurate to a value of one pKa. The combination of two or more indicators increases the accuracy of the measurement.
The indicators are used in a titration to show the completion of an acid-base reaction.
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