PH indicator
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A pH indicator is a chemical compound that is added in small amounts to a solution so that the pH (acidity or alkalinity) of the solution can be determined easily. Hence a pH indicator is a chemical detector for protons (H+). Normally, the indicator causes the color of the solution to change depending on the pH.
pH indicators themselves are frequently weak acids or bases. When introduced into a solution, they may bind H+ (Hydrogen ion) or OH- (hydroxide) ions. The different electronic configuration of the bound indicator causes the indicator's color to change.
Because of the subjective determination of color, pH indicators are susceptible to imprecise readings. For applications requiring precise measurement of pH, a pH meter is frequently used.
pH indicators are frequently employed in titrations in analytic chemistry and biology experiments to determine the extent of a chemical reaction.
Tabulated below are several common laboratory pH indicators. Indicators usually exhibit intermediate colors at pH values inside the listed transition range. For example, phenol red exhibits an orange color between pH 6.6 and pH 8.0. The transition range may shift slightly depending on the concentration of the indicator in solution and on the temperature at which it is used.
| Indicator | Color at low pH | Transition pH range
(approximate) | Color at high pH |
|---|---|---|---|
| Methyl violet | yellow | 0.0-1.6 | blue-purple |
| Thymol blue (first transition) | red | 1.2-2.8 | yellow |
| Methyl yellow | red | 2.9-4.0 | yellow |
| Bromophenol blue | yellow | 3.0-4.6 | violet |
| Congo red | blue | 3.0-5.2 | red |
| Methyl orange | red | 3.1-4.4 | yellow |
| Litmus | red | 4.5-8.3 | blue |
| Bromocresol purple | yellow | 5.2-6.8 | violet |
| Bromothymol blue | yellow | 6.0-7.6 | blue |
| Phenol red | yellow | 6.6-8.0 | red |
| Thymol blue (second transition) | yellow | 8.0-9.6 | blue |
| Phenolphthalein | colorless | 8.2-10.0 | pink |
| Thymolphthalein | colorless | 9.4-10.6 | blue |
| Alizarin Yellow R | yellow | 10.1-12.0 | red |
| Indigo carmine | blue | 11.4-13.0 | yellow |
- Universal Indicator and Hydrion Papers are blends of different indicators that exhibits several smooth color changes over a wide range of pH values.
- Anthocyanins are a class of compounds that occur in many different plants; they appear red in acidic solutions and blue in bases. Extracting anthocyanins from red cabbage leaves to form a crude acid-base indicator is a popular introductory chemistry demonstration.
Natural pH indicators
- Beets
- Blueberries
- Carrots
- Cherries
- Curry Powder
- Delphinium Petals
- Geranium Petals
- Grapes
- Horse Chestnut Leaves
- Morning Glories
- Onion
- Pansy Petals
- Petunia Petals
- Poison Primrose
- Poppy Petals
- Purple Peonies
- Red cabbage [1]
- Red Radish
- Rhubarb
- Rose Petals
- Strawberries
- Tea
- Thyme
- Turmeric
- Tulip Petals
- Violet Petals
