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PH indicator
From Wikipedia, the free encyclopedia
- The correct title of this article is pH indicator. The initial letter is shown capitalized due to technical restrictions.
A pH indicator is a halochromic 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 hydronium ions (H3O+) (or Hydrogen ions (H+) in the Arrhenius model). 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 to H+ (Hydrogen ion) or OH- (hydroxide) ions. The different electron configurations 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 6.8 and pH 8.4. 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 | Low pH color | Transition pH range | High pH color |
|---|---|---|---|
| Gentian violet (Methyl violet) | yellow | 0.0–2.0 | blue-violet |
| Leucomalachite green (first transition) | yellow | 0.0–2.0 | green |
| 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 | purple |
| Congo red | blue-violet | 3.0–5.0 | red |
| Methyl orange | red | 3.1–4.4 | yellow |
| Bromocresol green | yellow | 3.8–5.4 | blue-green |
| Methyl red | red | 4.4–6.2 | yellow |
| Azolitmin | red | 4.5–8.3 | blue |
| Bromocresol purple | yellow | 5.2–6.8 | purple |
| Bromothymol blue | yellow | 6.0–7.6 | blue |
| Phenol red | yellow | 6.8–8.4 | red |
| Neutral red | red | 6.8–8.0 | yellow |
| Naphtholphthalein | colorless to reddish | 7.3–8.7 | greenish to blue |
| Cresol Red | yellow | 7.2–8.8 | reddish-purple |
| Thymol blue (second transition) | yellow | 8.0–9.6 | blue |
| Phenolphthalein | colorless | 8.2–10.0 | reddish-purple |
| Thymolphthalein | colorless | 9.3–10.5 | blue |
| Alizarine Yellow R | yellow | 10.2–12.0 | red |
| Leucomalachite green (second transition) | green | 11.6–14 | colorless |
Indicators that are not on this table:
- 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.
- Beets
- Blackcurrant juice
- Blueberries
- Carrots
- Cherries
- Curry powder
- Delphinium petals
- Geranium petals
- Grapes
- Horse Chestnut leaves
- Hydrangea
- Morning Glories
- Onion
- Pansy petals
- Petunia petals
- Poppy petals
- Red cabbage
- Rhubarb
- Rose petals
- Strawberries
- Tea
- Thyme
- Turmeric
- Tulip petals
- Violet petals