pH indicator

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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 colour of the solution to change depending on the pH. Solutions with a pH value above 7.0 are basic, and solutions with a pH value below 7.0 are acidic. Solutions with a pH value of 7.0 are neutral.

Theory

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, which allows the pH to be determined by the different colors.

Application

pH indicators are frequently employed in titrations in analytic chemistry and biology experiments to determine the extent of a chemical reaction. 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.

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
Leucomalachite green (second transition) green 11.6–14 colorless
Thymol blue (first transition) red 1.2–2.8 yellow
Thymol blue (second transition) yellow 8.0–9.6 blue
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 orange
Bromocresol green yellow 3.8–5.4 blue-green
Methyl red red 4.4–6.2 yellow
Methyl red / Bromocresol green red 4.5–5.2 green
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 purple
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
Phenolphthalein colorless 8.3–10.0 fuchsia
Thymolphthalein colorless 9.3–10.5 blue
Alizarine Yellow R yellow 10.2–12.0 red

Commercial preparations

File:PH indicator paper.jpg
pH measurement with indicator paper.

Universal indicator and Hydrion papers are blends of different indicators that exhibits several smooth color changes over a wide range of pH values.

Naturally occurring pH indicators

File:Light mauve hydrangea.jpg
Hydrangeas can change colour with soil acidity

Many plants or plant parts contain chemicals from the naturally-colored anthocyanin family of compounds. They are red in acidic solutions and blue in basic. Extracting anthocyanins from red cabbage leaves or the skin of a lemon to form a crude acid-base indicator is a popular introductory chemistry demonstration.

Anthocyanins can be extracted from a multitude of colored plants or plant parts, including from leaves (red cabbage); flowers (geranium, poppy, or rose petals); berries (blueberries, blackcurrant); and stems (rhubarb). An exhaustive list would be beyond the scope of this article.

References


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