PH indicator

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The correct title of this article is pH indicator. The initial letter is shown capitalized due to technical restrictions.

pH measurement with indicator paper.

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.