Chemistry 38 - pH Indicator
A pH indicator is a halochromic chemical compound that is added in small amounts to a solution so that the pH (acidity or basicity) of the solution can be determined visually. 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. At 25° Celsius, considered the standard temperature, the pH value of a neutral solution is 7.0.
Solutions with a pH value below 7.0 are considered acidic, whereas solutions with pH value above 7.0 are basic. As most naturally occurring organic compounds are weak protolytes, carboxylic acids and amines, pH indicators find many applications in biology and analytical chemistry. Moreover, pH indicators form one of the three main types of indicator compounds used in chemical analysis. For the quantitative analysis of metal cations, the use ofcomplexometric indicators is preferred, whereas the third compound class, the redox indicators, are used in titrations involving a redox reaction as the basis of the analysis.

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 the solution and on the temperature at which it is used.

IndicatorLow pH colorTransition pH rangeHigh pH color
Gentian violet (Methyl violet 10B)yellow0.0–2.0blue-violet
Leucomalachite green (first transition)yellow0.0–2.0green
Leucomalachite green (second transition)green11.6–14colorless
Thymol blue (first transition)red1.2–2.8yellow
Thymol blue (second transition)yellow8.0–9.6blue
Methyl yellowred2.9–4.0yellow
Bromophenol blueyellow3.0–4.6purple
Congo redblue-violet3.0–5.0red
Methyl orangered3.1–4.4orange
Bromocresol greenyellow3.8–5.4blue
Methyl redred4.4–6.2yellow
Methyl redred4.5–5.2green
Azolitminred4.5–8.3blue
Bromocresol purpleyellow5.2–6.8purple
Bromothymol blueyellow6.0–7.6blue
Phenol redyellow6.8–8.4red
Neutral redred6.8–8.0yellow
Naphtholphthaleincolorless to reddish7.3–8.7greenish to blue
Cresol Redyellow7.2–8.8reddish-purple
Phenolphthaleincolorless8.3–10.0fuchsia
Thymolphthaleincolorless9.3–10.5blue
Alizarine Yellow Ryellow10.2–12.0red
Litmusred4.5-8.3blue



abstracted from: http://en.wikipedia.org/wiki/PH_indicator