PH indicator



         


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.

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Common pH indicators


Name Acid pH Range Alkali
Methyl violet Yellow 0.0 - 1.6 Blue
Malachite green Yellow 0.2 - 1.8 Blue/Green
Thymol blue (acid) Red 1.2 - 2.8 Yellow
Methyl yellow (in ethanol) Red 2.9 - 4.0 Yellow
Methyl orange-xylene cyanole solution Purple 3.2 - 4.2 Green
Methyl orange Red 3.2 - 4.4 Yellow
Bromophenol blue Yellow 2.8 - 4.6 Blue
Congo red Violet 3.0 - 5.0 Red
Bromocresol green Yellow 3.8 - 5.4 Blue
Methyl red Red 4.2 - 6.3 Yellow
Azolitmin (litmus) Red 5.0 - 8.0 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
Thymol blue (base) Yellow 8.0 - 9.6 Blue
Phenolphthalein (in ethanol) Colourless 8.2 - 10.0 Red
Thymolphthalein Colourless 8.3 - 10.6 Blue
Alizarin yellow R Yellow 10.1 - 13.0 Orange/Red


All of the above are 0.1% H2O solutions besdes those with information in brackets.

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Natural pH indicators






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