What is an acid-base indicator?
An acid-base indicator is a weak acid or a weak base. The undissociated form of theindicator is a different color than the iogenic form of the indicator. An Indicator doesnot change color from pure acid to pure alkaline at specific hydrogen ionconcentration, but rather, color change occurs over a range of hydrogen ionconcentrations. This range is termed the
color change interval
. It is expressed as apH range.
Some Common Acid Base Indicators
Several acid-base indicators are listed below, some more than once if they can beused over multiple pH ranges. Quantity of indicator in aqueous (aq.) or alcohol (alc.)solution is specified. Tried-and-true indicators include: thymol blue, tropeolin OO,methyl yellow, methyl orange, bromphenol blue, bromcresol green, methyl red,bromthymol blue, phenol red, neutral red, phenolphthalein, thymolphthalein, alizarinyellow, tropeolin O, nitramine, and trinitrobenzoic acid. Data in this table are forsodium salts of thymol blue, bromphenol blue, tetrabromphenol blue, bromcresolgreen, methyl red, bromthymol blue, phenol red, and cresol red.
is a measure of the acidity or basicity of a solution. It approximates butis not equal to
, the negative logarithm (base 10) of the molar concentration of dissolved hydrogen ions (H
); a low pH indicates a high concentration of hydrogen ions,while a high pH indicates a low concentration. Crudely, this negative of the logarithmmatches the number of places behind the decimal point, so for example 0.1 molar hydrochloric acid should be near pH 1 and 0.0001 molar HCl should be near pH 4. Purewater is neutral, and can be considered either a very weak acid or a very weak base(center on the pH scale), giving it a pH of 7 (at 25 °C (77 °F)), or 0.0000001
. For an aqueous solution to have a higher pH, a base must be dissolved in it, which bindsaway many of these rare hydrogen ions. Hydrogen ions in water can be...