Acid and Base Indicators

Acid and Base Indicators

Acid - Base Indicators:

The most common method to get an idea about the pH of solution is to use an acid base indicator. An indicator is a large organic molecule that works somewhat like a " color dye". Whereas most dyes do not change color with the amount of acid or base present, there are many molecules, known as acid - base indicators , which do respond to a change in the hydrogen ion concentration. Most of the indicators are themselves weak acids.

The most common indicator is found on "litmus" paper. It is red below pH 4.5 and blue above pH 8.2.

Color	Blue Litmus	Red Litmus
Acid	turns red	stays same
Base	stays same	turns blue

Other commercial pH papers are able to give colors for every main pH unit. Universal Indicator, which is a solution of a mixture of indicators is able to also provide a full range of colors for the pH scale.

A variety of indicators change color at various pH levels. A properly selected acid-base indicator can be used to visually "indicate" the approximate pH of a sample. An indicator is usually some weak organic acid or base dye that changes colors at definite pH values. The weak acid form (HIn) will have one color and the weak acid negative ion (In^-) will have a different color. The weak acid equilibrium is:

HIn --> H+ + In^-
For phenolphthalein: pH 8.2 = colorless; pH 10 = red

For bromophenol blue: pH 3 = yellow; pH 4.6 = blue

See the graphic for more indicators, colors, and pH ranges.

from: http://www.elmhurst.edu/~chm/vchembook/186indicator.html