1. are substances which change colour with pH
Indicators
Definition: A substance that indicates the presence, and
sometimes the concentration of a certain constituent
Acid-base indicators
Redox indicators (see lab.)
Metallochromic indicators (see lab.)
are substances which change colour with pH

2. Acids and bases- Indication of aqueous solution acidity
The most common way to determine the pH of an aqueous solution rapidly is to use an acid-base indicator. These chemical dyes respond to the presence of H2O+ generating a colour change. Litmus, often in the form of litmus impregnated paper, is blue above pH 8.5 and red below pH 4.5, neutral paper therefore, appears mauve.
Other indicators, Phenolphthalein for instance, have differing ranges and it may transformed from colourless to pink over a range of pH 8.2 to pH 10.0.
Ind H+  H3O+ (aq) + Ind
In an electronic digital world pH meters that work on potential differences set up across selectively permeable membranes, (selective to H+ ions) give immediate accurate readings of pH of, say, small quantities of biological fluids.
(Colourless)
(Coloured)

3. HInd(aq) H+(aq) + Ind-(aq)
Indicators
HInd(aq) H+(aq) + Ind-(aq)
IndH H3O+ + Ind
Indicator exponent
[H+][Ind-]
if [Ind-] = [HInd]
Kd =
[HInd]
Kd = [H+]
pH = -logKd = pi

4. Indicators 2. Colour-change interval, pH range: 0.1 ≤ ≤ 1.0
0.1 ≤ ≤ 1.0
(pi-1) (pi +1)
[Ind]
[HInd]
Intermediate colour

5. Indicators Indicator Acid colour pH range Alkaline colour Thymol blue
red
yellow
Methyl orange
Methyl red
Litmus
blue
Pheolphtalein
colourless
Alizarin yellow
lavender

6. Colours of Some Acid-Base Indicators at Various pH Levels