A pH indicator is a substance that changes its color at a certain pH. pH stands for power of Hydrogen. A low pH is acidic, and a high one is alkaline/basic. The pH is determined by the concentration of Hydrogen Ions in the solution. Hydrogen Ions are the protons of Hydrogen atoms that have been stripped off their electrons. The point at which an indicator changes its color is called its end point.

pH scale

The pH scale is used to measure the acidity or alkalinity of a solution. The scale goes between pH 0 to 14. pH 0-6 are acids, while pH 8-14 are either bases of alkalis (see below). A pH of 7 is considered neutral.

• Acids are solutions that produce Hydrogen Ions (H+), protons that have been stripped off their electrons. They are corrosive («eat away») and taste sour (acid comes from the Latin «acidus» meaning sour). Strong acids have more Hydrogen Ions and have a pH between 1-3. Weak acids have less Hydrogen Ions and have a pH between 4-6.
• Bases and Alkalis are both thought to mean the same thing, but there is a difference between the two. Both are substances that produce Hydroxide Ions (OH-), but bases are not soluble in water, unlike alkalis. Strong acids and alkalis can burn your skin. Strong bases and alkalis have more Hydroxide Ions and have a pH between 10-14, while weak bases and alkalis have a pH of 8-9 and less Hydroxide Ions.

The pH Scale
Color in Universal Indicator	pH	Examples
	0-3	Hydrochloric Acid, Lemon Juice
	4	Vinegar
	5	Black Coffee
	6	Milk, Saliva
	7	Water
	8	Sea Water
	9	Baking Soda
	10	Ammonia
	11-14	Oven Cleaner, Laundry Liquid), Lye (Sodium Hydroxide)

List of Indicators

Name	Acid Color (pH)	Neutral/Original Color	Alkaline Color (pH)	Picture (credit goes to the original creators)
Universal Indicator	See table on the pH scale above.
Litmus	Red	Red/Blue	Blue
Methyl Red, Methyl Orange, or Methyl Yellow	Red (4.4)	Red	Yellow (6.2)
Bromothymol Blue	Magenta (concentrated HCl) Yellow	Green	Blue (7.6)
Phenolphthalein	Vermilion (concentrated H2SO4) Colorless	Colorless	Pink (8.2-12) Colorless (12)	Phenolphthalein at pH 9, exhibiting a pink color.
Thymolphthalein	Red (>0) Colorless	Colorless	Blue
Congo Red	Indigo (3)	Red	Red (5.2)	Congo Red at three different pH values.
Thymol Blue	Red	Yellow	Blue
Anthocyanins (the pigment in a red cabbage indicator)	Red	Purple	Blue, Green, Yellow
Indigo Carmine	Blue	Blue	Yellow (12<)	In the foreground there is a gel composed of PVA glue and borax (4%) and coloured with blue and yellow food colourings in order to obtain green. In the background there is a yellow solution of the alkaline Sodium Hydroxide, indigo carmine and glucose.

Other less known pH indicators

Indicator	Low pH color	Transition low end	Transition high end	High pH color
Gentian violet (Methyl Violet 10B)	yellow	0.0	2.0	blue-violet
Malachite green (first transition)	yellow	0.0	2.0	green
Malachite green (second transition)	green	11.6	14.0	colorless
Bromophenol Blue	yellow	3.0	4.6	blue
Bromocresol Green	yellow	3.8	5.4	blue
Methyl Purple	purple	4.8	5.4	green
Bromocresol Purple	yellow	5.2	6.8	purple
Phenol Red	yellow	6.4	8.0	red
Neutral Red	red	6.8	8.0	yellow
Naphtholphthalein	pale red	7.3	8.7	greenish-blue
Cresol Red	yellow	7.2	8.8	reddish-purple
Cresolphthalein	colorless	8.2	9.8	purple
Alizarine Yellow R	yellow	10.2	12.0	red
Hydrangea flowers	blue			pink to purple

References and Notes

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