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.
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|
|0-3||Hydrochloric Acid, Lemon Juice|
|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.|
|Methyl Red, Methyl Orange, or Methyl Yellow||Red (4.4)||Red||Yellow (6.2)|
|Bromothymol Blue||Magenta (concentrated HCl)
|Phenolphthalein||Vermilion (concentrated H2SO4)
|Congo Red||Indigo (3)||Red||Red (5.2)|
(the pigment in a red cabbage indicator)
|Red||Purple||Blue, Green, Yellow|
|Indigo Carmine||Blue||Blue||Yellow (12<)|
Other less known pH indicators
|Indicator||Low pH color||Transition
|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|
|Alizarine Yellow R||yellow||10.2||12.0||red|
|Hydrangea flowers||blue||pink to purple|
References and Notes
- This will depend on whatever type of Universal Indicator you use: paper or liquid form. Liquid forms mostly have orange end points at around pH 5 and are green at pH 7, but most paper forms are yellow at pH 7 and are only dark green (as opposed to purple) at pH 13-14.