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PH indicator

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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[edit]

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[1]

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[edit]

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.
Skala boja 2.JPG
Litmus Red Red/Blue Blue
1024px-Litmus paper.JPG
Methyl Red, Methyl Orange, or Methyl Yellow Red (4.4) Red Yellow (6.2)
Color transition of Methyl red solution under different acid-base conditions.jpg
Bromothymol Blue Magenta (concentrated HCl)

Yellow

Green Blue (7.6)
800px-Bleu bromothymol teintes.jpg
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
Tymolftalein.jpg
Congo Red Indigo (3) Red Red (5.2)
Congo Red at three different pH values.
Thymol Blue Red Yellow Blue
Color change of thymol blue at different pH.jpg
Anthocyanins

(the pigment in a red cabbage indicator)

Red Purple Blue, Green, Yellow
Indicateur chou rouge.jpg
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[edit]

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[edit]

  1. 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.


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