pH indicators are organic compounds that change color based on the pH of a solution. They are used to check pH and indicate the endpoint of reactions. Common indicators include litmus, which is red below pH 4.5 and blue above pH 8.2, and phenolphthalein, which is colorless below pH 8.2 and pink above pH 10. In a demonstration, phenolphthalein was added to solutions containing sodium hydroxide, turning them pink, but when one solution was then mixed with acetic acid, the indicator turned colorless due to the drop in pH caused by neutralization.

1. pH INDICATOR

2. “The compounds (mostly organic) which
change their color when pH of the solution
changes”
Indicators are used to;
1) Check the pH of the solution.
2) Indicate the end-point i.e. completion of
the reaction.

3. At 25 °C, considered the standard temperature, the pH
value of a neutral solution is 7.0. Solutions with a pH value
below 7.0 are considered acidic, whereas solutions with pH
value above 7.0 are basic (alkaline). As most naturally
occurring organic compounds are weak carboxylic acids and
amines, pH indicators find many applications in biology and
analytical chemistry.

4. Types of indicators
The choice of the indicator depends upon the nature of
the reaction. There are three basic types of indicators.
1) Acid- base indicator
2) Redox indicators
3) Precipitation indicators
For the quantitative analysis of metal cations, the use of
complexometric indicators is preferred.

5. Acid – Base indicators:
These are the organic compounds which are used in acid
base titrations. Acid - base indicators, respond to a change
in the hydrogen ion concentration. Most of the indicators
are themselves weak acids other are weak bases which
responded to change hydroxyl ion concentration.
Color Blue Litmus Red Litmus
Acid turns red stays same
Base stays same turns blue
The most common indicator is found on "litmus" paper. It is red below pH 4.5 and blue above pH 8.2.

6. An indicator is usually some weak organic acid or base dye that
changes colors at definite pH values. The unionized form of 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

7. Magic Pitcher Demonstration:
Phenolphthalein is an indicator of acids (colorless) and bases (pink).
Sodium hydroxide is a base, and it was in the pitcher at the
beginning, so when added to the phenolphthalein in beakers 2 and
4, it turned pink (top half of the graphic).
Explanation:
Equilibrium: HIn --> H+ + In-
colorless pink
The equilibrium shifts right, HIn decreases, and In- increases. As the
pH increase between 8.2 to 10.0 the color becomes red because of
the equilibrium shifts to form mostly In- ions.

9. The third beaker has only the NaOH but no phenolphthalein, so it
remained colorless. The first beaker contain acetic acid and is skipped
over at first.
After pouring beakers 2, 3, 4 back into the pitcher it give a pink solution.
Bottom half of the graphic: When the pitcher is then poured back into
beakers 2, 3, 4 it is a pink solution.
In the first beaker, a strange thing happens in that the pink solution
coming out of the pitcher now changes to colorless. This happens
because the first beaker contains some vinegar or acetic acid which
neutralizes the NaOH, and changes the solution from basic to acidic.
Under acidic conditions, the phenolphthalein indicator is colorless.
Neutralization: HC2H3O2 + NaOH --> Na(C2H3O2) + HOH

10. The simplified reaction is:
H+ + OH- --> HOH
As OH- ions are added, they are consumed by the excess of acid
already in the beaker as expressed in the above equation. The
hydroxide ions keep decreasing and the hydrogen ions increase, pH
decreases.
See lower equation: The indicator equilibrium shifts left, In- ions
decrease. Below pH 8.2 the indicator is colorless. As H+ ions are
further increased and pH decreases to pH 4-5, the indicator
equilibrium is effected and changes to the colorless HIn form.
Equilibrium: HIn --> H+ + In-
colorless red

12. pH range of indicator
INDICATOR COLOR CHANGE pH RANGE
PHENOLPHTHALEIN COLORLESS to PINK 8.3 – 10.0
METHYL ORANGE RED to ORANGE 3.1- 4.4
METHYL RED RED to YELLOW 4.4 – 6.0
BROMOTHYMOL BLUE YELLOW to BLUE 6.0 – 7.6
LITMUS RED to BLUE 5.0 – 8.0

13. What is End Point?
In an acid- base titration, the base solution is gradually
added from a burette into an acidic solution in a titration
flask. When the amount of base added neutralize to the
amount of base added in the flask “equivalence point or
End point” will reached. The end point of the titration is
shown by color changes of an indicator , previously added
to the acidic solution in the titration flask.

14. pH – CURVE (coming soon…..)