Acid Base Titration
Indicators and PH meters
. Acid –Base Indicators:
Are weak acids and weak bases that change color in acidic and basic media.
Examples.
Indicator
Color in acidic
medium
Color in basic Used in titration of
medium
Methyl red
Pink
Yellow
Bromothymol blue
Yellow
Methyl Orange
Red
Yellow
Strong acid with weak Base
Bromophenol Blue
Yellow
Pink
Strong acid with weak base
Phenolphthalein
Colorless
Pink
Weak acid with strong base
Phenol red
Yellow
Pink
Weak acid with strong base
Blue
Strong acid with strong base
Strong acid with strong
base
. Measuring PH of a solution.
PH can be measured by two methods in the laboratory.
1. Using the universal indicator paper:
It is a mixture of indicators that cover the PH from 0 to 14.
2. Using PH-meter:
it is an electronic device that measures the PH of a solution by
measuring the in the solution.
. Each indicator has a PH range called Transition Interval and is
defined as the PH range over which the indicator changes color.
. Examples of substances and their PH values:
Substance
PH
Substance
PH
Battery acid
0
Anti acid
8
Stomach acid
2
Baking soda
9
Apple juice
3
Hand soap
Black coffee
5
House hold ammonia
Pure water
7
Drain cleaner
9-10
11-12
14
. Titration:
is the controlled addition and measurement of the amount of a solution of
known concentration required to react completely with a measured amount
of a solution of unknown concentration.
Example . We can use titration to find the concentration of a sample of acid
of unknown concentration using a suitable indicator, and vise versa.
. Apparatus used in titration.
.
. The steps of titration.
1. Fill the burette with a solution of NaOH of known concentration (example
0.01 M ) called standard or known solution.
2. Pour a known volume (10 ml) of an acid solution in a conical flask.
3. Add one or two drops of indicator to the acid solution.
4. Add the alkaline solution( NaOH solution) from the burette gradually to the
acidic solution in the conical flask and shaken lightly to mix.
5. Keep adding the NaOH solution until the color of indicator changes.
6. Take the reading of the burette example 25 ml ( the volume of NaOH that
neutralized the acid ) (.then use the following equation.
Ma X Va = Mb X Vb
Ma X 10 = 0.01X 25
Ma = 0.01X25 / 10 = 0.025 M is the concentration of the acid.
. Example: Calculate the concentration of KOH solution if 15 ml
of it needed 17.5 ml of 0.02 M HCl. Suggest an indicator for this
titration.