ACID-BASE INDICATORS
An acid-base indicator is a weak acid or a weak base.
Examples of indictors used in acid base reactions
-Litmus
-Phenolphthalein
-Methyl orange
thymol blue, tropeolin OO, methyl yellow, methyl orange, bromphenol
blue, bromcresol green, methyl red, bromthymol blue, phenol red,
neutral red, phenolphthalein, thymolphthalein, alizarin yellow,
tropeolin O, nitramine, and trinitrobenzoic acid.
The pH range of indictors
Indictors dose not change colour sharply at one
particular pH, they change over a narrow range of pH
indctors
pK nd
pH
litmus
6.5
5-8
methylorange
3.7
3.1-4.4
phenophthaline
9.3
8.3-10.0
i
Litmus
• Indicators as weak acids Litmus
• Litmus is a weak acid. It has a seriously
complicated molecule which we will
simplify to HLit. The "H" is the proton
which can be given away to something
else. The "Lit" is the rest of the weak acid
molecule.
The un-ionised litmus is red
The ionised litmus is blue
What happened when
• Adding hydrogen ions
• Adding hydroxide ion
If the concentrations of HLit and Lit
- are equal:
• At some point during the movement of the position of equilibrium,
the concentrations of the two colours will become equal. The colour
you see will be a mixture of the two.
•
red
blue
brown
Phenolphthalein
•
is a chemical compound with the formula C20H14O4 (often written as "HIn" or
"phph"). Often used in titrations, it turns colorless in acidic solutions and pink in basic
solutions
Collarless acidic
Pkind value
indictor
Pkind
litmus
6.5
methylorange
3.7
phenolphthaline
9.3
• The importance of pKind
• Think of what happened through the color change.at this point the
concentration of the acid and its ion are equal
• pH=pkind
• This means that the end point for the indictor depends entiry on
what pKind value is
Methyl orange
What is methyl orange? How is it made?
Methyl orange is an intensely colored compound used in dyeing and printing
textiles. It is also known as C.I. Acid Orange 52, C.I. 13025, helianthine B,
Orange III, Gold orange, and Tropaeolin D. Chemists use methyl orange as an
indicator in the titration of weak bases with strong acids. It changes from red
(at pH 3.1) to orange-yellow (at pH 4.4):
pH-related color changes result from changes in the way electrons are
confined in a molecule when hydrogen ions are attached or detached. Methyl
orange in acidic solution.
Methyl orange in basic solution.
Methyl orange in acidic solution.
Titration curves for strong base with strong acid
indctors
pK nd
pH
methylorange
3.7
3.1-4.4
phenophthaline
9.3
8.3-10.0
i
Both of ph.ph and M.O. are useful
Titration curves for strong acid v weak base
• This time we are going to use hydrochloric
acid as the strong acid and ammonia
solution as the weak base.
indctors
pK nd
pH
methylorange
3.7
3.1-4.4
phenophthaline
9.3
8.3-10.0
i
MO is useful and ph.ph is useless
Titration of weak base with strong acid
indictor
Methyl orange
pKind pH
3.7 3.1-4.4
Phenolphaline
9.3
8.3-10.0
M.O is useful phph is useless
Choosing indictor for titration
Titration of weak base with
strong acid
indictor
Methyl orange
pKind pH
3.7 3.1-4.4
Phenolphaline
9.3
8.3-10.0
Both phph and M.O are useful
Titration curves for weak base v strong acid
indctors
pK nd
pH
methylorange
3.7
3.1-4.4
phenophthaline
9.3
8.3-10.0
i
Ph.ph is useful M.O is useless
Titration curves for weak base with week
acid
indctors
pK nd
pH
methylorange
3.7
3.1-4.4
i
phenophthaline 9.3
8.3-10.0
Both phph and MO are useless
Titration of sodium carbonate with
HCl
Phph useful for detect first
endpoint
MO is useful for detect the
second end point
A summary of the important
curves
The way you normally carry out
a titration involves adding the
acid to the alkali. Here are
reduced versions of the graphs
described above so that you
can see them all together