Acid-Base Indicators
Green & Damji Chapter 8, Sections 18.4
Chang Chapter 16
pH
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Indicators
http://depts.uwc.edu/chemistry/helpful_files/indicators.jpg
4.3
Acid-Base Indicators
16.5
An Indicator…..
• is a different color in acidic and alkaline solutions
– usu an organic dye
• is a weak acid / base
HIn (aq)
litmus red
in acid
H+ (aq) + In- (aq)
blue
in base
• can be used to detect the end point of a titration
16.5
An Indicator’s color …..
• depends on pH
• depends on Ka
HIn (aq)
+] [In-]
[H
Ka =
[HIn]
H+ (aq) + In- (aq)
[HIn]
=
[In ]
[H+]
Ka
When [H+] = Ka (pH = pKa), the two colored forms ([HIn]
& [In-] ) will have the same concentration… and the indicator
will be in the middle of its color change.
So – if you need an indicator for a titration that will reach its
end point at pH = 7, find an indicator with a pKa around ____
If you need an indicator for a titration that will reach its
end point at pH = 8.7, find an indicator with a pKa around ____
16.5
This table is available in your data booklet:
An Indicator’s color …..
When
[HIn]
[In-]
[HIn]
[In-]
HIn (aq)
H+ (aq) + In- (aq)
 10
Color of acid (HIn) predominates
 10
Color of conjugate base (In-)
predominates
Many indicators will change color over a region of 2 pH units….
centered on the pKa
Property
Phenolpthalein
Methyl Orange
pKa
9.6
3.7
pH Range
8.3 – 10.00
3.1 – 4.4
Color in acid
Colorless
Red
Color in base (alkali)
Titrations involving
strong base with
weak acid
Titrations involving
strong acid with
weak base
16.5
Selecting an effective indicator
for a titration:
• Color change (end point) must occur rapidly
at the equivalence point (stoichiometric
equivalence)
• Strong acid / Strong Base – large pH change centered
around pH of ___... Many indicators will work
• Strong acid / weak base – smaller pH change centered
around pH of ___; methyl orange is a good choice
• Strong base / weak acid – smaller pH change centered
around pH of ___; phenolpthalein is a good choice
The titration curve of a strong acid with a strong base.
16.5
Which indicator(s) would you use for a titration of HNO2
with KOH ?
16.5
Which indicator(s) would you use for a titration of HNO2
with KOH ?
Weak acid titrated with strong base.
At equivalence point, will have conjugate base of weak acid.
At equivalence point, pH > 7
Use cresol red or phenolphthalein
16.5
TEXTWORK: Section 18.5
Read Section 18.5 pp 228-229
Do Ex 18.5 pp 229-230 # 1,2,3,4