Pharmaceutical Analytical Chemistry (PHCM223-SS16)
Lecture 5
ACID- BASE EQUILIBRIUM-V
“pH indicators”
Dr. Rasha Hanafi
PHCM223,SS16
Lecture 5, Dr. Rasha Hanafi
1
LEARNING OUTCOMES
By the end of this session the student should be able to:
1.
2.
3.
4.
Define indicators.
Predict range of color change of an indicator.
Select an Indicator for a certain titration.
Compare between the selection conditions of indicators in
strong versus weak acids/ bases titrations
5. Select indicators for titrations of polyprotic acids.
6. Compare between difficulty of end point detection in
different titrations.
7. Determine the exact concentrations a mixture of bases using
a double indicator method (Warder titration).
PHCM223,SS16
Lecture 5, Dr. Rasha Hanafi
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Indicators are organic
compounds with an
extensive conjugated
system that causes the
absorption of colored light
waves.
anthocyanins
PHCM223,SS16
Lecture 5, Dr. Rasha Hanafi
A conjugated system
consists of a series of
connected p-orbitals with
delocalized electrons. This
has the effect of increasing
stability and reducing the
energy gap between the
HOMO (highest obtained
molecular orbital) and the
LUMO (lowest unoccupied
molecular orbital).
This moves the absorption
from the UV to the visible
region giving it color.
3
REMINDER
•
•
•
•
•
The equivalence point in an acid-base titration is defined by the stoichiometry and not by
the pH (i.e. 50 ml 0.1M will neutralize 50 ml 0.1 M HCl, and will also neutralize 50 ml 0.1M
CH3COOH).
The equivalence point occurs when enough titrant has been added to react exactly with the
acid or base being titrated.
The pH value at the equivalence point is affected by the acid strength. i.e. the weaker the
acid, the greater the pH value at the equivalence point.
In approaching calculation of pH during titration curve should think first about the major
species in solution and decide whether a reaction occurs that runs to completion.
Run stoichiometry and equilibrium calculations a to get the pH.
pH upon
Weak acid vs. strong base
Strong acid vs.
strong base
0 mL
added
[H+] = √(Ka [weak acid])
[H+] = [acid]
Some mL
added
Henderson Hasselbach
[H+] = [remaining
acid]
End point
pH of salts of weak acid
7
xss titrant
PHCM223,SS16
[OH-]
Lecture 5, Dr. Rasha Hanafi
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WHAT ARE ACID –BASE INDICATORS?
•
Indicator is usually a weak organic acid or base that has distinctly different
colors in its non-ionized and ionized forms.
HIn(aq)
H+(aq) + In-(aq)
+
OH-
−
𝑯 [𝑰𝒏 ]
[𝑯𝑰𝒏]
+
[𝑯𝑰𝒏]
[𝑰𝒏]
•
𝒌𝒊𝒏 =
•
If the indicator is in a sufficiently acidic medium, the equilibrium, according to
Le Châtelier’s principle, shifts to the left and the predominant color of the
indicator is that of the nonionized form (HIn).
In a basic medium the equilibrium shifts to the right and the color of the
solution will be due mainly to that of the conjugate base (In-).
Not all indicators change color at the same pH, so the choice of indicator for a
particular titration depends on the nature of the acid and base used in the
titration (that is, whether they are strong or weak).
•
•
 𝑯
H+
= 𝒌𝒊𝒏. [𝑰𝒏−]  pH= pKin + log [𝑯𝑰𝒏−]
Amount HIn
Amount In-
Color of indicator
10
1
of its acidic form
1
10
of its basic form
PHCM223,SS16
Lecture 5, Dr. Rasha Hanafi
[In  ]
1

[HIn] 10
[In  ]
10

[HIn]
1
5
For a typical acid-base indicator with dissociation
constant Kin, the color changes over a range of pH
values given by the pkin 1.
[𝑰𝒏]
pH= pKin + log [𝑯𝑰𝒏−]
The end point of an
indicator does not occur
at a specific pH; rather,
there is a range of pH
within which the end
point will occur.
A good indicator should have a
pKin value that is close to the expected
pH at the equivalence point.
PHCM223,SS16
Lecture 5, Dr. Rasha Hanafi
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CHOICE OF INDICATOR FOR A CERTAIN TITRATION
The equivalence point also lies on the steep part of the titration curve, hence the end point of
the indicator should also lie on this steep part.
For a strong acid–strong base titration, the choice of the indicator is easy due to the very large
change in pH that occurs around the equivalence point.
In contrast, using the wrong indicator for a titration of a weak acid or a weak base can result
in relatively large errors.
Strong base (analyte) versus strong acid
Weak base (analyte) versus strong acid
Both phenolphthalein and methyl orange can be used to determine the end point fort
the strong base titration. While for the weak acid only Methyl Red can be used.
PHCM223,SS16
Lecture 5, Dr. Rasha Hanafi
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CHOICE OF INDICATOR FOR A CERTAIN TITRATION
The [In−]/[HIn] ratio
changes from 0.1 at a pH
1 unit below pKin to 10 at
a pH 1 unit above pKin.
Ex: pKin of PhPh is 9.5,
the color change takes
place in the range of 8.2
to 10.
pKin of MR is 5, the color
change takes place in the
range of 4.95 to 6.
Most indicators change
color over a pH range
of about 2 pH units.
PHCM223,SS16
Lecture 5, Dr. Rasha Hanafi
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DOUBLE INDICATOR TITRATIONS
Need of 2 indicators
Mixture of 2 monobasic acids
Polyprotic acids
Ex: HCl + CH3COOH
H3PO4
HCl + H3BO3
Condition: Difference in pKa of the acids >4
Ka(boric acid) = 5.5x10-10, Ka(acetic acid) = 1.8x10-5.
Under this condition, H+ of HCl suppresses the ionization of the other weak acid,
by common ion effect, so that the base neutralizes HCl first.
1. Methyl orange is used as indicator for titration of the strong acid (pH 7).
2. The weak acid is then determined using phenolphthalein as indicator (pH>7).
PHCM223,SS16
Lecture 5, Dr. Rasha Hanafi
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Test yourself
The pKin values of phenol red, bromophenol blue, and
phenolphthalein are 7.4, 4.1, and 9.5, respectively. Which
indicator is best suited for each acid–base titration?
• Titrating a solution of Ba(OH)2 with 0.10 M HCl
phenol red
• Titrating a solution of trimethylamine with 0.15 M HNO3
bromophenol blue
PHCM223,SS16
Lecture 5, Dr. Rasha Hanafi
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Diprotic acids
• For a diprotic acid, K2 is always smaller than K1. The acid can be titrated
stepwise with an alkali provided that the ratio of K1/K2 >104, so that well
separated breaks in pH occur as titration progresses from one step to the other.
• If K1/K2 is less than 104, the inflections in the pH titration curves are smeared
out and only one sharp break is observed corresponding to the neutralization
of both hydrogen ions.
0.1M solution of maleic acid titrated with 0.1M solution of strong base. pKa1=1.83,
pKa2=6.07.
PHCM223,SS16
Lecture 5, Dr. Rasha Hanafi
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TRIBASIC ACID: PHOSPHORIC ACID
At equal acid and conjugate base
concentrations, pH=pKa.
There are three such points for
phosphoric acid. These points are
important in the prediction of the
titration curves. They correspond
to points where half of an
equivalent of proton has been
consumed by addition of strong
base.
Thus, the point where pH=pKa1 is
halfway to the first equivalence
point. Where pH=pKa2 is halfway
between the first and second
equivalence points, etc.
The equivalence point is halfway between the
two points with maximum buffer capacity
The solution has maximum
buffer capacity at these points.
PHCM223,SS16
Lecture 5, Dr. Rasha Hanafi
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TRIBASIC ACID: PHOSPHORIC ACID
1. At the first equivalence point, K1/K2 = 1.2x105 
stepwise titration is possible. pH=
1/2(pK1+pK2)=4.77. Methyl orange or
bromocresol green are the most appropriate.
2. At the 2nd equivalence point, pH= 1/2(pK2+pK3)
= 9.7. The only suitable indicator is
thymolphthalein (pKin = 10.0) . With
Phenolphthalein (pKin = 9.2) the error would be
high.
For the second stage of dissociation, K2/K3 =
1.2x105 , so that separate titration of the 2nd
hydrogen is possible. However, the 2nd end point
is less sharp than the first one since the solution
is well buffered by the HPO42- present.
3. At the 3rd equivalence point, the 3rd
hydrogen, being very weakly dissociated (pK3 =
12.3), no indicator can serve to show this point,
that even after adding large excess of titrant, the
acid remains mainly in the form of HPO42-.
PHCM223,SS16
Lecture 5, Dr. Rasha Hanafi
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WARDER TITRATION
NaOH- both solid and dissolved - easily reacts with atmospheric CO2 , it is usually contaminated
with Na2CO3. It is not a problem to determine sum of hydroxide and carbonates concentration by
titration with a strong acid. What are the main species in a solution of NaOH and Na2CO3 to be
titrated with HCl?
NaOH + HCl -----> NaCl + H2O ....................... (1)
Sodium carbonate reacts with hydrochloric acid in a similar way but the reaction comes in 2
steps:
Na2CO3 + HCl ----------->NaHCO3 + NaCl............. (2)
NaHCO3 + HCl ----------->NaCl + H2O + CO2.......... (3)
The pH for equivalence of reactions (1) and (2) at a pH value higher than 7.
Reaction (3) takes place at a pH value lower than 7.
So, using the 2 acid-base indicators, the end points for the reactions can be found.
Phenolphthalein turns from pink to colorless as the pH value changes from 10 to 8.
Methyl orange turns from yellow to orange when the pH value drops from about 4 to 3
Soln. of 0.1M NaOH and 0.1M Na2CO3 titrated with 0.1M solution of strong acid. Carbonic acid dissociation constants: pKa1=6.37, pKa2=10.25.
PHCM223,SS16
Lecture 5, Dr. Rasha Hanafi
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WARDER TITRATION
• During titration of the mixture we first add phenolphthalein to the
solution and titrate it till pink color disappears, noting the first end point
volume (let's call it V1). Then we add methyl orange and titrate solution to
the intermediate color of the indicator. Let's call the second end point
volume V2.
• V1 is the volume of titrant that was necessary for completion of two
reactions:
NaOH + HCl → NaCl + H2O
Na2CO3 + HCl → NaCl + NaHCO3
• V2-V1 is the hydrogen carbonate protonation:
NaHCO3 + HCl → NaCl + H2O + CO2
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Lecture 5, Dr. Rasha Hanafi
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REFERENCES
1.
2.
3.
4.
5.
6.
7.
Chemistry, 10th ed., Raymond Chang, ISBN 978-0-07-017264-7,
McGraw Hill. Chapter 14 and 16
Lecture 5 - PHCM223, Prof. Rasha Elnashar, GUC, SS15.
Home-made acid/ base indicator
(https://www.youtube.com/watch?v=4phd0XRMKHU)
https://mylespower.co.uk/2012/04/06/homemade-ph-indicator/
http://www.foothill.edu/psme/larson/resources/1C-Lecture-andLab/Chapter-17-Part-3-Titration-Curves
https://saylordotorg.github.io/text_general-chemistry-principlespatterns-and-applications-v1.0/s20-05-acid-base-titrations.html
http://www.titrations.info/acid-base-titration-polyprotics-andmixtures
PHCM223,SS16
Lecture 5, Dr. Rasha Hanafi
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