Chemistry 205
Acid - Base Titration
purpose
• To learn the concept and technique of
titration.
• To standardize a sodium hydroxide (NaOH)
solution against a primary standard acid.
• To determine the concentration of an
unknown acid by titration with the
standardized base solution.
Titration
In titration a solution of accurately known concentration,
called a standard solution, is added gradually to another
solution of unknown concentration (or vice versa) until the
chemical reaction between the two solutions is complete.
Equivalence point – the point at which the reaction is complete.
Indicator – substance that changes color at (or near) the
equivalence point
Slowly add base
to unknown acid
UNTIL
the indicator
changes color
Titration (Cont’d)
• End Point: the point in the titration at which the
indicator changes color (visually determined).
• Conditions for a good titration
o Rapid and complete reaction.
o Reaction of known stoichiometry (no side
products).
o End point easily detected (use proper
indicator).
Acid – Base Titration
Involves a neutralization reaction which is
the complete reaction between an acid and a
base.
Acid
+
base
HCl(aq) + NaOH(aq)
H+(aq) + OH (aq)
Salt
+ water
NaCl(aq) + H2O(l)
H2O(l) Net ionic equation
What volume of a 1.420 M NaOH solution is
Required to titrate 25.00 mL of a 4.50 M H2SO4
solution?
WRITE THE CHEMICAL EQUATION!
H2SO4 + 2NaOH
2H2O + Na2SO4
n NaOH = 2 n H2SO4
(M x V) NaOH = 2 (M x V) H2SO4
1.420 mol/L x VNaOH = 2 (4.50 mol/L x 25.00 x 10-3L)
V NaOH = 0.158 L = 158 mL
Standard Solution
• Is a solution of accurately known concentration.
• prepared by dissolving an exact amount of the solute,
followed by dilution, to form a definite volume of solution.
Standard solution (cont’d)
• Properties of a good primary standard
o High molar mass
o Stable
o Not hygroscopic (do not absorb water readily)
o Highly pure, cheap, and available
• Example: potassium hydrogen phthalate,
KHC8H4O4, molar mass = 204.23 g / mol.
O
abbreviated as KHP,
C
OH
Structure:
OK
C
O
Standard solution (cont’d)
• Sodium hydroxide is not a good primary standard solution.
Solid NaOH absorbs water from air (hygroscopic), and its
solution reacts with carbon dioxide.
• In this experiment a sodium hydroxide (NaOH) solution will be
standardized by titration with a primary standard acid (KHP)
as follows:
KHC8H4O4(aq) + NaOH(aq) → KNaC8H4O4(aq) + H2O(l)
or
KHP(aq) + NaOH(aq) → KNaP(aq) + H2O(l)
or
HP-(aq) + OH-(aq) → P2-(aq) + H2O(l)
net ionic equation
The Ion Product of Water
H2O (l)
H+ (aq) + OH- (aq)
[H+][OH-]
Kc =
[H2O]
[H2O] = constant
Kc[H2O] = Kw = [H+][OH-]
The ion-product constant (Kw) is the product of the molar
concentrations of H+ and OH- ions at a particular temperature.
At 250C
Kw = [H+][OH-] = 1.0 x 10-14
[H+] = [OH-]
Solution Is
neutral
[H+] > [OH-]
acidic
[H+] < [OH-]
basic
pH – A Measure of Acidity
pH = -log [H+]
Solution Is
neutral
[H+] = [OH-]
At 250C
[H+] = 1.0 x 10-7
pH = 7.00
acidic
[H+] > [OH-]
[H+] > 1.0 x 10-7
pH < 7.00
basic
[H+] < [OH-]
[H+] < 1.0 x 10-7
pH > 7.00
pH
pOH = -log [OH-]
[H+][OH-] = Kw = 1.0 x 10-14
-log [H+] – log [OH-] = 14.00
[H+]
pH + pOH = 14.00
The pH of rainwater collected in a certain region of the
northeastern United States on a particular day was 4.82.
What is the H+ ion concentration of the rainwater?
pH = -log [H+]
[H+] = 10-pH = 10-4.82 = 1.5 x 10-5 M
The OH- ion concentration of a blood sample is 2.5 x 10-7 M.
What is the pH of the blood?
pH + pOH = 14.00
pOH = -log [OH-] = -log (2.5 x 10-7) = 6.60
pH = 14.00 – pOH = 14.00 – 6.60 = 7.40
Strong Acid-Strong Base Titrations
Example: Addition of 0.100 M NaOH solution (from a buret ) to
an erlenmeyer containing 25.0 mL of 0.100 M HCl.
NaOH (aq) + HCl (aq)
H2O (l) + NaCl (aq)
Indicators
• Indicators are used to detect the end point.
• They are substances that have distinctly
different colors in acidic and basic media,
they are weak organic acids or bases that
change their color at the equivalence point:
H3O+ + In
HIn + H2O
(acidic)
(Basic)
+
−
Ka = [ H 3 O ][ In ]
[HIn ]
Ka= Acid dissociation
constant
Indicators (continued)
[ HIn]
• Rearranging the above equation [H3O+] = Ka
−
[
In
]
• Taking the -log of both sides
- log [H3O+] = - log Ka - log [HIn]
pH = pKa + log
If
If
[In ]
−
[In− ]
[HIn]
[In ]
≤
[In ]
≥ 10 The color of the base ( In ) predominates
−
[HIn]
−
[HIn]
1
The color of the acid (HIn) predominates
10
Therefore pH range of indicator is: pH = pKa ± 1
Indicators (continued)
• Example: If pKa of the indicator = 6.2, then the
indicator pH range = 5.2 – 7.2
Indicators (continued)
choose the indicator whose color range includes the
equivalence pH or that lies on the steep part of the titration
curve.
Experimental
Four steps are involved:
1- Preparation of a primary standard potassium
hydrogen phthalate (KHP) solution.
2- Standardization of 0.1 M sodium hydroxide solution
with KHP.
3- Titration of an unknown monoprotic acid with the
standardized sodium hydroxide solution.
4- Blank Titration
1- Preparation of KHP Primary Standard
• Weigh KHP bottle using the analytical balance.
• Transfer the KHP to a 250 mL volumetric flask
using a funnel.
• Add some water, dissolve ,dilute and shake,
make up to the mark and homogenize.
• Weigh the empty bottle
• Calculate molarity of KHP
n
mass
M= =
V Molar mass x volume
mass ( g )
M=
204.23 ( g / mol ) x 250.0 x10 −3 mL
2- Standardization of 0.1M NaOH
• Rinse the buret with NaOH.
• Fill it to the mark, make sure that there are no air
bubbles in the tip.
• Pipet 25.00 mL of KHP into an Erlenmeyer flask and
add two drops of phenolphthalein indicator.
• Add 20mL of distilled water.
• Titrate against KHP to the end point (light pink color).
• Repeat the titration 3 times.
• Calculate M of NaOH.
(MxV) of NaOH = (MxV) of KHP
3- Titration of an Unknown Acid
• Proceed as in part 2.
• Use the unknown instead of KHP
• Titrate against the standardized NaOH
solution using phenolphthalein as an
indicator.
• Repeat three times.
• Calculate the molarity of the unknown.
Note: The unknown is monoprotic acid.
4- Blank Titration
• Blank titrarion is done to correct for
possible acids other than the acid being
titrated. (Distilled water and phenolphthalein
are acidic).
• Proceed as in part 2, use distilled water
instead of the KHP.
• Subtact the volume of NaOH needed for
the blank titration from the average volume
of NaOH used in each of parts 1 and 2
before you start your calculations.
Practice exercises
1. What is the concentration of OH- ions in a HCl
solution whose hydrogen ion concentration is
1.3 M?
Answer: 7.7 x 10-15 M
2. A 0.2668 g sample of a monoprotic acid
neutralizes 16.4 mL of 0.08133 M KOH
solution. Calculate the molar mass of the acid.
Answer: 202 g/mol