Acid Base Indicators

1
Experiment 6
Acid Base Indicators
2
What are acids and bases?
There are many different definitions for
classifying a substance as an acid or a base.
Definitions are based on different theories
Arrhenius theory of acids and bases
Lowry-Bronsted theory of acids and bases
Lewiss theory of acids and bases
3
Arrhenius theory of acids and bases
Proposed in 1884 by Svante Arrhenius
An acid is a substance that contains hydrogen and
produces H in aqueous Solution. A base is a
solution that contains the OH group and produces
Hydroxide ions OH-, in aqueous solution
Neutralization is defined as the combination of
H ions with OH- ions to form H2O molecules.
H (aq) OH-(aq)
H2O(l)
Very Limited in Scope!!
4
Lowry-Bronsted theory
Proposed in 1923 by N. Bronsted and T.M.Lowry
An acid is defined as a proton donor, H, and a
base is Defined as a proton acceptor
An acid-base reaction is the transfer of a proton
from an acid to a base
The complete ionization of hydrogen chloride, HCl
in water is an acid-base reaction
Step 1 HCl(aq) H(aq) Cl-(aq)
(Arrehnius)
Step 2 H (aq) H2O(l) H3O
Overall HCl(aq) H2O(l) H3O(aq)
Cl-(aq) (LB)
5
Overall HCl(aq) H2O(l) H3O(aq)
Cl-(aq) (LB)
conjugate acid
conjugate base
acid
base
Overall NH3(aq) H2O(l) NH4(aq)
OH-(aq) (LB)
conjugate acid
conjugate base
base
acid
Whether water can act as an acid or base depends
on the other species present
6
Autoionization of water
H2O (l) H2O(l) H3O
(aq) OH-(aq)
Simplified notation
H2O(l) H (aq)
OH-(aq)
Water is said to be amphiprotic
7
Strength of acids
Acids and bases are classified as strong or
weak depending on the extent to which they can
ionize.
Higher ionization Strong
Strong acids and strong bases are completely
ionized
Weak acids and weak bases are partially ionized
Strong acid does not necessarily mean that it is
the most corrosive acid. HF is a weak acid but
it is one of the most corrosive acids.
Strength of some acids HClO4 gt HI gt HBr gt HCl gt
HNO3
H2SO4 gt HNO3
8
Lewis theory
Proposed in 1923 by G. N. Lewis
An acid is any species that can accept a share in
an electron pair. A base is any species that can
make available or donate a share in an electron
pair.
A more broader definition. There is no need for
the molecule to have hydrogens.
9
What happens when salts are dissolved in water?
HCl(aq) NaOH(aq) NaCl(aq) H2O(aq)
Acid
Base
Salt
Water
NaCl(s) H2O NaOH(aq) HCl(aq)
Salt
Water
Base
Acid
Depending on the strength of the acid and base
that are formed in solution, the medium will be
either acidic or basic
10
How do I know if the resulting solution is
acidic or basic?
Measure pH of the solution
What is pH?
It is the measure of hydrogen ions in solution
11
What is pH?
pH14.0
Basic
Neutral
pH7.0
Acidic
pH1.0
12
What are some of the ways to measure pH?
1. Using an acid-base indicator
2. Using pH paper
3. Using pH meter
4. ..
13
What are acid-base indicators? How do they work?
An organic dye, whose color depends on the
concentration of H3O or pH
HIn(aq) H2O(l) H3O(aq) In-(aq)
conjugate acid
conjugate base
acid
base
When the indicator is added to the solution of
interest, depending on the pH of the solution,
the indicator stays in either its acid form or
its conjugate form.
14
Indicator "Acidic" Color "Basic"
color (MO)methyl orange Red below pH
4.0 Yellow above pH 4.4 (BG)bromcresol green
Yellow below pH 4.5 Blue above pH
4.8 (BB)bromthymol blue Yellow below pH
6.9 Blue above pH 7.3 (TB)thymol blue
Yellow below pH 8.0 Blue above pH
8.4 (P)Phenolphthalein Colorless below pH
9.3 Pink above pH 9.7 (AY)alizarin yellow
Yellow below pH 10.8 Red above pH 11.2
Indicators cannot give a particular numerical
value for pH. They can only give a range for the
pH of the solution.
If we are to pick a suitable indicator for
following the pH change in the reaction, We have
to pick one that is suitable over the pH range
of the reaction.
15
Ionization of acids
All acids undergo ionization. Strong acids ionize
more than the weak ones. Strong acids are
almost 100 ionized. Since the weak acids are
partially ionized, the measure of the extent of
ionization is given by an expression called acid
ionization constant, Ka.
In our experiment today, we would like to
measure the Ka for a weak monoprotic acid.
16
What is a monoprotic acid?
Usually acids have one or more hydrogen atoms
(H) which they end up donating as protons (H).
of H atoms Name Example
1 monoprotic acid (HA) HCl
hydrochloric acid CH3COOH -
acetic acid
2 diprotic acid (H2A)
H2SO4 sulfuric acid
3 triprotic acid (H3A)
H3PO4 Phosphoric acid
17
Ionization constant of an acid
For a monoprotic weak acid (HA) dissolved in
water,
HA(aq) H2O(l) H3O(aq) A-(aq)
conjugate acid
conjugate base
acid
base
18
Ionization constant of an acid
Ionization constant of the acid
19
Ionization constant of an acid
Taking log of the equation on both sides,
20
Ionization constant of an acid
Multiplying both sides of the equation by -1
21
Ionization constant of an acid
Henderson-Hasselbach equation
In the above equation, if we can make the A-
HA
22
Ionization constant of an acid
23
Vinitial
Vfinal- Vinitial Vused
Add Vused amount of water to the solution in the
volumetric flask
0.2 M NaOH
Vfinal
Mix the solution in volumetric flask to
the Solution in the erlenmeyer flask
100mL
1.0 g of HA In 100mL of water
Measure the pH of the final solution with
indicators and pH paper
250mL
250mL
50 mL of HA 2 drops phenolphthalein