Experiment #6. Acid Base Indicators. 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.
Acid Base Indicators
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
Lewis’s 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
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!!
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)
Overall: NH3(aq) + H2O(l) NH4+(aq) + OH-(aq) (LB)
Whether water can act as an acid or base depends
on the other species present
H2O (l) + H2O(l) H3O+ (aq) + OH-(aq)
H2O(l) H+ (aq) + OH-(aq)
Water is said to be amphiprotic
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 > HI > HBr > HCl > HNO3
H2SO4 > HNO3
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.
dissolved in water?
HCl(aq) + NaOH(aq) NaCl(aq) + H2O(aq)
NaCl(s) + H2O NaOH(aq) + HCl(aq)
Depending on the strength of the acid and base
that are formed in solution, the medium will be
either acidic or basic
solution is acidic or basic?
Measure pH of the solution
What is pH?
It is the measure of hydrogen ions in solution
1. Using an acid-base indicator
2. Using pH paper
3. Using pH meter
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)
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.
(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.
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
In our experiment today, we would like to measure
the Ka for a weak 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
For a monoprotic weak acid (HA) dissolved in water,
HA(aq) + H2O(l) H3O+(aq) + A-(aq)
Ionization constant of the acid
Taking log of the equation on both sides,
Multiplying both sides of the equation by -1
In the above equation, if we can make the [A-] = [HA]
Vfinal- Vinitial =Vused
Add Vused amount of
water to the solution in
the volumetric flask
Mix the solution in
volumetric flask to the
Solution in the erlenmeyer
1.0 g of HA
In 100mL of
Measure the pH of the
final solution with
indicators and pH paper
50 mL of HA
+ 2 drops phenolphthalein