Week 2, Lesson 1
Chapter 4 – Analysing Acids and
Bases
Acid Base Chemistry Revisited
• Acids are proton donors.
• Bases are proton acceptors.
• Acid-base reactions involve the transfer of a
proton from an acid to a base.
Strong and Weak
• Strong Acid: ionises almost completely in
water.
• Weak Acid: does not ionise completely in
water.
• Strong Base: readily accept protons.
• Weak Base: accepts protons only to limited
extent.
pH
• The concentration of H3O+ ions in a solution is
referred to as the solution’s acidity.
• Acidity is measured using a logarithmic scale,
called the pH scale.
• pH = -log[H3O+]
Indicators
• An indicator is used during acid-base titration
to identify the equivalence point of the
reaction.
• An acid-base indicator is a substance whose
colour depends on the concentration of H3O+
ions in solution.
• Indicators are acids with their acid form being
one colour and the conjugate base being
another.
Indicators & their Range
Indicator
Colour of Acid
Form
Colour of Base
Form
pH Range
Phenolphthalein
Colourless
Pink
8.2-10.0
Methyl Orange
Pink
Yellow
3.2-4.4
Bromothymol Blue
Yellow
Blue
6.0-7.6
-The indicator must be chosen carefully to ensure that the point during the titration
where the indicator changes colour, the end point, closely matches the equivalence
point of the reaction.
- If we look at the pH changes that take place between a strong acid and a strong
base, we would see a graph similar to 4.4a (page 40). At the end point, addition of a
very small volume of strong acid produces a large change in pH. This is referred to
as the sharp end point.
-In the case between a strong acid and base, the equivalence point falls close to 7.0
Indicators & their Range cont…
• When a weak acid is titrated with a strong base, or vice
versa, there is a more gradual change in pH around the end
point.
• In this situation, only indicators whose colour changes
within the end point range can be used.
• When a strong acid reacts with a weak base, the
equivalence point occurs between pH 3 and 7.
• Methyl orange changes colour over this range and as such
would be a suitable indicator.
• The equivalence point for a reaction between a weak acid
and a strong base occurs between pH 7 and 11.
• A suitable indicator for this would be phenolphthalein.
Week 2, Lesson 2
Back Titration
• Some acids and bases are so weak that they do not produce a sharp colour
change at the end point of a titration.
• A technique called back titration is used to overcome this problem.
• The procedure of a back titration has two parts:
1. If the substance to be analysed is a weak acid, it is mixed with an excess of
strong base. The original amount of the base is known. All of the weak
acid reacts in stoichiometric proportions, leaving some of the strong base
unused.
2. The unused strong base is titrated as normal with a standard solution of a
strong acid. Knowing the original amount of strong base and the amount
of strong base left unused, it is possible to work bask to find the amount
of weak acid present.
The same procedure can be used to determine the concentration of a weak
base by adding an excess of a strong acid.
Back Titration cont…
Determining the Concentration of Ammonium
Ion in Lawn Fertiliser
- Nitrogen-containing ions, such as ammonium ions,
are required by plants to synthesise materials
needed for healthy growth.
- For this reason, lawn fertilisers often contain a high
proportion of ammonium sulfate or ammonium
nitrate.
- The ammonium ion (NH4+) acts as a very weak acid,
and a direct titration with a base does not give a
sharp end point.
Determining Concentration cont…
1.
A measured mass of fertiliser is added to a solution containing an excess
amount of sodium hydroxide. The ammonium ions react with hydroxide
ions:
NH4+(aq) + OH-(aq)  NH3(aq) + H2O(l)
The mixture is boiled to ensure complete reaction of the ammonium
ions. Some of the the hydroxide ions are consumed by the reaction but
others remain unreacted.
2. The quantity of excess hydroxide ions is found by titrating the solution with
a standard solution of hydrochloric acid:
HCl(aq) + NaOH(aq)  NaCl(aq) + H2O(l)
Knowing the original quantity of sodium hydroxide and the amount in excess
after reaction with the fertiliser, we can calculate, by subtraction, the
quantity of hydroxide ions that reacted with the fertiliser. The
concentration of ammonium ions in the fertiliser can then be calculated
by working backwards.
Back Titration Example…
A 1.50g sample of lawn fertiliser was boiled with 25.00mL of
0.9987M sodium hydroxide solution. When no further
ammonia gas was evolved from the mixture, it as cooled and
titrated with 0.2132M hydrochloric acid, using
phenolphthalein as an indicator. A titre of 19.78mL was
required. Calculate the percentage of ammonium ions in the
fertiliser.
STEP 1: Find the original amount of NaOH used.
STEP 2: Find the amount of NaOH that did not react with NH4+
ions.
STEP 3: Find the amount of NaOH that reacted with NH4+ ions.
STEP 4: Find the amount of NH4+ in the fertiliser sample.
STEP 5: Find the percentage of NH4+ in the sample.