Acid Base Titration

What is Acid-Base Titration?

Acid base titration is an analytical technique for estimating the concentration of an acid or base by neutralizing it with a known-concentration standard solution of base or acid. A pH indicator is used to monitor the acid-base reaction’s progress. The molarity of the analyte solution can be calculated if the acid dissociation constant (pKa) of the acid or the base dissociation constant (pKb) of the base in the solution is known.

Alternately, if the concentration of the analyte solution is known, the pKa can be measured by generating a titration curve.

Learning Objective

To determine the concentration of an acid or base by measuring the volume of titrant (of known concentration) that reacts with it according to a stoichiometric proton-transfer reaction.

Theory

In an acid-base titration, strong or weak acids or bases are used. In particular, an acid-base titration can be calculated to assess the following:

1. The acidic or basic concentration
2. How powerful or weak an unknown acid or base is.
3. pKa of the unidentified acid or pKb of the unidentified base.

Consider an acid-base reaction that is occurring in the presence of a proton acceptor. Typically, the proton is solvated in water as H3O+. Adding H2O to the base causes it to lose (OH–) or gain (H3O+). Reversible acid-base reactions can occur.

Reactions are shown below.

HA + H₂O → H₃O⁺ + A^– (acid)

B^– + H₂O → BH + OH^– (base)

Here 

[A^–] is the conjugate base, B-H is conjugate acid. 

Acid + Base ⇋ Conjugate base + Conjugate acid

Hence

It is easy to express [H+] in terms of KA, KB, and Kw for mixtures of different types of strong and weak acids or bases.

Some terminologies

Titration- It is a method for determining the concentration of an indicator in a treated sample by adding a solution of known concentration to a known volume of the sample.

Titrant – A solution of known concentration and strength used in titration.

Titrand – The titrand is any solution to which the titrant is added and which includes the ion or species being tested.

Titration curve — A graph of pH vs millilitres of titrant illustrating the relationship between pH and titrant during an acid-base titration.

Equivalence point — The point at which just enough reagent is added for material to react fully.

Buffer Solutions – A buffer solution is a solution that doesn’t change pH when a strong acid or base is added or when it’s diluted with water.

Types of Acid-Base Titration

Types	Examples	Analyte	Titrant
Strong acid-strong base	Hydrochloric acid and sodium hydroxide	Strong Acid	Strong Base
Weak acid-strong base	Ethanoic acid and sodium hydroxide	Weak Acid	Strong Base
Strong acid-weak base	Hydrochloric acid and ammonia	Strong Acid	Weak Base
Weak acid-weak base	Ethanoic and ammonia	Weak Base	Weak Acid

Titration Curve & Equivalence Point

In a titration, the equivalence point is the point at which exactly the same number of moles of hydroxide ions have been added as there are moles of hydrogen ions. In a titration, if the base is added from the burette and the acid has been accurately measured into a flask. The shape of each titration curve is typical for the type of acid-base titration.

• The pH does not change in a regular manner as the acid is added. Each curve has horizontal sections where a lot of bases can be added without changing the pH much.
• There is also a very steep portion of each curve except for weak acid and weak base where a single drop of base changes the pH by several units.
• There is a large change of pH at the equivalence point even though this is not centre on pH7. This is relevant to the choice of indicators for each type of titration.

Indicator choice

• Acid-base indicators are substances which change colour or develop turbidity at a certain pH. They locate equivalence points and also measure pH. They are themselves acids or bases are soluble, stable and show strong colour changes. They are organic in nature.
• A resonance of electron isomerism is responsible for colour change. Various indicators have different ionization constants and therefore they show a change in colour at different pH intervals.
• Acid-base indicators can be broadly classified into three groups.
  • The phthalein՚s and sulphophthaleins (eg; Phenolphthalein)
  • Azo indicators (eg; Methyl orange)
  • Triphenylmethane indicators (eg; Malachite green)

• The two common indicators used in acid-base titration is Phenolphthalein and methyl orange. In the four types of acid-base titrations, the base is being added to the acid in each case. A graph is shown below where pH against the volume of base added is considered. The pH range over which the two indicators change colour. The indicator must change within the vertical portion of the pH curve.

pH Range Over which the Two Indicators Change Colour

The Choice of indicators based on the type of titration is tabulated below.

Types of titration	Indicators
Strong acid-strong base	Phenolphthalein is usually preferred because of its more easily seen colour change.
Weak acid-strong base	Phenolphthalein is used and changes sharply at the equivalence point and would be a good choice.
Strong acid-weak base	Methyl orange will change sharply at the equivalence point.
Weak acid-weak base	Neither phenolphthalein nor methyl orange is suitable. No indicator is suitable because it requires a vertical portion of the curve over two pH units.

Equivalence Point Indicators

• Before you begin the titration, you must choose a suitable pH indicator, preferably one that will experience a color change (known as the “end point” ) close to the reaction՚s equivalence point; this is the point at which equivalent amounts of the reactants and products have reacted. Below are some common equivalence point indicators:
  • strong acid-strong base titration: phenolphthalein indicator
  • weak acid-weak base titration: bromothymol blue indicator
  • strong acid-weak base titration: methyl orange indicator the base is off the scale (e. g., pH>13.5) and the acid has pH>5.5: alizarine yellow indicator
  • the base is off the scale (e. g. , pH>13.5) and the acid has pH>5.5: alizarine yellow indicator
  • the base is off the scale (e. g. , pH>13.5) and the acid has pH>5.5: alizarine yellow indicator
  • the acid is off the scale (e. g. , pH<0.5) and the base has pH<8.5: thymol blue indicator

Solved Example

Problem:

A 1.2gm sample of a mixture of (Na₂CO₃ + NaHCO₃) is dissolved and titrated with 0.5N HCl. With phenolphthalein, the endpoint is at 15ml while after further addition of methyl orange a second endpoint is at 22ml. Calculate the percentage composition of the mixture.

Solution:

15 + 15 = 30ml acid is necessary to neutralize Na₂CO₃ completely.

Total volume needed = 15 + 22 = 37ml

(37-30) = 7ml acid is needed for neutralizing NaHCO₃

Therefore, Na₂CO₃ composition (%) is

= [(30 x 0.5 x 0.053)/1.2] x 100 = 66.25%

= (7 x 0.5 x 0.042 x 100) / 1.2 = 24.50% NaHCO₃

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