Titration

What is Titration?

Titration is the concept of stoichiometry applied to find the unknown concentration of a solution. The world of chemical analysis can be divided into two basic types.

• Qualitative Analysis: Where one finds at the composition of a compound i.e. to find what radicals are present in the salt.
• Quantitative Analysis: Where the interest is laid upon the concentration of an unknown solution.

There are some fundamental requirements which should be understood before one does the titration. The unknown concentration of a solution can be in the form an acid, base or salt of an acid or base. For any titration process, the method is similar except for a few differences.

Table of Content

The titration process can be divided in the following way;

• Gravimetric Analysis
• Volumetric Analysis
• Combustion Analysis
• Others (Spectroscopy)

Titrations can also be used to furnish purity of samples, calculation regarding PH, etc. Any calculations during the analysis can be done in two ways.

1. Using mole concepts (Required balanced chemical equations)
2. Using equivalent concepts (Balanced chemical equations are not required)

Therefore a sound knowledge of the concepts of moles and equivalents is mandatory before studying about titration. In this article, wherever required, the above said concepts are brushed for the convenience.

Types of Titrations

Depending at the types of reactions involved titrations can be classified as follows.

There can be cases where the titrate can have more than one component (For Example, Na2CO3 + NaHCO3). Therefore, based on the number of components in the titrate, The titrations can be divided as follows.

• Single Titration
• Double Titration

Acid-Base Titration (Acidimetoy or Alkalimetry)

Acids can be classified into strong or weak acids depending on the amount of dissociation to give H+ ions when dissolved in water.

If an acid solution of known concentration is titrated against a strong base, the concentration of acid can be calculated, considering the fact that the neutralization reaction reaches completion.

For the same fact, only a strong base is used for the titration process. So in this case, the acid solution is the titrate and the strong base is the titrant or the standard solution.

Procedure of Acid-Base Titration?

• The required volume of the base is taken whose concentration is known in a pipette ad is poured into the titrating flask.
• The acid whose concentration is unknown is taken in the Burette and is allowed to react with the base drop by drop.
• An indicator which is used for detecting the endpoint is also added in the titration flask.
• When the reaction reaches completion the colour the solution in the titration flask changes due to the presence of the indicator.
• The indicator used for this purpose can be phenolphthalein which forms pink colour in basic solution and colourless in acid and neutral solution.

Therefore the endpoint is detected when the pink coloured solution turns colourless.

How to choose an Indicator for an Acid-Base Titration?

When the acid is very weak the detection or obtaining the endpoint is difficult. For this purpose, the salt of the weak acid is titrated against a strong acid, because the conjugate base of a weak acid is a strong base.

For Example: CH3COOH is a weak acid. But CH3COONa is a strong base. EG of strong acids HNO3, HCl, H2SO4, HClO3

Bases again can be of two types strong and weak. The same process done in case of acid titration is done except that unknown solution (titrate) is the base and titrant is a strong acid. Indicators used in this case can be methyl red or methyl orange which is orange in acidic solution and yellow in basic and neutral solutions.

As in the case of acids, in order to obtain 100% completion of the reaction, a weak base is first converted into its salt and is titrated against a strong base.

If a salt of a weak acid and weak base like ammonium carbonate’s concentration has to be found out, it can be titrated against a strong acid or a strong base depending on the salt.

The analysis part in which the calculation of unknown concentration is illustrated in the following example.

A 2.0 g sample of an unknown monobasic acid is dissolved in 100 ml water 20 ml portion of this solution required 15 ml of 0.12m NaOH solution to reach the endpoint. If the molecular mass of acid is 122g/mol, determine the purity % of the acid.

Solution:

For 20ml acid solution: 15ml of 0.12m NaOH is required

Therefore, Number of equivalents of base = 12 × 15 = 1.8 × 10-3 equivalents

Therefore, in 20ml of the acid solution, 1.8 0 × 10-3 equivalents of acids are present

Therefore, if 20ml → to 1.8 × 10-3 equivalents for 100ml → X

Therefore, x = 9 × 10–3 equivalent since it a monobasic acid, molar mass acid equation mass is the same.

In 2g of sample:

Mass of acid = 9 × 10-3 × 122 = 1.098 g

% purity = 1098/2 × 100 = 54.9%.

Double Titration

In this type of titration, the titrate (unknown concentration) solution contains more than one component.

Examples can be a mixture of NaOH and Na2CO3 or Na2CO3 and NaHCO3

For finding the composition of the mixture or say to check the purity of a sample, titration of the mixture is done against a strong acid. But in this case, instead of one indicator, two indicators are used because there are going to be two endpoints during the titration.

Commonly used indicators are phenolphthalein and methyl orange. Before understanding how to calculate, knowing the reactions involved is important.

In case of a mixture of two basses like NaOH + Na2CO3 or Na2CO3 + NaHCO3,

First, the stronger of bases will react with the acid to reach the 1st endpoint and then only the weaker base.

Redox Titration

These types of titrations are almost similar to the volumetric acid. Base titrations except that here, the reactions involved are Redox reactions. Here, the interest is laid upon finding the unknown concentration of a reducing or an oxidising agent.

The oxidising or reducing agents are titrated against strong reducing or oxidising agents respectively. In most of the redox titrations, one of the reactants itself will act as an Indicator (self indicators), changing their colour wrt their oxidising states.

Equivalents weights of oxidising and reducing agents

Balancing a redox reaction is a tedious job and so using the concept of equivalents is very much preferred in redox titrations.

Oxidation Titration using KMnO4

KMnO4 is a strong oxidising agent which almost oxidises every other common reducing agent. It is purple in colour and changes to colourless when Mn2+ or $MnO_{4}^{2-}$ is formed and black when it forms MnO2.

Iodometric and Iodimetric Titration

This titration involves Iodine and Iodide solutions used as titrants. The use of Iodine or Iodide can be done in two ways:

Iodimetry Titration (Direct Method)

In this type of titration Iodine solution (I2) is used for titrations of reducing agents. The following are common reactions involving Iodine.

Common Reactions involved in Iodimetry Titration

In all the reactions I2 acts a weak oxidising agent.

Iodometry Titration (Indirect Method)

Here I acts as a reducing agent, which is oxidised to I2. The amount of I2 is titrated against standard hypo (thiosulphate) solution, which in turn is used for estimating the unknown concentration of reaction oxidising agent. The following are some common reaction involved.

Common Reactions involved in Iodometry Titration

Gravimetric Analysis

This is done for elements which can form insoluble salts in aqueous solution. It involves separation of ions from the compound in the form of precipitates. The following are the step involved in the process.

1. The sample of a certain amount is taken.
2. The required component in the sample is converted into a precipitate.
3. This precipitate is purified and tested and the required result (concentration or purity of the sample) is obtained.

Endpoints or equivalence point

This is the stage where the reaction attains completion. Endpoints or equivalence point can be also defined as a point in the reactions where the number of equivalents of titrant and titrate becomes equal.

To detect the endpoint, most of the times and “Indicator” is used. The indicator used depends on the type of reaction.

For example: phenolphthalein or methyl orange can be used for titrations involving acids and bases. The indicators indicate the endpoint by changing their colours at the endpoint, sometimes one of the reactants itself can act as indicators.

Volumetric Analysis Titration

This involves two reactants of known volumes, the concentration of one of the solution is known and the other is unknown.

The reaction involved in the titration process should satisfy the following criteria.

1. The skeletal equation of the reaction should be known.
2. The reaction should not be too slow or fast so that the calculations are free of error.
3. The endpoint (completion) of the reaction should be easily detectable. For this purpose, indicators are used which is discussed later.
4. There should not be any side reactions. If there is something which makes up the side reactions, that particular component should be removed before the titration process.

For any titration process, there are some basic definitions which are important.

1. Titre or titrate or analytic: This is the solution whose concentration is unknown
1. Titrant: This is the solution whose concentration is known.

Standard Solution: The titrant is actually taken from the standard solution whose concentration is known. Depending on how they are prepared they can be of two types.

Primary standard:

This is prepared by dissolving a measured amount of solute is a measured volume of water directly. For example Ferrous Ammonium sulphate, etc.

Secondary standard:

This is prepared by titrating a solution against a primary standard solution of KOH, NaOH or any other which can’t be stored for longer duration comes under the secondary standard.

A standard solution should satisfy the following criteria.

1. The purity of the sample should be ideally 100%
2. Stable at room temperature
3. Concentration should not change over a period of time
4. It should have a high molecular mass. This gives error-free calculations.