Titrations: Reactions of Acids | Chemistry for Grade 10 PDF Download

What is Titration?

Titration which is also known as titrimetry is a chemical qualitative analysis technique that is used to calculate the concentration of a given analyte in a mixture. Titration is an important technique in the field of analytical chemistry and is sometimes referred to as volumetric analysis also.

Types of Titrations

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

• Acid-base Titration
• Redox Titrations (KMNO₄, K₂Cr₂O₇, Iodometry, Iodimetry)
• Precipitation Titration
• Complexometric Titration

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)

• Acid-base titrations mainly depend on the neutralization between an acid and a base when mixed in solution. More significantly, the strength of an acid is determined by using a standard solution of a base. This process is also called acidimetry.
• 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.

The 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 that is used for detecting the endpoint is also added in the titration flask.
• When the reaction reaches completion the colour of 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 is 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 the 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.

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 is 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 the 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 oxidizing agent. The oxidizing or reducing agents are titrated against strong reducing or oxidizing 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 oxidizing states.

Equivalents weights of oxidizing 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.

Gravimetric Analysis Titration

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

• A sample of a certain amount is taken.
• The required component in the sample is converted into a precipitate.
• 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 points 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 SP ime 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 an indicator.
  • KMnO₄ (Purple)
    
  • K₂Cr₂O₇ (orange)

Volumetric Analysis Titration

This involves two reactants of known volumes, the concentration of one of the solutions is known and the other is unknown. The reaction involved in the titration process should satisfy the following criteria.

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

Basic Requirements for Titration;
For any titration process, there are some basic definitions that are important.

• Titre or titrate or analytic: This is the solution whose concentration is unknown
• 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 a longer duration comes under the secondary standard.
A standard solution should satisfy the following criteria.

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

Gas-phase Titration: These are titrations done in the gas phase, specifically as methods for determining reactive species by reaction with an excess of some other gas, acting as the titrant.
Complexometric titration: Complexometric titrations mainly are depended on the formation of a complex between the analyte and the titrant.
Zeta potential titration: Zeta potential titrations are those ones where the completion of the reaction is monitored by the zeta potential, rather than by an indicator, in order to characterize heterogeneous systems, such as colloids.
Assay: An assay is a type of biological titration that is mainly used for the determination of the concentration of a virus or bacterium.

Titration Curve

A titration curve is where we plot the pH of the analyte solution versus the volume of the titrant added as the titration progresses. In a titration curve the x-coordinate of which represents the volume of titrant added since the beginning of the titration. The y-coordinate represents the concentration of the analyte at the corresponding stage of the titration. In an acid-base titration, the titration curve mostly represents the strength of the corresponding acid and base.