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Types of Titrations – Acid-Base Titrations, Terms involved, Law of Equivalence, Practice Problems and FAQ

Titration is a significant analytical technique that has many uses in day-to-day life. In the pharmaceutical industry, acid-base titrations are frequently used to evaluate the purity of compounds. Acids or bases are commonly used as components or reagents in many medications. For instance, the purity of the medication ephedrine, which is frequently present in many cough syrups, can be checked using acid-base titration.

On this concept page, we will get to know more about acid-base titrations in detail.

TABLE OF CONTENTS

  • Titrations
  • Major Terms Involved in Titrations
  • Law of Equivalence
  • Types of Titrations
  • Acid-Base Titration
  • Acid-Base Titration – Types
  • Acid-Base Titration – Choice of Indicators
  • Acid-Base Titration – Titration curves
  • Practice Problems
  • Frequently Asked Questions – FAQ

Titrations

Titration is a technique used to find the concentration of a solution using a solution of known concentration.

Titration is a quantitative and volumetric method for calculating the concentration of an unknown solution from the concentration of a known solution in the presence of an indicator.

Some important points about titration are

  1. Titration, in general, is a sort of quantitative analysis that entails figuring out how much of a particular composition is present in a solution in a suitable solvent.
  1. This strategy makes use of the law of equivalence.
  1. The material that has to have its concentration determined is titrated against the titrant, which is a solution with a known concentration.
  1. By gradually introducing a certain extra component (often with a burette) until the reaction is complete, which is indicated by the indicator's colour changing, the concentration of a substance in a solution should be determined.

Major Terms Involved in Titrations

Titrant: A solution with known concentration or strength used in the titration.

Titrand (or) Analyte: Any solution that has the titrant added to it and contains the ion or species whose concentration is to be measured is called the titrand or analyte.

Titration curve: A plot of pH vs. millilitres of titrant demonstrating how pH varies with the volume of the titrant used in an acid-base titration.

Equivalence point: The point at which just an appropriate amount of reagent is required to completely react with a material. In other words, the point at which the equivalents of the titrant are equal to the equivalents of the analyte.

End Point: The point at which the analyte solution shows a change in colour or intensity. The endpoint implies that the titration is complete.

Buffer solution: A solution that resists pH changes even when a strong acid or basic is added or when it is diluted with water.

Indicator: A weak organic base or a weak organic acid that is added in drops to the analyte solution to indicate the endpoint in an acid-base titration.

Law of Equivalence

At the equivalence point, the number of equivalents of the acid is equal to the number of equivalents of the base in a neutralisation reaction.

Volume Normality = Number of Equivalents

In general, all the titrations are based on the law of equivalence. Generally, a colour change signifies the endpoint of a titration. However, in potentiometric titrations, the acid-base titration is based on a change in pH, and the pH change signifies the end point. Since the pH change signifies the endpoint, the addition of an indicator is not required.

Types of Titrations

There are various types of titrations, each with its own goals and methods. Redox titrations and acid-base titrations are the most used in quantitative chemical analysis. Titrations can be broadly classified as shown below.

In this concept page, we will discuss acid-base titrations in detail.

Acid-Base Titration

Acid-base titrations work on the principle that, the number of equivalents of an acid/base of unknown concentration should be equal to the number of equivalents of the acid/base of known concentration for complete neutralisation.

In a neutralisation reaction, an acid reacts with a base to form water and salt.

Acid + Base Salt + water

For example, the strength of acid of unknown concentration can be estimated by reacting it with a suitable base of known concentration. From the number of equivalents of base required to completely neutralise the equivalents of acid present, the concentration of the acid can be calculated.

  • In an acid-base titration, a known volume (say 100 mL) of the acid or a base, whose strength is to to be determined is taken in a conical flask.
  • To this acid/base, a few drops of a suitable indicator like phenolphthalein are added. Indicators exhibit different colours in different mediums.
  • For example, phenolphthalein is pink in colour in a basic medium and colourless in an acidic medium. A burette is filled with a suitable base of known concentration.
  • The titration starts when the base in the burette is added drop by drop to the analyte mixture in the conical flask.
  • The titration is complete when the colour of the analyte mixture shows a sharp change in colour.
  • At this point, the addition of the titrant is stopped.
  • From the volume and the strength of the base used for the neutralisation of the analyte, the strength of the analyte can be calculated using the principle of volumetric analysis.

V1N1=V2N2

Where V1 is the volume of the titrant

N1 is the normality of the titrant

V2 is the volume of the analyte

N2 is the normality of the analyte

N2=V1N1V2

 

We know that N=M ✕ n

 

Where, N is normality

M is molarity

n is the n-factor

Therefore, in terms of molarity the principle of volumetric analysis can be written as,

V1M1n1=V2M2n2

Where, V1 is the volume of the titrant

M1 is the molarity of the titrant

n1is the n-factor of the titrant

V2 is the volume of the analyte

M2 is the molarity of the analyte

n2 is n-factor of the analyte

M2=V1M1n1V2n2

Acid-Base Titration – Types

S.No.

Types

Examples

1

Strong acid - Strong base

HCl & NaOH

2

Weak acid - Strong base

CH3COOH & NaOH

3

Strong acid - Weak base

HCl & NH4OH

4

Weak acid - Weak base

CH3COOH & NH4OH

Acid-Base Titration – Choice of Indicators

It is highly important to choose a suitable indicator for a titration.

For titrations involving a strong acid and a strong base, the pH value of the solution changes from 3.3 to 10.5 near the end point with pH = 7 being the endpoint. Therefore, any indicator with a pHrange between 3.3 and 10.5 can be used as an indicator for this titration, but the most ideal would be an indicator which can show a colour change close to pH = 7. Bromomethyl blue having a pH range of 6.0 to 7.6 can be used as an indicator for a reaction involving a strong acid and a strong base.

In acid-base titrations, phenolphthalein, which has a pH range of 8.0 to 9.8 , and methyl orange, which has a pH range of 3.1 to 4.5, are the most commonly used indicators.

When the titrant is an alkali, phenolphthalein is a suitable indicator, and when the titrant is an acid, methyl orange is a suitable indicator.

Acid-Base Titration – Titration Curves

A titration curve is a graph that illustrates the pH of a solution during a titration.

  1. Each titration curve has a shape that is typical for the particular acid-base titration. The titration curve has the same general shape, but the pH at the equivalence point is different.
  1. There are horizontal stretches on each curve where a large amount of bases can be added without noticeably altering the pH. Each curve has a highly steep part, with the exception of weak acid and weak base, where a single drop of base affects the pH by several units.
  1. The equivalence point in a strong acid-strong base titration is reached when the moles of acid and base are equal and the pH is 7.
  1. The pH is higher than 7 at the weak acid-strong base titration's equivalence point.
  1. The pH is lower than 7 at the strong acid-weak base titration's equivalence point.

Practice Problems

1. Which of the following statements about titrand is true?

  1. A solution with a known concentration that is used in the titration is called a titrand.
  2. It is the solution to which the titrant has been introduced and contains the ion or species to be estimated.
  3. It is the solution of known strength that is poured into a predetermined volume of a treated sample that contains an indicator.
  4. The point at which only enough reagent is added to completely react with a material.

Answer: B

  • Titration is a process in which a solution of known strength is added to a solution of a predetermined volume of a treated sample of unknown strength and indicator.
  • Titrant is a solution whose concentration is known and is used to estimate the concentration of the titrand or analyte.
  • Titrand or analyte is any solution that contains the ions or species that has to be estimated.

Thus, the statement given in option B is correct.

So, option B is the correct answer.

2. Match the following types of titrations and the indicators used.

Types of titrations

Indicators

  1. Strong acid - Weak base
  1. Phenolphthalein
  1. Weak acid - Strong base
  1. Methyl orange
  1. Weak acid - Weak base
  1. No indicator
  1. a - (I), b - (II), c - (III)
  2. a - (I), b - (I), c - (III)
  3. a - (II), b - (I), c - (III)
  4. a - (I), b - (II), c - (I)

Answer: C

In acid-base titrations, phenolphthalein has a pH range of 8.3 -10. In basic solutions, the colour of phenolphthalein is pink, and in acidic solutions, it appears colourless. In the titration of weak acid vs strong base, the pH should be slightly greater than 7 as the solution is basic in nature due to the presence of a strong base. Phenolphthalein is a suitable indicator for this type of titrations as it shows a sharp change in colour near the equivalence point.

Methyl orange has a pH range of 3.1 to 4.5. It is yellow in alkaline and neutral solutions and turns red in acidic solutions. In the titration of strong acid vs weak base, pH should be slightly less than 7 as the solution is acidic in nature due to the presence of strong acid. Methyl orange is a suitable indicator for this type of titration as it shows a sharp change in colour near the equivalence point.

Both methyl orange and phenolphthalein are inappropriate in titrations of weak acids and weak bases because pH range in which they show deviation is very small (6.5-7.5). In this range, neither methyl orange nor phenolphthalein works.

Hence, the correct match is a - (II), b - (I), c - (III).

So, option C is the correct answer.

3. Calculate the normality of 50 mL of potassium hydroxide when it is titrated against 100 mL of 0.2 M sulphuric acid.

  1. 0.4 N
  2. 0.2 N
  3. 0.1 N
  4. 0.8 N

Answer: D

Solution:

According to the law of equivalence, in a neutralisation reaction,

Number of equivalents of acid = Number of equivalents of base

Volume Normality = Number of Equivalents

(Volume Normality)Acid = (Volume Normality)Base

V1N1=V2N2

Volume of acid H2SO4 (V1) = 100 mL

Volume of base KOH (V2) = 50 mL

Normality of acid H2SO4 (N1) = n-facor of H2SO4 Molarlity of H2SO4

= 20.2 = 0.4 N

Normality of base KOH (N2) = ?

V1N1=V2N2

100 mL0.4 N=50 mLN2

0.8 N=N2

Thus, the normality of potassium hydroxide used is 0.8 N.

So, option D is the correct answer.

4. Calculate the molarity of 60 mL of barium hydroxide when it is titrated against 150 mL of 0.3 N hydrochloric acid.

  1. 0.75 M
  2. 1.5 M
  3. 0.375 M
  4. 0.5 M

Answer: C

According to the law of equivalence, in a neutralisation reaction,

Number of equivalents of acid = Number of equivalents of base

Volume Normality = Number of Equivalents

(Volume Normality)Acid = (Volume Normality)Base

V1N1=V2N2

Volume of acid, HCl (V1) = 150 mL

Volume of base, Ba(OH)2 (V2) = 60 mL

Normality of acid, HCl (N1) = 0.3 N

Normality of base, Ba(OH)2(N2) = ?

V1N1=V2N2

150 mL×0.3 N=60 mL×N2

0.75 N=N2

Molarity of the base, Normality of Ba(OH)2 (N2)number of ionisabel OH-ions=0.752=0.375

Hence, the molarity of barium hydroxide is 0.375 M.

So, option C is the correct answer.

Frequently Asked Questions – FAQ

1. What are the factors that affect acid-base titrations?
Answer:
There are majorly three factors which affect acid-base titrations. They are:

  1. Concentration of acid and base
  2. Volume of acid and base
  3. Type of indicator used.


2. In an acid-base titration, why does the colour of the indicator change at the endpoint?
Answer: In general, indicators are organic weak acids or weak bases that have different colours in the ionised and unionised forms.

For example, phenolphthalein is a weak acid (the ionised form is pink and the unionised form is colourless). The colour change is caused by the ionisation of the acid-base indicator, as the unionised form is different from the ionised form.

3. Are titrations affected by adding too much indicator?
Answer: Addition of too much indicator has an effect on titration because if you add too much, it will change the concentration of the solution to which the titrant is added.

Let us assume, the indicator is acidic in nature. If you add it to the acid, it will make the acid more acidic than you calculated. The amount of base required to neutralise the acid will be greater, which will ruin your entire experiment because it will be inaccurate.

4. What are some titration errors that could happen?
Answer: Misreading volumes, selecting the wrong concentrations, or using poor technique are just a few of the things that might lead to errors in titration results. When using laboratory glassware like a burette or pipette to transfer a known concentration of solution into a particular volume of the unknown, care must be taken.

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