Types of Titration - Acid Base Titration

What is Titration?

Titration is a method commonly used in a laboratory for the quantitative estimation of an analyte i.e., using the method of titration the concentration or strength of a given chemical is determined.
Titration is basically used in volumetric analysis. Volumetric analysis can be used for many types of estimation and various types of titrations can be classified under the following categories:

  1. Acid- base titration
  2. Redox titration
  3. Precipitation titration
  4. Complexometric titration


Titration chemistry: In general, we can state that titration is a mode of quantitative analysis involving estimation of the quantity of a chemical species by measuring the volume of the solution of that particular species in a suitable solvent. This method is based on the Law of Equivalence. So, it can be said that titration is the process of determining the volume of the reagents by bringing about a definite reaction to just completion.
The solution used during titration whose accurate concentration is known is called titrant and the substance whose volume is to be determined is said to be titrate.

Acid Base Titration
The determination of the strength of a solution of an acid by titrating it with a standard solution of a base, or the determination of the strength of a solution of an alkali by the means of titration with a standard solution of acid, is termed as acid-base titration.

Titration chemistry in detail: The completion of the reaction between an acid and an alkali is termed as neutralization and it proceeds with the formation of a salt and water according to the general equation:

Acid + Base 🡪 Salt + Water

The detection of the end point of the titration is assisted by the addition of an indicator to the system under investigation. The indicator employed in these titrations is required to indicate the equivalent point rather than the true neutral point. At the point of equivalence, the pH of the solution could be equal to, greater than or less than 7, depending on the relative strengths of the acid and alkali. The salt formed is the solution at the end of the reaction may suffer hydrolysis to some extent and the pH of the solution will either be less than or more than 7. The solution is, thus, not truly neutral.

Indicators are used during an acid-base titration. The indicators employed are either weak organic acids or weak organic bases. Their degree of dissociation is greatly affected by any alteration in the hydrogen ion concentration of the solution. An acid indicator can be expressed by the general formula HIn and the basic indicators as InOH. Their respective dissociations can be expressed as:


The dissociated and undissociated forms form a dynamic equilibrium mixture of two tautomeric forms having different structural formulae and colors. One of these exists in acidic medium, while the other in an alkaline medium. The change in pH causes the transformation from one form to the other and vice-versa; and consequently, a change in color is seen.

Two such indicators are: Phenolphthalein
                                            Methyl orange

pH range of some common acid-base indicators:

S. No.IndicatorAcidAlkalipH range
1Thymol blueRed-yellowYellow-Blue1.2 - 2.88-9.6
2BromophenolYellow-Blue3.0 - 4.6
3Methyl orangeRed- Yellow3.1 - 4.4
4Methyl redRed-Yellow4.2 - 6.3
5LitmusRedBlue4.5 - 8.3
6Methyl purplePurple- Green4.8 - 5.4
7Bromo thymol blueYellowBlue6.0 - 7.6
8Phenol redYellowRed6.8 - 8.4
9PhenolphthaleinColorless -Pink 8.3 - 10.0
10Alizarin yellowYellow- Violet10.1 – 12.0


Usually, an indicator such as phenolphthalein is used for the titrations involving a strong alkali and an indicator like methyl orange is used for titrations involving a strong acid.


What is Acid-Base Titration?


In order to do the titration, 100ml of the acid/base, which is to be titrated, is taken in a conical flask and a drop of phenolphthalein is added. Suppose, we take the acid in the flask. The counter chemical i.e. the base is taken in the burette and the titration is done until a sharp change in the color is observed. Phenolphthalein gives pink color in basic medium. When the sharp change in color is seen, the addition of the base is stopped and the amount of base added is noted down. By using the concept of normality, and the law of equivalence, and using the known value of the concentration of the base, the unknown concentration of the analyte i.e., the acid that was titrated is determined. 

                                              
                                                                                      

  
                                              
                                                                                           

The above curve is known as the neutralization curve. The basic concept of acid-base titration is based on this neutralization curve. The calculations for the determination of the strength or concentration of the unknown analyte are done using the law of equivalence.

Law of equivalence:
For a neutralization reaction, the number of equivalents of acid must be equal to the number of equivalents of the base at the end point/equivalence point.

 Normality X Volume = Number of equivalents

Normality = No. of equivalents of the solute dissolved in 1L of the solution. And, no. of equivalents for an acid can be calculated as {(Molecular weight) / Basicity}. No. of equivalents for an acid can be determined as {(Molecular weight) / Acidity}.

The overall titration chemistry lays its foundation on this law of equivalence only.

In general, during titration, an indicator showing change is color is taken. However, in potentiometric titrations, change in pH is taken for the acid-base titration and in such titrations, there’s no requirement for the addition of an indicator because the endpoint is analyzed by the change in the pH of the solution.

This was all about acid-base titration where an acid and a base were involved, of which the concentration of one was known and the concentration of the other was unknown; the titrations are performed in order to determine the concentration of the unknown using the known solution.