Acid base titration Pharmaceutical Analysis Unit-II

 

Definitions: 

    Acid: Acid is the substance whose solution turns blue litmus paper red. They have a sour taste.

    Base: It is substance whose solution turns red litmus paper blue. They have a bitter taste and their solutions feel slippery like soap water.

    Acidimetry: It is acid base titration method used to determine concentration of basic substances by titration with a standard base solution. 

    Alkalimetry: It is acid base titration method used to determine concentration of basic substances by titration with a standard acid.

    Titrant: A substances or a reagent solution of precisely known concentration that is added in titration. It is generally filled in burette.

    Titer: A substance which is being analyzed in the titration, it is generally taken in reaction flask.

    Titration: The process, operation, or method used for finding out the concentration of substance in solution by adding to it a standard reagent of known concentration until reaction is complete.

    End point: It is the point where the titration ends in practice, the point when the indicator color changes.

    Equivalence point: The point when the number of equivalents mixed together is the same. The Equivalence point is where the reaction is theoretically complete. 

    Neutralization: A chemical reaction where an acid is reacted with an equivalent amount of base.

    Titration Curve: Plot the pH of the solution as a function of the volume of titrant added.

     Indicators: An auxiliary chemical compound that changes color and structure when exposed to certain conditions and therefore uses to detect the end point of the titration.

 Theories of Acids and Bases:

    The Arrhenius Theory:

  Arrhenius define, 

        Acid as a substance when dissolve in water gives rise to H+ ion which is association with solvent from hydrogen ion

    example: HCl (aq) → H+(aq) + Cl–(aq)

Base is define as the substance that when dissolve in water ionizes to gives hydroxyl ion (OH–)

    example: NaOH(aq) → Na+(aq) + OH–(aq)

Limitations of Arrhenius Theory: 

• Some compounds are basic in nature but they do not contain hydroxyl group(OH–) in its structure.
• Some of the compound do not contain hydrogen ions but still shows acidic property in aqueous medium, example : FeCl₃
• _{The Arrhenius acid base concept is applicable in aqueous state but not for the gaseous state.}
• _{Arrhenius theory says that H+ available freely in aqueous solution, but essentially H+ is always hydrated to form hydronium ion (H₃O⁺)}

The Bronsted-Lowry Theory

In 1923, Bronsted and Lowry  proposed an extension of Arrhenius theory based upon the idea of protonation of bases through the de-protonation of acid. 

According to this theory,

a) An acid is a substance which is rich with the protons H+ ions and donates it to the base.

b) A base is a substance which accept proton from any other substance.

Limitations of Bronsted-Lowry Theory 

• This theory could not explain the acid-base behavior in aprotic solvents like Benzene(C₆H₆${\mathrm{C}}_6{\mathrm{H}}_6$), etc.
• Since there is no proton transfer reaction between acid oxides, for example, Sulphur dioxide and Carbon dioxide respectively and basic oxides for example Calcium oxide(CaO) $\left(\mathrm{CaO}\right)$and Magnesium oxide(MgO) $\left(\mathrm{MgO}\right)$ respectively. It failed to explain this type of reaction.
• It failed to exhibit the acidic nature of substances like Aluminium chloride(AlCl3${\mathrm{AlCl}}_3$) and Boron trifluoride(${\mathrm{BF}}_3$BF₃) with no proton.
• Here, water(H₂O${\mathrm{H}}_2\mathrm{O}$) is acting as Bronsted Lowry acid and Ammonia(NH₃${\mathrm{NH}}_3$) is acting as Bronsted Lowry's base.

The Lewis Reaction

According to Lewis, 

An acid is a substance which accepts an electron pair.

An base is a substance which donates an electron pair.

example:

Limitations of Lewis concept: 

• The relative strength of acids and bases can no be explained by this concept.

Theories of Acids and base indicator:-

1. Ostwald's Theory: In 1894,Ostawald proposed the theory of indicator on the basis of ionization. According to this theory, the color change is due to ionization of the acid-base indicator. The unionized from has different color than the ionized form.                                                                       The ionization of the indicator is largely affected in acids and bases as it is either a weak acid or a weak base. In case, the indicator is a weak acid, its ionization is very much low in acids, due to the common H+  ions while it is fairly ionized in alkalis.                                                                Example: Considering phenolphthalein in an important indicator, Ostwald theory can be illustrated as follows:-                                                                                                                             
   
   
   The undissociated molecules of phenolphthalein are colorless while Ph- ions are pink in colour.
   
   

Salient Feature: 

• An acid-base indicator is usually a weak organic acid or base.
• If the indicator is weak acid, the color of its anions will be deeper than that of unionized form.
• Although acid-base indicators are weak electrolytes.

1. Quinonoid theory: 

According to this theory:

(a) The acid-base indicators exist in two tautomeric forms having different structures. Two forms are in equilibrium. One form is termed benzenoid form and the other quinonoid form.

(b) The two forms have different colors. The color change in due to the interconversion of one tautomeric form into other.

(c) One form mainly exists in acidic medium and the other in alkaline medium.

Thus, during titration the medium changes from acidic to alkaline or vice-versa. The change in pH converts one tautomeric form into other and thus, the colour change occurs.

Example: 

Phenolphthalein has benziod form in acidic medium and thus, it is colourless while it has quinonoid form in alkaline medium which has pink colour.

Methyl orange has quinonoid form in acidic solution and benzenoid form in alkaline solution. The color of benzenoid form is yellow while that of quinoniod form is red.

Salient Feature: 

• Each acid-base indicator exists in two or more tautomeric forms, one in acidic medium and the other in alkaline solution.
• Colours of the two tautomeric forms are different due to the difference in their structures.

$$\begin{gathered} {\mathrm{NH}}_3\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\rightleftharpoons {{\mathrm{NH}}_4}^{+}\left(\mathrm{aq}\right)+{\mathrm{OH}}^{-}\left(\mathrm{aq}\right) \\ \mathrm{base}\mathrm{acid}\mathrm{Conjugate}\mathrm{acid}\mathrm{Conjugate}\mathrm{bas} \end{gathered}$$