Study Notes On Conductometric Titrations (Chemical Science) - Download PDF!

Study Notes on Conductometric Titration

The determination of the endpoint of a titration by means of conductance measurements is known as conductometric titration. The conductance readings corresponding to various added amounts of titrant are plotted against the latter.

Note: (i) The titrant should be at least ten times as concentrated as the solution being titrated, in order to keep the volume change small.

(ii) Conductance due to weak acid, weak base or H₂O is very small because of their low value of dissociation constant.

(a) Strong acid with a strong base: When a strong alkali e.g., sodium hydroxide is added to a solution of a strong acid, e.g., hydrochloric acid, the reaction.

(H⁺ + Cl^–) + (Na⁺ + OH^–) → Na⁺ + Cl^– + H₂O

• During the reaction, highly conducting H+ are replaced by Na+ which has much lower conductance, so the addition of alkali to the acid solution is accompanied by a decrease of conductance.

• When neutralization is complete then the further addition of alkali results in an increase of conductance since the sodium ion and hydroxyl ion are no longer used up in the chemical reaction.
• At the neutral point the conductance of the system will have a minimum value.

(b) Strong acid with weak base:

When the strong acid is titrated with a weak base, e.g., an aqueous solution of NH3, the first part of the conductance titration curve representing the neutralization of the acid and its replacement by salt is very similar to the previous case since both NaCl and NH4Cl are strong electrolytes. However, the conductance will remain almost constant when the endpoint is passed due to the very low dissociation of NH4OH.

(H⁺ + CI^–) + (NH₄+ + OH^–) → NH₄⁺ + Cl^– + H₂O

(c) Weak acid with strong base:

CH₃COOH + Na+ + OH– → CH₃COO– + Na+ + H₂O

• A weak acid (CH₃COOH) has lower conductance than its salt (CH₃COONa), due to its lower dissociation w.r.t. salt, so the addition of base increases conductance (region ab in the graph).
• The reason for the initial decrease in conductance (region oa) is because the small addition of the base leads to common anion, i.e., CH3COO– ion formation which represses the dissociation of CH₃COOH which gives highly conducting H+ ions.

• After the end point there is a higher rate of increase in conductance because Na+ + OH– has higher conductance than Na+ + CH3COO– (region bc in Graph-I).

(d) Weak acid with weak base:

CH₃COOH + NH₄OH → CH₃COO– + NH₄+ + H₂O

• Graph II shows the titration of a weak acid with a weak base.
• The first two points of the explanation are the same as the above case.
• The region b'c’ of the graph is due to the weak nature of base, so conductance remains almost constant with the addition of weak base after end-point.

Note: (i) Endpoint determination in case of a weak acid with a weak base is easier than titration of a weak acid with the strong base because region ab and bc of the graph may have almost the same slope (due to closeness in conductance value of salt of weak acid and conductance of strong base).

(ii) Sometimes the regions oa and o‘a’ are so small that they are not shown in the graph.

(e) Mixture of the weak acid and strong acid with weak base:

• The 1st end-point is obtained when a strong acid is consumed.
• The 2nd end-point is obtained when weak acid is consumed.

(f) Mixture of weak acid and strong acid with strong base:

Similar explanation as the above case

(g) Precipitation reactions:

Type I: (a) Titration of KCI with AgNO₃:

(K⁺ + Cl^–) + (Ag⁺ + NO₃^–) → AgCI↓ + K+ + NO₃^–

(b) Titration of MgSO₄ with NaOH:

(Mg⁺² + SO₄^2–) +2(Na+ + OH^–) → Mg (OH)₂↓ + 2Na+ + SO₄^2–

• In reaction (a) the net result is the placement of CI– by NO3–. As both Cl– and NO3– have almost the same conductance so up to end point there is no change in conductance value.
• After the end-point, there is an increase in conductance due to the combined conductance of Ag+ and NO₃^– ions.

For reaction (b), the same explanation as above.

Type II: If both products of the reaction are sparingly soluble.

Example: (a) Titration of MgSO₄ with Ba(OH)₂
(Mg⁺² + SO₄^2–) + (Ba⁺² + 2OH^–) → Mg (OH)₂↓ + BaSO₄↓

Here conductance of solution decreases right from the commencement but increases after the end-point because of the free barium hydroxide.

(b) (1). Weak acid with strong base: CH3COOH with NaOH

CH₃COOH + Na+ + OH- CH₃COO- + Na+ + H₂O

• A weak acid (CH₃COOH) has lower conductance than its salt (CH₃COONa). Due to its lower dissociation w.r.t. salt. So, the addition of base increases conductance (region ab in the graph). 
• The reason for the initial decrease in conductance [region (oa)] is because small addition of base leads to the common anion, i.e., CH₃COO^- ion formation which represses the dissociation of CH₃COOH which gives highly conducting H⁺ ions.

• After the endpoint, there's a higher rate of increase in conductance because Na+ + OH- has higher conductance than Na+ and CH3COO- (region bc in the graph).

(2). Weak acid with weak base:

CH₃COOH + NH₄OH CH3COO- + NH₄+ + H₂O

• A weak acid (CH₃COOH) has lower conductance than its salt (CH₃COONa). Due to its lower dissociation w.r.t. salt. So, the addition of base increases conductance (region ab in the graph). 
• The reason for the initial decrease in conductance [region (oa)] is because small addition of base leads to a common anion, i.e., CH3COO- ion formation which represses the dissociation of CH3COOH which gives highly conducting H+ ions.

• A weak acid (CH3COOH) has lower conductance than its salt (CH3COONa). Due to its lower dissociation w.r.t. salt. So, the addition of base increases conductance (region ab in the graph). The reason for the initial decrease in conductance [region (oa)] is because small addition of base leads to a common anion, i.e., CH3COO- ion formation which represses the dissociation of CH3COOH which gives highly conducting H+ ions.

• The region b ‘c’ of the graph is due to the weak nature of the base. So, conductance remains almost constant with the addition of a weak base after the endpoint. 

Download Study Notes on Conductometric Titration - CLICK HERE←

Enrol for CSIR NET 2022 Crash Courses - Join Online Classroom Program

Get Access to Unlimited Test Series for CSIR NET 2022 Exam

Check Out:

• Complete Study Notes on Life Science - Download PDF

• Most Expected Questions for the Upcoming CSIR NET Life Science 2022

• Topic Wise Mind Maps for Life Science

BYJU'S Exam Prep

Download the BYJU’S Exam Prep App Now. 

The Most Comprehensive Exam Prep App.

#DreamStriveSucceed

App Link: https://bit.ly/3sxBCsm