Conductometry

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Conductometry

INTRODUCTION

Conductometry is the measurement of the electrical conductivity of a solution. The conductance is defined as the current flow through the conductor. In other words, it is defined as the reciprocal of the resistance. The unit for the conductance is Seimens (S) which is the reciprocal of Ohm's (Ω−1). This method is mainly used for the determination of the physico-chemical properties of the compounds.

PRINCIPLE

The main principle involved in this method is that the movement of the ions creates the electrical conductivity. The movement of the ions is mainly depended on the concentration of the ions.

A+B− + C+D− AD + C+B−

where A+B− is the solution of strong electrolyte; C+D− is the solution of the reagent.

Here the ionic concentration of A+ is determined by reacting the electrolyte solution with the reagent solution so that the A+ ions are replaced by the C+ ions. This replacement of the ions with the other ions shows the conductance increase or decrease. This is done mainly by the replacement of the hydrogen ion with other cation.

Ions movement

THEORY

The theory is mainly based on Ohm's law which states that the current (I) is directly proportional to the electromotive force (E) and inversely proportional to the resistance (R) of the conductor:

I = E/R

The conductance is defined as the reciprocal of the resistance. The resistance is expressed by the following equation:

R = ρl/a

where ρ is the resistivity; l is the length; a is the cross-sectional area of the homogenous material.

Therefore,

C = 1/R

 = k/la

where K is the conductivity; l is the length; a is the cross-sectional area of the homogenous material.

Then the molar conductivity is defined as the conductivity due to 1 mole and it is expressed by the following formula:

Λ = 1,000k/C

where K is the conductivity; C is the concentration of the solution in mol/l.

The sample solution is placed on the cell which is composed of platinum electrodes. These are calibrated with the help of known conductivity of the solution, for example, standard potassium chloride solution.

Cell constant is defined as the conductivity of the cell:

R = ρl/a

where ρ is the resistivity; l is the length; a is the cross-sectional area of the homogenous material.

Cell constant = specific conductivity/observed conductivity

Then the cell constant is determined by the substitution of the value of the specific conductivity of N/50 KCl solution at 25 °C. The value is 0.002765 mhos which is given by the Kohlrausch.

Cell constant = 0.002765/observed conductivity

Methods for the Conductance Measurements

The conductance of the sample solution is measured by the resistance measurement by the Wheatstone bridge.

The following are the different bridges used for the measurement of the conductance:

• Kohlrausch bridge: It consists of a meter bridge wire AB with a fixed resistance R on both the ends. To increase the length of the wire, it is connected to the resistance box R*, conductance cell C and the head phone D and a small induction coil I. All these are operated by the battery.
• Headphone is used for the detection of the conductance difference.
• 
• Kohlrausch bridge
• Conductivity cell: These conductivity cells are made up of glass. These are commonly employed by dipping in the analyte solution. It is composed of pair of electrodes placed at a constant distance. There are mainly three types of cells used as conductivity cells:

1. Type A: This consists of the electrodes placed at a large distance and is used for the measurement of the high conductance.
3. 
5. Conductivity cell
7. Type B: In this type, the cell is dipped in the sample solution to measure the conductance in the titrations.
9. 
11. Conductivity cell
13. Type C: In this type, large electrodes are placed with small distance. This type cell is mainly used for the measurement of the low conductance. They are made up of glass fitted with the platinum electrodes.
15. 
17. Conductivity cell

APPARATUS

The conductometric apparatus is composed of the following. The electrodes are made up of platinum sheets. These electrodes are fixed in a constant distance and are sealed in the connected tubes. To avoid the polarisation effect, these electrodes are coated with the platinum black. This is done by the 2–3% of the chloroplatinic acid solution. Then 0.02–0.03 g of lead acetate solution is taken into the cell. On passing the current, chloroplatinic acid under goes electrolysis and the electrodes are blackened. Then these electrodes are repeatedly washed with the distilled water and finally with the conductivity water. The conductivity water is the water obtained by treating the distilled water with small amount of sodium hydroxide and potassium permanganate. Here the induction coil is used for inducing current.

METHOD

The sample solution is placed in the conductivity cell at constant temperature. The temperature is maintained constant with the help of the thermostat. Then the cell is connected to the resistor box R and the alternating current is passed through the cell with the help of induction coil. Then the conductivity of the solution is measured by the following equation:

Conductivity of the solution = 1/resistance of the solution

Conductometer

FACTORS AFFECTING THE CONDUCTIVITY MEASUREMENTS

• Temperature: The conductivity of the electrolyte increases with the temperature increase. This is because of the ions mobility by increasing the temperature.
• Concentration of the sample solution: The concentration of the solution is inversely proportional to the conductivity of the sample solution. The conductance is decreased with the increase in the concentration. Hence diluted solutions are used for the conductivity measurements.
• Number ions present in the sample solution: This is mainly based on the dissociation of the compounds into ions. That is mainly of the number of ions present in the solution. The number of ions present in the solution is directly proportional to the conductance. Strong electrolytes completely dissociate into ions and have high conductance.
• Charge of the ions: Negative charge of the ions increases the conductivity where as the positively charged ions decreases the conductivity.
• Size of the ions: The conductivity is inversely proportional to the size of the ions. That is the increase in the size of the ions increases the conductivity.

Types of the Conductometric Titrations

1. Acid–base titrations: In this method, the conductance of the hydrogen ions and hydroxyl ions are compared with the conductivity of the sample solution.
• Strong acid with a strong base:
• For example, take the titration of the HCl with NaOH.
• [H+Cl−] + [Na+OH−] [Na+Cl−] + H2O
• The initial conductivity of the HCl solution is high because of the protons from the dissociation of the acid. Then titrating with NaOH dissociates into Na+ and OH−. This hydroxyl ion reacts with the H+ ions to form the water. This shows the decrease in the conductivity. After completion of the reaction, the excess addition of the NaOH shows the increase in the conductivity. The plot between the conductivity and the volume of the titrant shows the V-shaped curve.
• 
• Strong acid with strong base curve
• Strong acid with weak base:
• For example, titrations of the strong acid such as HCL with the weak base such as the ammonium hydroxide.
• HCl + NH4OH NH4Cl + H2O
• Same as the titration of the strong acid with strong base, it initially shows the increase in the conductivity because of the H+ ions. This conductivity is decreased by the addition of the weak base that is with the NH4OH that neutralises the H+ ions with the OH− ions and decreases the conductivity. The excess addition of the NH4OH does not show the change in the conductivity. Then the plot between the conductivity and the volume of the titrant shows the plateau.
• 
• Strong acid with weak base titration curve
• Weak acid with a strong base:
• The weak acid such as acetic acid is titrated with the strong base such as sodium hydroxide.
• CH3COOH + NaOH CH3COONa + H2O
• The acetic acid dissociates to produce the H+ ions which shows the high conductivity and is titrated with the sodium hydroxide which is dissociated to produce the OH− ions which shows slight increase in the conductivity by the formation of the CH3COONa at the equivalence point. Then it shows the gradual increase in the conductivity by the addition of excess titrant. Then plot the graph between the conductivity and the volume of the titrant which shows the plateau.
• 
• Weak acid with strong base titration curve
• Weak acid with weak base:
• The weak acid such as the acetic acid is titrated with the weak base such as ammonium hydroxide.
• CH3COOH + NH4OH CH3COONH4 + H2O
• The acetic acid is dissociated and it combines with the ammonium ion after dissociation of the ammonium hydroxide. This forms the ammonium acetate salt which shows the increase in the conductivity. After attaining the equivalence point, the addition of the titrant does not shows the conductivity change. The plot between the conductivity and the volume of the titrant shows the plateau.
• 
• Weak acid with weak base titration curve
2. Precipitation titrations:
3. When compared to the acid–base titrations, precipitate titrations are not that much accurate because of the more number of the interferences. These are also known as the replacement titrations. The precipitate formation is taken as the end point when the conductivity is measured.
4. Example: Potassium chloride is titrated with the precipitating agent such as the silver nitrate solution.
6. KCl + AgNO3 AgCl + KNO3
8. Initially the addition of the silver nitrate to the potassium chloride shows the stability in the conductivity and the excess of the silver nitrate addition increases the conductivity because of the formation of the single precipitate.
10. 
12. Precipitation titration curve
14. In another case, the titration will form two precipitates. For example, magnesium sulphate is titrated with the barium hydroxide and forms two precipitates: the magnesium hydroxide and the barium sulphate. Initially the plot shows the decrease in the conductivity and then shows the increase in the conductivity.
16. MgSO4+ Ba(OH)2 Mg(OH)2 + BaSO4
18. 
20. Precipitation titration curve
22. Redox titrations:
23. In this method, the decrease in the hydrogen ions concentration shows the decrease in the conductivity at the end point.
24. Example: The titration of the ferrous ions with the dichromate ions.
26. 6Fe+2 + Cr2O7 + 14H+ 6Fe+3 + 2Cr+3 + 7H2O
28. The hydrogen ions show sharp decrease in the conductivity. After the equivalence point, the addition of the excess of the titrant shows the stability in the conductivity.
30. 
32. Redox titration curve
34. Complexometric titrations:
35. This was first proposed by Werner. In this method, the two end points are observed. First is the formation of the metal ion and the second one is the formation of the complex.
36. Example: The titration of the potassium chloride with the mercuric chlorate.
38. Hg(ClO4)2 + 4KCl HgCl4−2 + 2K+ + 2KClO4
39. HgCl4−2 + 2K+ K2HgCl4
41. Initial end point is shown by the formation of the HgCl4−2 and the other end point is shown by the formation of the K2HgCl4.
43. 
45. Complexometric titration curve
47. Non-aqueous titrations:
48. These are carried out as same as the acid–base titrations by using non-aqueous solvents.
49. Example: The titration of weak acids in the methanol by using tetra methyl ammonium hydroxide.

ADVANTAGES

• Appropriate for the dilute solutions.
• Broad selectivity.
• End point is determined by plotting the graph.
• No need for the specific conductivity.
• No need of indicator.

DISADVANTAGES

• Less accurate when compared to other methods: Because the high concentrations are not measured by the conductometric titrations. The solutions are compulsory diluted for the measurements.
• Less satisfactory when compared to other methods.

APPLICATIONS

• Used in the determination of the basicity of the acids.
• The basicity is defined as the number of carboxylic acid groups attached to the molecules.
• Used in the determination of the sparingly soluble salts such as barium sulphate and lead sulphate.
• Used in the determination of the purity of the water.
• Used in the determination of the salinity of the sea water.
• Used in the determination of the ionic product of the water.
• Used in the quantitative analysis of the compounds.

REVIEW QUESTIONS

1. What is the principle involved in the conductometry?
2. What is the theory of conductometry?
3. Explain the concept of molar conductivity.
4. Explain the concept of cell constant.
5. What are the different methods of conductivity measurements?
6. What are the factors affecting the conductivity measurements?
7. What are the different types of conductometric titrations?
8. What are the different types of applications of conductometry?