Electrolytic conductivity is a measure of the ability of a solution to carry electric current. Electric solutions conduct electric current by the migration of ions under the influence of electric field. According to Ohm’s Law, the current strength flowing through a conductor is directly proportional to the potential difference (E) and inversely proportional to the resistance (ρ) of the conductor.
i.e. I = E/R or, R = E/I
Where the resistance (R) is the hindrance provided by the solution. The resistance of any conductor varies directly with the length and inversely with its area of cross-section.
R = ρ x (l/a)
Where, ρ is the specific resistance and it is the resistance of a unit length of conductor of unit cross-section. l/a is called cell constant.
The reciprocal of specific resistance is called specific conductance or conductivity. Specific conductance (k) = 1/ ρ = (l/a) x (1/R)
Conductance of electrolyte depends upon i) number of free ions, ii) changes on the free ions and iii) mobility of the ions on the substitution of one ion by another of different mobility (speed of ions). So, conductometric method can be used to determine the end point of ionic titrations like i) acidimetric titration, ii) precipitation titration, iii) titration involving the formation of complex ion.
When hydrochloric acid solution (HCl) is titrated with sodium hydroxide solution (NaOH), the highly mobile hydrogen ions (l°H+ = 350 ohm–1 cm–1) are progressively replaced by slower moving sodium ions (l°Na+ = 50 ohm–1 cm–1) and the conductance of the solution decreases. After the end point, the conductance of the solution rises sharply due to the presence of excess, highly mobile hydroxide ion (l°OH– = 198 ohm–1 cm–1). Thus the neutralization of a strong acid by addition of a strong base leads to a minimum conductance at the end points. This is due to the disappearance of H+ ions and their replacement by slower moving Na+ ions of the base followed by the presence of highly mobile OH– ions after the end point.Therefore the nature of the plot (conductance of the solution versus volume of base added) will be as given below:
Chemicals: HCl solution, Standard NaOH solution, Distilled water.
Apparatus: Burette, Pipette, Beaker, Digital conductivity meter & Conductivity cell.
- Rinse the conductivity cell a number of times with conductivity water or double distilled water.
- Pipette out 25 mL of HCl in a beaker and dip the conductivity cell in it, so that the cell should dip completely in solution.
- Note the temperature of the sample solution and accordingly set the temperature control or keep the cell in a thermostat at room temperature.
- Add small amount of NaOH solution (10 drops) from burette, stir it and measure the conductance after each addition.
- Take at least five readings beyond the end point.
Observation: Record the room tempeture = …….°C
Table 1:- Conductometric Titration
|No. of drops of NaOH added||Total no. of drops of NaOH added||Conductance|
Plot a graph between conductance and volume of titrant (NaOH solution). Two intersecting lines will be obtained (as given in the Figure 1) and the points of intersection of these lines represent the equivalent point.
Let, V2 be the volume of NaOH at the equivalent point (from graph) and the strength of acid is S1 and strength of NaOH solution is S2 = 0.1(N).
Then, 20 x S1 = V2 x S 2
S1 = (V2 x S 2/ 20) (N)
Conclusion: The strength of the acid is ______ (N)
- Electrical connection should be made carefully.
- Temperature during the experiment should be kept constant as conductance depends on temperature.
- Stirring should be done after each addition of titrant.
- To avoid the dilution effect, the concentration of the titrant should be 5-10 times more than that of the solution to be titrated.