CONDUCTOMETRIC TITRATION Aim:
To determine the strength of given HCl solution using a standard NaOH solution by performing a conductometric titration
Apparatus Required: 1. 2. 3. 4. 5. 6.
Conductometer CL-250 Burette (50 ml) Graduated Pippette (10 ml) Beaker (100 ml) Measuring Cylinder (100 mL) Beaker (250 ml)
Reagents Required: 1. Distilled Water (500 ml) 2. 1 NNaOH 3. HCl
Theory: The specific electrical conductivity and the electrical conductance are a measure of the ability of a solution, a metal or a gas – in brief all materials – to conduct an electrical current. In solutions, the current is carried by cations and anions whereas in metals it is carried by electrons. If a substance has a high electrical conductance G , the electrical or ohmic resistance R is low. The electrical conductance G is the reciprocal of the resistance R :
The unit of R is the Ohm and the unit of G is the Siemens. To measure the electrical conductance, a voltage is applied to the electrode pairs and the current that flows is measured. During this process, the cations migrate to the negative electrode, the anions to the positive electrode and the solution acts as an electrical conductor. A conductor is defined by its length and cross-section. The smaller the electrode gap l and the larger the electrode area A, the larger the measurable current at the same electrolyte concentration and same voltage. The electrical conductance G is given by the equation:
Where A is the electrode area, l the electrode gap, γ is the specific conductivity and ρ the specific resistance. γ and ρ are material constants with the units S/m and Wm. This equation also illustrates the relation between the specific conductivity γ and the conductance G . The quotient of the length and area is the cell constant
K
(resulting in the unit
-
m 1):
In this experiment, the conductivity of a solution is utilized as an indicator for determining the end-point of a strong acid-strong base titration. The base solution is standard while the concentration of acid is unknown. A fixed quantity of the solution of strong acid is taken in a beaker and its’ initial conductivity is recorded. Being a strong electrolyte, the conductivity value will be large. To this, if we start adding a strong base solution, we find that the conductivity falls slightly in the beginning. This is because the added strong electrolyte is consumed completely in the neutralization reaction, and hence the ionic concentration doesn’t appreciate much. On the other hand, dilution of the existing ions due to increase in volume causes the conductivity to decrease. However, as soon as the equivalence point is reached, the added ions of the strong base remain free in solution, and hence beyond this point, further addition of base leads to a sharp rise in the conductivity of the solution. To determine the end point, the observed conductivity of the solution is plotted against the volume added. Conductivity values follow two distinct linear trends before and after the equivalence point, as can be seen in the following schematic diagram:
Procedure: Preparation of Solutions: Preparation of 0.1N NaOH solution: Pipette out 10 ml of the given1 N NaOH solution and transfer it to a 100 mL measuring cylinder. Dilute it up to the mark with distilled water.
Performing the Titration: 1. Wash the burette with distilled water and mount it on the stand. 2. Fill the standard base solution (prepared earlier) into the burette. Open the stopper and allow the solution to flow back into the beaker to remove the air from the burette. Once a continuous solution column is achieved, close the knob and put the solution back into the burette. 3. Note the initial volume level in the burette. 4. Using a 100 mL measuring cylinder, take out 30 mL of the HCl solution into a 100 mL beaker. 5. Dip the conductivity cell into the beaker and turn the conductometer on. Measure the initial conductivity of the sodium carbonate solution. 6. From the burette, start adding the strong base solution in 1.0 mL increments. Note down the conductivity after each increment. 7. Continue with the previous step till about 45 mL and then empty the burette into a waste container.
8. Plot a graph between the observed conductivity value and the volume of acid added. Locate the end point as the intersection of the two lines (see figure in theory). 9. Calculate the strength using the data obtained.
Observation Table: Tabulate the volume added and the observed conductivity.
Calculations: Following is the relation applicable for calculating the Normality of HCl : N1V1 = N2V2 Here, N1= Concentration of base = 0.1N, V1= volume of base required for complete neutralization (read from graph), and V2= volume of acid taken = 30 mL. Once the normality of HCl is calculated, its strength in g/L can be calculated by multiplying the normality with the molecular weight: Strength (in g/L)= N 2 X 36.5
Result: Report the strength of HCl solution obtained as the final outcome.
Precautions: 1. In general, be very careful when handling expensive glassware. 2. Concentrated acids are very corrosive to human skin; exercise extreme caution while handling them. 3. Addition of acid to the base should be slow enough to keep the effervescence in control, otherwise it could cause spillage.
