PHYSICAL cHEMISTRY - I

Experiment 7

Aim of the Experiment

Determination of the concentration (strength) of a given sodium hydroxide solution by titrating it against a standard solution of oxalic acid.

Principle

In the titration of a strong acid with a strong base, the amount of acid and base becomes chemically equivalent at the end point and the chemical reaction is called neutralization reaction. Near the end point there is a sudden change in the pH of the solution. If after end point even a small amount of base/acid is added the solution would become slightly alkaline or acidic respectively. In the titration between oxalic acid (weak acid) and sodium hydroxide (strong base), following reaction takes place:

In this titration phenolphthalein (HPh) is used as an indicator. The concentration of unknown solution is calculated in g/L. Normality of the solution can be calculated by using the formula

N1 V1 = N2 V2

where N1, V1 are respectively normality and volume of acid used and N2 and V2 are normality and volume respectively of base used in the titration.

Materials required

Apparatus:

Burette (50 mL) ,Pipette (10 mL) ,Conical flask (100 mL), Burette stand ,Funnel ,White glazed tile, Measuring flask (100 mL)

Chemicals:

Oxalic acid ,Sodium hydroxide solution, Phenolphthalein indicator

Procedure

(A)Preparation of 0.1M Standard Solution of Oxalic Acid

If dihydrated from of oxalic acid is available then dissolve 6.3 g of the acid in one litre of distilled water to get 0.1 N oxalic acid solution.

(B) Titration of Sodium Hydroxide and Oxalic Acid Solution

(i) Clean the burette thoroughly, wash it with distilled water and finally rinse it with sodium hydroxide solution. (Always rinse the burette with the solution, which is to be taken in it). Clamp the burette vertically in a burette stand.

(ii) Fill sodium hydroxide solution into the burette through a funnel above the zero mark.

(iii) Remove the air gap, if any, from the nozzle of the burette by running the solution forcefully from the burette nozzle.

(iv) Remove the funnel before noting initial reading of the burette. Also while noting the reading, see that no drop of the liquid is hanging at the nozzle of the burette.

(v) Note the initial reading by keeping the eye exactly at the same level as the meniscus of the solution. (vi) Pipette out 10 mL of oxalic acid solution in a washed and dried conical flask. Always wash the pipette with water and rinse with the liquid to be measured before pipetting out the liquid.

(vii) Add 1-2 drops of phenolphthalein indicator to the conical flask. Place the flask over the glazed tile and Titrate the acid with sodium hydroxide solution till a very faint permanent pink colour is obtained. Add sodium hydroxide solution in small amounts initially and then dropwise.

(viii) Read the lower meniscus of the solution in the burette again and record it as final reading.

(ix) Repeat the procedure until three concordant readings are obtained. Record your readings as shown in Observation Table.

Observation table

Calculations

Normality of NaOH solution can be calculated by using the equation:

Oxalic acid Sodium hydroxide

N1V1 = N2V2 where, N1 and V1 are the normality and volume of the oxalic acid solution. N2 and V2 are the normality and volume of the sodium hydroxide solution.

Calculate the concentration of sodium hydroxide solution in g/L by using the equation given below. Concentration (strength) in g/L =Normality ×Equivalent wt.

Result

Concentration of NaOH solution is _____________ g/L.

Precautions

(a) Always rinse the burette with the solution, which is to be taken in it.

(b) Remove the air gap if any, from the burette before titrating the solution. Make sure that the nozzle of burette is also filled.

(c) Never forget to remove the funnel from the burette before noting the readings of the burette and ensure that no drop is hanging from the nozzle of the burette.

(d) Always read the lower meniscus for all transparent solutions and upper meniscus for coloured solutions.

(e) Do not blow out the last drop of the solution from the jet end of the pipette into the flask.

Reference Material

1. Practical Physical Chemistry Google EBOOK Link.

Questions

1.What is an indicator? Which indicator is used in the titration of oxalic acid vs sodium hydroxide? Can the titration be performed by using some other indicator?

2.What is the primary standard solution?