PREPARATION OF STANDARD SODIUM HYDROXIDE SOLUTION

Principle:

            When an acid reacts with an alkali, salt and water are formed and the reaction is known as neutralization. An indicator indicates the end point by its characteristic colour change, such as colourless to pink when phenolphthalein is used as indicator.

               NaOH + H₂C₂O₄                                       NA₂C₂O₄ + H₂O

                       Colourless                           Pink (Phenolphthalein indicator)

Procedure:

1.      Weigh by difference into a 250 ml beaker accurately about 1.575 g. of oxalic acid using a weighing bottle.

2.      Add some distilled water into beaker and dissolve the oxalic acid with the help of glass rod.

3.      Transfer the solution into a clean 250 ml volumetric flask through a funnel.

4.      Wash the beaker 3 - 4 times with small amounts of distilled water and transfer the washings into the volumetric flask.

5.      Make up the volume with distilled water to the mark.

6.      Shake the solution thoroughly.

7.      Take a clean burette and rinse it with the given sodium hydroxide solution.

8.      Fill the burette with sodium hydroxide solution and fix it in a burette stand.

9.      Pipette out 25 ml of the prepared oxalic acid solution into a 250 ml Erlenmeyer flask.

10. Add one or two drops of phenolphthalein indicator and mix the contents of the flask.

11. Run down the alkali solution from the burette 1ml at a time and mix the contents of the flask.

12. Note the volume of the alkali required to change the contents of the flask into a permanent pale pink colour.

13. Repeat the titration by adding the alkali till the last ml of the alkali required to change the color to pink is reached: then add the last ml drop wise mixing the contents well and complete the titration as in the previous case the given sodium hydroxide solution by titrating against the standard oxalic acid.

14. Repeat the titration till at least two concordant readings are obtained.

15. Record the results in a tabular form and take concordant readings for calculation.

Observation:

Weight of weighing bottle with oxalic acid before transferring                      = a g.

Weight of weighing bottle with oxalic acid after transferring              = b g.

Weight of oxalic acid transferred                                                              = (a – b) g.

Burette reading	Titration I	Titration II	Titration III
Final reading
Initial reading
Volume delivered in ml.

Volume of oxalic acid solution taken                   -           V₁ ml (25ml)

Volume of alkali solution required                                   -           V₂ ml

Normality of standard acid solution                     -           N₁ (0.1N)

Normality of alkali                                                  -           N₂ (to be calculated)

Calculation:

Volume of acid solution X normality of acid solution

=  Volume of alkali solution X normality of alkali

V₁ X N₁ = V₂ X N₂

             V₁ X N₁

N₂ =                                          = Strength of NaOH solution

                 V₂

                                    1000 X 0.1

1000 ml of 0.1 N =                        =   ‘X’   ml of given solution.

                                           N₂

Conclusion:

Take X ml of NaOH solution and dilute to 1000 ml to obtain 1 litre of 0.1N solution.