ESTIMATION OF SODIUM HYDROXIDE AND SODIUM CARBONATE IN THE GIVEN MIXTURE OF A SOLUTION (DETERGENT)

Principle:

The detergent used in Dairy contains a mixture of sodium carbonate (1.5%) and sodium hydroxide (0.5%). It is necessary to check carbonate and caustic alkalinity because NaOH is converted into Na₂CO₃ when it is exposed to atmosphere due to absorption of CO₂ from the atmosphere. The reaction of Na₂CO₃ with acid is a two-step process

 

I

            2 NaOH + H₂SO₄                       Na₂SO₄ + 2H₂O                                  

            2 Na₂CO₃ + H₂SO₄                    2 NaHCO₃ + Na₂SO₄                  

            2 NaHCO₃ + H₂SO₄                   Na₂ SO₄ + 2 CO₂                        II

When the solution containing both Na₂CO₃ and NaOH are titrated against standard acid using phenolphthalein as indicator entire NaOH and ½ of Na₂CO₃ (step – I) will be neutralized at the end point. Remaining ½ of Na₂CO₃ (step – II) can be titrated using methyl orange as indicator.

Procedure:

1.      Rinse a clean burette with the given N/10 sulphuric acid solution.

2.      Fill the burette with the sulphuric acid solution and fix it to the stand.

3.      Pipette out 25 ml of the mixture solution to a Erlenmeyer flask.

4.      Add 1 – 2 drops of phenolphthalein indicator and mix the contents of the flask.

5.      Run down the sulphuric acid solution from the burette and mix the contents of the flask.

6.      Note the volume of the acid required to change the pink colour to colourless.

7.      Now add a drop of methyl orange indicator and continue titration till the yellow colour just change to a light orange colour.

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

9.      Record the result in a tabular form.

Observation:

Burette reading	Titration I	Titration II	Titration III
With phenolphthalein indicator
Final reading
Initial reading
Volume delivered in ml.
With methyl orange indicator
Final reading
Initial reading
Volume delivered (ml)

Calculation:

For sodium carbonate

Normality of acid                             =          0.1 N

Volume of acid of second titration           =          V ml

Volume of acid required to neutralize all Na₂CO₃ in the given mixture = 2 X V ml.

                                                            2 X V X 0.1                         

Strength of sodium carbonate =     

                                                                     25

Weight per litre of sodium carbonate =  Strength  X  Equivalent weight

Strength  X   53.0  g / l or Percent of Na₂CO₃ =  Strength  X  5.3

For sodium hydroxide

Normality of acid = 0.1 N

Volume of acid required for first titration  – volume for second titration (V ml) = x ml

                                                x X 0.1

Strength of NaOH =  

                                                   25

Weight per litre NaOH = strength X equivalent weight = Strength  X   40

Percent of NaOH = Strength X 4.0.