Analysis of commercial caustic soda

1. SHREE MALLIKARJUN COLLEGE CLASS: TYBSC
SAFETY: Refer to MSDS of hydrochloric acid, sodium bicarbonate, sodium carbonate,
phenolphthalein and methyl orange
-Dr. Mithil S. Fal Desai
Aim: Determination of alkalinity of a given mixture of OH- and CO3-2
using phenolphthalein and methyl
orange indicator.
Chemicals: Hydrochloric acid, sodium carbonate, sodium hydroxide, methyl orange and phenolphthalein.
Apparatus: Standard volumetric flask, pipette, burette, conical flask, beakers, funnel and glass rod.
Theory: The aqueous solution of sodium carbonate (Na2CO3) and sodium hydroxide (NaOH) is basic. The
use of pH-sensitive indicators such as phenolphthalein [colourless pH(8.0-9.6) pink] and methyl orange
[red pH(3.1- 4.4) yellow] can differentiate these chemical species by neutralization with dilute mineral
acids. The product of acid and base titration is salt which usually has neutral pH. In an aqueous solution
Na2CO3 and NaOH exist as a carbonate (CO3
2-
(aq)) and hydroxide (OH-
(aq)) molecular ion. When an acid is
added to a solution of CO3, the bicarbonate ions are initially formed.
CO3
2-
(aq) + H+
(aq) → HCO3
-
(aq) (Reaction-1) phenolphthalein and methyl orange
HCO3
-
(aq) + H+
(aq) → H2CO3 (aq) → H2O (l) + CO2 (g) (Reaction-2) methyl orange
OH-
(aq) + H+
(aq) → H2O (l) (Reaction-3) phenolphthalein and methyl orange
The completion of Reaction-1 and 3 can be analyzed by methyl orange and phenolphthalein indicator.
However complete neutralization of bicarbonate (Reaction-2) cannot be studied using phenolphthalein
indicator as the solution will be colourless below pH 8. To determine the amount of carbonate and hydroxide
in a mixture, two titrations are performed using phenolphthalein and methyl orange. The former titration
endpoint (V1) corresponds to the neutralization of hydroxide and partial neutralisation of carbonate. The
titration endpoint (V2) with methyl orange corresponds to the complete neutralization of carbonate and
hydroxide (total alkali).
Then ‘V2-2(V2-V1)’ mL of acid corresponds to hydroxide and 2(V2-V1) corresponds to neutralisation of
carbonate.
Procedure: I) Standardisation of 0.1N (appx.) HCl (Reference-2)
II) Estimation of bicarbonate and hydroxide in the mixture.
1) Pipette out 10 mL of the mixture in a conical flask add 2 drops of methyl orange indicator.
2) Titrate against standardized 0.1N HCl (appx.) solution and repeat the titration for constant
readings.
3) Similarly, titrate the 10 mL of mixture against standardized 0.1N HCl using phenolphthalein
indicator
4) Repeat the titration until a constant reading is observed.
Further reading (available in the library)
1) Experiments in Applied Chemistry, S. Rattan, S. K Kataria and Sons.
2) Vogel’s textbook of quantitative chemical analysis, J. Mendham, R. C. Denney, J. D. Barns, M. Thomas,
B. Sivasankar, Pearson India Education, 6th
Edition.

2. SHREE MALLIKARJUN COLLEGE CLASS: TYBSC
SAFETY: Refer to MSDS of hydrochloric acid, sodium bicarbonate, sodium carbonate,
phenolphthalein and methyl orange
-Dr. Mithil S. Fal Desai
Questions
1) Which amongst the following is/are the primary standards? Why?
KH(C8H4O4), Na2B4O7, NaHCO3
2) What is the equivalent weight of Na2CO3 and NaOH?
3) Why an indicator is required in the volumetric determination of acid or bases?
4) Derive the value of x and y in equations 1 and 2 in terms of the burette reading V1 and V2.
5) Calculate the total alkalinity of the solution in terms of CaCO3.
Results: In the mixture,
1) Amount of sodium carbonate is ___g/L
2) Amount sodium hydroxide is ___ g/L.