Determination of the Amounts of Sodium Hydroxide Solution and Sodium Carbonate in a Mixed Solution

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Experiment 2

Determination of the Amounts of Sodium Hydroxide Solution and Sodium Carbonate in a Mixed Solution

Date of Experiment: 21/9/2005

Objective: To determine the percentage by mass of Sodium Hydroxide and Sodium Carbonate in a mixed solution.

Introduction

Sodium carbonate and sodium hydroxide are both alkaline in nature. When hydrochloric acid is added to the mixture, sodium hydroxide reacts like this:

NaOH + HCl -----> NaCl + H2O ....................... (1)

Sodium carbonate reacts with hydrochloric acid in a similar way but the reaction comes in 2 steps:

Na2CO3 + HCl ----------->NaHCO3 + NaCl............. (2)

NaHCO3 + HCl ----------->NaCl + H2O + CO2.......... (3)

The pH conditions in which reactions (1) and (2) take place are approximately the same: they both occur at a relatively higher pH (at a pH value higher than 7). Reaction (3) is quite different from the other 2 reactions. It takes place at a lower pH (at a pH value lower than 7).

So, using the 2 acid-base indicators, the end points for the reactions can be found. Phenolphthalein turns from pink to colorless as the pH value changes from 10 to 8. Methyl orange turns from yellow to orange when the pH value drops from about 4 to 3.

After adding phenolphthalein and then titrating the solution, the end point for reactions (1) and (2) can be found. If we further add methyl orange and titrate the mixture afterwards, the end point for reaction (3) is also found. By considering the amount of hydrochloric acid used for both titrations, the percentage by mass of sodium carbonate and sodium hydroxide can be found.

This is just one of the methods to determine the percentage by mass of sodium carbonate and sodium hydroxide. We can make use of barium chloride in this case. This is because when barium ions and carbonate ions come in contact, the following reaction occurs.

Ba2+ (aq) + CO32- (aq) ------------->BaCO3 (s)

How can we utilize this special point of barium ions? We can use this to precipitate out the carbonate ions. Then what would be left in the mixture would be sodium hydroxide only. Then, using phenolphthalein as an indicator, we could find out the number of moles of sodium hydroxide in the solution.

Then one may ask,' How do I find out the percentage by mass of sodium carbonate using the second method?' We can do so by titrating the pure solution (no barium chloride added) using methyl orange indicator. The end point indicates the amount of hydrochloric acid used to neutralize both sodium carbonate and sodium hydroxide. The total number of moles of both sodium carbonate and sodium hydroxide can be found. By this way, we can find the percentage by mass of the both substances.
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Procedure

. All the apparatus involved were washed beforehand.

Method 1

2. With a pipette, 25cm3 of the solution mixture was transferred into a conical flask.

3. Appropriate amount of hydrochloric acid of was poured into a burette, and the initial reading was taken.

4. Using phenolphthalein as an indicator, the solution in the conical flask was titrated by the hydrochloric acid. The titration stopped when the color of the indicator changes from pink to colorless. The reading was taken.

5. Methyl orange was further added to the solution. Titrate ...

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