Titration Of Sodium Carbonate With Hydrochloric Acid

You can use the technique of titration to determine the concentration of a sodium carbonate solution using a solution with a known concentration of hydrochloric acid, or vice versa. HCl gradually reduces the alkalinity of the solution until the pH is 7. Because the reaction between sodium carbonate and hydrochloric acid proceeds in two stages, you can use more than one indicator. Phenolphthalein is suitable for the first stage, and methyl orange is best for the second.

When you add a hydrochloric acid (HCl) solution to a solution of sodium carbonate (Na₂CO₃), the hydrogen ion in HCl switches places with one of the sodium ions in Na₂CO₃ to produce sodium hydrogencarbonate, also known as sodium bicarbonate (baking soda), and sodium chloride (salt).

Na₂CO₃(aq) + HCl(aq) → NaHCO₃(aq) + NaCl(aq)

Sodium hydrogencarbonate is basic, and it reacts with the HCl still in solution to produce sodium chloride, carbon dioxide and water.

NaHCO₃(aq) + HCl(aq) → NaCl(aq) + CO₂(g) + H₂O(l)

Phenolphthalein is a good indicator for the first reaction because it responds to the pH change caused by the formation of sodium hydrogencarbonate. It is pink in basic solutions and turns colorless as soon as the solution becomes acidic. Methyl orange, on the other hand, responds to pH changes associated with the formation of NaCl, changing from yellow to red as the solution becomes more acidic. At neutrality, it is a distinct orange color.

Titrations generally call for accurately graduated beakers and pipettes for transferring solution from one beaker to another. Wear goggles and gloves for protection from corrosive chemicals.

1. Prepare the Solutions

Measure out a suitable amount of a sodium carbonate solution of unknown concentration and a hydrochloric acid solution of known concentration in separate graduated beakers.

2. Add Phenolphthalein Indicator

Put a few drops of phenolphthalein in the sodium carbonate solution. The indicator will turn pink.

3. Transfer the Titrant

Carefully add HCl to sodium carbonate solution until the solution becomes colorless. Record the volume of HCl solution you added.

4. Calculate Concentration

Calculate the number of moles of HCl in the original solution and derive from this the number of moles of Na₂CO₃ in the target solution, keeping in mind that 1 mole of HCl reacts with 1 mole of Na₂CO₃. Determine the concentration of the Na₂CO₃ solution using a volumetric analysis.

5. Repeat the Procedure Using Methyl Orange

In this part of the titration, HCl is reacting with NaHCO₃, but the proportion is still one mole to one mole. After molarity calculations and a volumetric analysis, the results should be identical to those using phenolphthalein.