Sodium carbonate is a basic compound, meaning that it generates hydroxide ions (OH⁻) when dissolved in water. Hydrochloric acid is acidic, meaning that it releases protons (H⁺) when dissolved in water. When combined, aqueous solutions of sodium carbonate and hydrochloric acid generate an acid-base reaction. Chemists refer to this process as "neutralisation" and exploit it to determine the amount of acid or base in a variety of samples.
Sodium carbonate (also known as soda ash or washing soda) is a water-soluble ionic compound represented by the formula Na₂CO₃. Chemists classify it as ionic because it contains positive metal ions (sodium ion, Na⁺) and a polyatomic negative ion (carbonate ion, CO₃²⁻). In water, it separates into its respective ions in a process known as dissociation. The carbonate ion is responsible for the "basic" behaviour of sodium carbonate because it generates hydroxide ions by extracting a proton from two water (H₂O) molecules:
CO₃²⁻ + 2 H₂O ' H₂CO₃ + 2 OH⁻
Hydrochloric acid (also known as muriatic acid) is a strong acid with chemical formula HCl. The designation of "strong" acid results from HCl completely dissociating in water into protons (H⁺, the species responsible for acidic behaviour) and chloride ions (Cl⁻).
When acids and bases are combined, they produce a salt (an ionic compound) and water. In the case of sodium carbonate and hydrochloric acid, the salt produced is sodium chloride, and the water results from the decomposition of carbonic acid (H₂CO₃). This can be represented by a two-step process, the first being represented by
2 HCl + Na₂CO₃ ' 2 NaCl + H₂CO₃
The second step is the decomposition of the carbonic acid into water and carbon dioxide:
H₂CO₃ ' H₂O + CO₂
The overall reaction can therefore be represented by
2 HCl + Na₂CO₃ ' 2 NaCl + H₂O + CO₂
"Titration" refers to an analytical technique in which the concentration (the amount of substance per millilitre of solution) of a substance is determined. Generally, this involves a chemical reaction in which the titrant (i.e., a solution whose concentration is known exactly) is placed in a slender glass cylinder called a "burette," which is capable of delivering volumes of liquids with great precision. The analyte (the substance being analysed) is usually contained in a flask or beaker beneath the burette. The titrant is then added to the analyte until the reaction is complete. Determination of when the reaction is complete normally requires an indicator top be added to the analyte; the indicator is a chemical that changes colour when a slight amount of unreacted titrant is present in the flask.
The amount of sodium carbonate is a sample that can be determined by titration with hydrochloric acid using bromocresol green as an indicator. The bromocresol green transitions from blue to green when the reaction flask contains a slight excess of hydrochloric acid. A variation of this technique is used to determine the amount of carbonate ion in water samples from rivers, lakes, streams, wells, municipal water supplies and swimming pools.