Sodium hydroxide (\(\text{NaOH}\)), known as lye or caustic soda, is a strong base used frequently in chemical processes, particularly acid-base titrations. Preparing an \(\text{NaOH}\) solution requires creating a reagent with a precisely known concentration, a process called standardization. \(\text{NaOH}\) is categorized as a secondary standard, meaning its exact concentration cannot be determined solely by weighing the solid and dissolving it in water. This is because the solid is chemically unstable, requiring an extra analytical step to confirm its true molarity before reliable use.
Inherent Instability of Solid Sodium Hydroxide
The primary issue with solid sodium hydroxide is its highly hygroscopic nature, meaning it rapidly absorbs moisture from the surrounding air. When weighing a specific mass of \(\text{NaOH}\) pellets, a significant portion of that measured mass is actually absorbed water. This water is incorporated into the solid structure (deliquescence), meaning the mass of pure \(\text{NaOH}\) is less than the measured value.
This absorbed water immediately flaws the initial calculation of the solution’s molarity, as the mass of the solute is overestimated. Furthermore, commercial \(\text{NaOH}\) is not 100% pure, often containing trace impurities like chlorides or other sodium salts. Since the calculated concentration assumes a pure, dry solid, the resulting solution’s actual concentration is inherently inaccurate.
Reaction with Atmospheric Carbon Dioxide
Beyond absorbing water, both solid and aqueous sodium hydroxide readily react with carbon dioxide (\(\text{CO}_2\)) present in the atmosphere. Dissolved \(\text{CO}_2\) acts as a weak acid, reacting with the strong base \(\text{NaOH}\). This chemical transformation converts the active ingredient, sodium hydroxide, into sodium carbonate (\(\text{Na}_2\text{CO}_3\)).
The reaction is \(2\text{NaOH} + \text{CO}_2 \rightarrow \text{Na}_2\text{CO}_3 + \text{H}_2\text{O}\). Sodium carbonate is a much weaker base than sodium hydroxide. This conversion continuously reduces the effective concentration of the strong base over time, especially if the solution is not sealed. The presence of sodium carbonate also interferes with titration accuracy by altering the mole ratio and shifting the expected endpoint.
Standardizing the Solution Using a Primary Standard
To overcome these physical and chemical instabilities, the \(\text{NaOH}\) solution must be standardized against a primary standard. A primary standard possesses high purity, chemical stability, a known molecular weight, and non-hygroscopic properties. These characteristics ensure that the weighed mass is an accurate representation of the moles of substance present.
The compound most commonly selected is Potassium Hydrogen Phthalate, or KHP (\(\text{KHC}_8\text{H}_4\text{O}_4\)). KHP is a stable, non-hygroscopic, solid acid that can be weighed accurately. The standardization process involves precisely weighing a KHP sample and titrating it with the prepared \(\text{NaOH}\) solution of unknown concentration.
The reaction between \(\text{KHP}\) and \(\text{NaOH}\) is a simple one-to-one mole ratio. By accurately measuring the mass of \(\text{KHP}\), the exact number of moles of acid is known. The volume of \(\text{NaOH}\) solution required to neutralize those moles is precisely recorded. Using this data, the true molarity of the sodium hydroxide solution can be calculated with high precision, converting the \(\text{NaOH}\) from an unstable secondary standard into a reliable reagent for accurate quantitative analysis.