By Kevin Beck
Updated Aug 30, 2022
In laboratories worldwide and within the human body, chemical reactions consistently obey precise molar ratios—discrete numbers of atoms or molecules—rather than arbitrary mass proportions. While the mass ratios can be derived from the known molecular weights of the reactants, the underlying stoichiometry remains fundamentally about countable particles.
Consider the neutralization of hydrochloric acid by sodium hydroxide: HCl + NaOH → NaCl + H₂O. The balanced equation tells us that one mole of acid reacts with one mole of base to produce one mole of salt and one mole of water, despite the differing masses of each species.
Titration is a quantitative analytical technique that determines the concentration of an unknown solution by reacting it with a second solution of known concentration (the titrant). By adding the titrant incrementally until the reaction reaches its equivalence point, the stoichiometric relationship allows calculation of the unknown concentration.
Accurate titration hinges on the titrant’s concentration being known with high precision. Any uncertainty in the reference solution directly propagates into the calculated concentration of the analyte.
A primary standard solution is prepared from a primary standard substance—a pure, stable compound whose purity and stoichiometry are well defined. This solution serves as the benchmark for calibrating other solutions.
Primary standard substances must satisfy four critical criteria:
1. Purity – Impurities distort the calculated molarity. A purity of ≥99.9 % (999 parts per 1,000) is acceptable. Sodium carbonate (Na₂CO₃) commonly meets this requirement.
2. Availability and Cost – The substance should be readily obtainable and inexpensive. While NaOH is widely available, it is hygroscopic and prone to absorbing atmospheric moisture, complicating accurate weighing.
3. Well‑Defined Formula – The compound must have a single, known chemical formula. Dissolving nitric acid (HNO₃) in water can produce nitrous acid (HNO₂) as a contaminant, thereby compromising the reaction stoichiometry.
4. Stability During Weighing – The substance must retain its mass and purity when handled. Physical contact during weighing should not alter its composition or introduce contaminants.
These attributes collectively ensure that a primary standard substance can reliably serve as the foundation for precise analytical measurements.
