In chemistry, accurate analysis depends on precise measurement, which requires the use of materials known as standards. These standards are substances of high purity and known concentration that serve as the baseline for all quantitative work. They are the reference materials used to calibrate instruments and determine the unknown concentrations of other solutions in the laboratory.
Defining the Secondary Standard
A secondary standard is a chemical solution whose exact concentration is determined by comparing it against a more fundamental reference material, known as a primary standard. The secondary standard’s concentration is derived from this comparison. Secondary standards are typically used when the compound needed for analysis does not meet the stringent purity or stability requirements of a primary standard. Common laboratory reagents like sodium hydroxide (NaOH) or hydrochloric acid (HCl) are generally prepared as secondary standards.
The concentration of a secondary standard solution is established through calibration, transferring the accuracy of the primary standard to the working solution. This concentration is determined not solely from the mass of the solute used, but from a precise chemical reaction. The resulting solution provides a known concentration that can be used routinely for various analytical tests. This preparation method allows relatively unstable or impure substances to be utilized as reliable measurement tools.
Key Differences from a Primary Standard
The core distinction between a secondary standard and a primary standard lies in their inherent chemical properties and preparation requirements. A primary standard must possess extremely high purity (typically above 99.98%) and be highly stable, resisting decomposition or reaction with air and moisture. In contrast, a secondary standard is generally less pure and often exhibits characteristics like hygroscopicity (readily absorbing water) or volatility, which causes its concentration to change.
For example, solid sodium hydroxide is a secondary standard because it rapidly absorbs water and carbon dioxide from the air, making it impossible to weigh an accurate, pure mass. Due to this instability, the concentration of a secondary standard solution is not constant and must be verified periodically. This verification process, known as restandardization, is necessary to maintain the solution’s accuracy for ongoing analytical measurements. Secondary standards are often cheaper and easier to prepare in large volumes for routine testing, unlike primary standards which are often expensive and difficult to prepare.
The Process of Standardization
The exact molarity of a secondary standard solution is determined through standardization, typically involving a titration. Titration is a technique where a solution of known concentration (the titrant) is incrementally added to a solution of unknown concentration (the analyte) until the reaction is complete. When standardizing a secondary standard, the primary standard serves as the titrant or the substance against which the secondary standard is reacted.
The process begins by accurately measuring a known mass or volume of the primary standard, such as potassium hydrogen phthalate (KHP) for an acid-base titration, to establish a precise number of moles. This primary standard is then reacted with a measured volume of the secondary standard solution, typically placed in a burette. The reaction continues until the equivalence point is reached, which is the specific point where the exact stoichiometric amount of titrant has been added to completely react with the analyte.
To confirm the equivalence point, a chemical indicator, such as phenolphthalein, is added to the reaction mixture. This indicator changes color sharply when the reaction is complete, signaling the titration endpoint. By recording the precise volume of the secondary standard solution required to reach this endpoint, the exact molarity of the secondary standard can be calculated using stoichiometric principles. This standardization process ensures the secondary standard is sufficiently accurate for subsequent analyses.
Common Uses in Analytical Chemistry
Secondary standards are widely employed in analytical chemistry, particularly where speed, efficiency, and cost-effectiveness are prioritized over the highest initial purity. They are routinely used in industrial quality control laboratories to quickly analyze the composition of products and raw materials. For example, a standardized sodium hydroxide solution might be used daily to determine the acidity of a food product or the concentration of an acid in a manufacturing process.
Clinical laboratories rely on secondary standards for testing patient samples, and environmental monitoring agencies use them for the routine analysis of pollutants in water and soil. In redox titrations, which involve the transfer of electrons, secondary standards like potassium permanganate (KMnO4) are commonly used because the primary standard is impractical for the required reaction. Using standardized secondary solutions allows labs to perform routine measurements rapidly without the need to constantly prepare highly pure primary standards.