Why NaOH is not a primary standard?

When we talk about primary standards in chemistry, we're referring to substances that are highly pure, stable, and can be used to determine the concentration of solutions accurately. Sodium hydroxide (NaOH) is commonly used in titrations and other chemical processes, but it doesn't fit the criteria to be classified as a primary standard. Let’s delve into the reasons behind this.

Characteristics of a Primary Standard

To understand why NaOH is not a primary standard, it’s essential to first outline what makes a substance a primary standard. A primary standard should possess the following qualities:

• High Purity: It should be available in a pure form, ideally with a purity of 99.9% or higher.
• Stability: The substance must remain stable over time and not react with moisture or carbon dioxide from the air.
• Non-hygroscopic: It should not absorb water from the atmosphere, which can alter its mass and concentration.
• Known Stoichiometry: The reaction it participates in should have a well-defined stoichiometry.

Why NaOH Falls Short

Now, let’s examine why NaOH does not meet these criteria:

• Hygroscopic Nature: Sodium hydroxide is hygroscopic, meaning it readily absorbs moisture from the air. This property can lead to changes in its concentration and mass, making it unreliable for precise measurements.
• Reactivity with Carbon Dioxide: NaOH can react with carbon dioxide in the air, forming sodium carbonate (Na2CO3). This reaction alters the composition of the NaOH sample, further complicating its use as a primary standard.
• Purity Issues: While commercially available NaOH is often of high purity, it can still contain impurities or variations in concentration that affect its reliability as a standard.

Alternatives to NaOH

In laboratory settings, chemists often use other substances as primary standards for titrations involving bases. Common examples include:

• Sodium Carbonate (Na2CO3): This compound is stable, non-hygroscopic, and can be accurately weighed to prepare standard solutions.
• Potassium Hydrogen Phthalate (KHP): KHP is another excellent primary standard for acid-base titrations due to its high purity and stability.

Practical Implications

In practice, when preparing a solution for titration, using a primary standard ensures that the concentration of the solution is known with high accuracy. This is crucial for obtaining reliable results in quantitative analysis. If NaOH were used as a standard, the variability in its concentration due to environmental factors would lead to inaccuracies in the titration results.

In summary, while NaOH is a valuable reagent in many chemical processes, its hygroscopic nature and reactivity with carbon dioxide prevent it from being classified as a primary standard. Instead, chemists rely on more stable and pure substances to ensure the accuracy of their analytical procedures.