Which substance is usually standardized against sodium hydroxide solution?

Potassium acid phthalate is commonly used as a primary standard for acid-base titrations, particularly in the standardization of sodium hydroxide solutions. This is due to its stable and high purity, as well as its well-defined stoichiometry in reactions with bases. When it dissolves in water, it does not exhibit significant moisture absorption, which helps maintain its accurate concentration.

In titration procedures, potassium acid phthalate reacts with sodium hydroxide to form water and potassium sodium phthalate. Because the reaction proceeds to completion and the acid can be accurately measured, this substance provides a reliable means of standardizing sodium hydroxide solutions, which can vary in strength and require precise calibration for accurate titration results.

While other substances listed, such as acetic acid, can also be involved in acid-base reactions, they do not possess the same level of stability and reliability for standardization purposes as potassium acid phthalate. Sodium chloride and sulfuric acid likewise do not serve the role of a primary standard for sodium hydroxide solutions; therefore, potassium acid phthalate's characteristics make it the most suitable choice for this process.