Final answer:
Sodium hydroxide cannot be used as a primary standard because it is hygroscopic and absorbs water from the air, altering its mass and making it impossible to maintain an exact concentration.
Step-by-step explanation:
One characteristic of sodium hydroxide that prevents it from being used as a primary standard is its hygroscopic nature, meaning it readily absorbs water from the air. This absorption makes it impossible to determine an exact concentration because the sodium hydroxide's mass continually changes as it absorbs water. For substances to be used as primary standards in titrations and other quantitative analyses, they must be stable, have a known purity, and be non-hygroscopic. Therefore, the hygroscopic nature of sodium hydroxide disqualifies it as a primary standard.