why is NaOH a poor primary standard ?

Because it is next to impossible to secure a sample of NaOH free of impurities. The most prominent impurity is that solid NaOH pellets absorb CO2 and H2O from the air almost as fast as you can weigh it. You CAN prepare some CO2 free NaOH in SOLUTION form but that still must be standardized so it isn't a primary standard.

Sodium hydroxide (NaOH) is considered a poor primary standard due to its hygroscopic nature, meaning it readily absorbs water from the atmosphere. This property makes it challenging to obtain a reliable and accurate measurement of its exact concentration.

When using a substance as a primary standard, it should possess certain characteristics, such as high purity, stability, and the ability to be accurately weighed. These features make it easier to prepare a standardized solution of known concentration.

However, NaOH pellets, commonly used for laboratory purposes, inevitably absorb moisture from the surrounding environment, leading to an increase in weight. This moisture uptake introduces uncertainty into the precise amount of NaOH being measured, resulting in inaccurate concentration calculations.

To overcome this limitation, substances like potassium hydrogen phthalate (KHP) or sodium carbonate (Na2CO3) are commonly used as primary standards. These materials exhibit higher stability and are less prone to moisture absorption, making them more suitable for accurately determining the concentration of a solution.