Why concentrated HCl is not considered as primary standard material?

Concentrated HCl is not considered a primary standard material because it tends to be hygroscopic, meaning it absorbs water from the atmosphere, leading to inaccuracies in the weighing process. Additionally, concentrated HCl can be prone to decomposition over time, which affects its stability for accurate standardization. For these reasons, it is more common to use a certified primary standard like potassium hydrogen phthalate or sodium carbonate for acid-base titrations.