1. It's not a pure compound: Ammonium hydroxide is actually a solution of ammonia (NH3) in water. This means the concentration of NH4OH in the solution is variable and not easily reproducible.
2. It's volatile: Ammonia is a volatile gas, meaning it readily evaporates from solution. This evaporation leads to a change in the concentration of the solution over time, making it unreliable for accurate titrations.
3. It's hygroscopic: Ammonium hydroxide readily absorbs moisture from the air, further altering its concentration and making it unsuitable as a primary standard.
4. It's unstable: The equilibrium between ammonia and ammonium hydroxide is influenced by factors like temperature and pH. This instability makes it challenging to maintain a consistent concentration.
In contrast, primary standards should have the following characteristics:
* High purity: They should be available in a very pure form, with a known and stable composition.
* Non-hygroscopic: They should not readily absorb moisture from the air.
* Stable: They should not decompose or react with the atmosphere.
* High molecular weight: This minimizes weighing errors.
* Easy to dissolve and handle: They should be readily soluble in the solvent used for the titration and easy to work with.
Due to these limitations, ammonium hydroxide is not suitable as a primary standard. Instead, it is typically standardized against a primary standard like potassium hydrogen phthalate (KHP).
