What characteristics make potasssium hydrogen phthalate (KHP) a good primary standard?
Answer:
"A primary standard is a reagent that is extremely pure, stable, has no waters of hydration, and has a high molecular weight."
Molar mass of KHP is 204.23, and after it's toasted in an oven, it can be massed out on a balance for highly precise titrations.
It is stable
It is non toxic
It is solid and therefore easily measured
It can be conveniently dried (removing water for accurate meaasurement)
Potassium hydrogen phthalate, often called simply KHP, is a white, colorless, ionic solid that is the monopotassium salt of phthalic acid. The hydrogen is slightly acidic, and it is often used as a primary standard for acid-base titrations because it is solid and air-stable, making it easy to weigh accurately. It is also used as a primary standard for calibrating pH meters because, besides the properties just mentioned, its pH in solution is very stable...
Potassium acid phthalate has 3 desireable characteristic for a primary standard-
1. A high equivalent weight
2. It is stable on drying
3. It is readily available
It's main negative is that it is a weak acid so it is generally suitable for standardization of carbonate-free base solutions only. Special precautions are necessary to remove carbonates from NaOH solutions and to maintain the solutions carbonate-free.
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