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Why is potassium hydrogen phthalate (KHP) a primary standard but sodium hydroxide is not?

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One can buy KHP in essentially 100% purity and after drying at 105 C for a couple of hours to make sure it contains no moisture, one knows exactly how many grams (thus how many moles) KHP are there. Unfortunately, NaOH absorbs BOTH CO2 and H2O from air. In the unlikely event that one could purchase 100% NaOH (certainly not likely), by the time it was weighed it would not be pure. It would be contaminated with CO2 and water. So we make up an approximate NaOH solution, standardize with KHP (or some other primary standard), and use that. By the way, any NaOH solution must be restandardized if it stands too long because it reacts slowly with glass (storing in plastic bottles helps that problem) and it also absorbs CO2 from the air (the space between the liquid level in the bottle and the cap is filled with air). One can solve that problem by allowing air into the bottle only after it has passed over a mixture that absorbs CO2 and water vapor.

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