Why is potassium hydrogen phthalate (KHP) a primary standard but sodium hydroxide is not?
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.
Potassium hydrogen phthalate (KHP) is considered a primary standard because it meets certain criteria that make it ideal for use in acid-base titrations. On the other hand, sodium hydroxide (NaOH) is not typically used as a primary standard due to some inherent challenges.
To understand why KHP is a primary standard and NaOH is not, let's discuss the characteristics of a primary standard:
1. High Purity: A primary standard should have a high degree of purity, meaning it is free from impurities that could affect the accuracy of the analysis. KHP can be obtained in a highly pure form, making it suitable for accurate measurements.
2. Stability: A primary standard should be stable, meaning it does not undergo significant changes in composition over time. KHP has excellent stability, which allows for reliable and consistent results.
3. Large Molar Mass: An ideal primary standard should have a large molar mass to minimize weighing errors. KHP has a relatively high molar mass (204.22 g/mol), enabling precise measurement of its mass.
4. Ease of Standardization: The standardization process involves determining the exact concentration of the primary standard by reacting it with a known amount of another substance. KHP can be easily standardized since it reacts with a strong base, such as sodium hydroxide, in a 1:1 stoichiometric ratio.
Now, let's consider why sodium hydroxide is not commonly used as a primary standard:
1. Hygroscopic Nature: Sodium hydroxide readily absorbs water from the atmosphere, making it challenging to obtain a precise and stable sample. Its high affinity for moisture causes it to vary in weight, impacting the accuracy of the analysis.
2. Reactivity: Sodium hydroxide is a highly reactive substance, especially with carbon dioxide in the air, leading to carbonate formation. This reactivity makes it more challenging to maintain its purity and consistency.
3. Deliquescence: Sodium hydroxide is deliquescent, meaning it absorbs moisture from the air and becomes a solution. This property makes it difficult to handle and store, as its concentration can change over time.
Due to these factors, potassium hydrogen phthalate (KHP) is a preferred primary standard in acid-base titrations, while sodium hydroxide (NaOH) is not commonly used for this purpose.