Question

why do we not need to determine the exact volume of water needed to make the solution with the KHP in the standardization titration?

Answer 1

Hi,

First we will understand why KHP is used

Many analytical procedures involve comparison of an unknown with a standard. A standard is usually a solution whose composition (concentration) is known very precisely (and accurately!). Unfortunately, NaOH is not a suitable primary standard. Solid NaOH is highly hygroscopic (it absorbs water from the air) and thus it cannot be accurately weighed. It also absorbs carbon dioxide from the air, forming sodium carbonate and thereby reducing the amount of sodium hydroxide present. This means that a NaOH solution of known molarity cannot be directly prepared without resorting to an air-and-water-free environment. We can only prepare a solution of NaOH that has the approximate concentration we will need. In cases like this we need to use an acid as the primary standard to determine the exact concentration of the NaOH solution. The primary standard we will use is potassium hydrogen phthalate (KC8H5O4, often abbreviated KHP). It is available as a pure, stable, crystalline solid that can be accurately weighed. A KHP sample of known mass (and, therefore, known moles) can be titrated with the NaOH solution to determine very precisely the concentration of the NaOH. This procedure is called standardizing the NaOH solution. Once this concentration is determined, the NaOH solution can serve as a standard for further work.

Standardization of Base Using KHP

The amount of the base (NaOH) in a solution can be determined by measuring very accurately the amount of solution required to react with a known amount of acid (KHP). One mole of KHP contains one mole of H+ that can react with one mole of sodium hydroxide, which contains one mole of OH-. Notice that early in a titration of KHP with OH-, not much OH- has been added, so it is the limiting reagent and KHP is left over (the solution is acidic). At the equivalence point stoichiometric amounts of KHP and OH- have been mixed. The product KP- is a weak base (the solution is basic). Just past the equivalence point, KHP is limiting, and so OH- is left over (the solution is basic). The point at which equivalent amounts have been mixed is determined using an indicator that changes color as the solution changes from acidic to basic. Phenolphthalein is the most commonly used indicator (and the one we will use in this experiment), although many other indicators would also work for this titration.

Now, as you have understood KHP is a primary standard and it is used to determine molarity of secondary standard, therefore it becomes critial to determine the exact volume of water needed to make the solution with the KHP in the standardization titration.