[srinivas23578]

To determine the normality of NaOH solution we are using Potassium Hydrogen Pthalate (PHP) but why to use this?

[Borek]

KHP, not PHP.

This is one of primary standards.

[Kaushey]

PHP (Potassium Hydrogen Pthalate)is also known as KHP (K salt of hydrogen pthalate).
One of the -COOH group is neutralised with K+ ion.So only one -COOH group is available for neutralisation with NaOH solution being standardise.
It is very stable compound in normal laboratory conditions.You can dry it in oven at 100-110 deg.c for removing absorbed moisture for accurate standardisation.That is why it is known as preliminary standard.

[Alpha-Omega]

It is usually impossible to obtain NaOH of sufficient purity to use it as a primary standard.  So an indirect method, such as one that utilizes a primary standard such as KHP, is more practical for obtaining a standard solution of NaOH.

Sodium hydroxide is not available as a primary standard. When dissolved in water, it is so basic that
it reacts with carbon dioxide from the air almost immediately. In fact, hydroxide ion can even react
with the silica (SiO2) in the glass, producing silicate in solution. That’s why it is stored in plastic
bottles.

Reagent grade NaOH is supplied in two ways: as pellets or as a 50% w/w solution. When pellets are
used to make a solution, the surface is washed off rapidly with boiled water, which removes any
sodium carbonate and sodium bicarbonate. When a 50% w/w solution is prepared, sodium carbonate
is essentially insoluble and sinks to the bottom of the container. So long as the material at the bottom
of the container is not disturbed, the liquid remains virtually carbonate-free.

Sodium hydroxide is standardized by titrating it against an acid available in a primary standard
grade, such as potassium hydrogen ortho-phthalate (usually shortened to KHP). Primary standards
are reagents with a number of useful qualities. A Primary standard should

* be of high purity
* remain unchanged in air during massing and remain stable during storage
* have a high molar mass to reduce massing errors
* react with the solution to be standardized in a direct, well-defined reaction

They are readily available in a highly purified form,
inexpensive, known to react in fixed stoichiometries, stable indefinitely, and easily handled. Primary
standard KHP is at least 99.99% pure.


NaOH(aq) + KHC₈H₄O₄(aq) ---> KNaC₈H₄O₄(aq) + H₂O(l)

Reactants:  ortho-phthalate ion, C₆H₄(CO₂)₂ ^- and water, H₂O

Products:  hydrogen ortho-phthalate ion, HC₆H₄(CO₂)₂^-  hydroxide ion, OH^-


The balanced chemical equation for the reaction between hydrogen ortho-phthalate anion and hydroxide anion is as follows:

HC₆H₄(CO₂)₂^–(aq) + OH^–(aq) 6 C₆H₄(CO₂)₂^–(aq) + H₂O(l)

At the end of the titration, all of the acid has been consumed, and the addition of excess hydroxide
causes a sharp increase in the basicity. In other words, you are switching from an acidic solution to
an alkaline solution at the stoichiometric point or equivalence point. These two terms are
synonymous and refer to the exact spot where the number of H⁺ ions is equal to the number of OH^–ions. In other words, there are stoichiometric or equivalent amounts of each reactant.

In conclusion:

The presence of carbon dioxide is one reason why one cannot weigh out pure NaOH and use it as a primary standard. Water which is in contact with the atmosphere can readily absorb carbon dioxide. Dissolved carbon dioxide acts as a weak acid in aqueous solutions, as shown in the following equation.

CO₂ (g) + H₂O ------> HCO₃^- + H⁺

If basic solutions are prepared using water which contains dissolved carbon dioxide, a portion of the base reacts with the carbon dioxide to yield hydrogen carbonate and carbonate. For this reason, we will need to use water from which all carbon dioxide has been removed to prepare any sodium hydroxide solutions which will be standardized.

Another reason why standard sodium hydroxide solutions cannot be prepared by weighing is that the solid reagent often contains significant amounts of impurities, and is highly hygroscopic. Usually, sodium hydroxide solutions are prepared by diluting a 50% aqueous solution of sodium hydroxide to approximately the desired concentration, followed by standardization of the solution by titration of an acidic primary standard. In 50% solutions of sodium hydroxide, dissolved carbon dioxide precipitates as sodium carbonate.

Potassium Hydrogen Phthalate (C₈H₅O₄K, called KHP for short sometimes see KHPh) is used as the primary standard. The KHP standard is a stable, pure, well-characterized material.

ACRONYMS-OH MY!!!! 

The acronym PHP makes no sense and could/might lead to alot of CONFUSION...it could stand for a million things....there would be no way to differentiate between the P = potassium and P = Phthalate...and P is not a chemical symbol/acronym for potassium, but K is.

For the sake of argument:  What if I had Phosphorous Hydrogen Phthalate...which P would be phosphorous and which would be phthalate. And then maybe I would really have Potassium Hydrogen Phosphorous...or maybe Phosphorous Hydrogen Potassium...I mean how would I know...

And then going a step further, how would I know the difference between potassium hydrogen phthalate and phosphorous hydrogen phthalate...since then they could both conceivably be named PHP. 

So if I had a jar with the label PHP on it....I would not trust it contained Potassium Hydrogen Phthalate....