Question

How to perform a pH titration of 25 mL of 50 mM Phosphoric acid from pH 1 to pH 13 with 0.5 M NaOH (including standardizing the NaOH)? Note: May need to look up analytical stuff & reflect on amino acid titrations for help.

Write in detail how to preform this, with math work shown for each step. Points will not be given to answers that do not fufill the whole question stated above.

Answer 1

In this reaction a buret is used to administer one solution to another. The solution administered from the buret is called the titrant.
The solution that the titrant is added to is called the analyte. In a titration of a Weak Acid with a Strong Base the titrant is a strong base
and the analyte is a weak acid. In order to fully understand this type of titration the reaction, titration curve, and type of titration problems
will be introduced.

The Reaction
The titration of a weak acid with a strong base involves the direct transfer of protons from the weak acid to the hydoxide ion.
The reaction of the weak acid, Phospheric acid, with a strong base, NaOH, can be seen below. In the reaction the acid and base react in a one to one ratio.

           H3Po4 + OH- -----> H2O + H2Po4 -

The titration curve is a graph of the volume of titrant, or in our case the volume of strong base, plotted against the pH.
There are several characteristics that are seen in all titration curves of a weak acid with a strong base. These characteristics are stated below.

1) The initial pH (before the addition of any strong base) is higher or less acidic than the titration of a strong acid
2) There is a sharp increase in pH at the beginning of the titration. This is because the anion of the weak acid becomes
a common ion that reduces the ionization of the acid.

3) After the sharp increase at the beginning of the titration the curve only changes gradually. This is because the solution is acting as a buffer.
This will continue until the base overcomes the buffers capacity.

4) In the middle of this gradually curve the half-neutralization occurs. At this point the concentration of weak acid is equal to the concentration of its conjugate base.
Therefore the pH=pKa. This point is called the half-neutralization because half of the acid has been neutralized.

5) At the equivalence point the pH is greater then 7 because all of the acid (HA) has been converted to its conjugate base (A-) by the addition of NaOH and now the
equilibrium moves backwards towards HA and produces hydroxide, that is: A?+H2O?AH+OH?

6) The steep portion of the curve prior to the equivalence point is short. It usually only occurs until a pH of around 10.
The image of a titration curve of a weak acid with a strong base is seen below. All of the characteristics described above can be seen within it.