1) Why are NaOH and HCl not used as primary standards for acid–base titrations?

Question

1) Why are NaOH and HCl not used as primary standards for acid–base titrations? What properties

should a primary standard possess?

2) In order to standardize a solution of NaOH, 1.05 mL of NaOH is titrated with 0.100 M oxalic acid(H2C2O4). If 295 µL of oxalic acid is needed to reach the endpoint, what is the molarity of the

NaOH solution?

3) The NaOH solution in Question 2 is used to standardize an unknown solution of HCl. It requires134 µL of the NaOH solution to titrate 1.00 mL of the HCl to the end point. What is the molarityof the HCl?

Explanation / Answer

1) Primary standard should have the following properties

Easy to weigh the material.

Should be stable under normal conditions

The concentration of the solution of standard should not change upon standing due to moisture or air

Now, NaOH and KOH are hygroscopic salts that tend to absorb moisture upon standing and thus, the there solution concentration changes over time.

2. moles of oxalic acid used = 0.1 M x 0.295 ml = 0.0295 mmol

molarity of NaOH = 0.0295 mmol/2 x 1.05 ml = 0.014 M

3. moles of NaOH used = 0.014 M x 0.134 ml = 0.002 mmol

molarity of HCl = 0.002 mmol/1 ml = 0.002 M

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