In an experiment to determine the total hardness of a water sample containing both calcium and magnesium ions, a solution of the reagent edta (ethylenediaminetetraacetic acid) in the form of its disodium salt (represented by Na2H2Y) was titrated against a sample of the water using a suitable indicator - Leaving Cert Chemistry - Question 1 - 2004

1. Rinse the burette with distilled water.
2. Rinse the burette with a small amount of the edta reagent, ensuring that the reagent flows through the tap.
3. Fill the burette with the edta reagent using a funnel but remove the funnel after filling.
4. Eliminate any air bubbles by tapping the burette gently and ensuring liquid is flowing freely from the tip.

The solution that must be added is a buffer solution. The purpose of this buffer is to keep the pH above 9 to ensure a sharp (accurate) end-point during the titration.

To calculate the total hardness, we note the following:
(i) The total hardness in moles per litre is calculated as follows:

$ext{Hardness} = \frac{100 \times 0.0010}{8.10} = 0.0001 \text{ mol/L}$

(ii) To convert to grams per litre in terms of CaCO₃:

$\text{Hardness (g/L)} = 0.0001 \text{ mol/L} \times 100.09 \text{ g/mol} = 0.0081 \text{ g/L}$

(iii) The pH to be maintained is approximately 10 to ensure accurate endpoint readings.

The balanced equation for the reaction of the whitish deposit (calcium carbonate, CaCO₃) with hydrochloric acid is:

$\text{CaCO}_3 (s) + 2\text{HCl} (aq) \rightarrow \text{CaCl}_2 (aq) + ext{H}_2 ext{O} (l) + ext{CO}_2 (g)$