Water: EDTA

Identify the two ions that most commonly cause hardness when dissolved in water

Calcium ions / Ca²⁺ //

Magnesium ions / Mg²⁺

Explain the term total hardness

Sum of permanent and temporary hardness

Name a suitable indicator for this titration

Eriochrome Black T

What colour change was observed in the conical flask at the end point?

Wine red → Blue

What pH does the buffer solution maintain in the mixture in the conical flask?

10

Why is it important to maintain this pH during the titration?

To ensure accurate end-point /

to ensure all calcium ions (magnesium ions) react with edta

Describe the correct procedure for preparing a burette, that had been previously rinsed with deionised water and some 0.010 M edta solution, for this titration

Clamp vertically //

fill above zero mark //

fill part below tap //

fill with clean dry funnel //

remove funnel //

ensure no bubbles //

bottom of meniscus on zero mark //

read at eye-level

Calculation 2022 Q1 part e)

Identify a compound of calcium that is one of the main causes of permanent hardness in water

CaSO₄

Describe the procedure used to measure out 50.0cm³ of hard water from a beaker into a conical flask

Use 25cm³ pipette twice //

previously rinsed with deionised water //

rinsed with the hard water sample //

bottom of meniscus on mark reading at eye level //

shake last drop from pipette //

wait until pipette fully empty

Calculation 2018 Q1 part e)

Suggest a way to determine if this water supply contained temporary hardness

Boil water //

scale (scum, precipitate, deposit) (as a result of boiling)

What is the full name of the edta reagent?

Ethylenediaminetetraacetic acid

A small volume of another solution was added to the water samples before commencing the titrations.

i) Identify this solution

ii) Why was this solution added?

i) pH 10 buffer

ii) To ensure accurate end-point / to ensure all calcium ions (magnesium ions) react with edta

What is the general purpose of buffer solutions?

To stabilise the pH of a solution

What problems would have been encountered in this titration if the wrong buffer were used?

Poor (inaccurate) end-point

Explain the two operations, involving the titration flask and its contents, carried out as the edta was being added from the burette during the titrations

Swirl to mix // allow time after addition from burette for reaction // wash down sides with deionised water // on white surface

State and explain whether the water having passed through the deioniser is suitable for use as deionised water in the laboratory or if the deioniser needs to be changed or regenerated

Unsuitable because hardness remaining /

deioniser needs to be replaced because hardness remaining after passing through the deioniser

Calculation 2010 Q1 part f)

A whitish deposit is often found on the insides of kettles in hard water districts. If some of this deposit is scraped into a test tube and dilute hydrochloric acid is added a reaction is observed. Write a balanced chemical equation for this reaction

CaCO₃ + 2HCl → H₂O + CO₂

Calculation 2004 Q1 part d)