7. To determine the total hardness in a water sample using ethylenediaminetetraacetic acid (EDTA)

Theory

• Hardness in water is caused by the presence of Ca2+ ions/Mg2+ ions

• The concentration of Ca2+ and Mg2+ and hence the harness of a water sample can be determined by titrating the water sample against a standard solution of EDTA

EDTA

EthyleneDiameneTetraAcetic acid

EDTA (represented by H2X2-) is a compound that reacts with metal ions such as Ca2+/Mg2+ (represented by M2+) - forming an edta-metal complex (MX2 –)

H2X2– + M2+ → CaX2– + 2H+

edta + Ca2+/Mg2+ in had water → edta-metal complex (CaX2-

Equation

H2X2- + M2+ → MX2- + 2H+

(Burette) + (conical flask)

edta + Ca2+/Mg2+ in hard water → edta-metal complex

Procedure

1) Making the EDTA up into a solution (if required)

2) Filling the burette with the EDTA solution

3) Transferring an exact volume of hard water into a conical flask

4) Carrying out the titration

Transferring an exact volume of hard water into a conical flask

At this point, the following is added to the hard water in the conical flask:

➢ ̴ 2-3 cm3 Buffer solution pH = 10

➢ ̴ 30 mg Erichrome Black T indicator

• A wine-red colour forms in the conical flask in the presence of Ca2+/Mg2+ ions

Suitable indictor for this titration

Erichrome Black T

Note: Erichrome black T indictor is also known as Solochrome black

Justification for this indicator

• Erichrome Black T is a wine-red colour in the presence of Ca2+/Mg2+ ions.

• Erichrome Black T turns blue in the absence of Ca2+/Mg2+ ions.

Colour change observed at end point

Wine/Red → Blue

(Presence of Ca2+/Mg2+ ions) → (Absence of Ca2+/Mg2+ ions)

What is hard water? What is the cause of hard water?

• Hard water is water that will not easily form a lather with soap

• Hard water is caused by the presence of calcium ions (Ca2+) or magnesium ions (Mg2+)

What is meant by temporary hardness? In what compound are Ca2+ and Mg2+ ions in temporary hard water present?

Temporary hardness is hardness that can be removed by boiling

Ca2+ - Ca(HCO3)2 - Calcium hydrogencarbonate

Mg2+ - Mg(HCO3)2 - Magnesium hydrogencarbonate

What is meant by permanent hardness? In what compound are Ca2+ and Mg2+ ions in permanent hard water present?

Permanent hardness is hardness that cannot be removed by boiling

Ca2+ - CaSO4 - Calcium sulfate

Mg2+ - MgSO4 - Magnesium sulfate

Explain the term total hardness?

Total hardness refers to the sum of the permanent hardness and temporary hardness

Suggest a way of determining if a sample of water contained temporary hardness and how the concentration of temporary hardness could be calculated

• Titrate the water sample against EDTA and find the value for hardness (Total hardness)

• Boil the water to remove the temporary hardness

• Titrate the boiled water sample against EDTA and find the value for hardness (Permanent hardness only)

Temporary hardness = Total hardness - Permanent hardness

Why is a standard solution of EDTA used in this titration?

EDTA reacts with metal ions and forms an edta- metal complex - during this titration , it will react with the Ca2+/Mg2+ ions in the hard water and use them up – The end point is the moment where all Ca2+/Mg2+ ions are used up; detected by a colour change so the concentration of Ca2+/Mg2+ in a hard water sample can be determined

Why is EDTA stored in a plastic container? (Glass)

EDTA reacts with the metal ions in glass if stored in a glass container for a long period of time

What are buffer solutions?

Buffer solutions are solutions that resist changes to pH and keeps the pH of an environment at a desired value

Why is buffer solution pH - 10 required in this titration?

1. A pH of 10 (alkaline conditions) is needed for EDTA to react with Ca2+/Mg2+ (Group II metal ions) and form an edta-metal complex

(At lower pH values EDTA may react with and form an edta metal complex with other ions that do not cause hardness i.e. non alkaline earth metal ions)

2. The Erichrome Black T indicator changes colour only at a pH of 8-10 – an inaccurate end point will be obtained at a pH other than ̴10