Total Hardness of water using edta

Question 1

What causes the hardness of water?

Answer 1

The presence of calcium and magnesium ions dissolved in the water

Question 2

What principal is involved in the measuring of the hardness of water?

Answer 2

Measuring the hardness of water involves measuring the concentration of calcium and magnesium ions in a sample of water

Question 3

What is the total hardness of water?

1/2

Answer 3

The total hardness of water = calcium hardness + magnesium hardness

The sum of the concentrations of calcium and magnesium ions in the water

Question 4

How is the hardness of water expressed?

Answer 4

The hardness of water is expressed in terms of mg per litre (ppm) of CaCO3

Question 5

What chemical is used to determine the concentration of calcium and magnesium ions in water?

Answer 5

Ethylenediaminetetraacetic acid (edta)

Question 6

Describe the structure of edta with reference to its constituent parts
(3)

Answer 6

Ethylene refers to the CH2CH2

Diamine refers to the two nitrogen atoms in the molecule

Tetraacetic refers to the four ethanoic acid (acetic acid) molecules

Question 7

What form of edta is usually used during titrations?

Answer 7

The disodium salt of edta

Question 8

Why is the disodium salt form of edta used instead of pure edta?
(2)

Answer 8

As the disodium salt version is more soluble

The disodium salt is a primary standard

Question 9

How can the disodium salt be represented?

Answer 9

NA2H2X

Question 10

How does edta work to form a complex?

Answer 10

By wrapping itself around the CA^2+ and MG^2+ ions

Question 11

What is a complex?

Answer 11

A compound in which a molecule such as edta forms coordinate bonds (dative bonds) to a metal ion

Question 12

What type of ion is CA^2+ referred to as before its bonding with edta?

Answer 12

It is called a simple ion

Question 13

What type of ion is Ca^2+ referred to as after it bonds with edta?

Answer 13

It forms a complex ion with edta

Question 14

Give the reaction that occurs between edta and Ca^2+

Answer 14

H2X^2- + Ca2+ –> CaX^2- + 2H+

edta ion………………..edta complex

Question 15

What is the ratio of edta to Ca2+?

Answer 15

1:1

Question 16

What indicator is used during this titration?

Answer 16

Eriochrome Black T

Question 17

What is Eriochrome Black T also called?

Answer 17

Solochrome Black

Question 18

What colour is eriochrome black T/solochrome black T in the presence of Ca2+ and MG2+ ions?

Answer 18

Wine-red colour

Question 19

What colour change takes place if sufficient edta is added to the water?

Answer 19

Changed from wine-red to blue

Question 20

What is the end point of this experiment?

Answer 20

The indicator changes from wine-red to blue

Question 21

What does a colour change of wine-red to blue tell us?

Answer 21

That all of the Ca2+ and Mg2+ ions have been converted to the corresponding edta complexes

Question 22

What type of titration is this?

Answer 22

A complexometric titration

Question 23

What is a complexometric titration?

2

Answer 23

A titration involving the formation of a complex between metal ions and a reagent such as edta

In this type of titration the end point is marked by a sharp decrease in the concentration of free metal ions

Question 24

Why is it necessary to keep the pH around 10?

2

Answer 24

Eriochrome Black T indicator requires a pH of 8 to 10 for the colour change to occur

The reaction between Ca2+ and Mg2+ ions and the edta is pH dependent and the edta may not fully complex with these ions if the pH is not kept constant at pH = 10

Question 25

What is added to keep the pH at around 10?

Answer 25

A buffer solution

Question 26

What is a buffer solution?

Answer 26

A buffer solution is a solution that resists changes in pH, i.e. it keeps the pH constant

Question 27

What would happen if an incorrect buffer was used in this experiment?

Answer 27

An inaccurate end point will result

Question 28

Why would the use of an incorrect buffer result in an inaccurate end point?
(2)

Answer 28

This is because there would be incomplete complexing of the edta with calcium and magnesium ions

The indicaor would also not work satisfactorily at the end point

Question 29

Why is edta stored in a plastic container?

Answer 29

The edta must be stored in a plastic container because edta reacts with the ions in glass if it is stored in a glass container for a long perios

Question 30

Why is the solution wine-red at the beginning of the procedure?

Answer 30

As Ca2+ ions and Mg2+ ions are present in the water sample

Question 31

What does the blue colour at the end of the titration indicate?

Answer 31

It indicates that all the calcium and magnesium ions have been converted to edta complexes