Edta - Possible Questions

Question 1

Name a suitable indicator

Answer 1

Eriochrome Black T

Question 2

What do the letters edta stand for?

Answer 2

E = ethylene 
D = diamine 
T = Tetraacetic 
A = acid

Question 3

What is the purpose of the buffer solution?

Answer 3

Keep the pH at 10 which ensured accurate end point is reached

Question 4

What colour change is observed at the end point?

Answer 4

Red-wine to blue

Question 5

After the experiment is completed, the water may be boiled and then the calculations may be done again.

What was the purpose in the boiling the water?
(2)

Answer 5

To find the permanent hardness of the water i.e. the hardness which cannot be removed by boiling the water

To remove the temporary hardness i.e. the hardness which can be removed by boiling

Question 6

State the type of hardness invovled

Answer 6

Permanent hardness

Question 7

How can permanent hardness be removed?

1/3

Answer 7

Distillation
Washing soda
Ion-exchange resins

Question 8

What colour change indicates the end point of the titration?

Answer 8

A change of colour from red-wine to blue

Question 9

Name the instrument used to place the sample of water in the conical flask

Answer 9

Pipette

Question 10

What pH buffer 10 did you use?

Answer 10

Ammonium chloride in ammonia solution

Question 11

Describe the indicator you used.

Answer 11

Black powder

Question 12

What precautions are normally taken when storing edta solution?

Answer 12

The edta solution is stored in a plastic container because edta reacts with the ions in glass if it is stored in a glass container for a long period

Question 13

What is the function of the buffer solution?

2

Answer 13

To keep the pH at 10 and therefore ensure an accurate end point is reached

Without a pH buffer or the incorrect buffer there would be incomplete complexing of the edta with calcium and magnesium ions and the indicator would not work satisfactory at the end point

Question 14

Why is the edta solution not stored in glass?

Answer 14

The edta solution is stored in a plastic container because edta reacts with the ions in glass if it is stored in a glass container for a long period

Question 15

Why does a colour change take place?

Answer 15

The indicator changes from wine-red to blue when all of the Ca2+ and Mg2+ ions have been converted to the corresponding edta complexes

i.e. it takes place because off the Ca2+ and Mg2+ ions have been converted into their corresponding edta complexes

Question 16

Why is this type of titration called a complexometric titration?
(2)

Answer 16

A complexometric titration involves 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 ions

Question 17

Distinguish between temporary hardness and permanent hardness

Answer 17

Temporary hardness = hardness that can be removed by boiling the water

Permanent hardness = hardness that cannot be removed by boiling the water

Question 18

Why is dionised water used to rinse the equipment?

Answer 18

As deionised water does not contain ions which means it will not affect the concentration of reactant in the flask