EDTA

Question 1

What is the full name of the EDTA reagent

Answer 1

Ethylenediamine tetraacetic acid

Question 2

Name an indicator suitable for this titration and state the colour change observed at the end of the titration

Answer 2

Eriochrome Black T

Wine red - Blue

Question 3

What is the general purpose of buffer solutions

Answer 3

Stabilise PH of a solution

Question 4

What buffer was required for this titration

Answer 4

PH 10

Question 5

What problem might arise if the wrong buffer is used

Answer 5

Inaccurate end point, EDTA complexing with other ions

Question 6

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

Answer 6

Swirl flask constantly to ensure homogenous solution

Wash down sides with deionised water

Question 7

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

Answer 7

Water is unsuitable -> ions still present after passing through deioniser
(Deioniser needs to be replaced because ions are still present)

Question 8

The addition of a small quantity of another solution to the water in the conical flask is essential before commencing the titration. What solution must be added and what is its purpose

Answer 8

Buffer solution - Keep PH at 10 and ensure sharp end point (allow indicator to work satisfactorily)

Question 9

Describe the correct procedure for rinsing the burette and filling it with edta reagent

Answer 9

Rinse with deionised water and with solution to contain (EDTA)
Clamp vertically to retort stand
Use funnel when adding reagent / remove funnel after filling
Open tap to fill below tap and set bottom of meniscus on graduation mark

Question 10

Why is EDTA solution not stored in glass

Answer 10

It reacts with the ions in glass

Question 11

What colour change is observed during the titration? Explain the reason for the colour change

Answer 11

Wine red - blue. In the presence of Eriochrome Black T indicator, calcium and magnesium ions have a wine red colour. As the EDTA solution is added from the burette the edta forms a complex with the calcium and magnesium ions. When all the calcium and magnesium ions have reacted the indicator changes to a blue colour since there are no calcium and magnesium ions left in solution

Question 12

Why is this type of titration called a complexometric titration

Answer 12

The titration is called a complexometric titration because edta works by wrapping itself around the calcium and magnesium ions in the water

Question 13

How can the total hardness of a water sample be estimated?

Answer 13

By titration with a standard solution of edta

Question 14

If the buffer solution was not added, would the PH of the solution increase or decrease and why

Answer 14

Decrease, there are H+ ions produced as the reaction proceeds

Question 15

Why is the end point difficult to detect?

Answer 15

Because it is a complexometric titration

Question 16

How is the hardness of a sample of water measured?

Answer 16

By measuring the concentration of the magnesium and calcium ions in the sample of water using EDTA

Question 17

How is the

i) pipette
ii) burette rinsed

Answer 17

i) deionised water and hard water sample

ii) deionised water and EDTA

Question 18

Write an equation for the reaction that took place when the water was boiled

Answer 18

Ca(HCO3)2 -> CaCO3 + H2O + CO2

Question 19

Why is it important that the reaction between the edta and the metal ions in solution is (i) rapid and (ii) go to completion?

Answer 19

If the reaction is not almost instantaneous the colour change of the indicator will lag behind the end point and too large a titre would be recorded.
If the reagents do not react completely, no conclusion about the concentration of one of the solutions can be obtained from the volume of it that reacts with a known concentration and volume of the other.

Question 20

The water sample could contain metal ions other than Ca2+ and Mg2+. How would the reliability of the result be affected if this were the case? Suggest two other metal ions that could be present in the water.

Answer 20

Since alkali metal ions such as sodium or potassium ions do not complex with edta reagent, the results would be unaffected by their presence in the water sample. If, however, there were, for example, iron or aluminium ions present, the value recorded for total hardness by this method would be expected to be too high.

Question 21

Suggest a method of establishing the amount of permanent hardness in a water sample.

Answer 21

A known volume of hard water is boiled to precipitate the temporary hardness-causing hydrogencarbonate compounds as carbonates. These are removed by filtration. The filter paper is washed with deionised water. The filtrate is made up to an exact volume with deionised water and the edta titration carried out again, The result is used to calculate the permanent hardness of the water sample.

Question 22

What is the appearance of the indicator

Answer 22

Grey/blue solid

Question 23

What is the significance of the colour change?

Answer 23

All Ca2+ and Mg2+ ions have been converted to the corresponding EDTA complexes

Question 24

CALCULATIONS check sheet!

Answer 24

M/L -> G/L [x100] -> ppm [x1000]

Question 25

Name the homologous series that edta is a member of

Answer 25

Alkenes??? ASK