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Flashcards in EDTA Deck (25):

What is the full name of the EDTA reagent

Ethylenediamine tetraacetic acid


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

Eriochrome Black T
Wine red - Blue


What is the general purpose of buffer solutions

Stabilise PH of a solution


What buffer was required for this titration

PH 10


What problem might arise if the wrong buffer is used

Inaccurate end point, EDTA complexing with other ions


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

Swirl flask constantly to ensure homogenous solution
Wash down sides with deionised water


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

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


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

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


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

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


Why is EDTA solution not stored in glass

It reacts with the ions in glass


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

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


Why is this type of titration called a complexometric titration

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


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

By titration with a standard solution of edta


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

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


Why is the end point difficult to detect?

Because it is a complexometric titration


How is the hardness of a sample of water measured?

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


How is the
i) pipette
ii) burette rinsed

i) deionised water and hard water sample
ii) deionised water and EDTA


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

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


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

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.


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.

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.


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

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.


What is the appearance of the indicator

Grey/blue solid


What is the significance of the colour change?

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


CALCULATIONS check sheet!

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


Name the homologous series that edta is a member of

Alkenes??? ASK