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
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
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
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]