Total Hardness of water using edta Flashcards
What causes the hardness of water?
The presence of calcium and magnesium ions dissolved in the water
What principal is involved in the measuring of the hardness of water?
Measuring the hardness of water involves measuring the concentration of calcium and magnesium ions in a sample of water
What is the total hardness of water?
1/2
The total hardness of water = calcium hardness + magnesium hardness
The sum of the concentrations of calcium and magnesium ions in the water
How is the hardness of water expressed?
The hardness of water is expressed in terms of mg per litre (ppm) of CaCO3
What chemical is used to determine the concentration of calcium and magnesium ions in water?
Ethylenediaminetetraacetic acid (edta)
Describe the structure of edta with reference to its constituent parts
(3)
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
What form of edta is usually used during titrations?
The disodium salt of edta
Why is the disodium salt form of edta used instead of pure edta?
(2)
As the disodium salt version is more soluble
The disodium salt is a primary standard
How can the disodium salt be represented?
NA2H2X
How does edta work to form a complex?
By wrapping itself around the CA^2+ and MG^2+ ions
What is a complex?
A compound in which a molecule such as edta forms coordinate bonds (dative bonds) to a metal ion
What type of ion is CA^2+ referred to as before its bonding with edta?
It is called a simple ion
What type of ion is Ca^2+ referred to as after it bonds with edta?
It forms a complex ion with edta
Give the reaction that occurs between edta and Ca^2+
H2X^2- + Ca2+ –> CaX^2- + 2H+
edta ion………………..edta complex
What is the ratio of edta to Ca2+?
1:1
What indicator is used during this titration?
Eriochrome Black T
What is Eriochrome Black T also called?
Solochrome Black
What colour is eriochrome black T/solochrome black T in the presence of Ca2+ and MG2+ ions?
Wine-red colour
What colour change takes place if sufficient edta is added to the water?
Changed from wine-red to blue
What is the end point of this experiment?
The indicator changes from wine-red to blue
What does a colour change of wine-red to blue tell us?
That all of the Ca2+ and Mg2+ ions have been converted to the corresponding edta complexes
What type of titration is this?
A complexometric titration
What is a complexometric titration?
2
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
Why is it necessary to keep the pH around 10?
2
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
What is added to keep the pH at around 10?
A buffer solution
What is a buffer solution?
A buffer solution is a solution that resists changes in pH, i.e. it keeps the pH constant
What would happen if an incorrect buffer was used in this experiment?
An inaccurate end point will result
Why would the use of an incorrect buffer result in an inaccurate end point?
(2)
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
Why is edta stored in a plastic container?
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
Why is the solution wine-red at the beginning of the procedure?
As Ca2+ ions and Mg2+ ions are present in the water sample
What does the blue colour at the end of the titration indicate?
It indicates that all the calcium and magnesium ions have been converted to edta complexes
