Edta Flashcards
What is a ligand
Latin “to bind”
A molecule or ion that has at least one pair of electrons the can be donated
Lewis base
What is a coordination number
The number of ligands bound to the Lewis acid
What is mono bi tri dentate ligands
One atom of the ligand bonded to the Lewis acid
2 ligands
Three
What is the chelate effect
What is an example of a chealating agent
Acid bound to mores ring ligands
Give the compound enhanced stability
EDTA
What is the fully protonated of EDTA
What is the neutral form
What is the most important form
H6Y(2+)
H4Y
Y(4-)
When does the Y(4-) form of EDTA dominate
After ph 10.5
At high ph
What does the fully deprotonated from (Y4-) of edta bind with
Forms 1:1 complexes with most metal ions
What is the effective concentration of edta given by
It’s fraction (*Y4-) that is fully deprotonated
What does edta not bind
K+ Na Li
The indicator in an edta titration
Binds less strongly to the metal than the edta
What is the concentration of [y4-] dependent on
pH it’s crucial to buffer to a basic ph
What are the zones of the edta titration curve
- Before EP the [metal] is determined by excess unreactes metal in solution
- At EP equal amount of EDTA and metal, [mwtal] is found by dissociation of product and conditional formation constant
- After ep excess EDTA, use excess [EDTA] in k’f expression to calc [metal]
What happens to the endpoint if the k’f is larger
What happens if the pH is lowered
The endpoint is more distinct
Ve point becomes less distinct, k’f lowers, endpoint less sharp
What is an auxiliary complexing agent
A ligand that binds to the metal string enough to prevent metalOH from being formed
This is because at high ph metals try forming product with OH which prevent titration with EDTA
What does the presence of auxiliary complexing agents do
Lowers amount of free metal
Decreases sharpness of endpoint
Does not change the ep
What are metal ion indicators
Have a diff colour when bound to a metal
Must bind to metal less less strongly then edta
Also acid base indicator so need to control ph
Can only be use in certain ph ranges
What is meant by indicator blockage
How do we fix it
Metal doesn’t diss isolate from and indicator, so the metal blocks the indicator
Do a back titration:
Add excess EDTA to the analyte
Add the indicator
Titrate excess edta with another standard solution that has another metal
Why are back titrations necessary
If the analyte precitptates with out edta present
Analyte reacts too slowly with edta
Analyte block the indicator
The metal ion for the back titration shouldn’t displace analyte from edta
What are displacement titrations
Metal displacement
The solution with the analyte metal is titrated with excess EDTA which is complexed to the other metal
What is an indirect titration
Used to determine anion concentrations
Anion is precipitated with excess metal ion (concentration known)
The precipitate is filtered then washed
The excess metal in the filtrate is titrated with standard EDTA
What is meant by hardness
The total concentration of alkaline earth ions in water (mainly ca and mg)
