how would you calculate the EDTA normality, from weight and assay?
Molar mass = 292.24, normality = molarity, but EDTA must be properly dried which is not a trivial task (but perfectly doable given enough time and good lab dryer).
no!no!no!
Normality = molarity only for monoacids or monobase : NaOH or HCl can exchange only one proton
Normality is two time the molarity for diacids : sulfuric or malonic acids have two protons per molecules
So Normality of EDTA (ethylenediamineTETRAacetic acid have 4 protons) is four time higher than molarity (1 mol edta = 4 mol proton)
Molar mass = 292.24, normality = molarity, but EDTA must be properly dried which is not a trivial task (but perfectly doable given enough time and good lab dryer).
EDTA always forms a 1:1 complex with metal ions. for this reason, its normality is equal to its molarity
It seems I failed to explain the argument :-[
Even if it acts on one ion; we don't count by ions.Normality = n * Molarity
Where n is the number of reacting units.
The reference says we count by the number of reacting units --> in complexation reactions by the number of electron pairs...
let me give you a nice argument.
Consider the reaction of sulphuric acid with calcium hydroxide.H2SO4 + Ca(OH)2 :rarrow: CaSO4 + 2H2O
they react 1:1, but I don't think you can claim that normality of sulphuric acid is the same as molarity :)
Of course, normality will be DOUBLE that of molarity. (just multiply the molarity by the reacting units; here it's acid-base --> count the protons)
Normality of reactants depends on the reaction they take place in
I know; I was giving you an example from other type of reaction.
Can you cite me a reference saying how we count the reacting units in complexation reactions?
and in the second phrase you state it doesn't (complexing agent normality doesn't depend on the reaction, depends only on the number of electron pairs)
To determine normality, we have to count the number of reacting units
You may use this number in calculations but the results will be off.I would suggest six for the other partner in the reaction as well. Notice that our difference is only VERBAL... I say 1 mol EDTA reacts with 1 mol calcium; corresponds to 6 N EDTA react with 6 N calicium (In that reaction; it's also 1:1). I am following a definition which won't affect the calculation cause in the very end they give the same result.
For a complexation reaction, the reaction unit is the number of electron pairs that can be accepted by the metal or donated by the ligand.
In this particular case it is molecule that is reacting unit
Please understand that you can't use your references blindly, especially when they give simplified rules that contradict general definition.
there is no change in the oxidation state of either reactant, nor are there relevant changes due to the release of protons from
the conjugate acid of the ligand which nay be used as the titrant
Note that 6N calcium you propose is an artficial construct prepared just to save weak definition.
In this case, normal solutions of the reactants have the same concentrations as molar solutions
of the same reacting species. There is clearly no advantage here in using the
concept of a normal solution.
so that's how they define it using oxidation states.
u got one depnding on the oxidation states...
A development of profound importance in practical analysis was the realisation that titrimetric procedures could be carried out with greater speed and convenience if the concentrations of the two reacting solutions were such that the reaction with the analyte was complete when comparable volumes of sample and titrant solutions had been brought together. More specifically, if volumes VA and VB of these solutions were mixed the reaction would be stoichiometric when NAVA = NBVB where 'NX' the 'normality' of the solution designated the number of 'gram equivalents' per litre.
you would find the concept of electron pairs useful in other complexation reactions not involving EDTA; where the concept of oxidation states would look useless...
According to you:
- in complexation reactions where reactants react 1:1, the reacting unit is the whole molecule, we say normality equals molarity
- in complexation reactions where reactants react otherwise, we count the number of reacting units, which are now the electron pairs.
this is less systematic. By definition, this is a weak definition :)
in the reaction each EDTA molecule donates two protons to neutralize one divalent calcium cation (Ca++)
EDTA is not used for acid/base titrations, but for complexometric analysis. As a complexant EDTA always reacts on a 1:1 basis with metal ions, thus it's normality equals molarity.
However, for practicality sake, to conform to the adopted convention
Complexation reaction can't be linked neither to protons nor electrons exchanged.