[pasky]

how would you calculate the EDTA normality, from weight and assay?

[Borek]

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).

[akami]

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)

[Borek]

no!no!no!

yes!yes!yes!

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)

You are partially right but wrong on the whole.

Normality is not connected with acid/base reactions only and for one substance can be different depending on what is the second reactant. 1M FeCl3 solution in redox reactions is has normality 1, but if used for Fe(OH)3 precipitation it's normality is 3.

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.

[pradeep]

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).

If you require it on assay, then divide weight by factor. For ex. If EDTA is 99% u require to weigh 292.24/0.99 g EDTA to prepare 1M -1000 ml solution.

[ksr985]

Borek is correct. EDTA always forms a 1:1 complex with metal ions. for this reason, its normality is equal to its molarity.

In fact, akami, EDTA is generally used as the disodium dihydrogen salt. It is not used as the tetrasodium salt. I believe the reason for this is that the tetrasodium salt is so basic that it absorbs CO2 from the atmosphere, leading to incorrect titre values.

Moreover, it is not wise to generalize the N factor between normality and molarity. One must look at individual reactions separately, to know exactly how the substance we're concerned with is behaving, in a particular reaction.

[Arabisch]

Dear all,

EDTA always forms a 1:1 complex with metal ions. for this reason, its normality is equal to its molarity

I think we need to return to the definition of normality. I checked "Modern Analytical Chemistry; Harvey 2000":

The number of equivalents, n, is based on a reaction unit, which is that part of a chemical species involved in a reaction.

(Then, the book starts mentioning different types of reactions)

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.

So, we can't just say that normality is molarity, as in our case EDTA might donate up to six pairs (and so the ligand binding to it would receive that much)...

Thanks all, correct me if I am mistaken

[Borek]

EDTA is a polydentate ligand, meaning it may act as several ligands at once - but on one ion. So it still reacts 1:1 and its normality equals its molarity.

[Arabisch]

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 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)

[Borek]

It seems I failed to explain the argument 

You failed, because you are wrong.

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...

You may repeat is as many times as you want, but EDTA normality in complexation reactions will still equal its molarity, because of the way it reacts. If you take other ligands - like ammonia of cyanide - they will react using 1 electron pair. If you take ligand like ethyldiamine - it has two electron pairs, so in the case of cation that needs 6 pairs three molecules of amine will be needed. EDTA has 6 electron pairs, but it always react in the 1:1 ratio - it uses as many electron pairs as needed and due to its geometry it never reacts with more than one ion. Your reference either doesn't say whole truth, or uses too simplified approach.

let me give you a nice argument.

Consider the reaction of sulphuric acid with calcium hydroxide.

H₂SO₄ + Ca(OH)₂ 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)

Nice example, but missing the point. Normality of reactants depends on the reaction they take place in. In this case for acid/base reaction their normality is twice the molarity, but for CaSO₄ precipitation both normalities equal molarity.

[Arabisch]

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?

thanks a lot

[Borek]

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?

No, I can't (I don't have easy access to literature, and what I have at hand is in Polish, so will be of no use for you).

Note, that you are contradicting yourself. You are stating in one phrase that you know that normality depends on the reaction (like normality of sulfuric acid being 1 or 2 depending on circumstances), 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). You can't be right in both cases at the same time.

I feel this discussion leads nowhere.

[Arabisch]

determining Normality depends on the type of reaction

To determine normality, we have to count the number of reacting units (which MIGHT differ according to the reactions).

I went through more than 5 references and no one states what you say!

You just need to find a system to define normality... If you have a referenced one, then it's ok for me... but according to the definition I found in all the references I have, we count in complexation reactions the number of electron pairs...

I also think the discussion is going nowhere except if we:
1- succeed to find a definition for calculating normality in complexation reactions
2- apply that definition

I got the definition from my reference! You might have a different definition, for sure and then I would agree

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)

According to reaction, you choose the way of counting reacting units... In acid-base --> protons; in complexation --> electron pairs; in redox --> electrons ... etc...

I don't think this takes us anywhere as well... Only because you can't reply in a better way respecting your opponent...

[Borek]

While the general idea (about counting eletron pairs) works for many compounds, it doesn't work for EDTA. EDTA has 6 electron pairs so normality of 1M solution should be 6. You may use this number in calculations (V_MeN_Me = V_EDTAN_EDTA) but the results will be off.

To determine normality, we have to count the number of reacting units

Which in the case of EDTA complexation is always 1, regardless of the number of electron pairs present in the molecule. In this particular case it is molecule that is reacting unit, not an electron pair. Same in the case of DCTA, DTPA and many other compounds (even citrate).

Please understand that you can't use your references blindly, especially when they give simplified rules that contradict general definition.

[Arabisch]

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.

If we agree to call the Kg a "half-k" it won't be a problem at all with any calculation, as we generalize it...

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.

That's the definition I got... Which goes for BOTH the EDTA and the one reacting with it. Of course, I don't mean that 6 N EDTA react with 2 N calcium (and I apologize if that what I managed to tell with my poor language). It goes for the donator AND the acceptor...

I asked the head of analytical chemistry department about this; how to calculate the normality of EDTA? she first told me that we don't need to calculate normality for EDTA as it reacts 1:1 then she said exactly what I stated

In this particular case it is molecule that is reacting unit

My friend, I don't mind this at all but it needs to be a convention known somewhere (that's why I asked for a definition or reference). If there is a reference saying so, or a convention following so, I really don't mind

You know, it's like 7 years ago when I argued about the definition of a "meter" in a physics class. The instructor laughed and said: "Define any meter you like, but others use another definition "

I am just afraid that conventionally the definition is different

Please understand that you can't use your references blindly, especially when they give simplified rules that contradict general definition.

Thanks for the advice. You might help me by giving me the general definition; may be I am mistaken. Please help me with this definition and mention the reference

This general definition, of course, should apply to all examples. It will really be helpful for me

Thanks a lot