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Author Topic: EDTA normality  (Read 32312 times)

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Borek

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Re: EDTA normality
« Reply #15 on: December 15, 2008, 09:30:03 AM »

Normality of complexing agent (read end of page 333 and the beginning of page 334):

http://www.iupac.org/publications/pac/1978/pdf/5004x0325.pdf

Normality is a concept based on equivalents, equivalents are defined by the reaction:

http://www.iupac.org/publications/analytical_compendium/Cha06sec3.pdf

Note that 6N calcium you propose is an artficial construct prepared just to save weak definition.
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Arabisch

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Re: EDTA normality
« Reply #16 on: December 15, 2008, 10:25:44 AM »

ok... let's see...the first reference states:

Quote
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

so that's how they define it using oxidation states.

Quote
Note that 6N calcium you propose is an artficial construct prepared just to save weak definition.

 ;D ... my friend, it's not a "weak definition"... We just need to agree on something and follow it :) I got a definition depending on the electron pairs; u got one depnding on the oxidation states... I just wanted to make sure there is such a referenced definition.

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...

but nice thing I would like to quote as well (from the first reference. Thanks by the way; it's excellent):

Quote
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.

Which my professor told me; no need to calculate normality AT ALL in this case

Thank you :)
« Last Edit: December 15, 2008, 10:43:54 AM by Arabisch »
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Arabisch

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Re: EDTA normality
« Reply #17 on: December 15, 2008, 10:40:14 AM »

and by the way, we studied in philosophy that a weak definition is the less applicable one on available cases.

If you define according to the change in oxidation states, then this is the real weak definition because it will not be applied on many many complexation cases. You must add another constraint; which is: the reaction should remain 1:1. Adding constraints decreases the applicability of a definition; making it less general and weaker...

Consider for instance complexing Ag+ with ammonia:

Ag+ + 2NH3  :rarrow: Ag(NH3)+2

no change in the oxidation states :) but if you think in term of electron pairs contributing, then we say n for Ag is 2 and for NH3 is 1.

The approach you and they adopted is logic and nice, I agree, but it is not systematic.

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 :)

But it was a nice chat anyway :)
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Borek

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Re: EDTA normality
« Reply #18 on: December 15, 2008, 12:07:29 PM »

so that's how they define it using oxidation states.

No, they don't. They refer to the earlier discussion in which they eplained how to deal with acid base reactions and redox reactions.

Quote
u got one depnding on the oxidation states...

You have obviously not read the documents I linked to. They define normality based on the equivalents, and equvalents are based on the reaction equation:

Quote
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.

For acid base reactions equivalents can be linked to protons, for redox reactions equivalents are linked to number of electrons exchanged (hence change in the oxidation state). Complexation reaction can't be linked neither to protons nor electrons exchanged. To some extent they can be linked to number of electron pairs - but this approach doesn't work in all cases.

Quote
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...

This is a classic strawman argument - nobody ever said that oxidation states are usefull for complexation reactions, so large part of your discussion doesn't make sense.

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 :)

This is not how I calculate normality, that was my explanation why your approach is not universal. Again, strawman, and you are twisting my words.

To calculate normality you should start always with the reaction equation. This is the strongest approach that covers complexation, acid-base, redox, precipitation and all possible other types of the reactions.
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enahs

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Re: EDTA normality
« Reply #19 on: December 16, 2008, 07:52:30 AM »

Seeing is how even to this day I see in papers and books where people use normality wrong (see this whole thread), I vote we get ride of the whole concept of normality. I can do math, I would much rather have to do the occasional simple math problem then instead of reading a paper or book that refers to normality go tracking down if it was used correct or not...
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dastgir

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Re: EDTA normality
« Reply #20 on: April 14, 2009, 03:46:05 PM »

All dears,

Why so much argument on the normality. Normality is very simple concept which derived from gram
equivalents of reactant. One gram equivalent is that weight of a reactant which releases / contributes electrons equal to avagadro number ( 6.023 X 10^23). Moreover gram molecular weight of any species
have no. of that species equal to avagadro number. That is why normality varies from reaction to reaction.

e. g. in case of HCl when it is reacting with NaOH its gram molecular weight and gram equivalent is same therefore its normality = molarity.

HCl (6.023 X 10^23)(1 gm mol. wgt.) = H(+)(6.023 X 10^23) + Cl(-)(6.023 X 10^23)

But if we replace HCl with H2SO4 the whole thing changes like below

H2SO4(6.023 X 10^23)(1 gm mol. wgt.) = 2H(+)(6.023 X 10^23)+SO4(2-)(6.023 X 10^23)

Since here 2 mol of H(+) (2 X6.023X10^23 nos.) are being released therefore for reaction with NaOH
the normality will be doubled of molarity, since as per definition equivalent wgt. of H2SO4 will be that
wgt. which releases only 6.023 X 10^23 nos. of H(+) and that will be half of its molecular wgt.

But this is the case when we are considering the only that part of reaction which involve H(+) cations.
For the other half reaction wherein SO4(2-) is taking part, the molarity and normality will be same
since here released species is 6.023 X 10^23 which indicate Mol. wgt. = Equ. wgt.

Regards,

Dastgir
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Metal Man

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Re: EDTA normality
« Reply #21 on: September 20, 2013, 07:52:11 AM »

Normality is overrated.

The normality of EDTA, where water hardness is concerned, is twice the molarity according to the widely accepted  definition of normality. 

http://environmentalchemistry.com/yogi/chemistry/MolarityMolalityNormality.html

While it is true that one molecule of EDTA reacts with one molecule of calcium, in the reaction each EDTA molecule donates two protons to neutralize one divalent calcium cation (Ca++).  So, even though there is a 1:1 molecular equivalence using molarity, the correct normality for EDTA in this reaction is, again by definition, twice the molarity.  All the available literature I could find on the subject bears this out.

You chemist-philosophers out there can carry on the debate, while the rest of us zero the burette and get on with the next titration. 
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Borek

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Re: EDTA normality
« Reply #22 on: September 20, 2013, 10:32:43 AM »

in the reaction each EDTA molecule donates two protons to neutralize one divalent calcium cation (Ca++)

Complete nonsense.
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Metal Man

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Re: EDTA normality
« Reply #23 on: September 24, 2013, 08:26:52 AM »

Borek, please explain specifically where you find error in my reasoning.  I stand by my explanation, based on the definition of normality.  Furthermore, you will want to share your explanation with the Labchem folks too.

LabChem Catalog # LC13850-1
Product description:  EDTA, 0.01M (0.02N), Titrant for Water Hardness, (1mL = 1mg CaCO3)

I welcome you to check out their fine product line for yourself...

Again, here is the widely accepted definition of normality:

Normality = molarity x n (where n = the number of protons exchanged in a reaction).
 
In short, while there is a relationship between the normality of a solution and the molarity of a solution, the normality can only be determined by examining the reaction, determining the proton exchange and multiplying molarity by that number.

Returning to the question at hand, the hardness test:

Ca+2 + EDTA-4  :rarrow: CaEDTA-2

We can agree that 1 mole of EDTA neutralizes 1 mole of Calcium as CaCO3 (it’s a 1:1 equivalence).  Here is where normality confuses things, since EDTA molarity is not equivalent to normality in this case.  It is very common to see the hardness calculation based on the normality of EDTA, even though in this instance molarity would be much more straightforward since we simply want to convert back to hardness in mg/L.  In the hardness reaction each EDTA molecule exchanges two protons to chelate one divalent calcium cat ion (Ca++).  So, even though there is a 1:1 molecular equivalence using molarity, the EDTA normality is, by definition, twice the molarity for the hardness test.    Again, given that EDTA donates two protons per molecule in the balanced equation for hardness determination, you must multiply EDTA molarity by two to get the correct normality.   For this very reason you don't see standardized EDTA available for purchase as a “normal” solution (unless specified "for hardness" as in the example I gave above).  EDTA can potentially donate up to four protons, depending on the reaction.  The textbook definition of “normality” means that a standardized “normal” solution of EDTA could be very misleading.

The EDTA hardness calculation using a “molar” solution is:

Hardness as CaCO3 mg/L =    Vol (mL) titrant x EDTA (Mol) x 100g/mol CaCO3 x 1,000mg/g   
                                                                                Vol (mL) Sample


This simplifies to:

Hardness as CaCO3 mg/L =   V1 x M1 X 100,000 / V2


To convert the calculation to use a “normal” solution, N/2 must be substituted for M1. Factoring out the 2 in the N/2 term with 100,000, you get:

Hardness as CaCO3 mg/L =   V1 x N X 50,000 / V2
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Borek

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Re: EDTA normality
« Reply #24 on: September 24, 2013, 08:42:07 AM »

I was not referring to the reasoning, but to exact wording of your post. Protons donated by EDTA don't neutralize Ca2+.

There is a problem with normality. Formal IUPAC definition refers to reactions with H+ and/or e-, but practical definition - at least as shown in analytical chemistry books from my neck of the woods - usually shows normality as reaction dependent. This produces quite a mess.
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Metal Man

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Re: EDTA normality
« Reply #25 on: September 24, 2013, 10:33:17 AM »

Quote
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.

So do you now redact the above statement? 

I agree normality is a somewhat contrived concept and very confusing in this context.  However, for practicality sake, to conform to the adopted convention, you must use the computation (normality = molarity * 2) for determination of EDTA hardness.  I believe this is much more germaine to the OP than the semantic dispute that followed.
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Borek

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Re: EDTA normality
« Reply #26 on: September 24, 2013, 12:03:18 PM »

However, for practicality sake, to conform to the adopted convention

Problem is, there is no single, adopted convention. There are two conventions - one in which normality depends on the reaction and one in which normality is defined always by reaction with H+ or e-. IUPAC tries to get rid of the problem by making the concept of normality deprecated.

I think I am going to lock the thread, it doesn't make sense to repeat the same discussion every few years just because someone googled it and necroposted.
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Metal Man

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Re: EDTA normality
« Reply #27 on: September 25, 2013, 05:39:33 AM »

Quote
Complexation reaction can't be linked neither to protons nor electrons exchanged.

Why do you assume this?  Why not treat it like an acid/base reaction?  The two protons from EDTA could, for the purpose of discussion, be said to associate with carbonate to form carbonic acid in the balanced reaction.

All that aside, there may be more than one convention, but dozens of chemical manufacturers (several of which distribute internationally) market standardized EDTA solutions as "0.01M (0.02N)".  You need only to Google "0.02N EDTA" to confirm this.  That does not make it ideal or right, but that formulation is in wide use.  While we can agree that the application of normality in this case is at best unhelpful and ought to be "deprecated", the convention above is the one used by water quality professionals in my experience and my hope is that my response is helpful to them.

I have enjoyed the discussion and leave closing remarks up to you.

Thanks.
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cpicsonly

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Re: EDTA normality
« Reply #28 on: January 11, 2014, 08:50:26 AM »

@Metal Man
U have saved my day.
I have wasted a huge time searching for answer on Normality and Molarity of edta.
Got relation and by the citation, it seems to be correct to follow.

Let me just get confirm
As per this manual on soil salinity
(Diagnosis and. Improvement of. United States Salinity Laboratory Staff. Contributing Authors: L. E. Allison. L. Bernstein. C. A. Bower. J. W. Brown. M. Fireman.)
take 2 gm of EDTA aprox normality is 0.01 N
where as we know 3.72 gm edta =0.01 M   
(Is it close to 0.02N)
this is refered in estimation of Calcium and Mg (both diavalent) by versenate method with specific note of checking standardisation with std. Ca+ solution.
Can u plz say that it is correct?
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