[kwikwi]

Hi all,

I'm having a problem figuring out one solution needed for sequential extraction of soil metals.
It's the fourth fraction of Zeien and Bruhmmer method (1989), where 0,025M NH4-EDTA is used.

It might be my lack of general chemistry knowledge (I'm a biologist), but among a variety of EDTA solutions and mixtures  to be ordered from supply companies (in Europe), I can't seem to find ammonium-EDTA...

What am I doing wrong?
I feeel like it's something notoriously obvious that I am missing...

Thanks,
viki.

[Borek]

I am not surprised you can't find it - EDTA is usually sold as as either acid or disodium salt. You can prepare ammonium salt just by dissolving EDTA in ammonia solution (watch for stoichiometry).

[kwikwi]

Thanks!

I suspected that, but was uncertain if it actually should be 0.025 mole of NH3 and 0.025 mole of EDTA in one litre. Should it? (is that what you mean by - watch for stectiometry)

And then again if  a tetraammonium edta is potentially formed, does that mean that as long as it is 1:1 in moles, it should stay what it is stated in the procedure - NH4-EDTA, with one ammonium attached?

I really appreciate your reply, I found no better answer from a number of coleageues around me.... thanx.
kwi.

[Borek]

It is not clear to me how to read NH₄-EDTA - EDTA is a tetraprotic acid, so there are 4 possible salts. If treated as a formula, that would mean monoammonium salt - and yes, that would mean you have to mix them in 1:1 molar ratio (although most likely small deviation won't hurt). Trick is, solutions of different salts have pH varying by several pH units (around 2.5 for a monoammonium salt, close to 9 for tetraammonium), and that is most likely a very important parameter.

[bambam]

Helpful thread! I`m concerned about the same topic.
the last step to obtain solid NH4-EDTA (monoammonium salt - required Substance for the Zeihen\Brummer extraction scheme) remains unclear for me. thank you in advance.

[Borek]

the last step (...) remains unclear for me

What is the last step and what is unclear for you? As much I would like to I can't read minds

[bambam]

unclear part was

Trick is, solutions of different salts have pH varying by several pH units (around 2.5 for a monoammonium salt, close to 9 for tetraammonium), and that is most likely a very important parameter.

and how to adapt the above statement in order to functionally adequate handle dissolving EDTA in ammonia solution. 1:1 molar ratio, adjust pH to aim, thats it?


aim is to yield 400ml 0.025M NH₄-EDTA p.a. (EDTA 7.31 g\L ) at pH 4,6 (with ammonia solution), which is required for the extraction of certain metals (EDTA-soluble \ organic-ligated fraction) from soil.

[Borek]

0.025M NH₄-EDTA p.a. (EDTA 7.31 g\L ) at pH 4,6 (with ammonia solution)

As stated it doesn't make much sense - but it is probably not your fault. NH₄H₃EDTA solution will have pH closer to 2.5 than to 4.6. To get higher pH you need to add more base, so it is either 1:1 ammonia/EDTA plus something else (like NaOH) or it is not 1:1 ammonia/EDTA. 2:1 ammonia/EDTA ((NH₄)₂H₂EDTA) will have pH close to what you need.

[bambam]

so the first option is definately not true. i read explicitly, that the pH later is to be reached by adjusting with diluted ammonia solution. Thank You- and thank the exclusion principle. hope, this acid is meant by NH₄-EDTA in all the descriptions.