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Re: EDTA chemistry lesson


David J Snell <dsnell at erols_com> wrote, Sat, Apr. 13

.........<snipped>........

>1.  How does one make FeEDTA or FeDTPA? I have found  several
>chemical supply companies that sell EDTA and DTPA, but what
>Iron chemical(s) do I mix with it.  Or is there a source I don't
>know about that will sells it premixed?

Get DTPA, because it is superior to EDTA as a chelator.  Get ferric
sulfate, FeSO4.7H2O.  It is easy to work with because it doesn't absorb
water from the air and become slushy.  Ferric chloride is horrible in that
respect.  Mix a molar eqivalent of the iron sulfate and the DTPA together.
The molecular weight of the FeSO4.7H2O is 278.02, and the molecular weight
of the DTPA is 393.35.  I mix it at 0.01 molar, about 200 mls at a time.
At that strength, 1 ml per gallon of water gives you about 0.15 ppm iron.
Two hundred mls of 0.01 molar solution would contain .002 moles of the
FeSO4.7H2O, or 0.556 grams(.002 moles x 278.02 grams per mole) and .002
moles of the DTPA, or 0.787 grams(.002 moles x 393.35 grams per mole).
Dissolve both the iron and the DTPA, and then heat the mixture to boiling.
If mold starts growing in the solution later, you can re-boil, or add some
hydrochloric acid, as does Kevin Conklin to his PMDD recipe.  By the way, I
once tried to make 0.1 molar FeDTPA, but a good portion precipitated out,
when the mixture cooled.
>
>2.  Based on whatever Iron chemical is used, how much of each
>chemical does one mix together in distilled water:
>        X Iron mixed with Y EDTA = what concentration of FeEDTA  or
                X + 1.05 X if X is weight of FeSO4.7H2O
>        X Iron mixed with Y DTPA = what concentration of FeDTPA  or
                X + 1.41 X if X is weight of FeSO4.7H2O
>        X Iron mixed with Y EDTA and Z DTPA = what concentration.
                X +(A x 1.05 X ) + (B x 1.41 X) (where A is the fraction of
EDTA
                and B is the fraction of DTPA you want.  [A+B should =1]).  I
                don't know what the advantage of having a mixture of the two
                chelators is. I would vote for having all iron DTPA, which
lasts
                in the water for me a lot longer than iron EDTA.)

Paul Krombholz                  Tougaloo College, Tougaloo, MS  39174

I ought to be doing my incom tax!