EDTA first or iron ?! - precipitation (Jun/01/2007 )

hi guys,

i'm starting to prepare my MS medium...i'm preparing a stock solution containing Na2EDTA.2H2O and FeSO4.7H2O...it's recommended to add the first (containing EDTA) to the iron containing compound and not versa to avoid precipitation...
it's chemistry, but anyone knows why?!!!

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precipitation can be avoided if any of the solution is added in little portions under well stirring;

similar as in the case of adding Ca2+ to phosphate buffer or vice versa

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preparation is under stirring, but precipitation occurs if the iron compound is added to the EDTA!!!!!!!

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what is the pH? you need acidic pH, add very little amount of conc H2SO4 to improve solubility of FeSO4 and minimize precipitation...

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usually, we don't care for pH at this stage (it's just one stock)...during medium preparation, we add the stock solutions and then adjust the pH

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I don't know exactly what happens but I guess it could be like this (this is just chemistry guess, since I think a number of chemical reactions happen in the same way):
- If you put Fe2+ solution in Na2EDTA solution, since the amount of Fe2+ go in is much less at the given moment compared to EDTA, the reaction happens made Fe salt, which probably is difficult to be dissolve and thus cause precipitation. Every Fe2+ added in will be precipitated.

- On the other hand, when EDTA is added instead into Fe2+ solution, the amount of EDTA is much less compared to soluble Fe salt at given moment. So that when reaction occurs and create Fe precipitation, this can immediately interact with other soluble Fe salt around and become soluble again.

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could it be that Fe (if added first) is completely soluble, so by adding the EDTA, reaction will occur and no preipitation happens
and if EDTA is added first, then the iron compound is added, there is no time for Fe to dissolve!!!!!!!!!!!!

i think it's something to do with EDTA!!!

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Sorry, my mistake, I thought that it is solution into solution. But yes, FeSO4 is soluble. However, the principle is still the same, I think. It is likely not EDTA per se, but the Fe.EDTA salt that would form which may be more difficult to dissolve. I imagine it looks like:
Na2EDTA + FeSO4 <--> Na2SO4 + FeEDTA (which I suppose harder to dissolve)
Beside this reaction (which could cause precipitation), there of course would be more, since all the salts could cross-react, but those reactions by themselves would not be counted as they are easily reversible and would not cause anything ultimately (no precipitation, vaporable compound or H2O made from them). However, as the amount of FeEDTA is very small (when EDTA solution added onto FeSO4 compound), although it is harder to dissolve, the amble amount of soluble FeSO4 around (which is solublizing in H20) would likely help it to solubilize easier. That is, the reaction above will happen less than the other (should be unimportant) reactions and the FeEDTA compound will have the chance to stabilize in the soluble form. On the other hand, when FeSO4 is added into EDTA solution, any added FeSO4 will interact with Na2EDTA and become FeEDTA, with no chance to exchange ion with other FeSO4. The reaction above would become the main reaction, FeEDTA will precipitate.

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no, you are right, that's what i meant
what i can't understand is that why it differs if u add one solution before the other, reaction is the same in both cases

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Chemical reactions are reversible. However, the reversal direction would be severely affected when at least one of the products is removed from the equation, due to either precipitation, vaporization, or products such as H2O. The reactions that would produce either of those products is steadily happened along 1 main direction, forming new compound, and thus is 'counted' as the main, or 'real' reaction when you mix two solutions or compounds together. All other reactions will just occur in both ways and so do not make anything new in the end.

As for the reactions between EDTA and FeSO4 solution: The reactions are the same in both cases, but the limit factors or the environtment where the reaction occurs (or some words with the same effect) are different. Same as you should add H2SO4 into H20 instead of the other way around. The main point here is that the main so call 'real' reaction (of forming FeEDTA) will produce a product that is difficult to dissolve. If this product is allowed to precipitate (due to lack of amble soluble FeSO4), then the reaction will proceed via the direction to form more precipitated FeEDTA.

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