EDTA first or iron ?! - precipitation (Jun/01/2007 )
Almasy, so u mean that the previous presence of FeSO4 is important for the solubility of FeEDTA!!!! 
but , HOW ?!! 
I don't know the exact details, but the idea is about the saturated ions in the solution (INTO which you are going to add another solution). This is important, I think, with ion that has more than one free bond, which can interact with other components contained in the solution (like the OH- or H+), beside the desired ion partner, and thus may, due to those interactions, to form either colloid or precipitation or soluble compound. Say, when you add Ca2+ into phosphate salt solution, the PO43- ion will be the saturated one, and they will surround all Ca2+ that come in. In the other way, Ca2+ will be the saturation factor and surround the PO43-. I am not very sure which way will form what, but in one way, it would facilitate the formation of Ca3(PO4)2, which is precipitated down, while in the other way, formation of colloid is possible, or maybe there could be many other combinations between Ca2+ and phosphate and other ions besides Ca3(PO4)2 --> no precipitation. I guess your case is similar.
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?!!!
Strawberry,
The forward and backwards reactions are indeed the same; however, chemistry here is the answer. The reason you need to add the EDTA first is simply because if you add the FeSO4 first, at mid-range pH's, the Fe will immediately begin precipitating out as iron hydroxide (which is extremely insoluble in mid-range pH's). The precipitation is actually quite slow, but it is fast enough to cause problems. This is the same reaction that occurs in your toilet tank (when you pop open the top of the tank and see the nasty red formations).
Iron hydroxide usually takes many days to precipitate out completely, but at pH's around 7 or 8, its solubility is in the ppb range. You don't see it right away because the precipitation is so slow, but that is just a kinetic factor. Thermodynamically, if you add your FeSO4 first, and wait, the iron hydroxide will form...Slowly...But quickly enough to change your measurements.
If you add the EDTA first, the reaction to complex with the EDTA is MUCH faster than hydroxide formation/precipitation.
Thus, the reason for the order of addition.
Hope this helped.
Good luck!
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?!!!
Strawberry,
The forward and backwards reactions are indeed the same; however, chemistry here is the answer. The reason you need to add the EDTA first is simply because if you add the FeSO4 first, at mid-range pH's, the Fe will immediately begin precipitating out as iron hydroxide (which is extremely insoluble in mid-range pH's). The precipitation is actually quite slow, but it is fast enough to cause problems. This is the same reaction that occurs in your toilet tank (when you pop open the top of the tank and see the nasty red formations).
Iron hydroxide usually takes many days to precipitate out completely, but at pH's around 7 or 8, its solubility is in the ppb range. You don't see it right away because the precipitation is so slow, but that is just a kinetic factor. Thermodynamically, if you add your FeSO4 first, and wait, the iron hydroxide will form...Slowly...But quickly enough to change your measurements.
If you add the EDTA first, the reaction to complex with the EDTA is MUCH faster than hydroxide formation/precipitation.
Thus, the reason for the order of addition.
Hope this helped.
Good luck!
Hi, raeganro01. Thanks, that is nice to know. I have asked a few chemistry people, but they could not tell me exactly what happened, although they believe that it is not the same as with calcium phosphate. Thanks again.
thanx raeganro01 
could it be due to the proportion between EDTA and iron, maybe iron is much more than EDTAs !?
It's the iron hydroxide as raegan said. What's surprissing is the many answers that were confidently offered and so wide of the mark.
If you're worried about the stoichiometry - do the math.