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Re: Aquatic Plants Digest V2 #591

> From: toado <toado at ihug_co.nz>
> Date: Thu, 20 Mar 1997 12:52:59 +1200
> Subject: Re: EDTA and blue water
> On EDTA..
> *****[begin chemspeak]
> Craig Bingman wrote:
> >> Uhm, Sodium Edetates do degrade appreciably if you chuck a little UV
> >> light at 'em. Lab suppliers package the stuff in opaque plastic or amber
> >> glass bottles for this reason.
> >Sorry, Don't think so.  Sodium EDTA is very stable in light.There is no
> I prefer to call it moderately stable; no problem for aquarists, but
> lousy as an analytical standard. 

Take it up with Hach, who seem to think it is stable enough to sell in 
natural polyethylene containers.
> >chromophore absorbing anywhere near the visible region, so I see no
> >mechanism for it to undergo a photochemical transformation with any
> >light that you are reasonably going to have indoors.  If you put it
> With a little spectroscopy exprience you should spot those C=O's PDQ
> (IR,NIR,UV) and give the near UV another try. Check out its spectrum if
> you're still mystified.

Those C=O's aren't absorbing anywhere near the visible region, and the 
light that makes it through window glass and is generated by fluorescent 
fixtures just isn't much of a problem.

> It seems wet-chem benches really have become a loosers game :).

Watch it.
> >Yes, light is pumping redox chemistry in the case of Fe:EDTA.
> >Relevance to planted tanks:  PMDD and other solutions containing Fe:EDTA
> >should be stored in the dark, or at least in amber containers.  If chem
> >whiz kids on this list make a stock of sodium EDTA, it is perfectly
> >stable in clear glass or plastic, at room temp, in the light.  Solutions
> >of EDTA and any transition metal should be stored in the dark.
> Not all transition metals are a problem (eg Cu)

Perhaps not, but I wasn't going through the entire list.  The 
conservative rec is to store all non-group I salts of EDTA in the dark.  
And if EDTA as light sensitive as the issue you are making of this, I'd 
think that internal consistency would indicate that you should rec that 
Cu:EDTA be stored in the dark as well, unless Cu somehow protects EDTA.

> Craig, you missed the relevance of the rest of my post.

The relevance of the rest of your post was irrelevant as far as the 
stablity of Fe:EDTA in light.

I'm not sure you have the chemistry quite right.  Fe:EDTA participates 
in Fenton chemistry slowly in the presence of oxygen, like a shot in the 
presence of hydrogen peroxide.  

> Back to blue water..
> I'm afraid I missed all this at the time so..

> As water has no obvious chromophores <g>, confirmed by its UV-Vis
> spectra, It is plain that *pure* water *should* appear colourless.

Wrong.  You can see the effect in even short path lengths with a very good 
visible spectrometer.  It should be very obvious at 10 cm.  Zero it in 
air.  Measure the cell, then the cell plus water.  There should be 
obvious absorption in the red, becoming stronger in the near IR.

Check out a book on marine optics.