[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: EDTA and blue water

*****[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. 

>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.

>under a UV lamp, sure, it will probably degrade.  Many suppliers sell
>EDTA in natural polyethylene containers.

Only 'volucon' and GPR, Not AR grades I Hope!. I doubt an accredited lab
can get away without freshly standardising EDTA if its used as a
It seems wet-chem benches really have become a loosers game :).

>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)
Craig, you missed the relevance of the rest of my post.
To spell it out:
Iron(2)/Ferrous is what we want not Iron(3)/Ferric ions. 
Adding EDTA makes the iron availiable but speeds up the oxidation *even
in the abscence of light*.
Ferrous tends to form Ferric even by disproportionation (self
Keeping the solution acidic will prevent any oxide from forming by
disproportionation. You can do this by using EDTA instead of its di- or
sodium salts, or by adding a little acid.
Dropping the temperature by 10C will roughly halve the reaction rate.
*****[end chemspeak]

So try to keep the Fe:EDTA soln. in the dark, cool (fridge) and acidic.
Better yet as Paul K. has pointed out more appropriate chelators than
EDTA are availiable. If you don't have lab access try

Back to blue water..
I'm afraid I missed all this at the time so..
Craig Bingman wrote:
>> >     and the bucket water is not yellow; it is colorless.
>If it appears colorless, then it has a small amount of yellowing
>substances present.
>> >     This information is available in any basic oceanography or limnology
>> >     text.
>You don't know what you are talking about.  Pure water in a white 5 gallon
>bucket has a discerably blue hue to it.  Period, end of story.
>If you have not seen that blue hue, then one of three things is true:
>1.  You don't have pure water.
>2.  You are colorblind.
>3.  Your light source is almost entirely devoid of blue content.
etc. ad nauseum.
As water has no obvious chromophores <g>, confirmed by its UV-Vis
spectra, It is plain that *pure* water *should* appear colourless.
Equally obvious is that many of us have noted an apparent blue hue under
various circumstances, and that gelbstoff can mask this. The blue
warrants an explanation.
I feel the most obvious explanation for the 'blue' with some, but not
white buckets is the buckets!. Most bright-white paints & plastics these
days use 'optical brighteners' as well as pigment. The same
'brighteners' are now found in almost all laundry detergents. These are
effectively dyes absorbing UV and fluoresing at the blue end of the
visible spectrum. They give that 'whiter-than-white' appearance we
consider so 'clean'. The appearance is greatly enhanced by UV found in
sunlight and actinic lamps.
Tap water can also appear blue with a few ppb of copper when ammonia or
ammines are present. The tetrahedral Cu complexes formed are *much* more
intense than normal copper salts. Possibly some old copper pipes and
decomposing chloramine could do the trick.
Craigs later mentions APHA colour units. These are an anachronism with
instruments so plentiful. However when staring down those nessler tubes
(or any fine bore glass) even triple distilled water tends to look blue.
Look closely and you notice the blue's mostly at the meniscus(edge).
Add a little surfactant and the effect disappears with the meniscus. The
effect is due to optics but I'll have to leave that to a physicist to

As Hollywood might put it: "often the obvious aint"