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Re: EDTA and blue water..#592
On Fri, 21 Mar 1997, Toado wrote:
> Subject:
> Craig Bingman wrote:
> on edta packaging:
> >Take it up with Hach, who seem to think it is stable enough to sell in
> >natural polyethylene containers.
>
> Hach products are designed for cheap, rapid analysis, requiring minimal
> skill, where errors of 30% or so are irrelevant. They are excellent for
> this purpose.
> Not a product typically found in an accredited analytical lab.
It doesn't matter. First, accredited anlytical labs don't use EDTA
titration to measure divalent cation concentration anymore. I'm not even
sure that EDTA titration is in standard methods anymore for Ca++, Mg++.
Secondly, when I measure standard seawater with my EDTA solutions that
are in natural poly containers, oddly enough the numbers continue to
agree within 3% of the concentrations found in natural seawater, and this
includes dilution errors, errors in the concentration of titrant, and
operator errors.
> on absorbance spectra and edta stability:
> >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.
>
> Experimental evidence is contrary to your position on this matter.
> Despite this, reaction mechanisms are more significant than absorbance
> in photochem. Reaction kinetics count!. Otherwise (assuming life were
> possible), chemists would all simply use phase diagrams, we'd be called
> physicists.
OK, show me an example of someone who kept a stock solution of Na:EDTA
for a year in a clear bottle, and used it to make up trace element
solutions, and the trace element supplement failed to perform because the
EDTA went off.
It doesn't make a damned bit of difference if the EDTA is 5% degraded for
this purpose.
> on redox chem:
> >The relevance of the rest of your post was irrelevant as far as the
> >stablity of Fe:EDTA in light.
> EDTA drops the redox potential of Fe(2)->Fe(3). This has direct
> consequences for the stability of Fe(2) with or without the presence of
> light. It means that a stock solution of ferrous edetate is less stable
> than one of say FeSO4. I apologise to all for not making this more
> obvious.
> >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.
>
> Fentons chem. involves the production of hydroxyl radicals. Totally
> irrelevant to the simple redox chem. I referred to.
I don't think that the single cycle of Fe+2 to Fe+3 makes iron EDTA
unstable in the light, or in air. And I sincerely doubt that the plants
much care whether or not it is Fe+2:EDTA or Fe+3:EDTA.
I do think that radical generation can destroy Fe:EDTA.
> on blue water:
> >> 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.
>
> The absorbtivity clearly is too small to be observed by the human eye
> over any reasonable path length. A good UV-Vis spectrophotometer uses
> dual beams, one measurement only required, otherwise instrument drift
> becomes too significant.
> More simply just look it up in a spectro' reference.
You are incredibly arrogant. You are talking with someone who has a Ph.D in
biochemistry, and who knows more about marine chemistry than the average
chemical oceanographry program product.
If you don't believe me, then make the measurement. I have. With a good
spec, you can see the effect over 1 cm.
> >Check out a book on marine optics.
>
> The absorbtion of red light by marine waters, or air (blue mountains in
> the distance) is predominantly due to light refraction by minute
> suspended particles, not by the water/air itself.
No. This is not a light scattering phenomenon. And with this sort of a
measurement, the transmitted light would seem more yellow, not more blue.
Please supply me with a reference indicating that the blue color of
seawater or pure water is due to light scattering. Then I'll refer you
to a text on marine optics that has a detailed discussion of the color of
pure water.
N.G. Jerlov. Marine Optics., Elsevier Scientific Publishing Company NY
1976.
There is a plot of the absorption curve for water in the IR. Transission
curves for pure water over various path lengths. There is a discussion
of scattering vs. absorption effects for pure water. And yes, pure water
does scatter light to some degree. As does just about anything except a
vacuum. The shape of the scattering curves are such that they would
imbue a bucket full of water with a yellowish cast, because in looking
into the bucket, you are seeing doubly transmitted light. The water
absorption effect is such that it makes pure water seem blue.
It has nothing to do with whiteners used in buckets or any other such
nonsense. If that were the case, the buckets would appear blue whether
or not they held water.
Craig
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