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Re: EDTA and blue water..#592 (fwd)
This is absolutely the last I have to say about either of these topics.
EDTA:
> 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
have been stored 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. So I am profoundly skeptical that over a
year or so, anyone's stock solution of pure NaX:EDTA would die to the
extent that it would affect anyone's mileage in making a trace element
supplement.
I think that is ultimately what discussions on this list are supposed to
be about, planted tanks and phenomena relevant to them.
Redox chemistry:
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, and this is my absolute bottom line on this:
-----------------------------------------------------------
> >> 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 both incredibly arrogant and simply wrong.
If you don't believe me, then make the measurement. I have. With a good
spec, you can see the effect over 1 cm. With your eye, you can see it
over 10 cm path lengths.
It seems that your are badly in need of a refresher course in
spectroscopy, if you don't know that water absorbs light in the IR.
> >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. The blue color of water is *not* a light scattering phenomenon. And
with this sort of a measurement, the transmitted light would seem more
yellow, not more blue.
The sky is blue due to light scattering. The sun looks more yellow, even
orange when it sets because blue light is scattered out of the directly
transmitted light, leaving more of the longer wavelengths.
When you look into a an opaque bucket, you are in transmission geometry.
Light goes through the water, reflects from the container wall, and comes
back out through the water. Two transmission events, one reflection
event from the wall of the bucket.
If a clear container were filled with a substance that scattered light, and
you observed the scatted light orthogonal to the beam, yes, it would be
more blue. If you looked at the transmitted beam, it would be more yellow.
Please consult the following reference:
N.G. Jerlov. Marine Optics. Elsevier Scientific Publishing Company, NY
1976.
There is a plot of the absorption curve for water in the IR. Transmission
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 is
stronger than the scattering effect in pure water, therefore, pure water
is 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.
The reason detergent kills the blue color in the Nessler tube is probably
because of light scattering effects. Detergent micelles do scatter
light. The meniscus effect is a red herring, because refraction is
causing you to see light that has been transmitted diagonally across the
tube, and hence has been through a longer path length, and is more
depleted in red light as a result.
Relevance to planted tanks:
I thought someone might be interested in knowing that pure water is blue.
Some of you use RO/DI to purify water. It should be very light blue when
held in a white polyethylene container. The presence of a blue color does
not assure that the RO/DI unit is removing all ions, but failure to
observe such color under reasonable seeing conditions may indicate that
there is a problem, especially if you have seen it before and it
disappears. This is also true for people who use activated carbon to
filter tap water.
I had no idea that such a simple and well understood fact would explode
into such a display of egos and conspicuous ignorance, and I apologize to
the list for this protracted discussion. Pure water is blue, if you don't
believe me, look at the book on marine optics which contains a detailed
discussion of this phenomenon.
Craig
P.S. Cynthia, please remove my name from the list. I came here
primarily to learn, and the list simply isn't meeting my needs.
Thanks.