[Prev][Next][Index]
Re: Diffusion coefficients
Craig Bingman responded to Olive K. Charlsey thusly:
> > > Which ions are more mobile? Perchance, is there a table giving
> > > mobility parameters?
> >
> > Well, really, their mobility is dependent on their charge and
> > their masses. Table? Well, you don't really need one...
>
> In any event, what the questioner wants to know or have is a table of
> diffusion coefs
Yup! :-)
> Although it is not apparent in the chopped up replies, the original
> statement was regarding conductivity, that various ions have different
> mobilities (even beyond charge and mass of the ion itself, because
> hydration plays a major role)
Yes, my question was really a splinter thread in a new direction since
somebody raised the issue of different ions having different mobilities.
> Diffusion coefficients and ion mobility in an electric field are related.
> Of course this charge business plays a dominant role in the latter.
>
> > However, to answer your question... O2 and CO2 diffuse rather
> > slowly in and out of the substrate - as do other nutrients.
>
> Thanks for that, the answer is "slowly." Its cafe au lait time.
Well, I hate to be picky but I've heard answers from ranging from
"slowly" to "really slowly" to "very very slowly" to "hundreds of
years for oxygen to diffuse a few feet in unstirred water". ;-)
Now we know the latter statement is an exaggeration since the transition
zone for redox potential in lake sediments is a few centimeters thick
and this is from oxygen diffusing into the otherwise anaerobic
substrate. (ignoring the role plants play in introducing oxygen
into the mud and in inducing circulation by drawing water into their
roots for the moment)
What I'm getting at is that when you first submerge a substrate,
there will be a certain amount of phosphates and other nutrients
which leach out and into the aquarium water where they can cause
complications like green water algae blooms. How much depends upon
several factors like:
1) how fast phosphate complexes and ions diffuse
2) how much dissolvable phosphate is in the soil/mud
3) what ratio the soil/mud is mixed with inert material like sand or gravel
4) how deep that layer is
5) how thick an overlying layer of inert material there is
6) how fine the inert material is (gravel < sand < clay)
7) how fine, compact or porous the soil layer is
7) any special procedures for preparing the substrate to avoid
mixing water containing phosphates upon submergence.
8) the presence of any stray electrical fields ;-)
It seems like phosphates leach out quickly (or maybe the aquarium
reacts to even a small concentration) whereas oxygen diffuses into
a substrate slowly. What I'm curious about is if there is any
motivating force for diffusion of polarized ion complexes aside
from kinetic jiggling? Is there some sort of energy potential
which gets minimized as an ionic solution reaches equilibrium
that would provide motivating forces for diffusion? (recall that
water molecules are polarized; that's what makes it good at
dissolving things)
I apologize for my lack of precise scientific terminology; if I
knew the right terminology I could probably answer my own question! ;-)
BTW, feel free to launch into a discourse about the true nature of
dissociated salts in solution. It might be fun to read (well for some
people)
I wonder if there is anything we could make use of an electric field
for in an aquarium like maybe to pull nutrients around with? Maybe
it would make the plants grow better!
Steve (off on a tangential thread) ;-)