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Re: H ion modelling


> Date: Sat, 9 Oct 1999 09:43:30 -0600 (MDT) 
> From: "Roger S. Miller" <rgrmill at rt66_com> 
> Subject: Re: H ion modelling
> ...

> He concluded specifically that there was a need for hydrogen ions when 
> plants grow on nitrate.  Conversely there was also a need for hydroxyl 
> ions when plants grow on ammonium. 
>  
> His conclusion is based on balancing all the various reactions necessary 
> for a plant to assimilate nitrate and on specific experiments with 
> Hydrodictyon africanum, which I believe is a freshwater macroalgae.

Certainly if you're taking NO3- out of the solution, you have to take out
H+, or take out some positive ion, or add some negative ion.  Conversely
if you take out NH4+.  But there are so many choices here -- I wonder
just what the experiments did to show that removal of H+ (or OH-) was
the key.  But anyway --


> Raven estimated that plants needed about 1 mole of hydrogen ions for
> each mole of nitrate they assimilated. 

This is what you'd need just to balance the electric charge.  I don't
know whether that's his argument, though.  

>  This is half what you might get by 
> estimating from the nitrate to amino conversion.

With the rest presumably coming from CH2 or something.

>  Our planted tanks 
> commonly have nitrate concentrations in the range of 0.08 to 0.8 
> mmoles/liter,

That's. say, 5 to 50 parts per million.  Many fishkeepers assert, while
others deny, that 50 ppm is high enough to be bad for fish in the long
term.  In any case, 5-10 ppm (0.08 - 0.16 mmol/l) is the level that the
PMDD folk talk about for supporting plants.  An excess over that amount,
if present, isn't taken up by the plants.

> while hydrogen ion is there at only 0.00001 to 0.001 
> mmoles/liter.

I still fail to see why this is relevant.  The (bicarbonate + carbonic
acid) is 0.4 mmol/l at a _minimum_.

Specifically, if you have KH of 2, you've got about 35 ppm of CaCO3, or
about 25 ppm of bicarbonate + carbonic acid. Divide by 61, the ionic
weight of HCO3-. (Conveniently, nitrate and bicarbonate are of nearly
equal weight, so you can compare the 5 ppm NO3- and the 25 ppm H2CO3
directly, without even converting to moles.)

And a KH of 2 is pretty marginal if you want pH stability, regardless of
what NO3- uptake may or may not do. In fact, at neutral pH the plants
will be short of CO2 if the KH is only 2. (See the green and yellow chart
in The Optimum Aquarium.)  So the _amount_ of available H+ in a planted
tank is adequate for any NO3- absorption, so long as you don't overlook
the H+ that HCO3- will release upon demand and without severely raising
pH.

(Granted, if you really shifted the H2CO3 <-> HCO3- balance by 25%, it
would raise the pH noticeably.  That would be one of the various
pH-changing reactions, and probably less important than loss of CO2 to
the atmosphere, depending on your tank.)
 
As to the _rate_ at which it's made available -- 

  This big difference suggest that plants may have a hard 
> time getting hydrogen ions at a rate equal to the rate they can get 
> nitrate.  If they can't match the rates, then the hydrogen ion may become 
> growth limiting.

No problem.  The balance between H+, H2CO3, and HCO3- adjusts
instantly. There's a famously slow equilibrium between CO2 in the air and
CO2 in the water (partly CO2 and partly H2CO3); but once it's in
solution, the whole H2CO3 <-> HCO3- <-> CO3-- equilibrium is as close to
instantaneous as you could want.  (Put some bicarb in the water, and use
a pH meter to see the pH changing as fast as the powder dissolves.  Add
dilute acid and see the quick change in the other direction.  This is a
fast eqiulibrium.)


My conclusion, since this has been rambling: H+ usage by NO3- uptake
may be one of the things that contribute to a rising pH in the tank; if
all these things together raise the pH too much, then plant growth will
be limited, either because CO2 is less available or because the plants
just don't like the high pH.  The tiny amount of actual H+ ion(*) present
in the water at any time is not relevant when there's a buffer system
present to releast H+ on demand.  And your plants will not grow if
carbonate buffering is absent: they'll be starved for CO2.



(*) I know it's really H3O+, and that's an oversimplification too, OK?

-- 
Dan Drake
dd at dandrake_com
http://www.dandrake.com/