Re: Nitrates

["Jim Seidman" <js4 at seidman_net>]

Shireen Gonzaga writes:
>Meanwhile, I was browsing the archives and came across a post from
>Paul Sears that contradicts what you said. It's located at
>  http://fins.actwin.com/aquatic-plants/month.9903/msg00649.html

I read that post, and didn't see where it contradicts anything I said. If,
after reading this (very long) message, you still see a contradiction,
please be specific and I'll try to resolve it.

As Paul Sears says in that post:
> [H+][HCO3-]/["H2CO3"]  =  4.16 x 10^-7   (Temperature dependent)
The reason this is true is that there's a certain strength to the bond
between the H+ ion and rest of the carbonic acid. As these molecules and
ions go bouncing around the water, H+ and HCO3- ions are constantly bumping
into each other, and each time have a certain probability of combining to
form H2CO3. Similarly, as the H2CO3 molecule goes through the water, there's
a certain probability that the pull on the hydrogen by the water molecules
will break the H2CO3 apart into H+ and HCO3. Since these things are
happening constantly, you wind up with a certain equilibrium between the
concentrations.

So, the more H+ ions you have in the water (i.e. the lower your pH), the
more likely any given HCO3- ion is to bump into an H+ ion and turn into
H2CO3. So the lower the pH, the more H2CO3 you'll have compared to HCO3-.
Thus the pH is directly related to the ratio of [HCO3-] to [H2CO3]

Similarly, if you add a lot of H2CO3, you'll have more molecules to break
into H+ and HCO3-. Thus adding H2CO3 will increase both [H+] and [HCO3-]
until the above equation is correct again.

Lastly, if you add a lot of HCO3-, as you proposed, a lot of those
bicarbonate ions will hit H+ ions, converting them to H2CO3. So adding
[HCO3-] will reduce [H+] and increase [H2CO3] in order to make the equation
balance. Thus adding HCO3- will cause you to have fewer H+ ions, lowering
your pH. (This is actually only true until you get to a pH of 8.4, because
bicarbonate is a weak acid as well. There's a similar relationship between
[HCO3-} and [CO3--]. But that's another story.) Also, note that this is kind
of a special case, because the extra H2CO3 won't stick around, but will turn
into CO2 and dissipate or be consumed by plants.

If it makes more sense to look at it mathematically, rearranging Paul's
equation a little bit gives:
  [H+] = 4.16 x 10^-7 x [H2CO3]/[HCO3-]

Now, by definition,
  pH = - log [H+]
That is, pH is defined as the negative value of the log (in base 10) of the
concentration of hydrogen ions.

So, what that means is that in our carbonic acid/bicarbonate example:
  pH = - log (4.16 x 10^-7) x [H2CO3]/[HCO3-])

So, that's what I said before, that the ratio of your concentrations of
carbonic acid and bicarbonate is what determines your pH. The higher [HCO3-]
is in this equation, the higher [H2CO3] must be to lower to any given pH.
Again, this doesn't mean a lot of bicarbonate reduces the availability of
CO2 to your fish. [H2CO3] can still be high even if [H2CO3]/[HCO3-] is low.

Now, part of what may be confusing here is that while the ratio of carbonic
acid to bicarbonate determines the pH, changing the pH through some other
means (e.g. adding acid) will change the ratio. So if you add some acid
other than carbonic acid to change the pH, enough HCO3- will turn into H2CO3
in order to make the above equations correct.
 
But, what you were suggesting in your post of Aug. 25 was this:
>At the advice of Scott H, I'm going to start increasing
>my kH with baking soda, since it's only 2.0 in the tank
>(according to the tetra test kit). My pH (also measured
>with tetra) in the tank is 7.6. Hopefully the increased
>kH will stimulate more carbonic acid production to lower
>the pH. Right? Or am I completely misunderstanding it?

And as the above equations demonstrate, increasing [HCO3-] isn't going to
help you lower your pH at all. It'll make it harder to lower pH.

Perhaps the point that's making it look like there's a contradiction between
my posts and Paul's is Paul's comments that adding acid (nitric acid in his
example) will cause bicarbonate to turn into carbonic acid. That's
absolutely true. But I think the important point is that with a high KH this
effect doesn't cause a large pH change. So he was saying the same thing: a
high KH makes it harder to lower your pH.

As an additional note, biological processes add acid fairly slowly. So don't
add a lot of bicarbonate thinking that acid will convert into CO2 rapidly
enough to make a difference to your plants. The real issue is that this
process can cause you to slowly run out of bicarbonate. As you can see from
the above equations, as [HCO3-] gets very small, even a little additional
change will cause a big difference in the [H2CO3]/[HCO3-] ratio. That's the
real reason that KH is important.

Sorry this was so long, but I thought that maybe attacking the issue a
couple of different ways would help.

- Jim