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KH/pH/CO2 Relationship

To Paul Sears:


Robert Destard and I have been engaged in an offline conversation about the
KH/pH/CO2 relationship.  I've been trying to pass along some of the
information you were kind enough to teach me a few months back.  

I thought I would bring the conversation online because it might be helpful
to other newcomers (I found it a hard topic to nail down on the first go)
and I thought you might correct any mistakes I've made.  As a
non-scientist, I'm out on the limb as far as I dare should go.  


Steve Dixon

Here goes:

Robert's description of the problem:

The trickle filter definitely caused a significant rise in ph from water
with a ph of about 6.7 to water with a ph of about 7.3 (I have forgotten
the exact measures).  I have a possible theory as to why putting in the
trickle filter caused such a rapid outbreak of this [cyanobacteria] algae;
indeed so fast that almost no ammonia, nitrites, or nitrates could have
developed in such time.  I was searching on the net for cyanobacteria, and
I ran across a large faq on the stuff.  One person had suggested that
cyanobacteria do very poorly in acidic waters, but that they thrive in
alkaline waters.  I experienced a ph shift from acidic to alkaline right
before the outbreak of cyanobacteria arrived.

Regarding purchase of a CO2 test kit Steve Dixon wrote:

CO2.  I would skip this kit altogether.  With a KH kit and the Pinpoint
pH meter, you can measure the CO2 exactly anytime you want.  Once you know
the KH level ... you'll "see" the CO2 level every time you look at the pH
meter.  The most important thing I learned in the last year was the
CO2/KH/pH relationship.  In every [bi]carbonate buffered aquarium the chart
will be "true."  It's in the Krib and the archives and I've gone over it
... with Paul Sears on the APD.

Robert Destard wrote:

The problem with determining CO2 through the ph/kh table is that the table
is assuming that the water is having as much CO2 as possible dissolved into
it.  It is not taking into consideration outgassing of CO2, etc.

Steve Dixon wrote:

Not true....  Outgassing is taken into account "perfectly" by the chart. 
By that I mean that every bit of CO2 that outgasses reduces the dissolved
CO2 level in your tank with a corresponding increase in pH.  Had you known
the KH level in your tank, you could have calculated the exact drop in CO2
levels when you turned on the trickle filter and got a .6 rise in pH.  

I've got a copy of the chart here at work so let me make up an example: 
Assume you start with a dKH of 2 and a pH of 6.8.  Right there without
knowing anything else, you know for certain that you have 9 mg./l CO2
dissolved in your water.  Period.  (Booth and Sears got into a debate
several months back about whether this was absolutely true.  Both agreed
after a while that Sears was right and that so long as it's a [bi]carbonate
buffering system, it "has" to be exactly what the chart says it is. It's
some sort of chemistry law or other.) If you got a .6 rise in pH when you
turned on the trickle filter, the CO2 level (in my example) dropped from 9
to 2.4 mg./l.  Again, period.  You know this as a "fact."  You can debate
the cause of the drop (trickle filter effect, DIY reactor ran out of juice,
etc.) but not the fact of the drop.

The beauty of the chart (and especially since you already have the Pinpoint
monitor) is that you're only $6 away (Tetra KH kit) from knowing exactly
how much CO2 you're getting into your tank with the DIY reactor.

I'm still worried that the whole CO2 thing is just a coincidence and that
some other parameter caused your cyano problem, but who knows.  I'm
certainly not an expert of any sort on that question.

Steve Dixon wrote:

For a given supply of CO2, you might lower the KH (but not below
dKH 2 or so) which will lower your pH.

Robert Destard wrote:

True, but lowering kh also leads to lower CO2 levels, which hurts my

Steve Dixon wrote:

Not true.  Go back to the chart again (Paul Sears staightened me out on
this point on the APD).  A given supply of CO2 (so many bubbles per second
in whatever setup you have) will produce the same CO2 mg./l in your tank at
different KH levels.  I've verified this experimentally in my tank and once
you understand it, it is apparent from the chart as well.  Get your CO2
setup working in its "regular" steady state mode; then measure the KH; then
raise the KH a point or two with baking soda (or any bicarbonate); measure
the KH again so that you're sure of the rise in KH.  (Of course you will
also see a rise in pH as the KH increases.)  Go to the chart, and bingo!
you will see that your CO2 level is the same as it was at the lower KH. 
Solubility of CO2 is not affected by KH changes (at least those we deal
with in our aquariums).

Robert Destard wrote:

Let's suppose that I do have a dkh of 2 and a ph of 6.8.  Well, what if the
ph was held there not by CO2 but by acidic compound (ph-down) which I added
to the water (so if the CO2 was turned off, the ph would hold steady).
Wouldn't that produce a false CO2 reading?

Steve Dixon wrote:

No, it would not produce a false reading (with one exception which we'll
get to in a minute).  I'm not a chemist so I probably won't say this
exactly right, but this is what Sears and others say is happening:  The
acid will "eat up" the KH bicarbonates as it brings down the pH.  Both the
KH and the pH will drop together as this occurs.  Once the reactions have
stabilized (I think some CO2 is produced by the reactions which will
stabilize by outgassing and/or being used up by the plants) if you have the
same steady supply of CO2, you will have the same xx mg./l of CO2.  Same
CO2 level (let's use 9 mg./l as an example) whether your running at 2 dKH
at ph 6.8 or 5 dKH at pH 7.2.  If you're running at 5 dKH at pH 7.2 and you
start adding acid to bring the pH down, you can keep going until you are at
2dKH and the pH will be 6.8 and CO2 will be 9mg./l. Again, period.

To answer your specific question: if you turn the CO2 off, the pH will rise
as the CO2 outgasses to equilize to the CO2 levels in the air; the pH will
not be held in check, so to speak, by some other acid.  The CO2 produces
just another acid, carbonic acid, I think, which reacts with the KH
(dropping the pH slightly) and follows the chart (it's a log function of
some sort which I embarrassed to say I've forgotten about).

Now to the exception:  If the aquarist is using a phosphate buffer (which I
gather is sometimes used in marine setups?) rather than a bicarbonate
buffer, there may be false CO2 readings from the chart NOT because the
KH/pH/CO2 chart is wrong (it is correct) but because with a phosphate
buffer we can no longer get an accurate KH reading. In other words, the
phosphate buffer will interfere with the KH test kit which we aquarists
are likely to use.

(This is the conclusion of the Booth/Sears debate which I mentioned
yesterday.  The KH/pH/CO2 relationship is absolute, so to speak.)

Robert Destard wrote:

It seems to me that the chart is assuming that you are lowering the ph
strictly by CO2 and not by other chemical methods.

Steve Dixon wrote:

Not at all.  All the other acids and chemistry are accounted for.  We just
want to avoid the situation where we can't get an accurate KH reading! 
Stay with bicarbonate buffers, and all is well with the chart and you won't
need to buy a CO2 kit which is where we started this conversation, isn't
it? :)

Robert Destard wrote:

So you are in essence saying that if the kh decreases and the ph does too
as a result, the CO2 levels stay the same?  And, if the kh increases and
the ph does too as a result, the CO2 levels will not change?

Steve Dixon wrote:
For a given supply of CO2, that is correct.  The CO2 level will hold steady
once the reactions have stabilized.

I went through the same struggle to understand this as you are going
through a couple of months ago.  Once I really got it, it has been easier
for me to tinker with my aquarium in my head as I try to solve various
problems.  I feel like I'm building knowledge now, rather than just adding
more random "facts" to my "aquaspace" so to speak. :)

Apologies to all for the length of the post.

Regards, Steve Dixon