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I've posted a similar post in the CO2 and Temp thread, but nobody said what they think about it (I guess it was too far down the thread), so I'm posting a new thread and I hope that people will add their thought.

We tend to think that a higher KH means a good buffer so if and when we add CO2, it will help us avoid having a large pH swing in case we add too much CO2.

I am looking at a CO2 concentration chart like the one on the krib (http://www.thekrib.com/Plants/CO2/kh-ph-co2-chart.html the second chart) and I'm taking two test cases.

The first one is a tank with a KH of 1.5 and 7 pH. That tank has a CO2 concentration of 4 mg/l. If we add 24 mg/l of CO2 to a value of 28 mg/l, then the pH will go down to 6.2, a change of 0.8 in pH.

The second is a tank with KH of 15 and 8 pH. Again, we have a concentration of 4 mg/l CO2. Again, if we add 24 mg/l of CO2 to a value of 28 mg/l, the pH will go down to 7.2, which is the same change of 0.8 in pH like in the first tank.

My conclusion is that having a higher KH doesn't mean that if we add too much CO2 our pH won't have as large a swing as it would if we'd have a lower KH, but rather will end in the exact pH swing. The only difference is that by raising the KH we raise the initial pH so the final value of pH after adding CO2 will be safe for our fish (7.2 pH rather than 6.2 in those two cases).

This is how I interpret the chart. I want to know what other people think about it and whether they agree/disagree and why.

Hi,
Let's take a look at a chart. In your first example
(pH 7.0, KH 1.5 = 4mg/l) then inject to 28mg/l(@pH6.2
& KH 1.5) The pH "swings" through four steps to reach
your last reading.

In your second example (pH 8.0 & KH 15 = 4.0mg/l)
and then inject to reach 28mg/l (pH 7.2 and KH 15)
notice that it only "swings" through 2 steps.
It takes more cuft of gas to reach the two steps
(your rate of injection is higher) than the four
steps in the first example.

Generally speaking, this is the way I use the chart,
and of course it too is open to interpretation.

First I test my water to see what the pH and KH is in
the tank. The first time, I would test about
24 hours after a water change to allow for mixing and
degassing. I would look to see what saturation that
gives me. The next step is to see what I need to
inject to stay either within the "green" or to stay
within the limits of 15-30mg/l. Essentially I look
to see where I am, and where I want to be. Then
I look at the pH range that will give me that saturation
and also look at what KH range will give me that
saturation. I, personally, choose a value that is
7.0 and down. In my instance I chose 6.8 due to
the fish I keep, and that acidic water is more resistant
to some forms of algae and other pests. If my KH
were kept between 3.5 and 6.0 the saturation would vary
between 16 and 28mg/l. In my instance I shoot for
maintaining a KH of 5.0 and a saturation of 23mg/l.

Rather than constant testing, I use an electronic pH
meter and probe manufactured by Pinpoint which I
purchased from the Drsfostersmith catalog. After
a few years, I now know that for x number of gallons
changed out, such and such a KH after filling,
I need a 1/8 or 1/4 or 1/2 teaspoon
of bicarbonate of soda to maintain my KH at the
desired level. I have my CO2 injection set with the
needle valve to provide just over 1 bubble/second
and at that rate, I can expect my 10# tank to last
for 8 months. My pH "swings" from lights on
to lights off less than 0.2.

Essentially, I looked at where I was, and at where
I wanted to be, and what it would take to get the
tank where I wanted it. By setting the KH relatively
low, I use the least gas to reach the desired pH and
CO2 saturation.

Hope this helps, or at least gives you another viewpoint.

Frank


-->>> The Confidence of Amateurs, is the Envy of Professionals <<<--

Thanks Frank.

What I am trying to do with this post is realise whether the KH really acts as a buffer that keeps our pH from crashing if we add too much CO2.

As for the chart, I think it's a little misleading. That is because they've neglected to add the range of pH 7.6 and 7.8. If we would add the corresponding readings of CO2 to 7.6 and 7.8, then the change would still be 4 steps, or pH 0.8. I don't know why all charts that I've seen neglected to add those values.

I still think that the adding the same amount of CO2 to two different tanks will have the same effect on pH.

Hi,
I agree with your thoughts. In a given quantity of water,
with the same starting pHs and different KHs, it will take
more CO2 to shift the pH a given amount.
The injected CO2 combines with the water to form Carbonic
Acid (H2CO3).
The Carbonates and Bicarbonates tend to buffer the
formation of the acid so the higher the KH, the more
CO2 required to shift the pH downward.

Frank


Last edited by FRANK at 01-Dec-2004 17:44

-->>> The Confidence of Amateurs, is the Envy of Professionals <<<--