KH and CO2 levels

The only way to get more CO2 into solution with the water is to add more to the water. Raise the bubble rate, wait a few hours, see if the drop checker is green yet. If it isn't repeat the above. The only thing KH does is change the pH of the tank water for a given ppm of dissolved CO2. How much it changes it depends a lot on what other stuff is in the water, such as your KH buffer.

 

*Sigh*

Laka, I already replied to your thread on the APC forums. You are now shopping for the answer you want to hear. Fortunately, chemistry isn't going to change for you.

Like I said there and say here again. The only way to increase you ppm of CO2 is by adding more CO2. KH has nothing to do with it.

 

Well I thought that no matter what your kH, you are basically always driving your pH down by about 1.0... A quick look at a pH/kH chart for measuring CO2 confirms this. Your base water should measure ~3ppm of CO2 so with a kH of 2 your pH should be 7.3 before CO2 injection. From there you need to get to 6.3 for ~30ppm of CO2. At a kH of 5 your base pH should be 7.7 with a target of 6.7 to get your CO2 to 30ppm.

Any way you look at it, you are bringing your pH down -1.0. I don't think more or less kH will change that.

 

if you add CO2, the KH will raise a very very tiny bit. But this is not detectable with our testkits. So in practice, the KH will remain the same.

greets,

yme

 

To answer your question, yes. It requires less CO2 gas to be dissolved in water with a low KH to reach a certain ppm of available CO2, than with a high KH. So for your 2 tanks the one with the KH at 1 will achieve 30ppm CO2 before the one with a KH of 10 when the bubble rates are the same. It can be calculated using Henderson-Hasselbalch Equation. This is all very well in theory but in reality you'll be hard pushed to notice any difference with the KH levels we normally operate at.

Actually the chemistry of adding CO2 to water is quite interesting - if you're that way inclined

James

 

The drop checker is a good cross-checking tool and a great visual warning if there is something wrong with your CO2 level. I don't think it is absolutely necessary, but mine has let me know that my CO2 tank was empty on more than one occassion for example.

A few more reasons for a drop checker. - Apparently there are also other "things" that can throw off your kH/pH relationship in some cases, and of course if you have close to 0 kH.

Otherwise, if your tank water is coming from a controlled source (say reconsitituted RO), I would think one of those intank pH testers could be nearly as effective...

 

A drop of 1 pH unit doesn't equal 30ppm CO2 it equals a 10x increase in co2 concentration. So if the original concentration was 3ppm then a 1 pH unit drop will give 30ppm but if you were starting at 0.5ppm then you would only be at 5ppm after a 1 unit decrease.

The CO2 concentration of water at equilibrium with the atmosphere will depend on atmospheric CO2 concentration, atmospheric pressure, and temperature. This makes it very hard to track down exact figures for equilibrium CO2 concentrations although at typical aquarium temperatures it is probably around 0.5ppm.

There are several problems with using the pH drop method to try and determine CO2 levels in the aquarium. Most significant is that a very small difference in the CO2 concentration of the ‘degassed’ water relates to a large difference when the pH has been lowered by a given amount. Other related problems include: The CO2 concentration of air in a typical room with the windows closed may be several times higher than the air outside, the final stages of reaching equilibrium with air will be slow, and if the sample is left too long then evaporation may cause an increase in KH and hence pH.

When I have tried using the pH drop method I have used an air pump to aerate the water with air from outside. I found at the time that I needed a drop of around 1.6 pH units before there were any noticeable effects on any fish (e.g. increased respiration rate).

I wanted to include a table that gives CO2 concentrations (ppm) in relation to starting (equilibrium) concentration and pH drop but was not sure how to add it here.

Cheers
Neil

 

Neil - It seems that I may be learning something new as well.

I was under the impression that water at equilibrium with atmospheric CO2 would settle to 3ppm in most cases. I can't remember where I heard it first, but that has always proven to be true for me.

It sounds like you know a lot more than I do about the subject, outside of my own experiences, but I am intrigued by the comment about equilibrium at .5ppm in the tank instead of 3ppm...

 

I did some testing to see what ppm of CO2 my water would degas to. The problem I ran into is that it takes about 48 hours for the pH of the degassing water to stop rising, and the end point is closer to 1 or 2 ppm than it is to 3 ppm. I tried to limit evaporation of the water as I did this, but I'm sure some did occur. One thing it definitely proved to me is that letting a tank sample sit out over night does not yield a 3 ppm level of CO2. Like use of the KH/pH/CO2 tables, this method does give a number for ppm of CO2 but the number is extremely unreliable and variable. The drop checker gives as much accuracy as your ability to judge the color of the solution in the drop checker, which isn't great at best, so our 30ppm measurement with a drop checker is really about a 25 - 40 ppm actual reading. That is the best we can do without the next step in improving the drop checker, which I think is the addition of a reference 6.6 pH water solution of the same geometry as the test solution in the drop checker and with the same amount of pH reagent in it. That gives a color reference to compare to, viewed with the same light, same background, etc and the drop checker fluid. After that, we need to switch to a pH probe using a very small gas permeable membrane contained 4 dKH reference solution around the pH probe junction, to get a better accuracy. Two years from now we may all be using just that.

 

I totally agree with VaughnH that the pH drop method has serious limitations. This is really what i was trying to highlight in my original post along wiht a drop of 1 pH units not equalling 30ppm CO2. The use of a drop checker as well as careful observation of livestock are almost certainly the best methods at this stage.

While i doubt anyone will be rushing out to buy one of these for their tank i thought this sort of thing may be of interest...
http://www.amt-gmbh.com/pdf/CO2%20sensor%20english.pdf

cheers
Neil

 

That optical sensor is very interesting. It looks like a drop checker, using a membrane instead of an air gap, and a colorimeter to measure the color. I wonder why they didn't just use a pH probe instead of the colorimeter. I don't know what this costs, but somehow I can't see it being $20.

 

Very interesting. I have never actually taken tank water out and aerated it to off-gas the CO2 for measurement. Instead, I just measure the pH/kH relationship of my water source (i.e. tap water). It seems to match out to ~3ppm of CO2 before added to the tank. I use both a drop checker and a pH probe but have made the general observation in my case that I always seem to be dropping my pH 1.0 - 1.1 from my water source levels.

So, are you saying that there may be a significant difference between the ambient CO2 level of my source water and my tank water if I let it off gas all of the injected CO2? Sounds like there are a lot of variables at play.

I have stopped testing most things now-a-days, but I think I may start testing and measuring this again on my end to make sure that my own assumptions are still true...