Alternative method of intepreting CO2/KH relationship to measure CO2 concentration?

Hi,

I understand the CO2 and KH relationship well enough to read it off the charts.

And I read this off the "Chuck's Planted Aquarium Pages" (Chuck, if you're reading this, my apologies for not seeking your copyright beforehand)

Chuck's website

This calculator (and the chart based on this formula) will only work if your water is carbonate buffered. If your water contains high levels of phosphates, it will alter your water properties, and invalidate these CO2 calculations.

Of course, I assume that a 1-2 ppm of PO4 should not be an issue if we follow the EI.

In addition, I suspect there are other sources of acid, e.g., tannic acid from decaying leaves (?), which may distort the pH reading, thus giving us a false impression of the actual level of CO2. Just to state, my tank is 2 years old and mature.

I have problems with my CO2 reading because I simply don't believe it. My kH is about 9 and my pH reading is 6.7-6.8 (using a pH meter of 0.1 accuracy). This shows a reading of about 42-53 ppm, which is more than adequate. However, I'm still getting BBA outbreaks, which I suspect is due to inadquate CO2. I suspect my water is tainted by some substances (acids?) which is distorting my pH reading.

However, I came by another paragraph by Chuck, which gave me an idea.

[Myth: A Low KH results in a larger pH swing when adding CO2.
Many people are under the mistaken impression that a low KH results in large pH swings when adding CO2, while raising the KH will result in smaller pH swings. This is not the case. The KH will move the start and end pH values, but the pH swing will be the same for a given level of CO2. You can see this in the chart below, or using the calculation:

Case 1: Assume a KH of 15 degrees, and a starting CO2 level of 4.5ppm, which would result in a pH of 8.0. If we then add CO2, to increase the CO2 level to 28ppm, that would drop the pH down to 7.2, for a pH shift of .8.

Case 2: Assume a KH of just 1.5 degrees, and a starting CO2 level of 4.5ppm, which would result in a pH of 7.0. If we then add CO2, to increase the CO2 level to 28ppm, that would drop the pH down to 6.2, for a pH shift of .8, the exact same as in case 1. ]

Therefore, assuming the following:

kH = 9
Normal CO2 ppm without CO2 injection at 29 degrees C = 4.5 ppm should give me a reading of pH = 7.8
To hit target of 30ppm, the pH should be 6.9.
Hence, the pH swing should be 0.9 - 1.0.

So, even if my beginning starting pH value is NOT 7.9, all I need to do is to observe my starting pH value and aim for a corresponding reduction of pH 0.9 - 1.0

I did an experiment by taking a small tub of my tank water and left it in lying around for 24 hours. This should reduce the CO2 to 4.5 ppm (in equilibrium with the atmosphere, I assume). I then took the reading, which showed that my starting pH = 7.5 (NOT 7.8 as predicted, based on kH 9). I do not know what is causing my starting pH value to be lower, maybe unknown acids from decay?

In any case, my pH measurement after CO2 injection is 6.7 - 6.8, which is only a reduction of about 0.7 - 0.8, not the 0.9 - 1.0, which suggests that my CO2 is not at the desired 30 ppm, hence giving me the BBA.

Is there something wrong with the way I'm analysing the data? Thanks for reading.

Cheers
Boon Yong

 

Re: Alternative method of intepreting CO2/KH relationship to measure CO2 concentration?

Just to add, I was using the ADA aquasoil which reduces the pH. However, since the gravel is already 2 years old, I was assuming that whatever was causing the pH distortion has long been leeched out and exhausted. I could be wrong of course

Boon Yong