Hi,
I used to measure PH and KH to estimate CO2 levels in my tank, however after some time noticed that the PH seems to be quite high regardless of fairly high BPS.
To double check I have bought the Ferplast CO2 test.
It turned out that the CO2 levels measured with the Ferplast test are twice as high as the ones calculated using PH and KH.

I was not happy with such a huge difference and decided to give a drop checked a go (a glass one from ebay). Added around 4ml of 4KH solution (RO + baking soda) and 2 drops of Hagen LR PH test.

Here is the problem. The CO2 levels calculated using the KH and PH of the water in my tank are consistent with the levels shown by the drop checked, but the Ferplast tests suggests the concentration of CO2 is twice as high.

Does anybody have any experience with the Ferplast test?

cheers
amoeba

B.T.W. With the Ferplast test I use 20ml of tank water, 5 drops of solution1, and add drop by drop solution 2. The water turns pink after adding 14 drops, what according to the user’s manual equals 28ppm of CO2

Never used Ferplast but i had a Red Sea test that sounds very close to the steps you described. I found the results were pretty much crap. In my opinion a drop checker is the way to go.

I can't find anything online that describes what that test kit is - how it works. Without that I have no idea how good it might be.

Even their own website (?) has no info on ingredients or how it may work...

CO2 Energy Tester

That does not bode well IMO.....why trust a product that does not publish this info?

I would use a drop checker as it is reasonably accurate and much less expensive than a meter.......

I always have the hope that someone, somewhere, will design an inexpensive test kit that accurately measures ppm of CO2 in typical tank water. So, any time I see a test kit that looks like it might be different from the simple KH/pH test, I get interested. Any test that required measuring pH with drops of reagent, then comparing a color to a chart just isn't going to be accurate. This one is unique in having two test reagents, making me wonder just what it is testing. But, the manufacturer choses not to tell us anything at all about it.

Dont know if this will help but as i said i have a red sea that has very similar steps. I dug it up last night and the first bottle contains Ethanol Absolute the second bottle is Sodium Hydroxide.

Thanks for all the answers. For the time being I’m using a drop checker.

The really interesting thing about the ferplast test is that it always shows double the concentration suggested by a drop checker. Almost like there was a typo in the instruction (2ppm/drop instead of 1ppm/drop).

I have an electronic PH meter so I’ll set up a small experiment. I am going to inject some CO2 into my 4dKH solution and compare the results:
1. LR PH test + KH/PH/CO2 table
2. electronic PH test + KH/PH/CO2 table
3. the ferplast CO2 test

That should be a good test. Let us know what you find out.

As promised.

The experiment:
A 70ml of 4dH (+/- 0.25dKH) solution of baking soda (NaHCO3) has been injected with CO2.
Source of CO2 – my lungs (I promise I was totally sober, so C2H5OH did not affect the results ).

The CO2 levels have been measured in 3 ways:
1. electronic PH meter + Ph/KH/CO2 table
2. Hagen LR test + Ph/KH/CO2 table
3. Ferplast CO2 test

I expected to get anything between 30 and 60ppm CO2.

The results:
1. Electronic PH meter – 6.75 PH (+/-0.1 PH), what gives around 24ppm CO2
2. Hagen LR test – 6.7 PH (+/-0.1 PH), what gives around 24ppm CO2
3. Ferplast CO2 test – 5drops of solution1 + 62 drops of solution2, what according to the instruction gives 124ppm CO2

The analysis:
Both PH/KH/CO2 methods gave consistent results, what was expected.
For the ferplast test, the results were 6 times higher.

I can see the following possibilities here:
1. The ferplast test is Na sensitive
2. After injecting CO2, some of the CO2 stayed in the form of small bubbles (C02) and did not form H2CO3. If the ferplast test is able to “force” the CO2 to fully dissolve, then the differences in the results can be explained, because PH based tests are not sensitive to CO2 in the gas form, just the acid.
3. The test is simply totally inaccurate, or requires a special water composition
(e.g. higher levels of Ca or Mg). I’m not a chemist so it is hard for me to analyse that possibility further.
4. Even huge inaccuracies in the KH measurement do not explain the differences between the results.

I was unable to get more than 10 - 15 ppm of CO2 in water by blowing through a "straw" into water, so I am very sure that your test sample didn't have 124 ppm of CO2 in it. My conclusion would be that that test method is of no value. I have no idea what the principle is behind the test, so I have no idea why it didn't work.

EDIT: It occurred to me after writing this, that it doesn't matter how accurate the reading of CO2 is, as long as it is consistent, reproducible by others, and the numbers given are proportional to the actual ppm of CO2. Our hobby doesn't require accurate numbers, just repeatable ones. Further, I know the pH/KH tables and equation are based on the relationship: CO2 concentration = a constant times ppm of carbonate divided by 10 raised to the pH power. But, I'm not at all sure how the constant is determined, so it is possible that the Ferplast test kit gives the accurate results, but the pH/KH test gives reproducible, consistent results.