CO2 test kit... how accurate?

[pepetj]

I've been using the Hach CO2 test kit which uses Phenolphthalein solution at 0.1% as tritation reagent.

I wrote to the local distributor with some questions regarding impact of water parameters in this particular test (such as pH, KH, GH, Temp).

I have not received an answer so after waiting for a couple of months for this reply I though about asking you.

I realize this is likely an indirect way of testing for dissolved CO2 yet I'm hoping this is at least better than using drop checkers and the KH/ph charts.

I get readings as high as 45-47 ppm in some of my tanks after 4 hours of continuous-on photoperiod... is that value likely realistic or not? Most of my tanks are in the 36 to 41ppm range, according to these readings.

Thank you

Pepetj
Santo Domingo

 

[ShadowMac]

good question, I have a similar test that I do not use because I am concerned about accuracy.

I don't think those ranges are extraordinary nor out of possible range. I think I've heard of Tom having values higher in some tanks (around 60 ppm I think), and we can be sure he is testing correctly.

 

[pepetj]

I know that storage temperature is a major issue with the Hach CO2 test kit reagents. I keep them in the fridge at 10-15C.

A few weeks ago "someone" (I suspect my wife looking for something) took the Phenolphthalein solution bottle out and left it in the kitchen. I had to order a replacement since it's likely a damaged reactive; may perform a comparative reading with it to see how far (or not) it reads compared to the one that has been kept appropriately.

Pepetj
Santo Domingo

 

[ShadowMac]

this thread may answer some of your questions: http://www.barrreport.com/showthread.php/3754-Anyone-ever-use-a-CO2-test-kit

 

[Biollante]

Thar Lies Maddness

Hi Pepetj,

Assuming the Phenolphthalein solution is fresh and not oxidized and you are following the sampling and titration procedure correctly, your answer is very accurate.

EDIT: The Phenolphthalein solution being left out of the fridge will probably not present too much of a problem as long as it was sealed. Obviously I would not use the solution for that attempt at the “Louisa Gross Horwitz Prize for Biology or Biochemistry” or use in a paper for one of Tom Barr’s classes…


What it all means is up for grabs… Then that is the problem with observations, what are we observing?

The direct answer to the musical question, are readings as high as 45-47-ppm CO2 accurate or possible? The answer is yes to both.

I, however, suspect you have a carbonate component and maybe one, two or a dozen others to take into account.

The other slippery question is where did we sample and will we be surprised that the amount of dissolved CO2 may vary greatly from point-to-point, place-to-place, from time-to-time. :gw

Biollante

 

[Tom Barr]

The biggest issue is making a standard reference for CO2 in the range of interest.

We can easily do this for NO3 or PO4..........but not for CO2.
So there are few ways to ensure that we have an accurate reading.

We are okay with the pH/KH aspect, if the KH is solely from bicarb.

So there's one way, using a KH reference solution and a pH meter and bubble CO2 in there till we get a specific CO2 ppm and have it stabilize for several minutes, hour etc.