A simple way to assess your pH measurements

[arking_mark]

A couple of facts:

1. There is a mathematical model for pH, Alk, and CO2. Given any two of these you can calculate the 3rd.
2. Most pH meters and test kits have an accuracy of +/- 0.2.
3. A pH measurement below 7.8 is most likely a measurement error (or 7.6 due to the accuracy of your meter or test kit) for a well aerated tank with normal house-level CO2 and no additives that are lowering pH.

A simple cup aeration test can help you assess if you have a measurement issue. Just take a cup of your tank water and aerate it outside with an airstone for 1hr.

Outdoor CO2 levels are ~400ppm. So after aerating, your cup of tank water will be in rough equilibrium with the outdoor air.

If you Alk is between 7-11dKH your actual pH will fall between 8.22 and 8.38. If measured by a pH meter or test kit, one would expect 8.02 to 8.58. If your measurement is outside this range...you definitely have a measurement issue.

Given the accuracy of hobby Alk measurements are usually better than +/- 0.5, taking your Alk reading can give you a pretty precise pH to compare to your meter or test kit. I use this number to see how far off my meters are.

It's should also be noted that pH meters can quickly lose calibration and significantly drift.

 

[Randy Holmes-Farley]

Thanks for posting this.

 

[taricha]

oh, and another nice pH sanity check is borax and cream of tartar.

Adding still more DIY to this test method: I have found that I can use DIY pH calibration buffers made out of Borax and Cream of Tartar to calibrate my Pinpoint pH meter!

The use of Borax as a pH calibration solution has been discussed before in aquarium hobby articles, such as this: http://reefkeeping.com/issues/2005-02/rhf/index.htm, which refers to this: http://web.archive.org/web/20021015005420/www.animalnetwork.com/fish2/aqfm/2000/feb/bio/default.asp. Dr. Bingman's calculations suggest using 1/2 tsp. of Borax in 1 pint of water. My measurements suggest something more like 1 1/4 tsp. in 1 quart of water (I probably didn't compress the Borax as much). As Dr. Bingman says, though, the pH of the resulting solution isn't very dependent upon exact measurements. This solution does need to be protected from atmospheric CO2, so it should be kept in a closed container, hopefully with minimal headspace. It will have a pH of 9.23 at 20C, and 9.18 at 25C.

Another easy and reliable DIY pH calibration solution can be created using potassium hydrogen tartarate, aka Cream of Tartar. Both it and Borax are discussed on page 11 of this document: http://pac.iupac.org/publications/pac/pdf/1978/pdf/5011x1485.pdf. The Cream of Tartar calibration solution is discussed in depth in this document: http://nvlpubs.nist.gov/nistpubs/jres/47/jresv47n6p433_A1b.pdf. A saturated solution at 25C requires about 6.4 grams per liter, which by my measurements is about 1 1/2 tsp. of the powder. The nice thing about this pH calibration solution is that you just need to make a saturated solution. In other words, you don't really need to measure at all -- just add enough to whatever amount of RO/DI water to make sure that it won't all dissolve. Once you are certain that you have a saturated solution, let the remaining crystals settle out, and then pour the clear solution off for use, leaving the undissolved crystals behind. This solution will have a pH of 3.56 at 20C, and 3.55 at 25C.

 

[hunterallen40]

What is the actual model? I have a CO2 sensor in the room with my aquarium, and I'd be interested in playing with those numbers.

 

[arking_mark]

I use python pyCO2Sys library...

 

[JimWelsh]

What is the actual model? I have a CO2 sensor in the room with my aquarium, and I'd be interested in playing with those numbers.

Just google "CO2SYS" and find many links, e.g. https://cdiac.ess-dive.lbl.gov/ftp/co2sys/

 

[Hemmdog]

Great info!

Random question sort of related. Can pH meters for horticulture uses be used in saltwater and give accurate readings? I have this meter and have never thought to use it to check the tank.

 

[Randy Holmes-Farley]

Great info!

Random question sort of related. Can pH meters for horticulture uses be used in saltwater and give accurate readings? I have this meter and have never thought to use it to check the tank.

Any pH meter can work in seawater.