MadSci Network: Chemistry |
Dear Craig, Determining the CO2 concentration in water solutions has always given me problems in industrial settings. Laboratories can use gas chromatographs to measure the CO2 directly but home hobbyists must rely on less technical solutions. The formula you cited in your question: [CO2] = 3 * KH * 10^(7-pH) gave excellent correlation to values published in a table appearing in a Finnish aquaria magazine (Akvaariomaailma). The values do not exactly match at the extremes of the values, so I think the two methods are independent of each other. The information was origianlly posted by Pauli Hopea and referenced on the web site: http://www.thekrib.com/Plants/CO2/co2-booth-faq.html This is a site of frequently asked questions specifically dealing with carbon dioxide control in aquarium settings. There is also some very good practical information about CO2 control using a commercial CO2 injection system at: http://www.dupla.com/e008.htm Proper application of CO2 can be tricky. It must be applied slowly enough in water with sufficient total aklalinity to prevent rapid pH swings. You asked if CO2 is added to low KH water could result in rapid/large pH swings. This is absolutely true. The volume of your aquarium will also play a role. The larger the volume of water, the slower a change will occur. A pH controller may be valuable if you want to protect your fish but this can get expensive. I reviewed price information on the dupla site mentioned above and such a controller could cost several hundred dollars. Cheaper ones may not do an adequate job. While I have not answered your questions of the derivation of your aquarium equation for [CO2], if you read through the linked sites, it should get your started in the right direction.
Try the links in the MadSci Library for more information on Chemistry.