MadSci Network: Chemistry
Query:

Re: Determining CO2 in H2O soln using carbonate hardness and pH

Date: Sun Jan 21 13:12:11 2001
Posted By: Lon Brouse, Faculty, Chemistry, Challenge Charter School
Area of science: Chemistry
ID: 978335841.Ch
Message:

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.



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