Oxyguard solve CO2 measurement problems – Fishupdate.com

Oxyguard solve CO2 measurement problems Commercial Features Published:  01 June, 2005

Modern fish farming using advanced oxygenation methods leads to the accumulation of dissolved CO2. This can cause many problems. Skilled fish farmers have for some time wanted easy, accurate methods of measuring carbon dioxide in the water. Until recently they had to rely on chemical determination of samples using titration, but now they can use the OxyGuard CO2 and measure directly in the water.

The introduction of this instrument has not been without problems, these can be traced partly to the special way in which CO2 dissolves in water. Unlike oxygen, which just dissolves physically, carbon dioxide dissolves physically and chemically. The physical part is carbon dioxide in gas form and the chemical part is in the form of hydrogen carbonate and hydrogen ions.

The two parts balance:

CO2(g) + H2O « HCO3- + H+

The physical part, the CO2(g) is the CO2 that the fish are subjected to and react to. It is also the part that the OxyGuard CO2 reacts to and measures.

With the OxyGuard CO2

The OxyGuard CO2 probe contains a sensor that is selectively sensitive to the gas pressure of CO2. It is completely insensitive to oxygen, nitrogen, argon, water vapour or any dissolved gas found in the water other than CO2. The sensor is located behind a gas permeable membrane that does not allow any ionic species to pass, so the OxyGuard CO2 only reacts to the CO2(g). The OxyGuard CO2 thus measures the CO2 that affects the fish.

By Titration

Another part of the problem is due to the fact that the values obtained by chemical determination are often much higher than those obtained with the OxyGuard CO2.

Chemical methods for measuring dissolved CO2 are based on titration of a sample, either with base to pH 8.2 or by acid to pH 4.3. Both methods work well in pure water, but if the water contains anything else (in addition to CO2) that can affect a titration then too high a value will be obtained.

Aquaculture water contains many different substances that will interfere with a titration, either by consuming base or by consuming acid. Phosphates, ammonia and ammonium interfere strongly, but silicates, borates, humic acid, urea and other substances also interfere. There is, unfortunately, no easy way around this.

It should therefore not come as a surprise that lower values are obtained using the OxyGuard CO2 – but test after test has shown that it is the OxyGuard CO2 that measures correctly!