The story of liquid CO2 fertilisation – The CO2 Content

In the third part of the series “ The story of liquid CO2 fertilisation ” we look at the CO2 content of the “liquid CO2” products which are added to the aquarium.

Does glutaraldehyde contain carbon dioxide (CO2)?

No, as already mentioned, CO2 is not present in liquid form at room temperature and normal pressure. The substance glutaraldehyde has the molecular formula C5H8O2. This clearly shows that carbon and oxygen atoms are bound to hydrogen atoms. To “crack” this bond, energy is required. During this process not all compounds are transformed completely. During the degradation of this substance per glutaraldehyde compound (molecule) only 2 molecules are released. In the respiratory chain (key word: citric acid cycle or citrate cycle) glutaraldehyde is degraded, and this requires additional energy to create 2 CO2 molecules.

It is therefore a given that, with the usual dosing, as used in the aquarium, glutaraldehyde has no measurably proven enhancing effects on the CO2 content in the water. This can also easily be ascertained with permanent tests such as the JBL kit per test CO₂-pH Permanent or the JBL CO₂ Direct kit per test . Of course, marginal quantities of CO2 are generated during the degeneration process. But these are too low to be measurable and therefore below the CO2 limit needed biologically to promote plant growth.

The conventional 1 – 4% solutions on the market contain 10 – 40g/l glutaraldehyde. In case of a complete degradation, 1 mole glutaraldehyde (which corresponds to 100.12 g of the substance) would result in 5 moles of CO2. This would then result in 35.2 – 87.9 g CO2 per litre of the product. A 100 ml bottle could generate a mere 3.52 – 8.79 g carbon dioxide. Let’s see what the following chemical table can tell us:

gramme/mol
glutaraldehyde 100.12
carbon dioxide 44.01
gramme
complete degradation of 1 mole glutaraldehyde (100.12 g) results in 5 moles CO2 = [g] CO2 220.05
realistic degradation of 1 mole glutaraldehyde (100.12 g) results in 2 moles CO2 = [g] CO2 88.02
gramme
of a 4% solution contains per 100 ml glutaraldehyde [g] 4
mol
are moles glutaraldehyde 0.03995
result in moles CO2 (with complete degradation) 0.19976
result in moles CO2 (with realistic degradation) 0.07990
gramme
are g CO2 (with complete degradation) per 100 ml 8.79
are g CO2 (with realistic degradation) per 100 ml 3.52
gramme
1 ml of a 4 % solution contains glutaraldehyde 0,04
mol
are moles glutaraldehyde 0.00039952
result in moles CO2 (with complete degradation) 0.00039952
result in moles CO2 (with realistic degradation) 0.00079904
milligramme
are mg CO2 (with complete degradation) per 1 ml 87.9145026
are mg CO2 (with realistic degradation) per 1 ml 35.165801
1 ml of the 4% solution dosed per 50 l water results in mg/l
CO2 in mg/l (with complete degradation) 1.75829005
CO2 in mg/l (with realistic degradation) 0.70331602

The mole (unit symbol: mol) in chemistry is the unit of the so-called amount of substance. It corresponds to a precisely defined amount of particles. This unit can be used in a variety of quantitative calculations of chemical reactions and concentration calculations.

As a result we see that with a dosage of 1 ml of a 4% glutaraldehyde solution, 87.9 mg CO2 can theoretically be created, but realistically only 35.2 mg. These numbers then also need to be divided by the water volume; with 1 ml per 50 l water it needs to be divided by 50. This then results in 0.7 ml/g- max. 1.76 mg/l! From the low concentration of carbon dioxide thus generated it becomes clear why with over the counter tests for carbon dioxide ( JBL CO₂ Direct kit per test ) no increase of the CO2 content is perceptible. The low resulting amount lies below the measuring tolerance of the test kits.

Does glutaraldehyde affect the plants by the production of carbon dioxide or are there any other ways known to promote the plant growth?

The answer to this question is simple: we don’t know. An American manufacturer of a comparable glutaraldehyde-containing product has the following statement on their website:

To clarify the precise mechanisms (e.g. decomposition to CO2 or formation to longer molecules) further studies are required using radioactively marked carbon (C14).

You can find the other blog posts in this series here: The story of liquid CO2 fertilisation

© 03.07.2016
Matthias Wiesensee
Matthias Wiesensee
M.Sc. Wirtschaftsinformatik
JBL GmbH & Co. KG

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