Aquarium pH and dissolved CO2

An interesting and widely misunderstood concept is the distribution of carbonate species (DIC:  dissolved inorganic carbon) in an aquarium.  What you usually see is a form of Bjerrum plot where it looks very much like the amount of dissolved CO₂ goes to zero as the pH gets much higher than 7.  Where this goes wrong is that whilst the plot successfully illustrates the relative fraction of DIC species with respect to each other, what it misses is that in an open system (exposed to atmosphere) the total amount DIC is not constant with respect to pH.

Note that in this graph (and most others as well) the species listed as H₂CO₃ (nominally carbonic acid), is actually composite carbonic acid:  an equilibrium of about 99.7% dissoved CO₂ and 0.3% carbonic acid.  The equilibrium between dissolved CO₂ and carbonic acid is independent of pH. 

High pH converts carbonic acid to other forms of carbonate

Carbonic acid can be deprotonated under high pH conditions, converting first to bicarbonate, and subsequently to carbonate if the pH gets high enough.  This process is conveyed by the Bjerrum plot, shown above for closed systems.

Total DIC increases exponentially above pH 5.5

There is an essentially infinite supply of CO₂ in the atmosphere, so an aquarium that doesn’t have some active process modifying CO₂ levels will, within hours, reach an equilibrium between dissolved CO₂ in the water and CO₂ in the atmosphere.  The relative amounts of these are described by Henry’s law and will come out to something between 1 and 3 ppm (parts per million) CO₂ dissolved in the water, compared to indoor atmospheric CO₂ gas which is generally between 400 and 1000 ppm.  This equilibrium between dissolved and atmospheric CO₂ is pH independent – the horizontal green line in the ‘DIC forms vs. pH‘ figure below – it is always a constant amount. 

Above 5.5 the carbonic acid starts being converted to other forms of carbonate species (see the Bjerrum plot), but you never run out of carbonic acid when this happens because carbonic acid can always be replenished from atmospheric CO₂ (again, Henry’s law).  This means the bicarbonate and carbonate forms of dissolved inorganic carbon have no upper limit to how much can accumulate – how much accumulates and in which form is entirely driven by pH; these are the blue and orange lines in the ‘DIC forms vs. pH’ picture.  The total amount of all carbonate species shown by the light blue curve grows exponentially with increasing pH (note that the Y-axis is log₁₀ transformed).  The high-pH driven accumulation of carbonate forms collectively is the major component of aquarium alkalinity.

The Bjerrum plot is still correct

So what does it mean when from the Bjerrum plot at high pH it looks like the levels of dissolved CO₂ go to zero?  The key is that the Bjerrum plot shows relative percentages and not absolute amounts.  The percentage of dissolved CO₂ doesn’t actually go all the way to zero, it just becomes very small relative to the total of all DIC, which is entirely consistent with the exponential increase in total DIC at high pH.  It is not that the levels of dissolved CO₂ become small with increasing pH, those stay constant, instead it is the amount of the other carbonate forms of DIC that become very large.

Changing aquarium pH does not change dissolved CO₂

What that all means is that if you want better aquatic plant growth and you want to make that happen by increasing the amount of dissolved CO₂ available to the plants, you can’t accomplish that by maintaining a stable low aquarium pH.  You might have other reasons to prefer your water have a lower pH, but dissolved CO₂ levels won’t be affected.  If you want to reliably increased dissolved CO₂ in the water, your best way is to inject pure CO₂ gas directly into the water.  Injecting pure CO₂ gas is a non-equilibrium method; once you stop injecting CO₂ gas into the water, levels of dissolved CO₂ will gradually fall to those determined by Henry’s law.  Injecting CO₂ gas into the water will (temporarily) decrease the pH of the water, but that pH decrease is a consequence of increasing dissolved CO₂, rather than being the cause of an increase in dissolved CO₂.