The pH buffers in the aquarium

Depending on their level of expertise, aquarists tend to interfere more or less in the chemical composition of aquarium water. While these parameters can and often must be adjusted as needed, there are compounds among them, such as pH buffers, whose presence in an aquarium is essential and getting rid of them altogether is unwise.

The role of bicarbonates in the aquarium

Bicarbonates play a significant and dual role in any water body. To explain both phenomena, let’s start with theory. Bicarbonates are hydrogen salts of carbonic acid. We all encounter carbonic acid (H2CO3) on a daily basis. It is formed when carbon dioxide (CO2) combines with water (H2O ).

CO2 + H2O ↔ H2CO3

Carbonic acid is not a very stable compound. As a result of e.g. pressure reduction or temperature changes, it decomposes very quickly (bubble effect after unscrewing a bottle of sparkling water). The dissociation (breakdown) of an acid is always gradual. Here it occurs in an aqueous environment, so a water molecule is always present in the equation.

H2CO3 + H2O ↔ H3O+ + HCO3 – bicarbonate ion is formed

HCO3+ H2O ↔ H3O+ + CO32- – carbonate ion is formed

H3O+ is present in each of the above equations. It is an oxonium cation, a combination of H+ and H2O. By way of introduction, note that the pH scale many decades ago was set in the range of 0-14, although nowadays one can easily find compounds e.g. with pH = -2.

The H+ and OH ions are responsible for the acidic or basic reaction. The more H+ ions, the more acidic the environment. And the other way round, the more OH the higher the pH (alkaline environment). It is therefore obvious to conclude that the amount of H+ and OH exist in close correlation, and equilibration of their amounts means pH=7 (neutral).

Bicarbonate is commonly known among aquarists as transient hardness (KH) and only considered in this regard. In one aquarium you will find more of them, in another less. Due to their constant presence in the water and their chemical structure, they are an invaluable source of carbon dioxide (CO2) for plants, which I wrote about in the article Calcium deposits in the aquarium and biogenic decalcification. Therefore, today we will focus solely on the important benefit of their presence in water, which is pH stabilization. Because bicarbotanets are the basic pH buffers in the aquarium.

How buffers stabilize the pH of the aquarium water?

When most people think of correcting acidity, they think of using acids (increasing H+ ions) or alkalis (increasing OH).

Be aware that acidity (pH) is not temperature. An increase or decrease of the water temperature by 1 degree has little influence on the life of the aquarium biota. However, only the decimal logarithm applies in the definition of pH. This means that a pH adjustment of 1, e.g. from pH = 6.5 to pH = 7.5 is by definition a change of a whole order of magnitude!

Let us imagine the following situation: we are standing in the middle of a meadow. It’s beautiful, sunny weather, 20⁰C. Perfect, neutral, like a pH = 7 aquarium. Suddenly the weather changes. The temperature drops 10-fold, to 2⁰C. We didn’t have time to prepare for that. We’re still standing wearing a T-shirt and asking “where did the perfect weather go?”. Well, the acidity in the tank has just dropped to pH = 6. A theoretical change of 1 point, in practice is a huge gap. What effect do we get in the aquarium? The fish swim near the surface, have heavily reddened gills, and breathe with difficulty.

Careless and novice aquarist decides to save the situation by using a preparation increasing pH, but the manufacturer did not write on the package how much to use. It is not surprising since no manufacturer can predict by how much an aquarist will want to adjust the parameter. This should be done carefully and the situation closely monitored with a test.

What happens when we overdo it? Suppose the pH increases from 6.0 to 8.0. The temperature in the meadow at the moment rose from 2⁰C to 40⁰C. No doubt in humans such sudden fluctuations in temperature will cause a drop in immunity and flu or cold. The habitat of fish is water and in this situation many negative consequences of careless action will be caused.

Such situations also occur in lakes and rivers. For example: decaying organic matter lowers the pH, intensive photosynthesis raises it. Nature has therefore evolved mechanisms to prevent such fluctuations to some extent. Bicarbonates come to the rescue.

Buffering effect of bicarbonates

In the presence of strong acids and bases, they react with them to neutralize them. Importantly, only one compound is needed to react with both an acid and a base. The scheme of such a reaction is always identical and looks as follows:

Bicarbonate + acid/base → inert molecule (precipitate, gas) + salt + water

Ca(HCO3)2 + 2HCl → 2CO2↑ + CaCl2 + 2H2O (CO2 molecule “escapes” to the atmosphere or is used by plants in the aquarium)

Ca(HCO3)2 + NaOH → CaCO3↓ + Na2CO3 + 2 H2O (CaCO3 is a precipitate, very hardly soluble in water)

Special attention should be paid to the fact that, despite the participation of strong acids and bases in the reaction, as a result no H+ and OH ions are formed, on which the reaction (pH) of water depends. These ions, originating in the above reactions from HCl (strong acid) and NaOH (strong base) are immediately neutralized due to the presence of bicarbonate. This is a unique case and pH buffers have saved not one aquarium of a beginning aquarist.

When the water lacks bicarbonates…

Every home tank has what is called a “buffer capacity”. It means only that bicarbonate dissolved in water exists there in a certain amount, and therefore is able to neutralize a certain number of acid molecules. Once all the bicarbonates are used up, there is nothing to stop the acids or bases from drastically affecting the environment.

For this reason, lowering the pH of the tank must also be considered in two ways, as it is this process that causes the most problems among aquarists. We pour many drops into the tank, do several or a dozen tests, and the acidity does not drop. How many people then decided to pour the preparation “by eye” wanting to make their lives easier? That’s how this story came about.

Effects of phosphoric acid in the aquarium

Finally, a small digression: How does it happen that often in an aquarium with constantly lowered pH with acids we observe algae infestation after some time? Where did this effect come form?

Aquaristics is now shifting away from using the simplest acid to correct acidity, hydrochloric acid (HCl). Many manufacturers replaced them with phosphoric acid(V), popularly known as orthophosphoric acid, with the formula H3PO4. It is a strong oxidizing acid, but due to the presence of phosphorus in its composition, its effect can become assimilable by aquarium plants. After all, some of us are adding fertilizers with with phosphorus in the formulation to our tanks. Let us consider this example by substituting the above acid according to the equation shown earlier:

3Ca(HCO3)2 + 2H3PO4 → 6CO2↑ + Ca3(PO4)2↓ + 6H2O

The above reaction results in the formation of very hardly soluble Ca3(PO4)2. If a compound precipitates in our aquarium in the form of sediment, it begins to accumulate in the gravel, filter and other materials present in the water. What this means for us is that we now have a ready-made phosphorus storage facility which will be permanently present in the tank. Gradually, with the help of plants and bacteria, the compound will change to more soluble forms and begin to leak heavily into the water.

Michał Wyskiel

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