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Roger's Aquatics Pages |
This section is rather hard work, but it is a good idea to try and understand it, so that you know what is going on with your tank pH when you add various chemicals. If you don’t understand the Chemical Formulae, don’t worry you don’t need to, they just help make it a bit clearer for those readers that do.
When you add a strong acid, such as Nitric acid, to pure water the pH drops rapidly. Similarly, if you add a strong Alkaline to pure water the pH rises dramatically. Some chemicals added to the water do not effect this. Other chemicals or mixtures of chemicals damp out this effect, so that comparatively large amounts of acid and alkali need to be added to cause the pH to change. This effect is known as Buffering chemicals are known as Buffers.
The most common buffering effect seen in aquariums is Bicarbonate buffering. This is a single chemical buffering effect that is good at maintaining the pH high, but not so good at stopping it from going too high. Bicarbonates occur naturally in fresh water in the form of Calcium and Magnesium salts. Some Aquarists also add Sodium Bicarbonate (Bicarb or Baking Soda) to the water to buffer it. Bicarbonates are weak alkalis, that is they keep the pH above 7. When you add acid to a bicarbonate, it reacts with the acid to form salts of the acid and Carbon Dioxide. The Carbon Dioxide is given off to the atmosphere, and the acid salt is ph Neutral. As an example Nitric Acid is produced as a result of Nitrification. This is a strong acid which would pull the pH down quickly, instead it reacts with the Calcium Bicarbonate in the water to form Calcium Nitrate and Carbon Dioxide.
The important reaction is
HCO3- + H+- => CO2 + H2O
Bicarbonate reacts with Hydrogen ions (from the Nitric acid) to give Carbon Dioxide and Water. This reduces the number of Hydrogen ions and so increases pH.
If strong alkali, such as Sodium Hydroxide (Caustic Soda) is added to the water, the bicarbonate will help keep the pH down, as the bicarbonate with react with the hydroxide to give carbonate and water.
HC03- + OH- => H2O +CO32-
Whereas strong Acids, such as Nitric dissociate completely to give Hydrogen ions, and strong alkalis dissociate completely to and absorb hydrogen ions, the same is not true of weak acids and alkalis.
If you add 1 mole of Nitric Acid to 1 litre pure water is will all dissociate and take the water close to pH0 – 1 Mole of Hydrogen ions per litre.
If you add 1 mole of a weak acid, such a acetic acid (vinegar) to 1 litre of water, some of it will ionise to form Acetate and Hydrogen ions, but some will stay in the molecular form to give a pH of about 3.
What is happening in out Acetic Acid solution is that Water molecules occasionally bump into an Acetic Acid molecule in such a way as to cause it to split into ions. Similarly Hydrogen ions are occasionally bumping into Acetate ions in such a way as to cause them to join back together to form an acetic acid molecule. The probability of this happening depends on the concentration of hydrogen and acetate ions. If there are too many ions then the molecules form faster than they split, and vice versa.
Now life gets more interesting if we add an acetate salt to the mixture. Let us say we also add 1 Mole of Sodium Acetate to our litre of water containing 1 Mole of Acetic acid. What happens? The pH goes up. Why should this be? The sodium acetate also partially ionises, so the number of acetate ions goes up dramatically, and the probability of a Hydrogen ion hitting an acetate ion goes up – simply because there are more of them, so the rate of ionisation is much lower – this means less Hydrogen ions and a higher pH.
This mixture of a weak acid and a salt of that acid is an extremely powerful buffer. If you add a strong acid then number of hydrogen ions goes up, so the acetic acid molecules form faster. If you add a strong base, such as Sodium Hydroxide, it reacts with the Acetic acid to form more Sodium Acetate, so the pH does not rise very quickly.
This complex buffering is what you get when you buy the various mixes from the fish store to set your aquarium pH at a given level. These should be used with caution. Some of the cheaper ones use a Phosphoric Acid/Phosphate complex which can cause blooms of algae, or worse, blue-green algae – otherwise known as Slime Algae, or more correctly Cyanobacteria.
You get a similar effect when you acidify water by adding peat and then raise the pH slightly by adding Sodium Bicarbonate. Here Tannic Acids are released by the peat which react with the Sodium Bicarbonate to give Sodium Tannates. These together form a buffer, which stabilises the water and protects against pH crash.