waterchem

Chemestry of Water

pH

pH is the degree of acidity in the water. The acidity is determined by the proportion of hydrogen ions (H+) to hydroxyl (OH-) ions in the water. Ions are atoms or group of atoms with an electric charge. Acid water (pH 1.0) has very few hyroxyl ions but many free hrdrogen ions. Akline water (pH 14) has few free hydrogen ions and many free hydroxyl ions. Neutral water (pH 7) has equal porpotions of both.

The pH scale is from 1 to 14. A pH of 7 is neutral, meaning the water is not acid or alkaline. As the pH scale goes down the water becomes more acidic. As the pH scale goes up the water gets more alkaline. The pH scale is a logarithmic scale. What this means is that the pH changes at a tenfold level between each number. For example, a pH of 5 is 10 times more acidic than a pH of 6. So if your fish are supposed to be at a pH of 7, and the water level is 8, they are in water that is 10 times more alkaline than what they should be. If the pH is 9, then they are in water that is 100 times more alkaline than what they need. So it is easy to see why even a small change in required pH is stressful and potentially fatal to fish.

There are several different ways water pH is changed. Free hydrogen ions lower the pH increasing the acidity of the water. Free hydrogen ions are released by the filter system as a by product of the nitrogen cycle. Carbon dioxide released by living organisums in the water dissloves into carbonic acid lowering the pH. Green plats and algae remove carbonic acid and carbon dioxide thus rising pH. There are chemical additives that can be added directly to the water that will either raise or lower the pH. Some natural agents can be used to alter water pH as well. Peat in the tank or filter will acidify the water. Mineral salts like calcium that are found in limestone or in some shells will cause an increase in alkalinity and pH.

Fish are very sensitive to changes in pH, and rapid changes in pH can cause extreme stress and death. Fish should not be exposed to a change in pH greater than 0.3 in a 24-hour period. However they are able to adapt to a ph somewhat outside there range. Given aslow change in pH over a longer time no more than 0.3 in 24hrs. A pH of 6.5 and 7.5 is usualy fine and doesn't require adjustment.

There are many factors that exert an influance on pH that are counter acted by the buffering system. Another important consideration in altering the pH of water relates to the mineral content or hardness of the water.

Water is often called the universal solvent meaning that water can and readly dissloves almost anything. It is said that water even disloves into itself. Therefore water is rarley found as pure water (H2O). Water contains salts, minerals, mettals, nutrients, living things....etc. Fish have adapted to their water conditions in their natural habitat. It is important to macth these natural conditions in an aquarium for healthy fish.

There are three main parts of water chemerstry each one affecting and dependant on the other. Buffering capacity(KH), water acididy(pH), and water hardness (GH). KH,GH, and pH all interact with each other and is diffilcult to change one without affecting the other. I like to think of water hardness as minerals in the water, the buffering as how minerals are reacting , and pH as ameasure of what or how much minerals have reacted.

                            Buffering Capacity or The Buffering system

The buffering capacity can be defined as the ability of the water to resist changes in pH and is measured in KH. The buffer system is comprised primarily of bicarbonates and carbonates, giving the buffer system the alternative name of Carbonate Hardness. However, other bases such as hydroxides, silicates, phosphates, and borates contribute to the buffering capacity.
   Bicarbonates is a salt of carbonic acid containing HCO3- group; sodium bicarbonate NaHCO3 for ex.
   Carbonate is a ester of carbonic acid (CO3).
   Carbonic acid (H2CO3) is the acid formed when carbon dioxide is dissloved in water.
   Salt in this instance is a product formed by the neturalization of an acid by a base; or the removal of one or more hydrogen atoms.
   Ester is a compound produced by the reaction of an acid and an alchol with the elimantion of a molecule of water.

The bicarbonate ion is actually a buffer in a water, able to raise the pH if it is too low or to lower the pH if it is too high. How can it do that: The normal process in water is for the bicarbonate to keep pH stable by "absorbing" the acidic ions (H+) that are produced by the bacteria in the biological filter. When this happens, we get water and carbon dioxide. The carbon dioxide is driven out of the pond by aeration devices or plants and the water is left behind.
The chemical formula is: HCO3- + H+ <=> H20 + CO2 (bicarbonate + acid = water + carbon dioxide).

Notice what a nice balance there is. There are three oxygen atoms on the left side of the equal sign, three on the right side. Two hydrogen atoms on each side, and one carbon. Nothing is lost; we have as much of each after the chemical reaction of "absorbing" the acid ion as we had before. What is lost, however, is the bicarbonate ion. The bicarbonate has been transformed into carbon dioxide (CO2) and driven off as a gas. We also lost the acid, which had been produced by the bacteria in the biological filter, but because the bicarbonate is consumed in the reaction, we must continually replace it.

How can the bicarbonate act as a buffer-to go the other way, lowering the pH if it is too high? In that case, the bicarbonate ion would give up its hydrogen as an ion, thus adding acid, lowering pH, and becoming a carbonate ion (CO3--).
Thus, HCO3- <=> CO3-- + H+ (bicarbonate = carbonate + acid).

The carbonate ion tends to unite with a calcium ion (Ca++) becoming a solid, calcium carbonate. That is the stuff oyster shells and marble chips are made of. Notice again that everything balances on both sides of the equation, all the H, O, and C atoms are equal in number on each side.
A "<=> " symbol shows that the chemical reaction can go both ways. Whether they go from left to right or from right to left depends on the relative concentrations of the compounds involved and on the pH of the water (and on other things as well). Try adding oyster shells or marble chips to the filter in order to get the benefit of the last equation going from right to left--that is the oyster shells or marble chips tend to dissolve (absorbing a hydrogen ion in the process) and providing the needed bicarbonate ion to "absorb" another hydrogen ion..

Back