Chapter 4 About pH
The pH of a water sample is the measure which indicates whether a water sample is acidic or basic (alkaline). More precisely, pH indicates the hydrogen iron concentration in the water sample.
The relationship between pH and other water parameters is important in aquaculture and can affect the toxicity of various things such as ammonia and heavy metals as mentioned above; the toxicity of metals is relevant when we consider the overall water supply route to our ponds which may be via iron, copper, lead and other types of pipes.
A pH scale is used to measure the amount of acid in liquid such as water. Because acids release hydrogen ions, the acid content of a solution is based on the concentration of hydrogen ions and is expressed as “pH.”
The pH is measured on a scale from 0 to 14, a pH of 7 is considered to be “neutral”, below 7 is considered to be “acidic”, and above 7 is considered to be alkaline.
The pH scale is logarithmic; the numbers in a logarithmic scale represent increases by factors of ten. So a decrease in pH from 7 to 6 means that the sample is ten times more acidic. A decrease from 7 to 5 means the sample is 100 times more acid.
Although koi will live in water with a pH in the range 6.5>9.0 the preferred range is 7>8; this being close to the average blood pH of aquatic vertebrates. Fish blood comes into close contact with water (only a one or two cell separation) as it passes through the blood vessels of the gills and skin.
As mentioned above the pH of a water sample can be reduced if carbon dioxide is dissolved into it. Carbon dioxide gas readily dissolves into water but it can be encouraged to leave the water by strong aeration and the use of trickle type towers which expose the pond water to the atmospheric air. Waterfalls together with the other mentioned methods are also a good means of allowing carbon dioxide and some nitrogen gas to leave water and for oxygen to be absorbed; this process is termed de-gassing.
The pH of a water sample can be measured in a variety of ways as follows;
1) Liquid test kits where drops of a reagent are added to a measured amount of water to be tested and the value of the pH is determined by the colour of the sample when compared with a colour card provided with the test kit.
2) An electronic test meter with a test probe which when inserted into a water sample will give a reading of the pH value on an LCD screen. These meters need calibration with a special solution with a fixed and known pH value.
The Buffer System Explained
Before the importance of the buffer system can be understood it is essential to explain the definition and chemical basis of pH.
The pH is the degree of acidity in the water. The acidity is determined by the proportion of hydrogen ions (H+) to hydroxyl ions (OH-) in the water.
Extremely acid water (pH 1.0) has very few hydroxyl ions, but many free hydrogen ions (H+).
Extremely alkaline water (pH 14) has very few free hydrogen ions but many hydroxyl ions (OH-).
Neutral water (pH 7.0) has equal proportions of the two.
Thus 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 and carbon dioxide released by living organisms in the water dissolves, making carbonic acid, which lowers the pH. There are also many organic acids in the detritus of a pond which lower the pH.
The pH can rise due to removal of carbon dioxide (by plants and algae during photosynthesis as explained above). The pH of any body of water is in a dynamic equilibrium. In other words there are many factors that exert an influence on the pH, and these are counteracted by the buffer system.
The buffering capacity of pond water can be defined as the ability of the water to resist changes in pH.
The buffer system is comprised primarily of bicarbonates and carbonates, giving the buffer system the alternative name of Carbonate Hardness (KH). However other bases such as hydroxides, silicates, phosphates, and borates contribute to the buffering capacity.
Another name for the buffer system is the “Total Alkalinity”. All the above mentioned compounds are strong alkalis (they are termed bases – they have a high pH) so the total alkalinity is the sum of all these compounds.
HOW DOES THE BUFFER PREVENT pH CHANGE?
Free Hydrogen ions are effectively ‘mopped up’ by the buffer, taking them out of solution, and preventing them from causing a decline in pH (a rise in the acidity of the water). Conversely when the pH begins to rise too high, due to a lack of free hydrogen ions the buffer system will ‘release’ some to bring the pH back down to the correct level.
HOW CAN I MEASURE THE STRENGTH OF MY BUFFER SYSTEM (KH)?
The KH test kit.
Test kits are available which add measured drops of acid to a 5 ml sample of pond or aquarium water. Mixed in with the acid is an indicator chemical, which changes colour from blue to yellow at a precise pH. Thus the number of drops added to the sample needed to bring about the colour change is directly proportional to the strength of the buffer – or the ability of the water to resist a change in pH! Each drop is equivalent to 17.9 mg/L of Calcium carbonate – or put more conveniently one German degree of hardness (ºdH).