pH and alkalinity

Water quality method: the pH

Water quality and pH are often mentioned in the same sentence. The pH is a very important factor, because certain chemical processes can only take place when water has a certain pH. For instance, chlorine reactions only take place when the pH has a value of between 6,5 and 8.

The pH is an indication for the acidity of a substance. It is determined by the number of free hydrogen ions (H+) in a substance.
Acidity is one of the most important properties of water. Water is a solvent for nearly all ions. The pH serves as an indicator that compares some of the most water-soluble ions.
The outcome of a pH-measurement is determined by a consideration between the number of H+ ions and the number of hydroxide (OH-) ions. When the number of H+ ions equals the number of OH- ions, the water is neutral. It will than have a pH of about 7.
The pH of water can vary between 0 and 14. When the pH of a substance is above 7, it is a basic substance. When the pH of a substance is below 7, it is an acid substance. The further the pH lies above or below 7, the more basic or acid a solution is.
The pH is a logarithmic factor; when a solution becomes ten times more acidic, the pH will fall by one unit. When a solution becomes a hundred times more acidic the pH will fall by two units.
The common term for pH is alkalinity.

The word pH is short for "pondus Hydrogenium". This literally means the weight of hydrogen. De pH is an indication for the number of hydrogen ions. It consisted when we discovered that water consists of hydrogen ions (H+) and hydroxide ions (OH-).
The pH does not have a unit; it is merely expressed as a number. When a solution is neutral, the number of hydrogen ions equals the number of hydroxide ions. When the number of hydroxide ions is higher, the solution is basic. When the number of hydrogen ions is higher, the solution is acid.

Did you know that the pH of Coca-Cola is about 2? And did you know that it is useless to measure the pH of RO-water or demiwater? Both demiwater and RO-water do not contain any buffer ions. This means that the pH can be as low as four, but it can also be as high as 12. Both kinds of water are not readily usable in their natural form. They are always mixed before application!

Methods to determine the pH

There are several different methods to measure the pH. One of these is using a piece of pH indicator paper. When the paper is pushed into a solution it will change colour. Each different colour indicates a different pH-value. This method is not very accurate and it is not suitable to determine more exact pH values. That is why there are now test-strings available, which are able to determine smaller pH-values, such as 3.5 or 8.5.
The most accurate method to determine the pH is measuring a colour change in a chemical lab experiment. With this method one can determine pH values, such as 5.07 and 2.03.
All of these methods are not suitable to determine a pH development in time.

The pH-electrode

A pH electrode is a tube that is small enough to put it in sample jars. It is tied to a pH-meter by means of a cable. A special type of fluid is located within the electrode; this is usually "3M Kalium Chlorine". Some electrodes contain a gel that has the same properties as the 3M-fluid. In the fluid there are silver and platinum wires. The system is quite fragile, because it contains a small membrane. The H+ and OH- ions will enter the electrode through this membrane. The ions will create a slightly positive charge and a slightly negative charge in each end of the electrode. The potential of the charges determines the number of H+ and OH- ions and when this is determined the pH will appear digitally on the pH-meter. The potential is co-dependent on the temperature of the solution. That is why the temperature is also presented on the pH-meter.

Acids and bases

When acids enter the water, the ions will separate. For instance, hydrogen chloride will separate into hydrogen and chlorine ions (HCL à H+ + CL-).
Bases also undergo separation of their ions when enter the water. When sodium hydroxide enters the water it will separate into sodium and hydroxide ions (NaOH à Na+ + OH-).

When an acid substance ends up in water, it will give up a hydrogen ion to the water. The water will than become acid. The number of hydrogen ions that the water will receive determines the pH. When a basic substance enters the water it will take up hydrogen ions. This will raise the pH of the water.
When a substance is strongly acidic it will give up more H+ ions to the water. Strong bases will give up more OH-.

Here we have summed up a list of products and their pH:

pH	product
14	sodium hydroxide
13	lye
12.4	lyme
11	ammonia
10.5	manganese
8.3	backing powder
7.4	human blood
7.0	pure water
6.6	milk
4.5	tomatoes
4.0	win
3.0	apples
2.0	lemon juice
0	hydrochloric acid

For more information about the effects of changing in pH on freshwater ecosystems, take a look at
acids & alkalis in freshwater and for the correlation between pH en tempererature of pure water

For more information on water quality assessment, please check out our water quality assessment FAQ

For more information on water and water terminology move to our water FAQ or our water glossary

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