Tuesday, October 29, 2013

Introduction to Water Hardness

Water Hardness

Water hardness is an important parameter in establishing an ideal environment in fish aquarium tanks. In order to better understand what water hardness is, first, we need to be familiar with the water cycle process.

Understanding the Water Cycle

As you can see from the picture, the water cycle is an endless process where water that falls from the sky as rain accumulates in bodies of water such as lakes, rivers, and oceans where it will evaporate into the atmosphere to form clouds which will then fall back to the surface as rain.

Water is a universal solvent thus it can easily dissolve many substances to form a solution. It is this property which allows various salts and organic matter to be dissolved in natural water. The type and amount of these dissolved substances highly depends on the source of water and the chemical reaction that can occur as water comes into contact with soil and rocks.

As salts dissolve in water, a solution that contains various ions is formed. These ions include both negatively- and positively-charged ions. The negatively-charged ions, also called anions, include Chlorides (Cl-), sulfates (SO42-), carbonates (CO32-), and bicarbonate (HCO3-). The positively-charged ions, also called cations, include Calcium (Ca2+), Magnesium (Mg2+), Sodium (Na+) and potassium (K+). All these ions are often considered the most common ions which are dissolved in water. Other ions which can also be found dissolved in water include trace amounts of phosphate, nitrate, silicate and minerals such as iodine, copper and zinc as well as other metal ions. In order to ascertain and assess how much of these substances are present in water, you need to understand water hardness and how it can be assessed.

Water Hardness

As water falls back to earth as rain, it dissolves particulate matter and various gases as it and passes through the ground and into bodies of water. When water flows over or though rocks, minerals from the rocks are added thus making the water “hard”.

Water hardness reflects the amount of dissolved mineral salts present in water. For aquarists, there are two basic types of hardness that you should be familiar with—general hardness (GH) and carbonate hardness (KH).

What is General Hardness (GH)?

General Hardness (Fetermined based on the presence of calcium (Ca2+) and magnesium (Mg2+). Normally, (Ca2+) ions are three to ten times far more abundant than (Mg2+). Compared to carbonate hardness, GH cannot be removed from water by boiling. This is the reason GH is called “permanent hardness”. It is often measured by parts per million (ppm) and degree of hardness (dH or dGH).

Aside from Calcium (Ca2+) and Magnesium (Mg2+), other cations such as barium, strontium, iron, copper, zinc and other metallic ions may be present in trace amounts. However these cations exert minimal influence on water hardness compared to Ca2+and Mg2+.

Since cations cannot exist on its own, it must be balanced with an anion. Ca2+and Mg2+can combine with anions such as hydroxides (OH-), carbonate (CO32-) and bicarbonate (HCO3-) and some in smaller quantities of sulphates, chlorides, silicates, phosphates and borate to form various combinations of salts.

What is Carbonate Hardness (KH)?

Carbonate hardness (KH) reflects the presence of carbonate (CO32-) and bicarbonate (HCO3-) ions. Although it does not have a direct effect on the fish population, it serves as a buffer that can protect the aquatic environment from acid which can be produced or introduced into the fish aquarium tank.

Carbonate hardness is also known as “temporary hardness”. It is so-called because you can easily reduce water hardness by boiling water using an open container.

This chemical equation shows how boiling reduces water hardness:

2 HCO3 (bicarbonate) → CO32– (carbonate) + CO2 (carbon dioxide)

Have you noticed precipitations staining your aquarium glass and hood? These are actually insoluble salt precipitates which are formed when CO2 escape from water to the atmosphere leaving behind CO32– (carbonate) which react with (Ca2+) and Magnesium (Mg2+) to form insoluble calcium and magnesium carbonate salt.

To sum up what we have learned so far –

  • Water hardness is a measurement of the amount of Calcium (Ca2+) and Magnesium (Mg2+) cations dissolved in water.
  • Water hardness is measured using 2 units - General hardness (GH) and Carbonate hardness (KH)
  • General Hardness (GH) refers to the total salt listed in the chemical equation— the commons salts Ca2+ and Mg2+, and the trace sulphates, chlorides, silicates, phosphates and borate salts.

SOURCE: http://www.wetwebmedia.com/fwsubwebindex/fwh2oquality.htm

Water Hardness and Resulting Water Conditions
a) General hardness: the dH scale

b) Carbonate hardness: the KH scale


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