Water is a compound that is most widely present in the world. It forms the basis of life on Earth. It is also an essential part of our daily lives. In order for its use – especially for drinking purposes – to be safe, the parameters of water quality must meet some specific criteria. One of the indicators of the quality of water is its hardness, which is responsible, among other things, for the white sediment that accumulates on your dishes or causes failures of household appliances.
Why is water hard?
The hardness of water is determined by the number of milligrams of Ca2+ and Mg2+ ions contained in it. The portion of hardness that determines its total alkalinity (which is another indicator of water quality) is referred to as carbonate hardness, otherwise known as transient hardness. The quantitative excess over total alkalinity is otherwise termed as non-carbonate (solid) hardness. The total value of carbonate and non-carbonate hardness represents the total (general) water hardness. The higher the content of calcium and magnesium ions in the water, the greater the hardness. Moreover, the value of this indicator is indirectly affected by iron and manganese ions. Water becomes hard because during its circulation, it comes into contact with numerous factors such as rocks or soil, which donate various concentrations of elemental ions to it. Naturally occurring waters typically contain up to 10 mg Mg/l. The concentration of magnesium ions rarely exceeds 100 mg per litre. In natural water sources, the prevalent type of hardness is the one owed to a high content of calcium ions, also referred as calcium hardness.
In analytical and quality control laboratories, total hardness is determined using the complexometric EDTA method. Carbonate hardness (corresponding to total alkalinity) is still determined using the acid-base titration method with a visual (or alternatively potentiometric) means of determining the endpoint of titration. The EDTA method is based on the determination of the total content of calcium and magnesium ions. The sample to be analysed is titrated with a solution of disodium edetate (this is the sodium salt of ethylenediaminetetraacetic acid) at a pH of 9 to 10, against eriochrome black T as an indicator. At the endpoint of the titration, the colour changes from red to blue. The titrant used in this determination is referred to by the acronym EDTA. Disodium edetate in aqueous solutions undergoes electrolytic dissociation, and the resulting disodium edetate ion forms a complex with the ions analysed. You can also check the approximate hardness of the water on your own (in addition to ordering a water quality analysis). In order to do this, you need to buy appropriate test strips or drop tests. If you use them, you must follow the instructions that come with the tests.
Effects of high water hardness:
- Deterioration of the physical and chemical properties of water.
- Destruction of consumer electronics and household appliances running on hard water. Precipitate that is not always completely removable from the appliance. Over time, it causes damage and failure of your appliances.
- Scale build-up. It occurs in the form of a white precipitate of calcium carbonate and magnesium carbonate. It precipitates in the process of thermal processing of water, which is why it is most often found in kettles. The formation of scale causes significant energy losses.
- The increase in the surface tension of water, which causes, for example, reduced wetting properties. As a result, you need to use much more detergent for washing your fabrics, for example.
- Deterioration of heating systems. Heating boilers that come in direct contact with hard water are particularly affected. The accumulation of insoluble scale clogs pipes, resulting in a significant increase in heating costs.
Types of water hardness
Generally, the hardness of water is classified into:
- Carbonate hardness – a type of hardness easily noticeable on your dishes or in the kettle. Carbonate hardness is caused by calcium and magnesium carbonates and bicarbonates. However, they are easily removed from water using heat, for example, which is why this type of hardness is often called transient. These compounds precipitate from hot water as a white scale, which accumulates in the container.
- Non-carbonate hardness – is also referred to as solid hardness. It is difficult to remove from water. While salts of unstable carbonic acid are responsible for carbonate hardness, salts of other acids, such as sulphuric (VI), hydrochloric or nitric (V) acid, are responsible for non-carbonate hardness. They form stable sulphates (VI), chlorides and nitrates (V), which remain in the water even after it is boiled.
- Total water hardness – is the sum of carbonate hardness and non-carbonate hardness.
What are the various degrees of water hardness?
The degree of water hardness can be expressed in different units. These usually include German degrees, French degrees, English degrees or the content of salts that cause water hardness in millimoles per litre of volume or in millivales per litre. It is a common practice in laboratories to quote hardness values in German degrees (on). If water has one German degree of hardness, it contains such an amount of calcium and magnesium salts in 1 dm3 that corresponds to 10 mg of CaO.
If you know the value of water hardness, you can then classify water based on the result given in German degrees:
- very soft water: below 5.6,
- soft water: 5.6 – 11.2
- moderately hard water: 11.2 – 19.6
- hard water: 19.6 – 30.8
- very hard water: above 30.8.
How to reduce water hardness?
High water hardness can become quite a hindrance to your daily life. If you do not know the quality of the water you use or suspect that its physicochemical parameters are not compliant with the relevant standards, it is worth ordering a water analysis from a competent laboratory.
The primary method of removing hardness from water is thermal processing. However, it is only effective for transient hardness. The thermal method involves heating water to a temperature of about 80oC. As the temperature increases, calcium and magnesium bicarbonates precipitate in the form of insoluble calcium carbonate and magnesium carbonate. The resulting scale can be removed by filtration, for example. The thermal method is often the initial step to the further reduction of water hardness.
Calcium and magnesium ions can also be precipitated using chemical methods. It involves the addition of specific softeners, i.e. chemical compounds that are aimed at precipitating the relevant sediments or form complex compounds with them. Calcium and magnesium ions precipitated in the form of salts, are separated by filtration or decantation. Softeners can also be used at home. Today, many manufacturers of heating boilers offer the installation of central water softeners, which work as ion exchangers.
An immediate way to soften water is to use filtering water pitchers, which have become very popular in recent years. The water you pour into the pitcher passes through a special sorption bed, which acts as an ion exchanger. There is a considerable choice of filters for such pitchers on the market and you can even buy filters that will immediately enrich your filtered water with essential minerals.
Read also about water treatment and wastewater treatment.