Water hardness is a combination of the chemical and physical properties of water associated with the content of dissolved salts of alkaline earth metals , mainly calcium and magnesium (the so-called hardness salts ).
Content
Hard and soft water
Water with a high salt content is called hard water, with a low content - soft . The term “hard” in relation to water has historically been formed due to the properties of fabrics after their washing with the use of a soap based on fatty acids — a fabric that has been washed in hard water, which is tougher to the touch. This phenomenon is explained, on the one hand, by the sorption by the tissue of calcium and magnesium salts of fatty acids formed during washing at the macro level. On the other hand, tissue fibers have ion-exchange properties, and, as a consequence, the ability to sorb multivalent cations - at the molecular level. There are temporary (carbonate) stiffness caused by calcium and magnesium bicarbonates Ca (HCO 3 ) 2 ; Mg (НСО 3 ) 2 , and constant (non-carbonate) hardness caused by the presence of other salts that are not released when boiling water: mainly sulfates and chlorides of Ca and Mg (CaSO 4 , CaCl 2 , MgSO 4 , MgCl 2 ).
Hard water, when washed, dries the skin, foam is poorly formed in it when using soap . The use of hard water causes sediment ( scale ) to appear on the walls of the boilers, in pipes , etc. At the same time, the use of too soft water can lead to corrosion of the pipes, since, in this case, there is no acid - base buffering provided by hydrocarbonate (temporary) stiffness. Consumption of hard or soft water is usually not dangerous to health, but there is evidence that high hardness contributes to the formation of urinary stones, and low - slightly increases the risk of cardiovascular diseases. The taste of natural drinking water, such as spring water, is caused precisely by the presence of hardness salts.
The hardness of natural waters can vary quite widely and is unstable throughout the year. Stiffness increases due to evaporation of water, decreases in the rainy season, as well as during the melting of snow and ice.
Units
For the numerical expression of water hardness indicate the concentration of calcium and magnesium cations in it. The recommended SI unit for measuring concentration is mol per cubic meter (mol / m³), however, in practice, degrees of rigidity and milligram equivalents per liter (mEq / l) are used to measure stiffness.
In the USSR, until 1952, the degrees of hardness used coincided with the German ones. In Russia , the normal concentration of calcium and magnesium ions, expressed in milligram equivalents per liter (mEq / l), was sometimes used to measure stiffness. One mEq / l corresponds to a content of 20.04 milligrams of Ca 2+ per liter of water or 12.16 milligrams of Mg 2+ ( atomic mass divided by valence ).
From January 1, 2014, the interstate standard GOST 31865-2012 “Water. Unit of stiffness " [1] . According to the new GOST, the stiffness is expressed in degrees of rigidity (° F). 1 ° W corresponds to the concentration of alkaline earth element, numerically equal to 1/2 of its millimole per liter (1 ° W = 1 mEq / l).
Sometimes they indicate the concentration per unit mass , and not volume , especially if the water temperature can change or if the water can contain steam , which leads to significant changes in density .
Different countries used (sometimes still used) various non-system units - degrees of hardness.
| Degree | Designation | Definition | Magnitude | |
|---|---|---|---|---|
| ° W | mmol / l | |||
| Deutsch | ° dH (deutsche Härte), ° dGH (degrees of general hardness), ° dKH (for carbonate hardness) | 1 part calcium oxide (CaO) or 0.719 parts magnesium oxide (MgO) per 100,000 parts water | 0.3566 | 0.1783 |
| English | ° e | 1 gran CaCO 3 per 1 gallon of water | 0.2848 | 0.1424 |
| French | ° TH | 1 part CaCO 3 per 100,000 parts of water | 0.1998 | 0,0999 |
| American | ppm | 1 part CaCO 3 per 1,000,000 parts of water | 0,0200 | 0,0100 |
According to the total hardness, they distinguish soft water (up to 2 ° W), medium hardness (2-10 ° W) and hard (more than 10 ° W).
The water hardness of surface sources fluctuates significantly throughout the year; it is maximal at the end of winter, minimal - during the flood period (for example, the hardness of the Volga water in March is 4.3 ° F, in May it is 0.5 ° F [2] ). In groundwater, the stiffness is usually higher (up to 8–10, less often up to 15–20 ° F) and changes less during the year.
Elimination Methods
- Thermal softening . Based on the boiling of water, as a result of thermally unstable calcium and magnesium bicarbonates decompose with the formation of scale:
Boiling removes only temporary (carbonate) hardness. Finds application in everyday life.
- Reagent softening . The method is based on the addition of soda ash Na 2 CO 3 or hydrated lime Ca (OH) 2 to the water. At the same time, calcium and magnesium salts pass into insoluble compounds and, as a result, precipitate. For example, the addition of hydrated lime leads to the conversion of calcium salts to insoluble carbonate:
The best reagent for eliminating the overall hardness of water is sodium orthophosphate Na 3 PO 4 , which is part of most household and industrial products:
Calcium and magnesium orthophosphates are very poorly soluble in water, and therefore easily separated by mechanical filtration. This method is justified at relatively high water consumption, since it is associated with the solution of a number of specific problems: sediment filtration, accurate reagent dosage.
- Cationization . The method is based on the use of ion-exchange granulated loading (most often ion-exchange resins). Such a load on contact with water absorbs cations of hardness salts (calcium and magnesium, iron and manganese). Instead, depending on the ionic form, it gives away ions of sodium or hydrogen . These methods are respectively called Na-cationization and H-cationization. With properly selected ion-exchange loading, the water hardness decreases with single-stage sodium cationization to 0.05-0.1 ° W, with two-stage loading - to 0.01 ° W.
In industry, ion-exchange filters replace calcium and magnesium ions with sodium and potassium ions, producing soft water.
- Reverse osmosis . The method is based on the passage of water through semipermeable membranes (usually polyamide). Together with hardness salts, most of the other salts are removed. The cleaning efficiency can reach 99.9%.
There are nanofiltration (the nominal diameter of the holes of the membrane is equal to units of nanometers) and picofiltration (the nominal diameter of the holes of the membrane is equal to units of picometers). The disadvantages of this method should be noted:
- the need for pre-treatment of water supplied to the reverse osmosis membrane;
- the relatively high cost of 1 liter of water produced (expensive equipment, expensive membranes);
- low mineralization of the produced water (especially when picofiltration). Water becomes almost distilled .
- Electrodialysis It is based on the removal of salts from water under the action of an electric field. Removal of ions of dissolved substances occurs due to special membranes. Just as with the use of reverse osmosis technology, the removal of other salts, in addition to the hardness ions, occurs.
Completely clean the water from hardness salts can be distilled .
Clinical value
Continuous consumption of hard water can cause an imbalance of the body’s mineral balance (for example, urinary or kidney stones ) .
The use of hard water for feeding and bathing newborns increases the risk of atopic dermatitis and / or eczema in children. The average age of the first symptoms is 3 months. Moreover, the appearance of eczema triggers a mechanism for the development of autoallergies along the atopic march chain: from eczema to food allergies and asthma . [3]
Links
Notes
- ↑ Interstate standard GOST 31865-2012 “Water. Unit of stiffness .
- ↑ Chemical encyclopedia. - M .: Soviet Encyclopedia, 1990. T. 2. S. 145.
- ↑ Hard Water Linked to Infant Eczema