Hard water
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Hard water is water that has a high mineral content (water with a low mineral content is known as soft water). This content usually consists of high levels of metal ions, mainly calcium (Ca) and magnesium (Mg) in the form of carbonates, but may include several other metals as well as bicarbonates and sulfates. It is not generally dangerous. The simplest way to determine if water is hard or soft is the lather/froth test. If the water is very soft, soap will tend to lather up easily when agitated, whereas with hard water it will not. Toothpaste will also not froth well in hard water. More exact methods of 'hardness' detection use a wet titration method to determine hardness.
For many uses, including drinking (aside from the taste), it does not matter whether water is hard or soft. For instance, to put out fires, water a lawn, or wash the mud off the streets, water would have to be very hard in order to make much of a difference.
Total water 'hardness' (including both Ca++ and Mg++ ions) is reported as ppm w/v (or mg/L) of CaCO3. Water hardness usually measures the total concentration of Ca and Mg, the two most prevalent divalent metal ions, although in some geographical locations iron, aluminium, and manganese may also be present at elevated levels. Calcium usually enters the water from either CaCO3, as limestone or chalk or from mineral deposits of CaSO4. The predominant source of magnesium is dolomite, CaMg(CO3)2.
The deposit of calcium carbonate (also called lime, limescale, etc.) left after hard water has evaporated from a surface is often referred to as 'hard water' even though no water is present.
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[edit] Types of hard water
A common distinction is made between 'temporary' and 'permanent' hardness. There are also common types of hard water depending on the ion (eg. Mg or Ca) found in the water.
[edit] Temporary hardness
Temporary hardness is hardness that can be removed by boiling or by the addition of lime (calcium hydroxide). It is caused by a combination of calcium ions and bicarbonate ions in the water. Boiling, which promotes the formation of carbonate from the bicarbonate, will precipitate calcium carbonate out of solution, leaving water that is less hard on cooling.
It should be noted that the above explanation is an oversimplification of the process that is occurring. The following equilibrium reaction actually happens when calcium carbonate (CaCO3) is "dissolved" in water:
CaCO3 (solid) + H2O (liquid) + CO2 (gas) ⇌ Ca2+ (aqueous) + 2 HCO3- (aqueous)
Upon heating, less CO2 is able to dissolve into the water (see Solubility). Since there is not enough CO2 around, the reaction cannot proceed from left to right, and therefore the CaCO3 will not "dissolve" as readily. Instead, the reaction is forced to go from right to left (i.e. products to reactants) to reestablish equilibrium, and solid CaCO3 is formed. Heating water will remove hardness as long as the solid CaCO3 that precipitates out is removed. After cooling, if enough time passes the water will pick up CO2 from the air and the reaction will again proceed from left to right, allowing the CaCO3 to "redissolve" in the water.
For considerably more detail on the solubility of calcium carbonate in water, and how it is affected by atmospheric carbon dioxide, please see the calcium carbonate article.
[edit] Permanent hardness
Permanent hardness is hardness (mineral content) that cannot be removed by boiling. It is usually caused by the presence of calcium and magnesium sulfates and/or chlorides in the water, which become more soluble as the temperature rises. Despite the name this can be removed using a water softener, or ion exchange column.
[edit] Types of measurement
It is possible to measure the level of hard water by obtaining a free water testing kit. These are supplied by most water softening companies. There are several different scales used to describe the hardness of water in different contexts.
- mmol/L (millimoles per litre)
- mg/L calcium carbonate equivalent
- grains/gallon (gpg)
1 gr/U.S. gal = 17.11 mg/L - parts per million weight/volume (ppm w/v or ppm m/v)
- Various obsolete "degrees":
- Clark degrees (°Clark)/English degrees (°E)
- conversion to mg/L calcium: divide by 0.175
One degree Clark corresponds to one grain of calcium carbonate in one Imperial gallon of water which is equivalent to 14.28 parts calcium carbonate in 1,000,000 parts water. - German degrees (°dH)
- conversion to mg/L calcium: divide by 0.14
One degree German corresponds to one part calcium oxide in 100,000 parts of water. - French degrees (°f) (shares symbol with degree Fahrenheit, but in lowercase)
- conversion to mg/L calcium: divide by 0.25
One degree French corresponds to one part calcium carbonate in 100,000 parts of water. - American degrees
One degree American corresponds to one part calcium carbonate in 1,000,000 parts water (1 mg/L or 1 ppm) - Degrees of general hardness (dGH)
One degree of general hardness corresponds to 10 mg of calcium oxide or magnesium oxide per litre of water
- Clark degrees (°Clark)/English degrees (°E)
The precise mixture of minerals dissolved in the water, together with the water's acidity or alkalinity (pH) and temperature will determine the behaviour of the hardness, so single number on a scale does not give a full description. Descriptions of hardness correspond roughly with ranges of mineral concentrations:
- Soft: 0 - 20 mg/L as calcium
- Moderately soft: 20 - 40 mg/L as calcium
- Slightly hard: 40 - 60 mg/L as calcium
- Moderately hard: 60 - 80 mg/L as calcium
- Hard: 80 - 120 mg/L as calcium
- Very Hard: >120 mg/L as calcium
[edit] Problems caused by hard water
While hard water is not generally unhealthy (see below), it can cause many potentially costly problems.
Hard water causes scaling, which is the precipitation of minerals to form a rock-hard deposit called limescale. Scale can clog pipes and can decrease the life of toilet flushing units by 70% and water taps by 40%[citation needed]. It can coat the inside of tea and coffee pots, and clog and ruin water heaters.
In the home environment, hard water requires more soap and synthetic detergents for laundry and washing. It takes half as much soap for cleaning with soft water[citation needed]. Hard water and soap combine to form "soap scum" that can't be rinsed off, forming a “bathtub ring” on all surfaces, and it dries leaving unsightly spots on dishes[citation needed].
Using soap on the body in hard water can cause the formation of a scum often referred to as “curd.” The formation of scum and curd is caused when calcium and magnesium form insoluble salts with anions (usually the stearates and other higher carboxylates found in soaps. This curd remains on the skin even after rinsing, clogging pores and coating body hair[citation needed]. This can serve as a medium for bacterial growth, causing nappy rash, minor skin irritation and skin that looks dry and continually itches[citation needed].
Similarly, the insoluble salts that get left behind from using regular shampoo in hard water tend to leave hair rougher and harder to detangle. [1]
In industry, hard water contributes to scaling in boilers, cooling towers and other industrial equipment. In these industrial settings, water hardness must be constantly monitored to avoid costly breakdowns. Hardness is controlled by addition of chemicals and by large-scale softening with zeolite resins.
[edit] Health considerations
Some studies have shown a weak inverse relationship between water hardness and cardiovascular disease in men, up to a level of 170 mg calcium carbonate per litre of water. The World Health Organization has reviewed the evidence [1] and concluded the data were inadequate to allow for a recommendation for a level of hardness.
A later review [2] by František Kožíšek, M.D., Ph. D. National Institute of Public Health, Czech Republic gives a good overview of the topic, and conversely to the WHO, sets some recommendations for the maximum and minimum levels of calcium (40-80 mg/L) and magnesium (20-30 mg/L) in drinking water, and a total hardness expressed as the sum of the calcium and magnesium concentrations of 2-4 mmol/L.
Naturally very soft water is more likely to corrode (i.e. react chemically with) metal pipes in which it is carried, and as a result it may have elevated levels of cadmium, copper, lead and zinc [3]. This is partly because hard water deposits a layer of limescale on the interior of pipes carrying it, and this coating of limescale slows down or prevents the process of taking up the pipe material into solution, including the toxic ions listed.
[edit] Softening
A water softener works on the principle of cation or ion exchange in which ions of the hardness minerals are exchanged for sodium or potassium ions, effectively reducing the concentration of hardness minerals to tolerable levels.[2]
The most economical way to soften household water is with an ion exchange water softener. This unit uses sodium chloride (table salt) to recharge beads made of ion exchange resin that exchange hardness mineral ions for sodium ions. Artificial or natural zeolites can also be used. As the hard water passes through and around the beads, the hardness mineral ions are preferentially absorbed, displacing the sodium ions. This process is called ion exchange. When the bead or sodium zeolite has a low concentration of sodium ions left, it is exhausted, and can no longer soften water. The resin is recharged by flushing (often back-flushing) with saltwater. The high excess concentration of sodium ions alter the equilibrium between the ions in solution and the ions held on the surface of the resin, resulting in replacement of the hardness mineral ions on the resin or zeolite with sodium ions. The resulting saltwater and mineral ion solution is then rinsed away, and the resin is ready to start the process all over again. This cycle can be repeated many times.
Some softening processes in industry use the same method, but on a much larger scale. These methods create an enormous amount of salty water that is costly to treat and dispose of.
Temporary hardness, caused by hydrogen carbonate (or bicarbonate) ions, can be removed by boiling. For example, calcium hydrogen carbonate, often present in temporary hard water, is boiled in a kettle to remove the hardness. In the process, a scale forms on the inside of the kettle in a process known as "furring of kettles". This scale is composed of insoluble calcium carbonate.
Ca(HCO3)2 → CaCO3 + CO2 + H2O
Hardness can also be reduced with a lime-soda ash treatment. This process, developed by Thomas Clark in 1841, involves the addition of slaked lime (calcium hydroxide — Ca(OH)2) to a hard water supply to convert the hydrogen carbonate hardness to carbonate, which precipitates and can be removed by filtration:
Ca(HCO3)2 + Ca(OH)2 → 2CaCO3 + 2H2O
The addition of sodium carbonate also softens permanently hard water containing calcium sulfate, as the calcium ions form calcium carbonate which is insoluble and sodium sulfate is formed which is soluble. The calcium carbonate formed sinks to the bottom. Sodium sulfate has no effect on the hardness of water.
Na2CO3 + CaSO4 → Na2SO4 + CaCO3
It is desirable to soften hard water, as the latter does not readily form lather with soap. Soap is wasted when trying to form lather, and in the process, scum forms.
Hard water may be treated in other ways to reduce the effects of scaling and to make it more suitable for laundry and bathing. See residential water treatment
[edit] Hard water in Australia
Analysis of water hardness in major Australian cities by the Australian Water Association shows a range from very soft (Melbourne) to very hard (Adelaide). Total Hardness as Calcium Carbonate mg/L are: Melbourne: 11.3 - 14.0; Sydney: 39.4 - 60.1; Perth: 30 - 198; Brisbane: 100; Adelaide: 101 - 216; Hobart: 6 - 50; Darwin: 22 - 38 [citation needed].
[edit] Hard water in Canada
The Laurentian shield does not leach many minerals into the water, resulting in very soft source and surface water. However, the moraine material on which the prairie provinces are located (mainly Saskatchewan and Manitoba) contains high quantities of calcium and magnesium, often as dolomite, which are readily soluble in the groundwater that contains high concentrations of trapped carbon dioxide from the last glaciation. In these parts of Canada, the total hardness in mg/L calcium carbonate equivalent frequently exceeds 200 mg/L, if groundwater is the only source of potable water.
Some typical values are: Calgary 165 mg/L, Saskatoon < 140 mg/L, Toronto 121 mg/L, Vancouver < 5 mg/L [citation needed].
[edit] Hard water in England and Wales
Information from the British Drinking Water Inspectorate shows that drinking water in England is generally considered to be 'very hard', with most areas of England, particularly the East, exhibiting above 200 mg/L as calcium carbonate equivalent. Wales, Cornwall and parts of North-West England are softer water areas, and range from 0 to 200 mg/L [citation needed]. In the brewing industry in England and Wales, water is often deliberately hardened with gypsum in the process of Burtonisation.
[edit] Hard water in the US
According to the United States Geological Survey, 85% of US homes have hard water. The softest waters occur in parts of the New England, South Atlantic-Gulf, Pacific Northwest, and Hawaii regions. Moderately hard waters are common in many of the rivers of the Tennessee, Great Lakes, Pacific Northwest, and Alaska regions. Hard and very hard waters are found in some of the streams in most of the regions throughout the country. Hardest waters (greater than 1,000 mg/L) are in streams in Texas, New Mexico, Kansas, Arizona, and southern California.[3]
[edit] See also
Note: Hard water should not be confused with heavy water.
[edit] References
- ^ Body And Fitness Healthy Hair Tips
- ^ How does a water softener work? at Howstuffworks.com
- ^ Briggs, J.C., and Ficke, J.F.; Quality of Rivers of the United States, 1975 Water Year -- Based on the National Stream Quality Accounting Network (NASQAN): U.S. Geological Survey Open-File Report 78-200, 436 p. (1977)
