Hard water

From Academic Kids

Hard water is water that has a high mineral content. 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. While it is not generally dangerous to safety or health, it does generally cause potentially costly nuisance problems in the home and in industry. On the other hand, hard water is considered to be ideal for brewing certain styles of beer, and Kentucky's distinctive Bourbon whiskey in part owes its flavor to the high calcium content of its groundwater. Some hard water chemicals, particularly silicates and calcium carbonate, are also effective corrosion inhibitors, and can prevent damage to pipes or contamination by potentially toxic corrosion products.

The simplest way to know if water is hard or soft is the lather/froth test. If the water is very soft, when you wash your hands the soap will tend to lather up easily, 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 taste), it does not matter whether water is hard or soft. For instance, to put out fires, water your lawn, wash the mud off the streets or float your boat, water would have to be pretty hard to cause a problem. But for bathing, washing dishes and clothes, shaving, washing your car and many other uses of water, hard water is not as efficient or convenient as "soft water." Hard water can cause limescale deposits in kettles, washing machines and pipes. Soft water also tastes "sweeter" than hard water.

Contents

What is hard water?

Earlier generations coined the phrase "hard water" because it made cleaning difficult. Hardness is caused by compounds of calcium, magnesium, and a variety of other metals. All freshwater sources contain calcium and magnesium in varying quantities. Water adsorbs certain trace elements from many things that it contacts on its travels. For example peat will soften water where as lime will harden it.

Total water hardness (including both Ca and Mg) 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 from mineral deposits of CaSO4. The predominate source of magnesium is dolimitic limestone, CaMg(CO3)2.

Types of Hard Water

A common distinction is made between 'temporary' and 'permanent' hardness.

Temporary Hardness

Temporary hardness is hardness that can be removed by boiling or the addition of lime (calcium hydroxide). It is caused by dissolved calcium bicarbonate in the water.

Calcium carbonate is less soluble in hot water than in cold water, so boiling (which promote the formation of carbonate) will precipitate calcium carbonate out of solution, leaving water that is less hard on cooling.

Permanent Hardness

This is hardness (mineral content) that cannot be removed by boiling. It is usually caused by the presence of calcium and magnesium sulfates in the water, which are more soluble as the temperature rises.

Types of measurement

There are several different scales used to describe the hardness of water in different contexts.

  • Various obsolete "degrees":
    • Clarke degrees (°Clarke)/English degrees (°E)
      - conversion to mg/l calcium: divide by 0.175
      One degree Clarke corresponds to one grain of calcium carbonate in one (British/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)
      - 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)
  • mmol/l
  • 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)

The precise mixture of minerals dissolved in the water, together with the water's acidity or alkilinity (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

Problems caused by hard water

While hard water is not generally unhealthy (see below), it can cause many potentially costly nuisance problems.

Hard water also 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 faucets by 40%. It can coat the inside of tea and coffee pots, and clog and ruin hot 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. 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 your dishes.

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. This curd remains on the skin even after rinsing, clogging pores and coating body hair. This can serve as a medium for bacterial growth, causing diaper rash, minor skin irritation and skin that continually itches

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.

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 Organisation has reviewed the evidence [1] (http://www.who.int/water_sanitation_health/dwq/chemicals/en/hardness.pdf) and concluded the data were inadequate to allow for a recommendation for a level of hardness.

A later review [2] (http://www.szu.cz/chzp/voda/pdf/hardness.pdf) 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.

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.

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.

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 minerals for sodium. Artificial or natural zeolites can also be used. As the hard water passes through and around the beads, the hardness mineral ions attach themselves to it, dislodging the sodium ions. This process is called "ion exchange". When the bead or sodium zeolite has no 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 of sodium ions force the hardness ions off the resin beads. The excess sodium is rinsed away, and the resin is ready to start the process all over again. This cycle can be repeated many, many time before the resin loses it's ability to react to these forces.

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.

Hardness can also be reduced with a lime-soda ash treatment. This involves the addition of slaked lime (calcium hydroxide — Ca(OH)2) to a hard water supply to remove the carbonate hardness by precipitation, with the precipitate being removed by filtration.

Temporary hardness, caused by hydrogencarbonate ions, can be removed by boiling. For example, calcium hydrogencarbonate, 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 → CaC03 + C02 + H20

The addition of sodium carbonate also softens hard water 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 sufate has no affect 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 in the US

According to the US Geologic 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.

See also

References

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