Hard water

From Wikipedia, the free encyclopedia

(Redirected from Water hardness)
Jump to: navigation, search

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 (Ca2+) and magnesium (Mg2+) 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.

Total water 'hardness' (including both Ca2+ and Mg2+ ions) is reported as ppm w/v (or mg/L) of calcium carbonate (CaCO3). Water hardness usually measures the total concentration of calcium and magnesium, 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 as either calcium carbonate (CaCO3), from limestone or chalk, or from mineral deposits of calcium sulfate (CaSO4). The predominant source of magnesium is dolomite (CaMg(CO3)2).

Contents

  • 1 Types of hard water
    • 1.1 Temporary hardness
    • 1.2 Permanent hardness
  • 2 Problems caused by hard water
  • 3 Types of measurement
  • 4 Indices
    • 4.1 Langelier Saturation Index (LSI)
    • 4.2 Ryznar Stability Index (RSI)
    • 4.3 Puckorius Scaling Index (PSI)
    • 4.4 Other indices
  • 5 Health considerations
  • 6 Softening
    • 6.1 Process
    • 6.2 Why does my skin feel slippery after washing with softened water, unlike hard water?
  • 7 Regional Information
    • 7.1 Hard water in Australia
    • 7.2 Hard water in Canada
    • 7.3 Hard water in England and Wales
    • 7.4 Hard water in the US
  • 8 See also
  • 9 References
  • 10 External Links

[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.

The following is the equilibrium reaction when calcium carbonate (CaCO3) is "dissolved" in water:

{CaCO_3} {_{(s)}} + {H_2} {O} {_{(l)}} + {CO_2} {_{(g)}} {\rightleftharpoons} {Ca^{2+}} {_{(aq)}} + {2HCO_3^{-}} {_{(aq)}} \!\

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] Problems caused by hard water

Hard water causes scaling, which is the precipitation of minerals to form a deposit called limescale. Scale can clog pipes and can decrease the life of toilet flushing units. It can coat the inside of tea and coffee pots, and clog and ruin water heaters.

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 and ion exchange resins.

[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.

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:

DETERMINATION TOTAL HARDNESS OF WATER BY EDTA METHOD :

We know that sulphates, chloridedes bicarbonates and nitrates of CALCIUM and MAGNESIUM are PERMANENT HARDNESS causing salts for water while carbonates cause TEMPORARY HARDNESS. Now for industrial purposes we need to determine total hardness of water. Where, TOTAL HARDNESS = PERMANENT HARDNESS + TEMPORARY HARDNESS.

This process can be done in TWO ways : (1) Using Soap solution (2) Using EDTA

Soap solution is Normal method where we use soap solution to determine Total Hardness and using the definition that ‘Hardness is Soap consuming capacity of water’ we determine the total harness of water. This is Primary Method.

EDTA is method is very accurate and gives exact value of Total Harness of water. In this method, EDTA i.e. disodium salt of diamine tetra- acetic acid is used.

 Hence the molecule can be represented as HY – 2   This molecule is very good CHILATING AGENT. Ie its able to form complex with DIVALENT CATION like Zinc or Magnesium, Calcium. Now in hard water as these are present as soon as EDTA is added to this water, it forms comples\X with this

In addition to water sample (HARD) in conical flask EBT INDICATOR id added (Eriocrome Black T) along with Ammonical buffer to maintain ALKALINE CONDITIONS. Now this EBT is itself able to form complex with Hard water ie. With divalent cations. However this comples is weaker than the complex which EDTA with Hard water. Hence as soon as EBT is added to water, it forms a WINE RED COM PLEX with water. But when EDTA is added form burette drop by drop to this solution, EDTA starts taking away the metal atoms with which EBT has formed comples. However this copmplex is COLOURLESS. Hence at one stage all metal atoms are taken away and hence Indicator is now set free and hence it gets is original BLUE colour. Hence we can now stop as it indicates END POINT of titration. Hence by noting the Burette reading and further applying NORMALITY EQUATION ( N1 x V11 = N2 x V2) one can find the total hardness of water Note that for the above procedure STANDARDISATION PROCEDURE of EDTA is required as it is SECONDARY STD. substance

Hence by this method we can find TOTAL HARDNESS of water.

[edit] Indices

Several indices are used to describe the behaviour of calcium carbonate in water, or in water, oil and gas mixtures.[2]

[edit] Langelier Saturation Index (LSI)

The Langelier Saturation Index (LSI) (sometimes also Langelier Stability Index) is a calculated number used to predict the calcium carbonate stability of water; that is, whether a water will precipitate, dissolve, or be in equilibrium with calcium carbonate. Langelier developed a method for predicting the pH at which water is saturated in calcium carbonate (called pHs). The LSI is expressed as the difference between the actual system pH and the saturation pH.

LSI = pH - pHs

If the actual pH of the water is below the calculated saturation pH, the LSI is negative and the water has a very limited scaling potential. If the actual pH exceeds pHs, the LSI is positive, and being supersaturated with CaCO3, the water has a tendency to form scale. At increasing positive index values, the scaling potential increases.

[edit] Ryznar Stability Index (RSI)

The Ryznar stability index (RSI) uses a database of scale thickness measurements in municipal water systems to try and predict the effect of particular water chemistry.

[edit] Puckorius Scaling Index (PSI)

The Puckorius Scaling Index (PSI) uses slightly different parameters to quantify the relationship between the saturation state of the water and the amount of limescale deposited.

[edit] Other indices

Other indices used include the Larson-Skold Index[3], the Stiff-Davis Index[4], and the Oddo-Tomson Index[5].

[edit] Health considerations

The World Health Organization says, "There does not appear to be any convincing evidence that water hardness causes adverse health effects in humans."[6]

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 liter of water. The World Health Organization has reviewed the evidence and concluded the data were inadequate to allow for a recommendation for a level of hardness.[6]

In a review by František Kožíšek, M.D., Ph.D. National Institute of Public Health, Czech Republic gives a good overview of the topic, and unlike 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.[7]

Other studies have shown weak correlations between cardiovascular health and water hardness.[8][9][10]

Very soft water can corrode the metal pipes in which it is carried and as a result the water may contain elevated levels of cadmium, copper, lead and zinc [6].

[edit] Softening

It is often desirable to soften hard water, as it 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 to reduce the effects of scaling and to make it more suitable for laundry and bathing.

[edit] Process

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.[11]

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 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 precipitates out 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

[edit] Why does my skin feel slippery after washing with softened water, unlike hard water?

Some confusion may arise after a first experience with soft water. Hard water does not lather well with soap and leaves a "less than clean" feeling. Soft water lathers better than hard water but leaves a "slippery feeling" on the skin after use with soap. For example, a certain water softener manufacturer contests that the "slippery feeling" after showering in soft water is due to "cleaner skin" and the absence of "friction-causing" soap scum.

However, the chemical explanation is that softened water, due to its sodium content, has a much reduced ability to combine with the soap film on your body and therefore, it is much more difficult to rinse off.[12] Solutions are to use less soap or a synthetic liquid body wash.

[edit] Regional Information

[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: Canberra: 40[13]; Melbourne: 10 - 26[14]; Sydney: 39.4 - 60.1[15]; Perth: 29 - 226[16]; Brisbane: 100[17]; Adelaide: 134 - 148[18]; Hobart: 5.8 - 34.4[19]; Darwin: 31[20].

[edit] Hard water in Canada

Prairie provinces (mainly Saskatchewan and Manitoba) contain 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,[21] Vancouver < 5 mg/L [citation needed], Charlottetown PEI 140 - 150 mg/L.[22]

[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, Devon, 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, 89.3% 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.[23]