An Introduction To Water Hardness
Some Simple Chemistry
When ionic substances dissolve in water they are split into their constituent ions. For example when
NaCl (sodium chloride, common salt) dissolves in water it produces Na+ (the positive or cation) and Cl(the negative or anion). These ions stay in the solution unless something happens to make them
precipitate out as a solid; for example if water is evaporated from a salt solution, the solution becomes
saturated and salt crystals will precipitate. The process of dissolving is therefore not a chemical reaction
and can be reversed by physical, rather than chemical, means.
The Causes Of Hardness
Most hardness in water is caused by the presence of dissolved calcium (Ca2+) and magnesium (Mg2+)
ions. Other cations, such as Al3+ and Fe3+ can contribute to hardness, however their presence is less
critical.
The most common, and troublesome, form of hardness is caused by the presence of calcium
bicarbonate (Ca(HCO3)2) which is picked up by rain water passing through lime stone (CaCO3). As rain
water falls it dissolves carbon dioxide (CO2) from the air and becomes slightly acidic because carbonic
acid (H2CO3) is formed:
CO2 (g) + H2O (l) Ž H2CO3 (aq)
In the above equation g=gas, l=liquid and aq=aqueous (i.e. dissolved in water). In the following
equation s= solid.
CaCO3 is not very soluble in water however, when the dilute acid runs through the lime stone a reaction
occurs that creates calcium bicarbonate which is readily soluble:
CaCO3 (s) + H2CO3 (aq) Ž Ca2+ (aq) + 2HCO3- (aq)
Thus the rain water has picked up Ca2+ and HCO3- (bicarbonate) ions and become hard. When hard
water is heated the previous two reactions are reversed and calcium carbonate, water and carbon
dioxide are formed:
Ca2+ (aq) + 2HCO3- (aq) Ž CaCO3 (s) + H2O (l) + CO2 (g)
Since calcium carbonate is much less soluble in water than calcium bicarbonate it precipitates out of
solution as a solid known as scale or lime scale. Because this type of hardness is easily removed (i.e. by
simple heating) it is known as temporary hardness. Scale normally appears around heating elements
and hot water systems. However, if the water is exceptionally hard scaling may occur in cold water
pipes [3].
Other types of temporary hardness are caused by the presence of Mg2+ ions and the precipitation of
magnesium hydroxide (Mg(OH)2) can contribute to scaling problems.
Combinations of Ca2+ and Mg2+ ions with chloride (Cl-), sulphate (SO42-) and nitrate (NO32-) ions are
known as permanent hardness. For example in some areas CaSO4 may cause considerable hardness.
permanent hardness can not be removed by boiling.
The term hardness total hardness is used to describe the combination of calcium and magnesium
hardness. However, hardness values are usually quoted in terms of CaCO3 because this is the most
common cause of scaling. The standard classifications are given below [3]:
Hardness
mg/L as CaCO3
moderate
60-120
hard
120-180
very hard
more than 180
Problems Caused By Hardness
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Excessive soap is needed for washing (i.e. soap will not lather). Some modern detergents work less
efficiently because anions (also known as surfactants) which are meant to hold dirt particles in
suspension react with Ca2+ and Mg2+ instead [5].
Soap based on animal fats can react with Ca2+ and Mg2+ forming a precipitation that can ruin cloths
and irritate skin.
Some foods, particular dried beans and peas, become tough and rubbery when cooked in hard water.
Calcium ions cause cross-linking to occur between certain molecules within the beans, the
subsequent structure prevents water entering and the bean remains hard. A simple way to counteract
this effect is to ad baking soda (sodium bicarbonate (NaHCO3)) to the cooking water [1,11,12].
Scale can clog pipes and fittings. Also heating elements can become 90% less efficient with a 25mm
coating of CaCO3 [2].
Softening
There are three basic ways to soften water:
• Force insoluble substances, such as CaCO3 and Mg(OH)2, to precipitate before water enters the
system.
• Remove the Ca2+ and Mg2+ ions from the water.
• Prevent the Ca2+ ions from CaCO3 by complexing them.
A. Soda-Lime Softening
Lime (calcium hydroxide - Ca(OH)2) and soda ash (sodium carbonate – Na2CO3) is added to the water
causing CaCO3 and Mg(HO)2 to precipitate out.
In large municipal systems Ca(OH)2 is added and the solid precipitates are removed though
sedimentation however fine particles will remain, therefore carbon dioxide is bubbled through the water
to turn these back into soluble bicarbonates. Because some of the hardness is retained this process is
know as a partial softening process.
On a smaller scale Na2CO3 (also known as washing soda) can be added to hard water which is to be
used for washing cloths.
B. Cation Exchange
This process replaces the Ca2+ and Mg2+ ions with other ions which do not contribute to such as Na+ and
k+. This is achieved by running the water through a container filled with a resin that contains sodium or
potassium ions. The ions in the resin are exchanged for the hardening ions in the water. Once all of the
resin ions have been used up the resin can not remove any more Ca2+ and Mg2+ ions until it has been
'regenerated' using NaCl or KCl. This method is commonly used in small domestic supplies.
Cation exchange is a 'total softening' method because it will remove all of the hardness.
C. Complexing
Another method used in small domestic water softeners is to add polyphosphates (containing the
P6O186-) to the water. These ions surround the Ca2+ ions in solution and prevents them from precipitating
as CaCO3: a process known as complexing.
Electric And Magnetic Anti-scaling Devices
Using electrical and magnetic fields to prevent scaling is a very contentious issue. There is no
consistent data to prove how such methods work or indeed if they work at all. However, they do seem
to work to some degree under certain circumstances [7, 8, 9].
The idea behind anti-scaling devices is not to remove Ca2+ or CaCO3 but to precipitate CaCO3 in a form
that prevents it from making a solid, stubborn scale. It seems that after passing through these devices
the CaCO3 is supposed to either stay as very fine particles suspended in the water or to settle out as a
flaky, easily dislodged scale. One theory as to how this happens is as follows:
The water is passed through an electric or magnetic field which causes a localised increase in pH (via
an electro-chemical reaction involving an electron a O2 molecule producing an OH- ). The high pH
allows CaCO3 crystal nuclei to form in the body of water. These minuscule crystals clump together to
form colloids. As these colloids are carried though the water they will not stick to the pipe surface and
any further precipitation that occurs will deposit CaCO3 on the colloids and not the pipe. Thus the
CaCO3 is held in a suspension of fine particles and if sedimentation occurs ta very loose scale results.
Since this method requires an electrical current the water must be flowing through the magnetic field
(so that a current is induced), therefore the magnet will have no effect on static water. This seems to
have been verified experimentally [10].
These devices are usually fitted to heating and hot water systems. However, many domestic users seem
disappointed that they apparently have little effect, particularly on soap lathering. In the absence of any
sedimentation CaCO3 will not be removed from the water and the overall chemical composition of the
water will not change therefore the only way to determine the magnets effectiveness against scale build
up is long term observations. Interestingly industrial users, who continually pump the same water
through a heating system (and the magnet) seem the most satisfied with these devices [2].
Anecdotal evidence suggests that beans, which could not be cooked in extremely hard water, were
successfully cooked in water from the same source that had been passed through a magnetic device.
This may be explained by the fact that the device causes the Ca2+ ions, which prevent the beans
softening [11, 12], being removed from solution as very fine CaCO3 particles.
Since the process by which these magnetic devices work is not widely understood and since the
experiments have been largely unrepeatable (in different labs around the world) what works on one
water system may not work on another. Experiments performed by Cranfield University (UK) suggest
that the degree to which the magnetic devices work is greatly effected by the waters hardness, hardness
and alkalinity [2].
Over Softening
The over softening of water can have an adverse effect on the corrosion rate of cast iron and galvanised
steel pipes. Very soft water can also hasten the decay of concrete tanks [5].
Normally, the inside of ferrous pipes are protected by a layer which builds up as water flows, this layer
contains CaCO3 and Fe. If very soft (aggressive) water is passed down a galvanised pipe the zinc layer
is slowly removed exposing the steel. Also if there is not enough Ca2+ ions in the water calcium
compounds can leach out of concrete causing a deterioration in it's structural integrity.
Therefore care should be taken when using total softening methods with water that is to pass through
ferrous pipe work.
Common Chemical Names
Baking Soda – NaHCO3 Sodium Bicarbonate
Caustic Soda – NaOH Sodium Hydroxide
Chalk – A type of lime stone that is largely CaCO3 Calcium Carbonate
Lime* – CaOH calcium Hydroxide
Lime Stone – Compositions vary, largely CaCO3 and MgCO3 – Calcium Carbonate and Magnesium
Carbonate
Soda Ash – Na2CO3 Sodium Hydroxide
Washing Soda – Na2CO3 Sodium Hydroxide
Quick Lime* - CaO Calcium Oxide
*Both CaOH and CaO are sometimes called lime. When CaO is added to water CaOH is formed and
the resulting solution is called lime water (CaOH forms this way is also called slaked lime). CaO
formed by heating calcium in oxygen is called quick lime.