Transcript Bendroji chemija

Water Hardness
2015.07.21
prof. V. Paulauskas
1
Hard water - is water that has high mineral
content (in contrast with soft water)
Soft water - is water with low mineral content
2015.07.21
prof. V. Paulauskas
2
Hard water minerals mainly consist of:
calcium Ca2+ and
magnesium Mg2+ cations
(the two most prevalent divalent metal ions)
Other metal ions, such as
iron Fe2+,
aluminium Al3+ and
manganese Mn2+
may also be present at elevated levels in some
geographical locations
2015.07.21
prof. V. Paulauskas
3
TYPES OF HARDNESS
HG = HT + HP
HT
- temporary (or carbonate)
Ca(HCO3)2, Mg(HCO3)2
HP
- permanent (or non-carbonate)
CaCl2, MgSO4, Ca(NO3)2, Fe(NO3)2…
GENERAL HARDNESS (HG) – is total amount
of divalent Me ions (or their salts) dissolved in water
TEMPORARY HARDNESS
Temporary hardness is caused by a combination of calcium
(magnesium) ions and bicarbonate ions - Ca(HCO3)2 in the
water TH can be simply removed by boiling the water
The following is the equilibrium reaction when calcium
carbonate (CaCO3) is dissolved in water:
CaCO3(s)+ CO2(aq) + H2O ⇋ Ca2+(aq)+ 2HCO3-(aq)
Upon heating, less CO2 is able to dissolve into the water. Since there is not enough CO2 around, the
reaction cannot proceed from left to right, and therefore the CaCO3 will not dissolve as rapidly. Instead,
the reaction is forced to the left (i.e., products to reactants) to re-establish equilibrium, and solid CaCO3
is formed. Boiling the 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 "re-dissolve" into water.
PERMANENT HARDNESS
It is hardness (mineral content) that cannot be removed by
boiling
It is usually caused by the presence in the water of calcium
and magnesium sulfates, nitrates, also chlorides, which
become more soluble as the temperature rises
Despite the name, permanent hardness can be removed using
a chemical reagents (water softeners) or ion exchange column
GENERAL HARDNESS – is expressed in millimoles
of dissolved salts of divalent Me ions (including both
Ca2+ and Mg2+) per litre of water (mmol/L) - Total H
ToH<0.75 mmol/L
0.75-2 mmol/L
2-4 mmol/L
4-6 mmol/L
ToH>6 mmol/L
-
water is very soft
water is soft
water is of medium hardness
water is hard
water is very hard
ToH (incl. both Ca2+ & Mg2+ ions) can also be expressed
as parts per million (ppm) or weight/volume (mg/L) of
calcium carbonate (CaCO3) in the water
Very soft:
Soft:
Slightly hard:
Moderately hard:
Hard:
Very hard:
0-70 ppm
70-140 ppm
140-210 ppm
210-320 ppm
320-530 ppm
>530 ppm
0-4 dGH
4-8 dGH
8-12 dGH
12-18 dGH
18-30 dGH
>30 dGH
Parts per million (ppm) - usually defined as one milligram of
calcium carbonate (CaCO3) per litre of water
Degrees of General Hardness (dGH) – 1 dGH defined as 10
milligrams of calcium oxide per litre of water, which is equivalent to
17.848 milligrams of calcium carbonate per litre of water, or 17.848
ppm (German degrees)
Millimoles per litre (mmol/L) – 1 millimole of calcium (either Ca2+
or CaCO3) per litre of water corresponds to a hardness of 100.09 ppm or
5.608 dGH, since the molar mass of calcium carbonate is 100.09 g/mol
Parts per million (ppm) - defined as
one milligram of calcium carbonate (CaCO3)
per litre of water
1L ≈ 1kg = 1000g = 1000 000mg = 106mg
1mg of CaCO3 /1000000mg of water =
= 1ppm
Degrees of General Hardness (dGH) – 1 dGH
defined as 10 milligrams of calcium oxide per litre of water
1 dGH is equivalent to 17.848 milligrams of calcium
carbonate per litre of water, or 17.848 ppm
56 mg CaO – 100 mg CaCO3
10 mg CaO – x mg CaCO3
x = 17,85 mg CaCO3 = 17,85 ppm
1 dGH = 17.848 ppm
Millimoles per litre (mmol/L) – 1 millimole of
calcium (either Ca2+ or CaCO3) per litre of water
1 mmol/L – corresponds to a hardness of 100.09 ppm or
5.608 dGH (molar mass of calcium carbonate –100.09 g/mol
1mmol/L ≈ 100 mg/L CaCO3= 56 mg/L CaO
1 mmol/L = 100 ppm = 5.6 dGH
ORIGIN
Calcium and magnesium ions are acquired through
contact with rocks and sediments in the environment
Calcium usually enters the water as either calcium
carbonate (CaCO3), in the form of limestone
and chalk, or calcium sulfate (CaSO4), in the
form of other mineral deposits (e.g. gypsum)
The predominant source of magnesium is
dolomite (CaMg(CO3)2)
Negative effects
Hardness in water can cause water to form scales and a
resistance to soap
It can also be defined as water that does not produce lather
with soap solutions, but produces white precipitate (scum). For
example, sodium stearate reacts with calcium:
2C17H35COONa + Ca2+ → (C17H35COO)2Ca + 2Na+
Iron, if present, is important for causing the calcification to be
brownish (the color of rust) instead of white (the color of most
of the other compounds)
Hard water is generally not harmful to one's health
Negative effects
Hard water causes scaling (limescale), which is the left-over
mineral deposits that are formed after hard water had evaporated
Scale can clog pipes, ruin water heaters, coat the insides of tea
pots, decrease life of toilet flushing units and washing machines
Similarly, insoluble salt residues will remain in hair after
shampooing, clothes after washing, food products after cooking
In industrial settings, water hardness must be constantly
monitored to avoid costly breakdowns in boilers, cooling towers,
and other equipment that comes in contact with water
Negative effects
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
Softening
It is often considered desirable to soften hard water –
removal of divalent metal ions
A water softener works on the principle of cation exchange in
which ions of the hardness minerals (mainly calcium and
magnesium) are exchanged for sodium or potassium ions,
effectively reducing GH to tolerable levels
Hardness can be removed using a water softener
(chemical reagent) or ion exchange column
In drinking water, the recommended limits for total hardness
expressed as the sum of the calcium and magnesium ion
concentrations is – 2-4 mmol/L
Softening:
1. Thermal method
Boiling promotes the formation of carbonate from the
bicarbonate and precipitates calcium carbonate out of
solution, leaving water that is softer upon cooling
t
Ca(HCO3)2  CaCO3 + CO2 + H2O
Softening
2. Chemical precipitation
MgCl2 + Na2CO3  MgCO3 + 2NaCl
3CaCl2 + 2Na3PO4  Ca3(PO4)2 + 6NaCl
CaSO4 + 2NaOH  Ca(OH)2 + Na2SO4
Ca(HCO3)2 + Ca(OH)2  2CaCO3 + 2H2O
Divalent Me ions forms solid precipitates – are
removed from water
Softening:
3. Ion exchange
Natural or synthetic zeolites (alumosilicates) used as cationites
Na2R (cat.) + CaSO4  CaR (cat.) + Na2SO4
Na2R + Ca2+  CaR + 2Na+
This process is called ion exchange
Large-scale softening is caried out with:
• zeolites (Na2Al2Si2O8 . xH2O) or
• ion exchange resins
3. ION EXCHANGE COLUMN
Filter filled with cationic
material
3. INDUSTRIAL DEMINERALISATION UNITS
Process based on
ion-exchange
3. WATER DEMINERALISATION
Cationite
H+
All ions are removed from water
Product: deionised water
Anionite
OH-
Softening – ion exchange
Many zeolite minerals occur in nature,
but specialized ones are often made artificially
Softening – ion exchange
When 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
CaR (resin) + 2NaCl  Na2R (resin) + CaCl2
(recycling of cationite)
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
1 Problem:
0.2 g Ca(NO3)2, 0.15 g Ca(HCO3)2 and 0.05 g Mg(HCO3)2 are
dissolved in 0.8 L of drinking water. Calculate: general,
temporary and permanent water hardness.
800 mL water - 200 mg Ca(NO3)2 1 mmol - 164 mg Ca(NO3)2
1000 mL water - x mg Ca(NO3)2 x mmol - 250 mg Ca(NO3)2
X1  1.52 mmol/L
800 mL water - 150 mg Ca(HCO3)2 1 mmol - 162 mg Ca(HCO3)2
1000 mL water - x mg Ca(HCO3)2 x mmol - 188 mg Ca(HCO3)2
X2  1.16 mmol/L
800 mL water - 50 mg Mg(HCO3)2 1 mmol - 146 mg Mg(HCO3)2
1000 mL water - x mg Mg(HCO3)2 x mmol - 63 mg Mg(HCO3)2
X3  0.43 mmol/L
HP  x1  1.52 mmol/L
HT  x2 + x3  1.59 mmol/L
HG  HP + HT  3.11 mmol/L
2 Problem:
General water hardness - 4 mmol/L. Calculate the amount
of FeCl2 which is dissolved in 15 m3 of this water
1mM (iron chloride)  127 mg/mmol
1 mmol - 127 mg FeCl2
4 mmol x mg FeCl2
1 l water - 0.508 g FeCl2
15000 l water x g FeCl2
x  508 mg  0.508 g FeCl2
x  7620 g  7.62 kg FeCl2
3 Problem:
Calculate the amount of reagent (sodium carbonate) needed to
soften 20 cubic meters of water used in a thermoelectric powerstation as a cooling agent (H = 6 mmol/L)?
1 M (sodium carbonate)  106 g/mol
1 L water - 6 mmoles Na2CO3
20000 L water - x mmoles Na2CO3
x  120000 mmol  120 mol Na2CO3
1 mol - 106 g Na2CO3
120 mol x g Na2CO3
x  12720 g  12.72 kg Na2CO3
Cycle of Carbon Dioxide
Carbon dioxide reacts with water to form carbonic acid (1) which at ordinary environmental pH exists mostly
as bicarbonate ion (2). Microscopic marine organisms take this up as carbonate (4) to form calcite skeletons
which, over millions of years, have built up extensive limestone deposits. Groundwaters, made slightly
acidic by CO2 (both that absorbed from the air and from the respiration of soil bacteria) dissolve the
limestone (3), thereby acquiring calcium and bicarbonate ions and becoming "hard". If the HCO3–
concentration is sufficiently great, the combination of processes (2) and (4) causes calcium carbonate ("lime
scale") to precipitate out on surfaces such as the insides of pipes. (Calcium bicarbonate itself does not form
a solid, but always precipitates as CaCO3.)