Field Methods of Monitoring
Aquatic Systems
Unit 6 - Hardness
Copyright © 2006 by DBS
Major Cations
Ca2+
Mg2+
Fe2+
Na+
K+
Common cations of high enough concentration to be readily
monitored are good indicators of pollution events
Hardness Index
Hard water contains high concentrations of dissolved
calcium and magnesium ions
Soft water contains few of these dissolved ions.
Counter ions of
alkalinity ions
Hardness = [Ca2+] + [Mg2+]
Carbonate minerals:
limestone - CaCO3
dolomite - CaCO3.MgCO3
sulfates - CaSO4
Alkalinity is a
good indicator
of hardness and
vice-versa
(also Al3+, Fe3+, Mn2+ and Zn2+)
Effects
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Deposition of white solid CaCO3 or MgCO3 when water is heated
– ‘furring-up blocks pipes and lowers efficiency of industrial
processes
Formation of scum (insoluble ppt) with soap and water
Ca2+(aq) + H3C-(CH2)10-COO-(aq)
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[H3C-(CH2)10-COO-]2Ca2+(s)
– detergent action is blocked
Staining (due to transition metals)
A pipe with hard-water scale
build up
Types of Hardness
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Solid deposit = carbonate hardness or temporary hardness
CaCO3(s) + H2CO3 ⇌ Ca2+ + HCO3-
(removed via boiling)
– Causes deposit in pipes and scales in boilers
– Temporary hard water has to be softened before it enters the boiler,
hot-water tank, or a cooling system
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No solid = non-carbonate or permanent hardness
– Amount of metal ions that can not be removed by boiling
Total hardness = temporary hardness + permanent hardness
Beneficial Effects
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Alkalinity lowers solubility of toxic metals
Buffering action of CO32- lessens effects of acidic pollutants
Non-toxic essential nutrients
Reduces heart disease
More pleasant to drink
Analysis
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Complexometric titration for Ca2+ and Mg2+ using
ethylenediaminetetraaceticacid EDTA
M2+ + H2EDTA2- ⇌ M(EDTA)2- + 2H+
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Buffer to pH 10, indicator is Eriochrome Black T
Expressed as mg L-1 (even though not all CaCO3)
Conc. CaCO3
(mg L-1)
Description
0 - 50
Soft
50 - 100
Moderately soft
100 - 150
Slightly hard
150 - 200
Moderately hard
200 - 300
Hard
> 300
Very hard
EDTA Complex
1:1
Total Hardness
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Titration estimates total divalent (+2) metal as molar concentration
Convert to mg L-1
Need precise individual concentrations of Ca, Mg etc. which we don’t have
Solution: express as all CaCO3
e.g. for a 5.00 mL sample titrated with 15.24 mL of 0.0027 M EDTA
C1V1 = C2V2
15.24 mL x
1 L x 0.0027 mol L-1 = 5.00 mL x 1 L
x C2
1000 mL
1000 mL
C2 = 0.00823 M
0.00823 M (Ca2+ + Mg2+) x
100.09 g CaCO3 = 0.824 g L-1 = 824 mg L-1
1 mol (Ca2+ + Mg2+)
Question
Which of the following solutions give 50 mg L-1 total hardness?
(a) 50 mg L-1 MgCO3
(b) 21.1 mg L-1 MgCO3 + 25 mg L-1 CaCO3
(c) 50 mg L-1 CaSO4
(d) 55 mg L-1 CaCl2
(a) 0.59 mM , x 100 g mol-1 = 59 mg L-1 CaCO3
(b) 0.50 mM , x 100 g mol-1 = 50 mg L-1 CaCO3
(c) 0.37 mM , x 100 g mol-1 = 37 mg L-1 CaCO3
(d) 0.5 mM , x 100 g mol-1 = 50 mg L-1 CaCO3
84.32 g mol-1
100.09 g mol-1
136.14 g mol-1
110.99 g mol-1
Question
Two unpolluted waters have the same pH valu of 7.8 are contaminated
by approximately the same amount of acidic pollutant. The pH of one
drops sharply, while there is only a small drop in the second. Suggest a
reason for the difference and the analysis which could be performed to
confirm your suggestion.
pH values suggest hardness, perhaps from the presence of
limestone. Determine via EDTA titration. Solution having the
smaller pH change indicates a greater buffer capacity and so
would be expected to have the greatest hardness.
Text Books
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Rump, H.H. (2000) Laboratory Manual for the Examination of Water, Waste Water and Soil.
Wiley-VCH.
Nollet, L.M. and Nollet, M.L. (2000) Handbook of Water Analysis. Marcel Dekker.
Keith, L.H. and Keith, K.H. (1996) Compilation of Epa's Sampling and Analysis Methods.
CRC Press.
Van der Leeden, F., Troise, F.L., and Todd, D.K. (1991) The Water Encyclopedia. Lewis
Publishers.
Kegley, S.E. and Andrews, J. (1998) The Chemistry of Water. University Science Books.
Narayanan, P. (2003) Analysis of environmental pollutants : principles and quantitative
methods. Taylor & Francis.
Reeve, R.N. (2002) Introduction to environmental analysis. Wiley.
Clesceri, L.S., Greenberg, A.E., and Eaton, A.D., eds. (1998) Standard Methods for the
Examination of Water and Wastewater, 20th Edition. Published by American Public Health
Association, American Water Works Association and Water Environment Federation.