Objective
 to understand the chemical basis of water hardness, how it originates, and ways it can affect water distribution systems.
 to know the methods for measuring and expressing hardness, and its relationship with other water quality parameters.
 to know the significance of water hardness in
Environmental Engineering


Poor lathering of soap in water.
 Scale deposition in Boilers and Kettles

Importance

Economic

Soap Costs

Maintenance of Boilers

Health

Putative link between Magnesium and heart disease (negative correlation)

Soft Waters and Lead pipes

Strength of Bones


Soap Effects

Divalent Cations

Especially

Calcium

Magnesium
Ca 2+
Mg 2+
 To a lesser extent

Strontium

Iron

Manganese
Sr 2+
Fe 2+
Mn 2+
 What’s Happening
C18 Stearate molecules linked by divalent Metals - cause Scum
O
O -
M 2+
-
O
O


Scale Formation
 Ions start in solution
 Temperature alters Carbon dioxide solubility, CO
2 volatilises.

Equilibrium of bicarbonate shifts
Ca 2+ + 2HCO
3
CaCO
3 (s)
+ CO
2(g)
+ H
2
O
Scale


Hardwater
 Rainwater dissolves CO
2 from air to form carbonic acid.
H
2
O + CO
2
H
2
CO
3

Groundwater formed where soil is thick over limestone, CO
2 from soil respiration (microorganisms)

Limestone is dissolved
MgCO
3 (s)
CaCO
3 (s)
+ H
2
CO
3
+ H
2
CO
3
Mg 2+ + 2HCO
Ca 2+ + 2HCO
3
-
3
-

Softwater

Thin soil , little limestone

AIR
rain
CO
2
Dissolves
SOIL
Bacteria Release CO
2
(Respiration)
CO
2
+ H
2
O H
2
CO
3
CaCO
3
+ H
2
CO
3
Ca(HCO
3
)
2 (aq)
LIMESTONE
MgCO
3
+ H
2
CO
3
CaCO
3
+ H
2
CO
3
Mg(HCO
3
)
2 (s)
Ca(HCO
3
)
2 (s)

 Three Approaches
 Soap Method (Obsolete)
 Calculation (Sum of all Divalent Metal ions , M
2+
). AAS ICP

EDTA Method
Express result as: mg CaCO
3
/l
 Calculation Method
1. Obtain Analysis for each of the divalent metal ions
2. Express each metal ion as mg CaCO
3
/l

Hardness mg CaCO
3
/l = M
2+
(mg/l) x 50/EW
(M
2+
)

Sum all Individual values from 2.


Bottled Mineral Water (eg. Perrier, ASDA)

Gives amounts of several Cations and Anions (mg/l)
 Only divalent cations of interest
Ca
2+
48.0
Na
+
Mg
2+
8.5
K
+
1.2
13.3

Calculation

CATION
Ca
Mg
2+
2+
EW
20
12.2

Disadvantage

Individual analyses are required
 Expensive non-portable equipment
Hardness
48 x 50 x 1/20 = 120 mg CaCO
3
/l
8.5 x 50 x 1/12.2 = 34.8 mg CaCO
3
/l
Total Hardness 154.8 mg CaCO
3
/l


Simple Accurate

EDTA (Ethylenediamine tetraacetic acid)
 Chelating Agent - binds M
2+ very strongly
 Indicator Eriochrome
 binds M
2+ more weakly than EDTA

Is Coloured

Titrate with EDTA (competes for the M
2+ bound to Eriochrome).

When all M
2+ removed from Eriochrome colour changes
Eriochrome-M
2+
(Wine Red) Eriochrome (Blue-tint)
 Extended Method
 Calcium ions Ca 2+ with Eriochrome Blue at pH 12 - pH 13

Magnesium ions Mg 2+ with Eriochrome Black at pH 4


Specific Cations

Calcium or Magnesium Hardness

Total Hardness - Calcium Hardness = Magnesium Hardness
 Specific Anions
 Carbonate Hardness
(formerly called Temporary Hardness)

Divalent cations associated with carbonate or bicarbonate anions

Non-Carbonate Hardness (NCH) (formerly called Permanent Hardness)

NCH = Total Hardness - Carbonate Hardness

Divalent cations associated with sulphate, chloride, or nitrate anions.

Pseudo- Hardness

Salt Water Common Ion Effect (Na + with soap)

 the general condition is as follows :
Carbonate Hardness (CH) is equal to either the Total Hardness (TH) or the
Alkalinity , whichever is the lowest value .
When TH > Alkalinity
 there is more calcium and magnesium than carbonate and bicarbonate
 some Non-Carbonate Hardness must be present
Then CH =Alkalinity ( bicarbonate and carbonate )
When TH < Alkalinity
 there is less calcium and magnesium than carbonate and bicarbonate
Then CH = TH

 Range of Hardness
RESULT
(mg CaCO
3
/l)
0 - 75
75 - 150
150 - 300
> 300

Bottled Mineral Water
– Total Hardness
– Total Alkalinity
– Carbonate Hardness
– Non-Carbonate Hardness (TH - CH)
RELATIVE
HARDNESS
Soft
Moderately Hard
Hard
Very Hard
154.8 mg CaCO
3
/l
110 mg CaCO
3
/l
110 mg CaCO
3
/l
44.8 mg CaCO
3
/l


Predicts whether Scale will deposit from water.

Based on the pH at which water is saturated with CaCO
3
 LSI defined as:
LSI = pH - pH s
( pH s
)
LSI > 0
LSI = 0
LSI < 0
Water is supersaturated, CaCO
3
Scale tends to deposit.
Water saturated (equilibrium).
Water undersaturated, tends to dissolve CaCO
3
Scale .


A groundwater sample has the following analysis of the individual species.

Find the Carbonate Hardness, Total Hardness, and Non-carbonate Hardness. Also classify the Relative Hardness of the water.

If EW of Calcium is 20 g/equivalent, what would the Atomic Absorption analysis result
(mg/l) have been for this ion before its conversion to units of “mg CaCO
3
”. (EW CaCO
3 is 50 g/equivalent)
ION Concentration
Ca
Mg
Na
2+
+
2+
( mg CaCO
3
)
187
164
22
HCO
3
-
Cl -
SO
4
2-
246
14
113
/l Ca 5 mg 7 ard h ery , v /l.
3 O aC mg C ) 05 =1 46 -2 51 3 CH N 1, 35 4= 16 7+ 18 TH , 46 H 2 (C s swer An