WATER HARDNESS
WATER HARDNESS

Definition

Concerns



Scaling
Cleaning
Public Health
CAUSES
Cations causing
hardness
Ca2+
Mg2+
Sr2+
Fe2+
Mn2+
Anions
HCO3SO42ClNOSiO32-
Table 19.1. Principle cations causing hardness in
water and the major anions associated with them
SOURCES
From USGS
WATER HARDNESS RATING
mg/L as CaCO3
Degree of Hardness
0-60
Soft water
61-120
Moderately hard water
121-180
Hard water
>180
Very hard water
From USGS
METHODS OF DETERMINATION
1.


Complete Cation Analysis
Most accurate
Calculation of the hardness caused by each ion is
performed by use of the general formula:
Hardness (mg/L) as CaCO3 = M2+ (mg/L) X
50
EW of M2+
where M2+ represents any divalent metallic ion and
EW represents equivalent weight
EXAMPLE OF COMPLETE CATION ANALYSIS

Calculate the hardness of a water sample with the
following analysis:
Cation
Concentration
(mg/L)
Anion
Concentration
(mg/L)
Na+
20
Cl-
40
Ca2+
15
SO42-
16
Mg2+
10
NO3-
1
Sr2+
2
Alkalinity
50
EXAMPLE OF COMPLETE CATION ANALYSIS
Cation
EW
Hardness, mg/L as CaCO3
Ca2+
20.0
(15)(50)/(20.0) = 37.5
Mg2+
12.2
(10)(50)/(12.2) = 41.0
Sr2+
43.8
(2)(50)/(43.8) = 2.3
Total hardness = 80.8
METHODS OF ANALYSIS
2.

Titrimetric Method
Burette

A piece of glassware designed
to deliver known amounts of
liquid into another container
EDTA
Ethylenediaminetetracetic acid
 Chelating agent



Substance whose molecules can form several bonds to
a single metal ion
Multi-dentate ligands
Many clawed; holding onto the metal ion to form a very
stable complex
 4-6 bonds

EBT

Eriochrome Black

Indicator
A WORD ABOUT MOLES


A mole used in chemistry is something like the
term ‘dozen’ we use every day.
The atomic weight of any element, expressed in
grams, contains 6.02 x 1023 atoms.



There are 6.02 x 1023 atoms of oxygen in 16 g of
oxygen
There are 6.02 x 1023 atoms of hydrogen in 1 g of
hydrogen
A mole simply means that you have 6.02 x 1023 of
whatever you’re talking about

Avogadro’s number
MOLARITY

Defined as the number of moles of solute divided
by the number of liters of solution


Molarity (mol/L; M) = moles of solute
liters of solution
The molar concentration of EDTA in this
experiment is 0.005 mol/L.
EXAMPLE OF EDTA CALCULATION
50.0 mL tap water sample
 13.75 mL EDTA used in the titration

a)
b)
c)
Calculate the moles of EDTA used.
Calculate the molarity of metal ion present in the
tap water sample (Remember: EDTA binds to metal
ions on a 1:1 molar ratio).
Find the ppm calcium ion concentration in the
sample based on your results from question b).
EXAMPLE OF EDTA CALCULATION
a)
Calculate the moles of EDTA used
Molarity of EDTA = 0.005 mol/L
Moles of EDTA = (Molarity)(Liters of EDTA used)
EXAMPLE OF EDTA CALCULATION
a)
Calculate the molarity of metal ion present
(Remember EDTA binds to metal ions on a 1:1
ratio)
Molarity of metal ion = moles of EDTA
liters of water sample
EXAMPLE OF EDTA CALCULATION

Calculate concentration of CaCO3 in ppm
CaCO3 (ppm) =
(Molarity of metal ion)(FW of Ca2+)(1000 mg/g)
CaCO3 (ppm) =