Hardness
WQT 134
Aquatic Chemistry II
Standard Methods 20th ed
#2340 Hardness
Applied Water and Spent water Manual
Chapter 14
Week 5 Objectives
Reading assignment:
American Public Health Association (APHA), American Water Works Association
(AWWA) & Water Environment Federation (WEF). 1999. Standard Methods for
the Examination of Water and Wastewater, 20th edition
1. Understand the role and function of
hardness in water treatment.
2. Understand how to measure hardness
(SM #2340)
3. Relate hardness to total alkalinity
Hardness #2340
Carbonate hardness: Occurs when the hardness
numerically is equal to or less than the sum of carbonate and
bicarbonate alkalinity
Noncarbonate hardness: Occurs when the hardness
numerically is more than the sum of carbonate and
bicarbonate alkalinity.
Hardness #2340
Temporary or Carbonate Hardness : is hardness that
can be removed by boiling or hydrochloric acid. It is
caused by a combination of calcium ions and
bicarbonate ions in the water.
Permanent hardness: is hardness (mineral content)
that cannot be removed by boiling. It is usually
caused by the presence of calcium and magnesium
sulfates and/or chlorides in the water, which become
more soluble as the temperature rises.
Hardness #2340
What is hardness?
•This test is designed to measure Hardness in water
which is mainly due to the presence of ions of the
calcium (Ca 2+), magnesium (Mg 2+), and iron (Fe 2+).
Why do we care?
•USGS, 85% of US homes have hard water.
•Hard water require considerable amounts of soap to
produce a lather
• Hard water mineral deposits form an insoluble
precipitate such as scale (coats pipes, boilers,
fixtures, heaters, etc.)
•Soft water =poor coagulation
•Secondary MCl for aesthetic purposes
Hardness is defined as the sum
of the _____ and ____ ions,
although any divalent metal ion
can contribute to hardness.
an
d
Ca
l
c iu
m
m
ag
ne
siu
te
vi
St
ru
Su
. ..
...
d
an
M
M
an
d
c iu
m
Ca
l
1. Calcium and Magnesium
2. Magnesium and Sodium
3. Calcium and Sulfate
4. Struvite
a.
..
25% 25% 25% 25%
Hardness #2340
How is it done?
1. Sample needs to be buffered to pH 10 with a salt and an
organic amine. An organic dye or indicator (Eriochrome
Black T) is added which complexes the calcium and
magnesium and turns the water red.
2. The sample is then titrated with 0.01 M EDTA which
complexes all the free calcium and magnesium ions in the
sample. At the endpoint the free calcium and magnesium
are no longer present and the EDTA will produce a blue
color.
3. The volume of EDTA consumed is used to determine the
hardness in mg/L CaCO3 or the total calcium and
magnesium combined hardness. You go from a wine red
color to a sharp blue color.
Hardness #2340
How is it done?
Before w/
indicator
After EDTA titration
To endpoint
Hardness #2340
What are disadvantages of the procedure?
•Some metal ions and organics interfere by causing
fading or indistinct end points
•Suspended or colloidal organic matter also may
interfere with the end point.
•Must be done at room temperature
Hardness #2340
What are the units and conversions?
•hardness in mg/l as CaCO3
Calculations and Formulas?
Hardness as CaCO3 mg/L=
(ml of EDTA (sample) – ml of EDTA (blank))(0.01 M EDTA)(100 mg=CaCO3 milliMole)(1000 ml/L)
ml of sample volume titrated
Hardness #2340
Example Problem?
The results for the hardness of a sample are as follows:
Volume of sample titrated = 50 ml
Volume of EDTA consumed by the sample = 4.4 ml
Volume of EDTA consumed by the blank = 0.2 ml
Hardness as CaCO3 mg/L=
(ml of sample used – ml of blank)(0.01 M EDTA)(100 mg =CaCO3 milliMole)(1000 ml/L)
ml of sample volume titrated
Total hardness = (4.4 ml – 0.2 ml) (0.010M) (100) (1000)
50 ml
Total hardness = 84 mg/L as calcium carbonate
Hard Waters in the USA
Hardness #2340
What are typical values in nature?
Classification
mg/L
Soft
Slightly hard
Moderately hard
Hard
Very Hard
0 - 17.10
17.1 - 60
60 - 120
120 – 180
180 & over
Drinking water average is about 250 mg/L as calcium
carbonate hardness
Hardness #2340
Tips and Suggestions?
1. Titrate color blank VERY SLOWLY 1-3 drops!!!
2. Sample volume (~100 ml??); its trial and error
3. When making buffer solution do it in the fume hood
ammonium hydroxide= ammonia odor
4. Add EDTA slowly!
5. Keep track of EDTA added for the (blank and sample)!
6. Rely solely on indicators or can use pH probe!
Hardness and Total Alkalinity
Summary?
 Accuracy of total hardness as calcium and magnesium
from 1 to 800 mg/L CaCO3.
Calmagite turns blue in the
absence of these ions.
25% 25% 25% 25%
on
a
ca
rb
Al
l
an
d
m
So
di
u
te
s
Ca
l. .
.
...
d
an
m
ag
ne
siu
M
Ca
l
c iu
m
an
d
M
a.
..
1. Calcium and Magnesium
2. Magnesium and Sodium
3. Sodium and Calcium
4. All carbonates
This is the titrant used for the
Hardness analysis.
25% 25% 25% 25%
xi.
..
c. .
.
So
di
u
m
hy
dr
o
Hy
dr
o
0.
12
5
N
Su
lfu
N
0.
03
ED
TA
-A
Ch
el
a.
..
ri.
..
1. EDTA - A Chelating Agent
2. 0.03 N Sulfuric acid
3. 0.125 N Hydrochloric acid
4. Sodium hydroxide
Alkalinity and hardness are both
expressed analytically as this.
as
Ca
CO
3
Ca
lci
bo
n.
..
m
g/
L
Ca
l
c iu
m
ca
r
bo
n.
..
ca
r
c iu
m
Ca
l
Ca
l
c iu
m
Ca
r
bo
n.
..
1. Calcium Carbonate - mg/L
2. Calcium carbonate
3. Calcium carbonate, mg/L
4. mg/L as Calcium carbonate
5. CaCO3
u.
..
20% 20% 20% 20% 20%
A test on a water supply showed a
hardness of 232 mg/L. A certain dosage
of polyphosphates will theoretically
reduce this hardness by 21 percent.
What should the water hardness be after
treatment?
m
g/
L
25%
17
4
m
g/
L
25%
18
3
m
g/
L
25%
21
1
m
49
1. 49 mg/L
2. 211 mg/L
3. 183 mg/L
4. 174 mg/L
g/
L
25%
Alkalinity and hardness are both
analyzed by adding a known
reagent to the sample. This
process results in a ______
change.
• Color Change