Calcium Carbonate Saturation
WQT 134
Aquatic Chemistry II
Standard Methods 20th ed
#2330 Calcium Carbonate Saturation
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 function of the marble test
2. Understand the role of calcium carbonate saturation
in corrosiveness.
3. Understand how to measure calcium carbonate
saturation (SM #2330)
Calcium Carbonate Saturation
WQT 134
Environmental Chemistry II
STM. 20th edition. #2330
Calcium Carbonate “Marble
Test” Experimental Design
Part 1 Standardization of Titrant (Sulfuric acid)=purchased from Hach!
ml
pH
0.2
7.0
0.4
7.4
0.8
7.1
1.5
7.2
3.5
8.0
Part 2 Titration for Total Alkalinity
determine
determine
p alkalinity
T Alkalinity mg/L as CaCO3/L
pH above 8.3 = P
alkalinity
color change
0.02 N Sulfuric acid
P alkalinity
Blue Color change after
addition of Bromcresol
green/methyl red
3.8
Part 2 ADD 0.5 g CaCO3 + redue!
8.3
200 ml of sample + 5 drops
Phenolphthalein indicator no color
change Palkalinity =0 or titrate
to pH 8.3
pH 4.5 = T
alkalinity
red color change
endpoint
T alkalinity
+ 5 drops Mixed bromcresol greenpH Met4er
stir bar
methyl red titrate to pH 4.5
stir plate
Calcium Carbonate
Saturation #2330
What is Calcium carbonate saturation?
•The sample pH and total alkalinity are determined. Then the
sample is saturated with calcium carbonate and the
procedure is repeated. A comparison of the before and after
values is done by subtraction to determine if the system is
under saturated, saturated, or in equilibrium with calcium
carbonate.
Why do we care?
•This test helps to determine how much lime or
soda ash are needed to keep a distribution system
in good operating condition
Calcium Carbonate
Saturation #2330
Why is it important in wastewater industry?
the corrosiveness of the water leaving the plant.
Why is it important in water industry?
Aesthetic taste of water and to prevent
corrosiveness of lines.
Calcium Carbonate
Saturation #2330
What are some assumptions in procedure?
•View procedure as a qualitative guide to the
behavior of CaCO3 in aqueous systems
Calcium Carbonate
Saturation #2330
What are some interferences in procedure?
•Deposition from oversaturated waters is
inhibited by the presence of phosphates
(particularly polyphosphates), certain
naturally occurring organics, and
magnesium
Calcium Carbonate
Saturation #2330
Calculations and Formulas?
Step 1 Calculate pH(1) of original sample
Step 2 Calculate alkalinity alk (1) of original sample
Alk(1) T Alkalinity mg/L as CaCO3/L=
(ml of acid to 8.3=colorless+ ml to reach pH 4.5=pink)(Normality)(50mg/meq CaCO3)(1000 ml)
ml of original sample volume
Step 3 Calculate pH(2) of 0.5 g spiked CaCO3 sample
Step 4 Calculate alk(2) of 0.5 g spiked CaCO3 sample
(ml of acid to 8.3=colorless+ ml to reach pH 4.5=pink)(Normality)(50mg/meq CaCO3)(1000 ml)
ml of original sample volume
Calcium Carbonate Saturation
Calculations and Formulas?
A= samples prior to saturation
B= samples after saturation
Eq 1: A-B= Stability Index
Eq 2: Alk(1)-Alk(2)= +, -, 0
Eq 3: pH(1) – pH(2)= +, -, 0
Step 5
A>B- water is supersaturated with respect to carbonate and ma
forma precipitate scale
A<B water is undersaturated with respect to carbonate and ma
be corrosive
A=B water is in equilibrium with respect to carbonate
constituents
Calcium Carbonate
Saturation #2330
Tips and Suggestions?
1. Use ample sample volume (300 ml); its trial and error
2. Leave the lime behind
3. Add H2SO4 slowly!
4. Keep track of H2SO4 added each time!
5. Rely solely on indicators or can use pH probe!