Alkalinity, Hardness,
& Lime/Soda Ash Softening
By
Douglas Rittmann, Ph.D., P.E.
Water/Wastewater Consultant
Presented to
CE 5345
On
Sept., 2006
General Considerations
1. Its capacity to neutralize acids or its buffering capacity.
2. 3 major classes of materials
A. Bicarbonates, HCO3- - pH 4.0 ≤ pH 8.3
B. Carbonates, CO3- - >pH 8.3 < pH 10
C. Hydroxide, OH- - > pH 10.0
Sanitary Significance
1. Lime/Soda Ash Softening
2. Effects on Coagulant Dosing
3. Total Dissolved Solids compliance
4. Industrial Wastes Discharges
5. Calcium Carbonate Stability
Method of Analysis
1. Phenolphthalein Alkalinity
A. Phenolphthalein indicator
B. pH 8.3 endpoint of titration
C. Measures Carbonates and Hydroxide ions
2. Total Alkalinity
A. Bromcresol green or Methyl Orange indicator
B. pH 4.5 endpoint of titration
C. Measures Bicarbonates
Methods of Expressing
1. Phenolphthalein Alkalinity as CaCO3
P. Alk. = ml 0.02N sulfuric acid (1000/ml sample) = mg/L as CaCO3
2. Total Alkalinity as CaCO3
T. Alk. = ml 0.02N sulfuric acid X (1000/ml sample) = mg/L as CaCO3
3. Hydroxide, Carbonate, and Bicarbonate Alkalinity as CaCO3
Alkalinity Relationships
Titration
Result
OHAlkalinity as
CaCO3
CO3Alkalinity as
CaCO3
HCO3Alkalinity as
CaCO3
P=0
0
0
T
P<1/2 T
0
2P
T-2P
P=1/2 T
0
2P
0
P>1/2 T
2P – T
2(T- P)
0
P=T
T
0
0
Standard Solutions

General Considerations
1. Saves Time in calculating results
2. Selection of Proper Normality is convenient
* 1 mg/ml or 1000mg solution
* 1/eq.wt., example: Alkalinity as CaCO3 = 0.02 N
3. Preparation of Solution of Proper Normality
* Material of Known purity is weighed & transferred to
volumetric flask
* Purchase Solutions of known Normality
Preparation of 1N Acid
Solution
* Sulfuric acid used for Alkalinity Test
1 GMW = 98 g pure H2SO4 = 2.016 g H+
1 GMW/2 = 49 g pure H2SO4 = 1.008 g H+
Acid is 96% pure, then 49/0.96 = 51 g = 1.008 g H+
Make 5% stronger = 51 X 1.05 = 53.5 g
Procedure: Weigh about 53g of conc. acid into a small beaker on
Trip balance. Place 500 ml of distilled water in 1-liter graduated cylinder
And add the acid to it. Rinse the contents of the beaker into the cylinder
With distilled water, and add water to the 1-liter mark. Mix by pouring
Back and forth from the cylinder into a large beaker. Cool to room temp.
Primary Standard
* Sodium Carbonate is a convenient primary standard
MW = 106 of Na2CO3
1EW or 1N = 53g/L when reacting with H2SO4 to pH 4.5, T. Alk endpoint
Preparation of 0.02 N Acid or N/50 can be made from 1N based on
ml X N = ml X N
Example: ml X 1.0 = 1000 X 0.02
ml = 20
Introduction To Hardness
By
Douglas Rittmann, Ph.D., P.E.
Water/Wastewater Consultant
Presented to
CE 5345
On
Sept., 2006
Introduction to Hardness
* Causes & Sources of Hardness
Cations causing
hardness
Anions
Ca++
Mg++
Sr++
Fe++
Mn++
HCO3SO4=
ClNO3SiO3=
* Source – Rain contact with soil and rock formations
Sanitary Significance
* Reasons to Soften
1. Reduce Soap Consumption
2. Improve Aesthetics of Water
3. Hot Water Heaters last longer
* Reasons not to Soften
1. Expensive Process
2. May be less healthy
3. Competes with health related costs
Method of Analysis
* EDTA – Ethylenediaminetertraacetic Acid Method
* EDTA complexes Ca & Mg
* Eriochrome Black T serves as an indicator when
EDTA is in excess of the complexed hardness ions.
* Color change is from red to blue
Types of Hardness
* Calcium and Magnesium Hardness
* Total Hardness – Calcium Hardness = Magnesium Hardness
* Carbonate and Noncarbonate Hardness
* When alkalinity < Total Hardness, CO3 Hardness = T. Alkalinity
* When alkalinity ≥ Total hardness, CO3 Hardness = T. Hardness
* CO3 hardness removed by boiling or lime (Temporary Hardness)
* Noncarbonate Hardness (permanent) = T. Hardness – CO3 Hardness
* Pseudo-Hardness
* Associated with Na+ which causes soap consumption but not
considered part of hardness.
Effective Lime/Soda Ash
Water Softening
By
Douglas Rittmann, Ph.D., P.E.
Water/Wastewater Consultant
Presented to
CE 5345
On
Sept., 2006
Water Softening
I. Introduction
A. Reasons to Soften
1. Reduce Soap Consumption
2. Improve Aesthetics of Water
3. Hot Water Heaters last longer
B. Reasons not to Soften
1. Expensive Process
2. May be less healthy
3. Competes with health related costs
Water Softening
II. What is Hardness?
A. Hardness Classifications
a. Soft Water = 0 to 70 mg/L
b. Moderate Hardness = 71 to 150 mg/L
c. Hard Water = > 150 mg/L
B. T. Hardness, mg/L, as CaCO3 = (Ca X 2.5) + (Mg X 4.12)
(MW=100) (40 X 2.5 = 100) (24.3 X 4.12 = 100)
C. Carbonate Hardness as CaCO3 = T. Alkalinity as CaCO3
a. Removed by Boiling
b. Removed by Lime
D. Non-Carbonate Hardness = T. Hardness – T. Alkalinity
a. Unaffected by boiling
b. Removed by Soda Ash
Water Softening
III. Methods of Softening
A. Lime-Soda Ash Chemistry
1. 1st Stage Treatment (Lime only)
a. Carbon Dioxide Removal (< 8.3 pH)
* CO2 + Ca(OH)2
CaCO3 + H2O
b. Carbonate Hardness Removal
* Ca + 2HCO3 + Ca(OH)2
* Mg + 2HCO3 + Ca(OH)2
2CaCO3 + 2H2O(pH 8.3-9.4)
CaCO3 + Mg + CO3 + 2H2O(pH >10.8)
c. Magnesium Hardness Removal (>pH 10.8)
* Mg + CO3 + Ca(OH)2
CaCO3 + Mg(OH)2
* Mg + SO4 + Ca(OH)2
Ca + SO4 + Mg(OH)2
2. 2nd Stage Treatment (Soda Ash)
* Ca + SO4 + Na2CO3
* Ca + Cl2 + Na2CO3
Na2SO4 + CaCO3
CaCO3 + 2NaCl
IV. Chemical Analyses Interpretations
A. Calcium Alkalinity = Ca Hardness or T. Alkalinity whichever is smaller
B1. Magnesium Alkalinity = Mg. Hardness if T. Alkalinity > or = than total hardness
B2. Magnesium Alkalinity = Total Alkalinity – calcium hardness if total alkalinity is > than
calcium hardness but less than total hardness.
C. Sodium alkalinity = total alkalinity – total hardness
D. NCH = Total Hardness – Total Alkalinity ( If Mg Alkalinity present then no Ca NCH)
Analyses
Water #1
Water #2
Water #3
Total Hardness
300
300
300
Calcium Hardness
200
200
200
Mg Hardness
100
100
100
Total Alkalinity
150
250
350
Interpretations
Water #1
Water #2
Water #3
Calcium Alkalinity
150
200
200
Mg. Alkalinity
None
50
100
Sodium Alkalinity
None
None
50
Ca N.C. Hardness
50
None
None
Mg. N.C. Hardness
100
50
none
Water Softening
V. Theoretical versus Practical
A. Theoretical Solubility of Ca & Mg:
Mg(OH)2 = 9 mg/L Solubility
CaCO3 = 17 mg/L Solubility
Total = ~ 26 mg/L Solubility
B. Practical Minimum Total Hardness = 50 to 80 mg/L
Example: Calculate the hydrated lime (100%), soda ash, and carbon dioxide requirement to
Reduce the hardness of a water with the following analysis to about 50 to 80 mg/L by the excess
Lime-soda ash process.
Analyses: Total Hardness = 280 mg/L as CaCO3
Mg++
= 21 mg/L
Alkalinity
= 170 mg/L as CaCO3
Carbon Dioxide = 6 mg/L
Lime Requirement: Carbon Dioxide = (6) (56) / (44)
= 8
Alkalinity
= (170) (56) / (100) = 95
Mg ++
= (21) (56) / (24.3) = 48
Excess Lime =
= 35
Total CaO required = 186mg/L
Soda Ash Requirement: NCH = 280 – 170 = 110 mg/L
Soda Ash (Na2CO3) = (110) (106) / (100) = 117 mg/L