This document defines acidity and alkalinity in water samples. Acidity is the capacity to neutralize added alkalis and is determined by titration with a standard alkali solution. Alkalinity is the capacity to neutralize acids and is determined by titration with a standard acid solution. Both are usually reported as calcium carbonate equivalent. The document outlines the different types of acidity and alkalinity, how they are measured through titration, and how the results are calculated and reported. It also discusses some applications of acidity and alkalinity data.

1. Acidity and Alkalinity
Definitions
Alkalinity Measurement
----------- Task -----------
Acidity Measurement
Importance

2. What are Acidity and Alkalinity
 Acidity
 The capacity of a water sample to neutralise added alkali.
 Two Types
 Mineral Acidity from Strong Acids, and from Salts of Trivalent Metal Ions
H2SO4  2 H+ + SO4
2-
FeCl3 + 3 H2O  Fe(OH)3 + 3 H+ + 3 Cl -
 Carbon Dioxide Acidity (Weak Acidity)
CO2 + H2O  H2CO3
*  H+ + HCO3
-

3.  Alkalinity
 The capacity of a water sample to neutralise added acid.
 Mainly from:
1. The Carbonate - Bicarbonate buffering system.
2. The salts of weak acids
 CO2 + CaCO3 + H2O Ca2+ + HCO3
-
 Na Acetate, Na Propionate
3. Hydroxide
Ammonia
What are Acidity and Alkalinity

4. Equilibrium of CO2 , HCO3
- , CO3
2-
CO2 (aq) + H2O H2CO3 H+ + HCO3
- 2H+ + CO3
2-
CO2 (g)
pH 4 4.5 5 6 7 8 8.3 9 10
pKa = 6.3 pKa = 10.3
350ppm
10-5M
pH = pKa + log ([A-] / [AH])
When  reaction is 99% to the left then
pH = 6.3 + log (1/100)
= 6.3 - 2
pH = 4.3

air
water

5. How is Acidity Determined
 By titration with a Standard Alkali solution to a specific Endpoint.
 Mineral Acidity
 Indicator used is Methyl Orange (endpoint at pH 4.5 )
 Carbon Dioxide Acidity
 Indicator used is Phenolphthalein (endpoint at pH 8.3 )
In practice CO2 is the major weak acid. Others are possible, eg. Acetic

6. Acidity
Strong Acids
Weak Acids (CO2 Acidity)
P endpoint (pH 8.3)
M endpoint (pH 4.5)
P
M
M = Methyl Orange
P = Phenolphthalein
Titration with N/50 NaOH
pH
High
Low
starting pH
starting pH

7. How is the Acidity Calculated and Reported
 Titration Volume, Alkali Strength, Sample Volume
 Usually Report Acidity as Calcium Carbonate Equivalent
“ The Acidity to pH X = Y mg CaCO3 / litre “
or Methyl Orange Acidity / Mineral Acidity =
Phenolphthalein Acidity / CO2 Acidity =
If N/50 NaOH is used
Acidity (mg CaCO3) = Vt x 1000/ Vs
Vt = Titre volume (ml) Vs = Sample volume (ml)

8. How is Alkalinity Determined
 By titration with a Standard Acid solution to a specific Endpoint.
 Sample pH Values > pH 8.3
 Two Step Titration
 Step 1. Indicator used is Phenolphthalein (endpoint at pH 8.3 )*
 Step 2. Indicator used is Methyl Orange (endpoint at pH 4.5 )**
 Step 1. neutralises alkalinity due to strong bases (OH- , CO3
2-)
 Step2. neutralises alkalinity due to weaker bases (HCO3
-)
* chosen as CO3
2-  HCO3
- reaction is complete.
** chosen as HCO3
-  H2CO3 reaction is complete.

9. How is Alkalinity Determined
 Sample pH Values < pH 8.3
 One Step Titration
 Step 1. Indicator used is Methyl Orange (endpoint at pH 4.5 )
 only Bicarbonate present when pH < 8.3 at start
 Interpretation of Data
 When Sample pH value is > pH 8.3 a number of base species can
potentially exist together.
 1. Carbonate and Bicarbonate pH > 8.3
 2. Carbonate only pH > 10
 3. Hydroxide and Carbonate pH > 10
 4. Hydroxide only pH >>11

10. Alkalinity (5 cases)
Hydroxide
Carbonate
Bicarbonate
P endpoint (pH 8.3)
M endpoint (pH 4.5)
P
M
A
B
B
X
Y
X
M = Methyl Orange
P = Phenolphthalein
P = a + b
M = b + P
Hydroxide = a = 2P - M
Carbonate = 2b = 2M - 2P
P = x
M = 2x + y
Carbonate = 2x = 2P
Bicarbonate = y = M - 2P
Titration with N/50 H2SO4
P=1/2M
P>1/2M
P<1/2M
pH
High
Low
starting pH
starting pH

11. How is the Alkalinity Calculated and Reported
 Titration Volume, Acid Strength, Sample Volume
 Usually Report Alkalinity as Calcium Carbonate Equivalent
ie. mg CaCO3 / litre
 For each of the 3 Classes Hydroxide Alkalinity
Carbonate Alkalinity
Bicarbonate Alkalinity
If N/50 H2SO4 is used
Alkalinity (mg CaCO3) = Vt x 1000/ Vs
Vt = Titre volume (ml) Vs = Sample volume (ml)

12. Application of Acidity and Alkalinity Data
 Acidity
 Selection of new Water Supplies
 minimise treatment costs (Lime, NaOH)
 Industrial Wastewater
 Metal Pickling Liquor (phosphoric acid)
 Consent to Discharge
 Alkalinity
 Calculate safe levels of Ferric and Alum
 Biological WWT Plants - gives buffering capacity
 Potable - range 50 - 300 mg/l
 Industrial - consent to discharge