Acidity and Alkalinity
Definitions
Alkalinity Measurement
----------- Task ----------Acidity Measurement
Importance
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+ + SO42FeCl3 + 3 H2O  Fe(OH)3 + 3 H+ + 3 Cl -
 Carbon Dioxide Acidity (Weak Acidity)
CO2 + H2O  H2CO3*  H+ + HCO3-
What are Acidity and Alkalinity
 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
 Na Acetate, Na Propionate
3.
Hydroxide
Ammonia
Ca2+ + HCO3-
Equilibrium of CO2 , HCO3- , CO32CO2 (g)
350ppm
air
water
CO2 (aq) + H2O
H2CO3
10-5M
pH

H+ + HCO3-
pKa = 6.3
4 4.5 5
6
pH
7
2H+ + CO32pKa = 10.3
8 8.3 9
10
= pKa + log ([A-] / [AH])
When  reaction is 99% to the left then
pH = 6.3 + log (1/100)
= 6.3 - 2
pH = 4.3
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
Acidity
High
P endpoint (pH 8.3)
P
pH
starting pH
M endpoint (pH 4.5)
M
Low
starting pH
M = Methyl Orange
P = Phenolphthalein
Strong Acids
Weak Acids (CO2 Acidity)
Titration with N/50 NaOH
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
or
=
Y mg CaCO3 / litre “
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)
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- , CO32-)
 Step2. neutralises alkalinity due to weaker bases (HCO3-)
* chosen as CO32-  HCO3- reaction is complete.
** chosen as HCO3-  H2CO3 reaction is complete.
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.
2.
3.
4.
Carbonate and Bicarbonate
Carbonate only
Hydroxide and Carbonate
Hydroxide only
pH > 8.3
pH > 10
pH > 10
pH >>11
Alkalinity (5 cases)
High
P>1/2M
P=1/2M
starting pH
A
P
Titration with N/50 H2SO4
P<1/2M
B
M
starting pH
X
P endpoint (pH 8.3)
pH
X
B
Y
M endpoint (pH 4.5)
Low
M = Methyl Orange
P = Phenolphthalein
Hydroxide
Carbonate
Bicarbonate
P=a+b
M=b+P
P=x
M = 2x + y
Hydroxide = a = 2P - M
Carbonate = 2b = 2M - 2P
Carbonate = 2x = 2P
Bicarbonate = y = M - 2P
How is the Alkalinity Calculated and Reported
 Titration Volume, Acid Strength, Sample Volume
 Usually Report Alkalinity as Calcium Carbonate Equivalent
ie.
 For each of the 3 Classes
mg CaCO3 / litre
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)
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