pH and Buffering

1
pH and Buffering

• Aim
• to know the logarithmic scale of pH
• to understand how weakly dissociating acids can
  buffer the pH of an aqueous environment
• to know the importance of the carbonate -
  bicarbonate buffering system

2

• pH, The master variable

• Consumed and produced
• Enzyme/biological optima

Biological activity (enzyme activity)
4 5 6 7
8 9 10 pH
3
Dissociation of Water

By Convention H2O 1 therefore OH-
H 10-14 So, if H is known, OH- is
also known if H 10-5, then OH-
10-9 Dealing in H is cumbersome Deal in pH
(minus the log of the hydrogen ion
concentration) pH - logH if H 0.1
M or 10-1 M, then pH 1
4
pH is a log scale

pH

H
OH-
10-7
7
10-7
10-6
6
10-8
10-5
5
10-9
10-3
3
10-11
10-11
11
10-3
5
Measurement of pH

• pH meter and glass electrode
• quick
• easy
• accurate
• portable
• Indicators
• titrations
• phenolphthalein pink ? colourless below pH 8.3
• methyl orange red ? yellow above pH 4.3

6
Weak acids and strong acids

• An acid is substance produces H in water
• H2SO4 ? 2H SO42-
• A base produces OH- and/or accepts H
• NaOH ? Na OH-
• A strong acid dissociates completely
• 1 mole HCl ? 1 mole H 1 mole Cl-
• 1 mole H2SO4 ? 2 mole H 1 mole SO42-
• A weak acid dissociates only partially
• 1 mole CH3COOH ? 0.0042 mole H 0.0042
  mole CH3COO-
• The concentration of hydrogen ions H is
  therefore not always the same as the
  concentration of the acid

7

• Buffers

• Chemicals which resist pH change
• Acetic acid Acetate
• CH3COOH ? CH3COO- H
• Carbonate Bicarbonate
• CO32- H ? HCO3-
• Amphoteric chemicals
• e.g. Proteins and amino acids
• (have both ve and -ve charged groups on the same
  molecule)

8

• Buffering range of a buffering chemical is
  indicated by its pKa
• pKa is the pH at which the buffering chemical is
  half dissociated
• for HA ? H A-
• when HA H A-, then pH pKa
• therefore buffering greatest when pH pKa
• Buffering capacity is given by the amount of
  buffering chemical present

9
Carbonate-Bicarbonate Buffering

• Major buffering in aquatic systems
• CO2 (g) ? CO2 (aq)
• CO2 (aq) H2O ? H2CO3
• (carbonic acid)
• Difficult to distinguish between the two forms in
  water.
• H2CO3 CO2 H2CO3
• H2CO3 is a proxy for dissolved CO2 plus
  carbonic acid

10

"Carbonic acid" dissociates to form
bicarbonate H2CO3 ? HCO3- H pKa
6.3 Bicarbonate dissociates to form
carbonate HCO3- ? CO32- H pKa
10.3 Carbonate can also come from the
dissolution of carbonate containing
minerals MgCO3, Ca CO3 MgCO3 ? Mg2
CO32- CaCO3 CO2(aq) H2O ? Ca2
2 HCO3-
11
Carbonate / bicarbonate system in a particular
water depends on its contact with air (CO2) and
carbonate minerals. For a closed system with no
minerals or CO2 input, the species are

HCO3-
H2CO3
CO32-
Fraction as designated species
5
7
6
10
8
9
12
11
4
pH
pKa 10.3
pKa 6.3
12

• References

• Sawyer, McCarty, Parkin(1994)
• Chemistry for Environmental Engineering
• Snoeyink, V.L. and Jenkins, D. (1980) Water
  chemistry, Wiley.
• Stum, J and Morgan, J.J. (1981) Aquatic
  Chemistry, Wiley Interscience.
• Loewenthal, R.E. and Marais, G.V.R (1976)
  Carbonate Chemistry of Aquatic Systems,
  Butterworths.