Title: pH and Buffering
1pH 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- Consumed and produced
- Enzyme/biological optima
Biological activity (enzyme activity)
4 5 6 7
8 9 10 pH
3Dissociation 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
4pH 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
5Measurement 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
6Weak 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- 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
9Carbonate-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-
11Carbonate / 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- 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.