Water in Soil,Neutralization and
Watershed Buffering
pH of a buffer and buffer capacity
Example,For a buffer solution consisting of 0.1 M acetic
acid and 0.1M sodium acetate,The pH of the solution is
4.75.
75.475.4
10
1.0
1.0
10
][
][
][
][
]][[
??
?
?
??
??
???
??
?
??
Ac
H A c
KaH
H A c
AcH
Ka
AcHH A c
If an amount of hydrochloric acid,equivalent to 10% of the
acetate present,is added to the buffer,what is the new pH
of the solution?
pH of a buffer and buffer capacity (Continued)
After addition of HCl,the new [Ac-]=0.09M,[HAc]=0.11M
09.066.475.4
66.4
10
09.0
11.0
10
][
][
][
66.475.4
????
?
???
??
??
?
?
pH
pH
Ac
H A c
KaH
The addition of 0.01M strong acid to pure water would lower
pH by 5 units (from 7 to 2)!
pH of a buffer and buffer capacity (Continued)
? The pH of a buffer depends on the pKa of
the buffer acid,not its concentration,
][
][
lo g
][
][
][
][
]][[
?
?
?
??
??
??
??
?
??
A
HA
p K apH
A
HA
KaH
HA
AH
Ka
AHHA
pH of a buffer and buffer capacity (Continued)
? Buffer capacity is how much
acid or base the buffer can
tolerate while maintaining the
pH (within a 1.00 unit).
? Buffer capacity is determined
by the concentration of the
buffer acid and its conjugated
base,as well as their
concentration ratio.
Water pH and Well-being of Fish Species
Dashed line,lake pH,Solid line,upwind SO2 emission from
the U.S,industrial midwest,
The ability of a
water body to
support its
normal
complement of
biological species
can be critically
affected by the
pH of the water.
Water acidification from acid deposition:
pH decline lags behind acid deposition,why?
? Observation,In Big Moose Lake,pH dramatic decline
lagged behind in the rise in SO2 emissions by some
70 years.
? Reason,The watershed’s natural buffering capacity
delayed the onset of pH decline.
? Implication,Polluting activities may be far displaced
in time from their environmental effects,
Decline in soil solution pH over time in response to
atmospheric acid inputs
The time-scales over which the soil solution passes from one
buffering range to the next depends on the intensity of acid
deposition,the nature of soil,the size of watershed,and the
flow characteristics of the lake or groundwater.
Watershed buffering,carbonate buffering
MH C O
H C OCa
MCOH
COH
H C OCa
K e q
H C OCaCOHC a C O
00.3
3
3
2
9.4
32
41.4
32
2
3
2
3
2
323
10][
][][2
10][
10
][
]][[
2
??
??
?
?
??
??
?
?
?
??
???
37.897.140.6
][
][
l o g
10
3
32
4.6
][
]][[
332
32
3
???
??
??
??
?
?
??
??
H C O
COH
p K apH
Ka
H C OHCOH
COH
H C OH
Acidic rainwater can be neutralized by exposure to
calcareous soils,with a concomitant significant increase
in the concentration of calcium ion in solution.
That’s how underground caves are formed
Watershed buffering,cation exchange buffering
????
??
?
???
KNaMgCaM
S iOHMS iOMH xnx
,,,
)](2[)]([
22
44
The buffer capacity of clay soils is usually limited
because of the limited exchangeable sites occupied by
the cations Na+,K+,Mg2+,and Ca2+.
The exchangeable pool of cations on the surface is
tiny compared to the pool trapped inside the soil
particles.
Weathering reactions release trapped cations,but
they are relatively slow compared to the rate of
acidification.
Watershed buffering,Aluminum buffering
When pH drops below 4.2,H+ dissolves the Al-
containing minerals.
OHAlHOHAl
OHS i OAlHOHOSiAl
2
3
3
22
3
4522
33)(
5226)(
???
????
??
??
Al-containing minerals are abundant in soils,buffer
capacity in this range is rarely depleted.
Al3+ is toxic to plants and aquatic organisms.
Water acidification from acid mine drainage
? When pyrite-rich coal is mined,pyrite is exposed to air and
water,
? Oxidation of pyrite produces sulfuric acid.
2FeS2 + 7/2 O2 + 2 H2O ? Fe2+ + 2 HSO42-
Fe2+ + ? O2 + 1/2 H2O ? Fe3+ + OH-
Fe3+ + 3 H2O ? Fe(OH)3 + 3H+
? Overall reaction
FeS2 + 15/2 O2 + 7/2 H2O ? Fe(OH)3 + 2 HSO42-
Brown
precipitation
Streams receiving this drainage could
have a pH as low as 3.0!
Assisted by
bacteria
Acid mine drainage
? Solution,Neutralize with limestone
CaCO3 (s) + 2 H+ + SO42- ? Ca2+ + SO42- + H2O + CO2 (g)
Study questions
1,What chemical substances can serve as pH
buffers? What determines the buffer
capacity of a pH buffer?
2,Why is there not an immediate decline in
pH following atmospheric input of large
quantities of acidic substances?
3,Describe the three defense lines of
watershed in resisting pH changes,
Watershed Buffering
pH of a buffer and buffer capacity
Example,For a buffer solution consisting of 0.1 M acetic
acid and 0.1M sodium acetate,The pH of the solution is
4.75.
75.475.4
10
1.0
1.0
10
][
][
][
][
]][[
??
?
?
??
??
???
??
?
??
Ac
H A c
KaH
H A c
AcH
Ka
AcHH A c
If an amount of hydrochloric acid,equivalent to 10% of the
acetate present,is added to the buffer,what is the new pH
of the solution?
pH of a buffer and buffer capacity (Continued)
After addition of HCl,the new [Ac-]=0.09M,[HAc]=0.11M
09.066.475.4
66.4
10
09.0
11.0
10
][
][
][
66.475.4
????
?
???
??
??
?
?
pH
pH
Ac
H A c
KaH
The addition of 0.01M strong acid to pure water would lower
pH by 5 units (from 7 to 2)!
pH of a buffer and buffer capacity (Continued)
? The pH of a buffer depends on the pKa of
the buffer acid,not its concentration,
][
][
lo g
][
][
][
][
]][[
?
?
?
??
??
??
??
?
??
A
HA
p K apH
A
HA
KaH
HA
AH
Ka
AHHA
pH of a buffer and buffer capacity (Continued)
? Buffer capacity is how much
acid or base the buffer can
tolerate while maintaining the
pH (within a 1.00 unit).
? Buffer capacity is determined
by the concentration of the
buffer acid and its conjugated
base,as well as their
concentration ratio.
Water pH and Well-being of Fish Species
Dashed line,lake pH,Solid line,upwind SO2 emission from
the U.S,industrial midwest,
The ability of a
water body to
support its
normal
complement of
biological species
can be critically
affected by the
pH of the water.
Water acidification from acid deposition:
pH decline lags behind acid deposition,why?
? Observation,In Big Moose Lake,pH dramatic decline
lagged behind in the rise in SO2 emissions by some
70 years.
? Reason,The watershed’s natural buffering capacity
delayed the onset of pH decline.
? Implication,Polluting activities may be far displaced
in time from their environmental effects,
Decline in soil solution pH over time in response to
atmospheric acid inputs
The time-scales over which the soil solution passes from one
buffering range to the next depends on the intensity of acid
deposition,the nature of soil,the size of watershed,and the
flow characteristics of the lake or groundwater.
Watershed buffering,carbonate buffering
MH C O
H C OCa
MCOH
COH
H C OCa
K e q
H C OCaCOHC a C O
00.3
3
3
2
9.4
32
41.4
32
2
3
2
3
2
323
10][
][][2
10][
10
][
]][[
2
??
??
?
?
??
??
?
?
?
??
???
37.897.140.6
][
][
l o g
10
3
32
4.6
][
]][[
332
32
3
???
??
??
??
?
?
??
??
H C O
COH
p K apH
Ka
H C OHCOH
COH
H C OH
Acidic rainwater can be neutralized by exposure to
calcareous soils,with a concomitant significant increase
in the concentration of calcium ion in solution.
That’s how underground caves are formed
Watershed buffering,cation exchange buffering
????
??
?
???
KNaMgCaM
S iOHMS iOMH xnx
,,,
)](2[)]([
22
44
The buffer capacity of clay soils is usually limited
because of the limited exchangeable sites occupied by
the cations Na+,K+,Mg2+,and Ca2+.
The exchangeable pool of cations on the surface is
tiny compared to the pool trapped inside the soil
particles.
Weathering reactions release trapped cations,but
they are relatively slow compared to the rate of
acidification.
Watershed buffering,Aluminum buffering
When pH drops below 4.2,H+ dissolves the Al-
containing minerals.
OHAlHOHAl
OHS i OAlHOHOSiAl
2
3
3
22
3
4522
33)(
5226)(
???
????
??
??
Al-containing minerals are abundant in soils,buffer
capacity in this range is rarely depleted.
Al3+ is toxic to plants and aquatic organisms.
Water acidification from acid mine drainage
? When pyrite-rich coal is mined,pyrite is exposed to air and
water,
? Oxidation of pyrite produces sulfuric acid.
2FeS2 + 7/2 O2 + 2 H2O ? Fe2+ + 2 HSO42-
Fe2+ + ? O2 + 1/2 H2O ? Fe3+ + OH-
Fe3+ + 3 H2O ? Fe(OH)3 + 3H+
? Overall reaction
FeS2 + 15/2 O2 + 7/2 H2O ? Fe(OH)3 + 2 HSO42-
Brown
precipitation
Streams receiving this drainage could
have a pH as low as 3.0!
Assisted by
bacteria
Acid mine drainage
? Solution,Neutralize with limestone
CaCO3 (s) + 2 H+ + SO42- ? Ca2+ + SO42- + H2O + CO2 (g)
Study questions
1,What chemical substances can serve as pH
buffers? What determines the buffer
capacity of a pH buffer?
2,Why is there not an immediate decline in
pH following atmospheric input of large
quantities of acidic substances?
3,Describe the three defense lines of
watershed in resisting pH changes,
