Andrew Dickson: pH and Alkalinity

51 Pages·2009·2.35 MB·English

CO chemistry in sea water:
2
pH and alkalinity
Andrew G. Dickson
Tuesday, November 3, 2009 1
In your course syllabus is a section prepared for the
EPOCA Guide to Best Practices . . .
Tuesday, November 3, 2009 2
http://cdiac.ornl.gov/oceans/Handbook_2007.html
Tuesday, November 3, 2009 3
Tuesday, November 3, 2009 4
Relationship between pH and lg K

pH = – lg[H+ ] lg x is log x
10
for the acid-base reaction HA  H+ + A –

+ –
[H ][A ]
K(HA ) =
[HA]

( )
–
pH = – lg (HA ) + lg [A ] [HA]

For this to be true, pH and lg K(HA) must be defined

on the same pH scale
Tuesday, November 3, 2009 5
Recommended!!!
The total hydrogen ion concentration
pH scale (1)
+
pH = – lg [H ]

As mentioned yesterday, the brackets imply total
concentration, that is the sum of the free concentration
plus the concentrations of all complexes with the
background ionic medium
(H O, Na+, Mg2+, K+, Ca2+, Cl–, SO 2–)
2 4
For H+ these include complexes with H O,
2
and with SO 2–, i.e., HSO –.
4 4
Tuesday, November 3, 2009 6
The total hydrogen ion concentration
pH scale (2)
+ 2–
[H ] [SO ]
HSO– = H+ + SO2– K′(HSO– ) = F 4

4 4 4 –
[HSO ]

4
+ + –
[H ] = [H ] + [HSO ]

F 4
At low pH the term
( )
+ + 2– –
[H ] = [H ] 1+ [SO ] / K(HSO ) in parentheses is

F 4 4
not constant
( )
+ + –
[H ] = [H ] 1+ S / K(HSO )
 ⇒ S is a function
F T 4 T
of salinity
At pH > 5, these two equations
are essentially equivalent.
Tuesday, November 3, 2009 7
The total hydrogen ion concentration
pH scale (3)
( )
def
+ + –
[H ] = [H ] 1+ S / K(HSO )
F T 4
The advantage of this is that [H+] is
directly proportional to [H+] at all pHs.
F
Tuesday, November 3, 2009 8
Measuring total hydrogen ion concentration
using a pH cell
KCl solution electrode reversible
reference test
–3 +
electrode solution
(c ! 1 mol dm ) to H (aq)

measured emfs
in sample and standard
E − E
pH(X) = pH(S) −  X S
RT ln10 / F
pH of pH of
0.05916 V per pH unit
sample solution standard buffer
Tuesday, November 3, 2009 9
Measuring total hydrogen ion concentration
using a pH cell
E − E
pH(X) = pH(S) −  X S
RT ln10 / F
NEED
1. pH cell (glass / reference electrodes)
2. pH meter (high input impedance & ± 0.1 mV)
3. Standard bu f f e r (“Tris” buffer in synthetic s/w)
4. Temperatur e c o n t r o l (±0.1 °C)
Tuesday, November 3, 2009 10