CO2 POTENTIOMETRIC DETERMINATION

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• CO2 POTENTIOMETRIC DETERMINATION

Introduction Equipments Data treatment Gran
Plots Non-linear least squares procedures
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• TOTAL DISSOLVED INORGANIC CARBON CT (TIC)
  (TCO2)
• CT CO2 HCO3- CO32-
• CT can be measured
• Directly by acidifying the sample, extracting the
  CO2 gas that is produced and measuring its
  amount.
• A. Gas chromatography
• B. Infrared spectroscopy
• C. Conductivity
• D. Collecting the CO2 in a solution and
  titrating it coulometrically
• 2. Following the AT titration determination and
  computing CT as the difference between the first
  and second endpoints.

• CO2 POTENTIOMETRIC DETERMINATION

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TOTAL ALKALINITY concentration of all the bases
that can accept H when a titration is made with
HCl to the carbonic acid endpoint in one kilogram
of sample. AT HCO3- 2 CO32- B(OH)4-
OH- HF HSO4- HF H3PO4
SiO(OH)3- Mg(OH) HPO42- 2 PO43-
HS- NH3 A known seawater sample is
titrated with a solution of hydrochloride acid.
The acid is made up in a sodium chloride
background to approximate the ionic strength to
that of seawater (activity coefficients are
mantained constant) When the titration is made
in a closed cell, it can be assumed that the
total dissolved inorganic carbon remains constant
throughout the titration (apart from the effect
of dilution). The titration is followed by a
glass electrode / reference electrode pH cell.

• CO2 POTENTIOMETRIC DETERMINATION

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• CO2 POTENTIOMETRIC DETERMINATION

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The location of the endpoint is very demanding
e.g. to achieve a precision of 1µmol/kg in AT
from the titration of 100 ml of sample with 0.1
molar HCl, the endpoint has to be located exactly
to 1 µl (while volume increments are about
100- 200 µl per step). The location therefore has
to be done by mathematical means, any graphical
approach would not give sufficiently exact
results.
A typical titration curve of 100 ml (W Vs? in
kg SW) of S 36.127 seawater with NHCl 0.100
N HCl (0.6 in NaCl) shows two endpoints, V1 and
V2. The value of AT is determined from AT V2
NHCl / W
The difference between the first and second
endpoints can be used to determine the CT CT
(V2 V1) NHCl / W
It is desirable to know the internal volume Vs of
the cell accurately (with the plunger fully
depressed). Maintenance of the titration cell may
require replacing the electrodes, or plunger. As
a result, the volume of the cell can change and
will need to be measured again.
Total alkalinity is computed from the titrant
volume and e.m.f. data using a least-squares
procedure based either on a non-linear curve
fitting approach or on a modified Gran approach.
Total dissolved inorganic carbon can be computed
from such titration data.

• CO2 POTENTIOMETRIC DETERMINATION

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The systems are designed to be computer-controlled
. The cell contains a glass pH electrode and an
Ag/AgCl reference electrode ( separate electrodes
improve the stability of the e.m.f. reading), a
capillary tube that supplies acid from an
automatic burette and a plunger which is free to
move thus adjusting the volume of the cell as
acid is added and allowing the titration to be
carried out without a head-space. The cell is
rinsed thoroughly (twice) with seawater to remove
any acidified seawater from the previous
titration. The thermostated seawater sample is
pumped (manually or automatically) to the cell
avoiding any bubble formation. The plunger must
be fully depressed at the end without any
pressure excess inside the cell. Titration can
take place both at a fixed acid addition (150 µl
HCl 0.1 N) or at variable volume addition. In
this case, after reading the e.m.f for the
seawater previous to any HCl addition (it will
also allow determination of the pH of the
seawater sample), 50 µl HCl are initially added
and the e.m.f. change is recorded. A simple
linear relationship is applied to know how much
acid should be added to get a pre-fixed e.m.f.
increment.

• CO2 POTENTIOMETRIC DETERMINATION

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GRAN PLOT CT (V2 V1) NHCl / W The pHsws
of the initial solution before the addition of
HCl can be determined from pHsws -(E
E)/(2.303RT/F) k 2.303RT/F 59.16 mV at
25ºC V1, V2 and E are determined from the
e.m.f. (E) and the volume of added HCl (V) with a
Gran Plot. A reasonable approximation of V1, V2
and E can be determined by Hsws
10(E-635)/k Using the e.m.f. data, a function F2
is calculated for e.m.f. values after the V2
value F2 (Vs V) Hsws The values of F2
are then fitted to the linear equation V a
b F2 At F2 0, V V2 F1 (V2
V)Hsws V a bF1 At F1 0, V V1 E
E k log(V-V2)/(Vs V) NHCl

• CO2 POTENTIOMETRIC DETERMINATION

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emf
V
V
V
F2
F1

• CO2 POTENTIOMETRIC DETERMINATION

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emf
V
V2 2.5018 ml V1 0.2697 ml AT 2383.88 µmol
kg-1 CT 2127.99 µmol kg-1 E 634.90 mV pHsws
7.9666

• CO2 POTENTIOMETRIC DETERMINATION

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Modification of the GRAN Plot More accurate
calculations need to make corrections for side
reactions F2 (Vs V) HHSO4-HF-HCO3-
F1 (V2 V) (H2 K1K2)/K1H 2
K1K2) (Vs V) H HSO4- HF
B(OH)4- OH- H2 K1 H K1K2/NHCl
(K1 H 2 K1K2)

• CO2 POTENTIOMETRIC DETERMINATION

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NON-LINEAR LEAST-SQUARES APPROACH Proton
condition at the equivalent point is defined
considering AT definition HF HSO4-
HF H3PO4 HCO3- 2 CO32- B(OH)4-
OH- SiO(OH)3- HPO42- 2 PO43-
HS- NH3 At any point in the titration, the
analytical total concentration of hydrogen ion is
given by CH HF HSO4- HF H3PO4 -
HCO3- - 2 CO32- - B(OH)4- - OH- -
SiO(OH)3- - HPO42- - 2 PO43- - HS- -
NH3 The initial analytical concentration of
hydrogen ion in the solution is thus the negative
of the total alkalinity.

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When m g of acid of concentration C are added to
m0 g of sample And HF HSO4-
HF H3PO4 - HCO3- - 2 CO32- B(OH)4-
- OH- - SiO(OH)3- - HPO42- - 2 PO43- -
HS- - NH3 This relationship, together with
the Nernst equation that relates e.m.f. with the
total hydrogen ion concentration and the
expressions of the individual species
concentrations in terms of the total
concentration and equilibrium constants are the
basis of all computations. E E - (2.303 R
T/F) log H

• CO2 POTENTIOMETRIC DETERMINATION

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A computer program which allows the computation
of both total alkalinity and total dissolved
inorganic carbon from titration data for a closed
cell titration of seawater is presented in DOE
1994, SOP 3, based on a non-linear least squares
evaluation of the data similar to that used by
Dickson (1981) and by Johansson Wedborg
(1982). In the least squares procedure a
multiplier is defined f H / H Where
H is computed in the same way as for the
initial value in the Gran Plot. A Qbasic
computer version is going to be provided in this
training course.

• CO2 POTENTIOMETRIC DETERMINATION