CO2 COULOMETRIC DETERMINATION

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• CO2 COULOMETRIC DETERMINATION

• Introduction
• VINDTA system description
• Principle
• CO2 extraction system
• Carrier gas
• Labview program
• Operational procedure

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Reactions to be considered when carbon dioxide
dissolves in water K0 CO2(g) ? CO2(aq) (1)
K1 CO2(aq) H2O ? HCO3- H (2)
K2 HCO3- ? CO32- H (3) Ksp Ca2
CO32- ? CaCO3(s) (4) CO2 H2O ? CH2O
O2 (5) At equilibrium CO2 is 670 times
higher than H2CO3 and the stoichiometric
association constant for the first ionization
is K1 HTHCO3-T/CO2 CO2 CO2
H2CO3

• CO2 COULOMETRIC DETERMINATION

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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 endpoint.

• CO2 COULOMETRIC DETERMINATION

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CO2 COULOMETRIC TECHNIQUE SOMMA SYSTEM.
Developed by K. Johnson at the University of
Rhode Island. VINDTA system. Developed by
Ludger Mintrop at Marianda company, Kiel,
Germany

• CO2 COULOMETRIC DETERMINATION

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VINDTA system

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Principle A known amount of seawater (?20 ml) is
acidified with H3PO4 (10) in a glass stripping
chamber and the resulting CO2 gas is purged with
an inert gas (N2). The CO2 gas is dried in a
condenser at 1-5 ºC (peltier system) and is
determined by absorbing the CO2 in an absorbent
solution. The coulometric cell contains
mono-ethanolamine and a colorimetric pH indicator
in the cathode cell with a platinum cathode, and
a silver anode in the anode solution. The
hydroxyethylcarbamic acid formed is titrated
coulometrically with hydroxide ions generated by
the coulometer circuitry. The final titration
point is determined spectrophotometrically by
maintaining the transmittance of the solution at
a constant value.

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1.- Absorption of CO2 by the cathodic solution
(Cathode reaction) CO2 HOCH2CH2NH2 ?
HOCH2CH2NHCOOH monoethanolamine
hydroxyethylcarbamic acid The acid causes the
blue color indicator to fade and the T to
increase, and titration current is automatically
activated

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The titration current is measured continuously
and integrated to a selected unit on the digital
display. The law of Faraday is applied For every
faraday of electricity utilized, one gram
equivalent weight (1 GEW) of CO2 is titrated. One
faraday is equivalent to 96486 coulombs. 12.011
g C are titrated by 96486 coulombs 1 mg C needs
8.033 coulombs 1 coulomb 1 amperesec
(Asec) 1 µg C needs 8.033 mAsec The coulometer
is designed to convert the measured current into
a frequency count 1 count is assigned to 0.02
mAsec 1 µg C corresponds to 401.65 counts

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Cathode solution A mixture of water,
ethanolamine, tetra-ethyl-ammonium bromide and
thymolphthalein in a dimethyl sulfoxide solution
(DMSO) Anode solution A saturated potassium
iodide solution (add crystals of KI to ensure
saturation) in water and DMSO

• CO2 COULOMETRIC DETERMINATION

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CO2 extraction system The seawater sample is
added to a borosilicate glass stripping chamber
equipped with a drain for removing spent sample.
The solution is stripped of CO2 by bubbling the
carrier gas through a fine frit submerged in the
acidified sample.

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CARRIER GAS For the extraction of CO2 from the
sample, a carrier gas is needed. This can be
nitrogen (purity 5.0 or better) or dry, CO2-free
compressed air. A moisture/CO2 absorbent should
preferentially be installed in the gas line. The
gas line passes by a needle valve to the flow
meter. The gas pressure is set to about 100kPa (1
bar), so that the flow can be adjusted to about
250 ml/min, without fully opening the needle
valve. Increase of the carrier gas flow will
speed up the purging of the sample and reduce
titration time for DIC, but the liberated CO2 has
to react quantitatively with the ethanolamine.
Though this reaction is fast, it will set an
upper limit for the gas flow. A flow of 150
ml/min should therefore probably not be exceeded
when the maximum current is set to 50 mA

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LabVIEW program for DIC measurement The VINDTA
system requires the program LabVIEWTM from
National Instruments Inc. Most of the program is
written under version 5.1 (Professional
Development Suite). The program VINDTA
aquisition.vi is prepared to perform the complete
titration of a sample for both alkalinity and
total dissolved inorganic carbon measurement.
The program for the VINDTA 3C allows you to
choose measure alkalinity, measure DIC, or
both. When both is selected, the liquid
handling is changed so that the pipette for DIC
is filled first (only after successful clear
ready check) using a peristaltic pump P1, but at
a lower speed. This is done while rinsing the
titration cell. After transfer of the DIC sample
to the stripper, the alkalinity sample is loaded
in the pipette. Both titrations are terminated at
about the same time, depending on the performance
of the coulometer and the electrodes. After both
measurements are terminated, the next sample is
loaded.

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OPERATION PROCEDURE 1.- Calibration of the
pipette Calibration of the pipette is done the
same way as the pipette is used for titration,
except that the stripper is replaced by a weighed
beaker (in the lab) or screw cap bottle (at sea).
The pipette is filled either with distilled water
or with seawater of well-known salinity and
drained into the beaker or bottle. The pipette
and the water have to be thermostated carefully
to the calibration temperature (usually 25C).
The bottle/beaker is weighed and the volume of
the pipette, including tubing, is determined from
4-5 replicates. Temperature, air pressure and
humidity are noted to allow for buoyancy
correction (SOP 21, DOE 1994) . The use of
seawater of known salinity has the advantage of
compensating errors arising from different
draining characteristics of distilled water due
to the different viscosity. With some experience,
the pipette volume can be determined to 20µl or
better from 5-6 replicates.

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2.- Preparing a new cell The coulometric cell is
filled with the cathode solution (80-100 ml) and
with the anode solution (the level should be
slightly lower than the cathode solutions, add KI
crystals). The compartments are closed with the
corresponding cathode (platinum electrode) and
anode (silver electrode) caps. The cell color is
clear The cell is placed in the coulometer. The
electrical terminals are connected to the
coulometer and the main power supply switch is
turned on. The transmitance reading should be
adjusted to 100 percent transmittance using the
T adjustment knob. Rotate the cell until the
highest T reading is obtained. The cell current
switch is turned on, allowing the cell current to
titrate the coulometer cell to its endpoint. When
the transmittance reading is 29 and the counts
are stable, the cell is ready to be used.

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2.- Preparing a new cell Before starting to
analyze samples, a blank determination should be
carried out. Introduce the name of the cell and
indicate a new cell is being used. Run a blank in
the VINDTA acquisition program to calculate the
background titration rate. The system will
deliver 20 drops of acid into the stripper and
will follow the instrument drift. After 10 min a
blank will be determined. The number of counts
per minute (b coulometer reading for 10 min/
10) should be 30 or lower. If higher, the system
will carry out another blank determination. Before
running a sample it is better to run a test
sample.

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2.- Preparing a new cell The system is now ready
to start analyzing seawater samples. Run a
Certified Reference Material CRM sample to check
the system and use the value to prepare a control
chart. A new cell should be prepared after every
40-50 samples The cell must be cleaned properly
and left in the oven at 60ºC until the next
preparation.

• CO2 COULOMETRIC DETERMINATION