DISSOLVED INORGANIC CARBON
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SUMMARY: Seawater samples are collected at discrete depths
using CTD-rosette sampling protocols. Subsamples for
dissolved inorganic carbon are collected, immediately
preserved with HgCl2 and stored for subsequent analysis
in the laboratory using a commercial CO2 coulometer.
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1. Principle
The accurate and precise determination of dissolved inorganic
carbon (DIC) concentrations over annual and interannual time scales
is central to the achievement of GOFS objectives. In the central
ocean basins, DIC concentration in surface seawater is believed to
be controlled by air-sea exchange reactions. However, physical
processes and biological activity also influence the concentration
of DIC in surface waters. Beneath the mixed layer the concentration
of DIC increases as a result of the decomposition of organic material.
DIC concentrations are determined using a commercial CO2 coulometer.
The coulometric determination of carbon dioxide has the unique distinction
of performing with a high degree of both precision and accuracy while
maintaining relatively high sample throughput. The coulometric
determination of carbon dioxide involves the stripping of an acidified
seawater sample with a carbon dioxide-free air stream and subsequent
absorption of the carbon dioxide by a solution of ethanolamine.
The weak acid generated by carbon dioxide absorbed in ethanolamine is
titrated by a strong base produced electrolytically. The equivalence
point is detected photometrically with thymolphthalein as the indicator.
The number of coulombs required to reach the end-point is proportional
to the quantity of carbon dioxide evolved from the sample.
2. Precautions
DIC samples should be the first samples taken from the water bottle
unless dissolved oxygen (DO) or pH is also sampled from the same
hydrocast. In this case, DIC samples are collected immediately after
the DO and/or pH samples.
Careful subsampling is important for all dissolved gases. Samples
should be taken as soon as possible and in the same manner as DO samples
(i.e., no bubbles, low turbulence with adequate flushing; see Chapter 5).
3. Water Sampling
3.1. Drawing the sample
3.1.1. Samples are drawn into clean 300 ml glass reagent bottles as
soon as the rosette arrives on deck.
3.1.2. The drawing tube is completely filled with sample by raising
the end of the drawing tube. Bubbles are simultaneously
dislodged by manipulation of the tube. The drawing tube is
flushed and inserted to the bottom of the sample bottle.
3.1.3. The sample bottle is overflowed with at least two volumes of
sample.
3.1.4. The tube is slowly withdrawn from the bottle while the sample
is flowing so that the bottle remains brimful when the tube
is completely withdrawn.
4. Preserving the Sample
4.1. Some of the sample is removed from the reagent bottle using a
plastic pipette equipped with a rubber bulb. Enough water is
removed so that approximately 1 ml of air is contained in the
bottle when the glass stopper is inserted.
4.2. 100 µl of saturated HgCl2 is added to each sample. The tapered
ground glass bottle neck is dried with a Kimwipe wrapped on an
applicator stick. The bottle is sealed with a ground glass
stopper coated with a light covering of Apiezon grease. The
stopper is pressed firmly into the bottle to make a good seal.
The stopper is secured with polyethylene tape or a large rubber
band.
4.3. The samples are stored in a cool location, in the dark.
5. Coulometric Determination of DIC
5.1. Maintenance of extraction and analysis system and temperature control
5.1.1. The glassware used in the extraction system is combusted (450°C,
3 hours) on a regular basis in order to prevent the buildup of
organic films within the extraction system.
5.1.2. The titration cell and rubber stopper are dried overnight at 55°C
before use.
5.1.3. The air stream leaving the extractor is passed through a condensor
and then through a Balston air filter.
5.1.4. The temperature of the titration cell is maintained at 25°C with
circulating water.
5.2.Analysis
5.2.1. A 50 ml plastic syringe is rinsed with sample and then filled and
weighed on a microbalance.
5.2.2. Five ml of 6% phosphoric acid is added to the extractor and the
acid is purged for 2-5 minutes with carbon dioxide-free carrier gas.
5.2.3. The contents of the syringe are injected into the extractor through
a septum.
5.2.4. The syringe is weighed again and the mass of the extracted sample
determined.
5.2.5. The acidified sample is purged with carrier gas. Successive
coulometer readings are integrated at 1 minute intervals until
they differ by less than 0.01%
6. Determination of the Coulometer Blank
The coulometer blank is determined at intervals throughout each
day by allowing the coulometer to titrate a CO2-free air stream. The
blank is taken as the µg C per minute value detected by the coulometer
when a steady-state reading is achieved.
7. Calibration
Although the digital coulometer output is fairly accurate, the
coulometer response per unit carbon may vary with time. In order to
achieve maximum accuracy, it is necessary to calibrate the coulometer
with samples containing known quantities of inorganic carbon. We are
presently using anhydrous sodium carbonate standards. The dried (270°C
for 3 hours) reagent is carefully weighed on a microbalance to the
nearest 0.1 µg and introduced into a degassed acidified sample solution
in a combusted aluminum boat through a port in the side of the extractor.
Recoveries are generally slightly less than 100%.
8. Data Reduction and Calculations
In order to compute the absolute concentration of DIC in a water
sample, the integrated reading given at the titration endpoint must
be corrected for both the coulometer blank and the recovery of NaCO3
standards. These corrections are made by multiplying the blank µg C
min-1 by the time taken to reach the endpoint and subtracting this
value from the integrated reading. This value is then corrected for
the recovery of standards by dividing by the average percentage
recovery of known standards run on the day of analysis.
9. Precision and Accuracy
Three replicate samples from a single Niskin bottle generally yield
a coefficient of variation of less than 0.1% (+2 µmol/kg). The
precision of our replicate determinations of liquid standards averages
approximately 0.5 µmol/kg over periods of months. The accuracy of our
determinations is within 1 µmol/kg as determined by the routine analysis
of liquid standards provided by Dr. Andrew Dickson of Scripps Institution
of Oceanography under the U.S. Department of Energy Global Change
Research Program.
10. Quality Control
As a safeguard on the quality of our results, we maintain a set
of secondary seawater standards. These are made from a large surface
seawater sample, which is preserved with saturated HgCl2 and subdivided
into 300 ml bottles. These are sealed as described above and stored
in the dark. These secondary standards are analyzed relative to a
primary seawater standard provided by Dr. Andrew Dickson.
11. Equipment/Supplies
5011 Coulometer (UIC Inc.) and modified glassware
300 ml ground glass stoppered reagent bottles
kimwipes and applicator sticks
large plastic pipette
Apiezon grease
Cahn microbalance
volumetric flasks
analytical balance / pan balance
muffle furnace
carbon dioxide-free carrier gas
data acquisition system
water bath
12. Reagents
distilled deionized water
high purity sodium carbonate
potassium iodide
coulometer cathode solution (UIC Inc.)
coulometer anode solution (UIC Inc.)
orthophosphoric acid
13. References
Johnson, K. M., A. E. King and J. McN. Sieburth. 1985.
Coulometric TCO2 analysis for marine studies; An introduction.
Marine Chemistry, 16, 61-82.
Johnson, K. M., J. McN. Sieburth, P. J. LeB. Williams and L.
Brandstrom. 1987. Coulometric total carbon dioxide analysis
for marine studies: Automation and calibration. Marine
Chemistry, 21, 117-133.
Lindberg, A. O. 1978. Automatic coulometric titration with
photometric end-point detection. Part II. Coulometric
determination of nanomole amounts of carbon dioxide by non-
aqueous titration. Analytica Chimica Acta, 96, 327-333.
Robinson, C. and P. J. LeB. Williams. 1991. Development
and assessment of an analytical system for the accurate and
continual measurement of total inorganic carbon. Marine
Chemistry, 34, 157-175.
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