Dataset: tco2

 NOTE:  The depth, temp and salinity contained in this data set (as 
 submitted by the PI) were extracted from the preliminary version of the
 bottle data and are reported here to show what was used in calculating to
 gravametric units.  The final version of bottle data are reported by
 L. Codispoti.  (JGOFS Data Management Office)


----------------------


This file briefly discribes the methods that were used to make the carbon
dioxide system measurements in the Arabian Sea on the JGOFS Process #2 cruise
by the University of Miami, CO2 group.  More detailed sampling and analytical
methods descriptions can be found in the references given below.

Discrete Total Carbon Dioxide measurements:

The total inorganic carbon dioxide (TCO2) in a volume of seawater was
determined coulometrically after acidification with 8.5 % H3PO4.  The system
has been used in earlier studies by Goyet and Hacker (1992) and it is similar 
to the SOMMA (Single-Operator Multiparameter Metabolic Analyzer), contructed by
Johnson et al. (1987). The system is composed of five units: a DICE (Dissolved
Inorganic Carbon Extractor) which controlls the movement and delivery of acid
and sample to the stripper, a coulometer (UIC Inc., model 5011), a CO2 free N2
generator (Balston, model 74-5021), a personal computer and a printer. Nitrogen
gas from the Balston generator is split into two streams, one for pneumatic
controll of sample and acid movements, and the other for the carrier gas for
the CO2 stripped from the seawater sample. The sample is acidified (with 1 to 
1.5 mls of 8.5 % phosphoric acid) and the carbon dioxide is extracted with N2 
and introduced into the coulometric cell where the CO2 reacts quantitatively 
with ethanolamine producing hydroxyethylcarbamic acid.  Hydroxyethylcarbamic 
acid is titrated by electrochemically generated hydroxide ion.  The number of 
electrons utilized in generating the titrant is proportional to the amount of 
inorganic carbon in the original sample.
 
The life time of the coulometer cell is about twelve hours, after which the 
cell solutions need to be changed.  In addition to the cell solutions, the 
water trap was changed (Gelman, 0.2 (m PTFE ACRODISC). Changing the cell 
solution requires about 30 minutes.  After which about three hours are need 
for the system to stabalize, determine new blank values, and confirm the 
calibration with analyses of CRM's.   If the CRM value was not reproduced with
in 2 (mol/kg TCO2, the system was recalibrated with Na2CO3 standards.  A single
measurement takes about 25 minutes, and a 24 bottle station cast can be 
completed in eight hours. 

Calibration of the system:

The electrical calibration of the coulometer is not perfectly accurate and the
current efficiency of the electrode processes occurring in the coulometer cell 
has been shown to vary from 100 %.  Consequently, the system was calibrated 
using aqueous solutions of sodium carbonate which were treated as if they were 
samples. The consistency of the calibration was checked for each cell solution 
using  the Certified Reference Material (Dr. Andrew Dickson, Marine Physical 
Laboratory, La Jolla, California).

Alkalinity Titration system:

The alkalinity titration system is similar to the one used in our earlier 
studies (Millero et al., 1993).  The titration systems used to determined TA 
consisted of a titrator (Metrohm, model 665 Dosimat) and a pH meter (Orion, 
model 720A) that is controlled by a personal computer.  The temperature of 
both the acid titrant in a water jacketed burette and the seawater sample in a
water jacketed cell were controlled to a constant temperature of 25  0.1C with 
a constant temperature bath (Neslab, model RTE 221).  The plexiglass water 
jacketed cells used during the cruise were similar to that used by Bradshaw et 
al. (1988) except a larger volume (about 200 cm3) was used to increase the 
precision.  Each cell had a fill and drain valve which increased the 
reproducibility of the volume of sample contained in the cell.  A LabWindows-C 
program was used to run the titration, record the volume of the added acid and 
the emf of the electrodes using RS-232 communication interfaces.  Seawater 
samples were titrated  by adding HCl to exceed  the carbonic acid end point.  
During a typical titration the emf readings are recorded after the readings 
become stable ( 0.05 mV),and then a volume of acid is added to change the 
voltage to a pre-assigned increment (13 mV).  In contrast to the delivery of a 
fixed volume increment of acid, this method gives an even distribution of data 
points in the range of  rapid increase in the emf near the endpoint.  A full 
titration (25 points) takes about 20 minutes.  Using two systems a 24 bottle 
station cast can be completed in 4 hours. 

Selected References:

DOE (1994) Handbook of methods for the analysis of the various parameters of 
   the carbon dioxide system in sea water, version 2, A. G. Dickson & C. Goyet,
   eds.	
Goyet, C. and Hacker, S.D. (1992)  Procedure for calibration of a coulometric
   system used for total inorganic carbon measurements. Marine Chemistry 38,
   37-51
Millero, F. J., Zhang, J., Lee, K., and Campbell, D. M. (1993) Titration
   alkalinity of seawater.  Mar. Chem., 44:153-165.