Ionics Instruments 2004 Page 3-9 DLM 30007-08 Rev. A
the analyzer. Refer to the RAM CARDS, CARD READER, and RAMReader
SOFTWARE chapter for further information.
Data Outputs
The analyzer has four output ports: an RS-232 output, two analog outputs (4-
20 mA, and 0-1V or 0-10V), and a parallel printer port. Data can be
downloaded from the analyzer via the RS-232 port to a PC using
communication software.
CO
2
Sensor Measurement Cycle
The concentrations of CO
2
in the TC and IC sample streams are
independently measured during a 6-minute cycle. At the beginning of each
measurement cycle, a fresh pulse of DI water is introduced into the membrane
module, and then the DI water flow is stopped when the solenoid valve closes.
The acidified sample stream flows continuously through the sample side of the
membrane module. At a pH < 2, bicarbonate and carbonate ions are
converted to carbonic acid (H
2
CO
3
), which readily dissociates to aqueous
carbon dioxide (CO
2
(aq)
) and water. Carbon dioxide in the sample stream
diffuses across the membrane into the deionized water chamber and ionizes
to form bicarbonate (HCO
3
-
) and hydronium ions (H
3
O
+
). After 5 minutes, the
concentration of CO
2
(aq)
in the deionized water approaches equilibrium with
the concentration of CO
2
(aq)
in the flowing sample stream.
After equilibrium has been established, the solenoid valve opens to introduce
fresh DI water into the chamber and sweep the CO
2
(aq)
, H
3
O
+
, and HCO
3
-
into the Conductivity and Temperature Measurement Cell. The solenoid valve
then closes and the next measurement cycle commences.
The increase in conductivity of the deionized water is proportional to the
concentration of CO
2
(aq)
in the deionized water stream, which is proportional
to the concentration of CO
2
(aq)
(and total carbon or inorganic carbon) in the
sample stream. From the measurement of total carbon and total inorganic
carbon concentrations, the TOC is computed as the difference.
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