OXYGEN AND CARBON DIOXIDE INSTRUMENTATION 
by JOHN W. KANWISHER, Research Associate 
Woods Hole Oceanographic Institution 
Woods Hole, Massachusetts 
The sum of the biological processes going on 
in the oceans is frequently considered in terms 
of the opposing processes of photosynthesis and 
respiration. In most cases we can consider these 
phenomena as reciprocally exchanging equivalent 
amounts of oxygen and carbon dioxide (0, and cO,), 
with the environment. For every CO, molecule 
removed from solution in sea water au ine the 
ph tosynthetic production of carbohydrate an 0 
molecule is released. An opposite exchange oc~ 
curs when this carbohydrate is later oxidized in 
the respiration of some plant or animal. We can 
represent this schematically as: 
LIGHT PHOTOSYNTHESIS 
0 SUGAR 
. , eee. 
RESPIRATION 
A quantitative knowledge of this gas exchange in 
a water mass is basic to any estimation of the 
overall biology taking place. The resulting vari- 
ation of these gases in the water is also 
frequently of hydrographic interest because of the 
way biological activity types a water mass. In 
fact, 0, content probably follows temperature and 
salinit? as the most measured hydrographic vari- 
able. 
One would of course like to have as complete 
a picture as possible of the distribution of any 
variable being studied. With 0, we must current- 
ly be satisfied with a mencedety few points in 
the vastness of the oceans. Most of these points 
have been determined only once or twice, so our 
knowledge of time variations is equally hazy. 
In addition, the only method currently in use 
involves a chemical titration and is surrounded 
by considerable uncertainty. Direct measure- 
ments of CO, are nearly nonexistent. Some new 
techniques Which allow continuous recording of the 
0, and CO, dissolved in sea or fresh water will 
be described, 0, has been determined polaro- 
graphically and C has been monitored by infra- 
red absorption of a gas phase which has been 
equilibrated with the water. The methods are 
applicable to both laboratory and field studies. 
Some of both will be covered. 
I. OXYGEN ELECTRODE RECORDING 
Recent references describe a polarographic, 2 
electrode suitable for recording dissolved 0 a 
It consists of a platinum electrode separated from 
the medium being measured by a layer of polyethy- 
lene or teflon, both of which are markedly perme- 
able to molecular 0,. As this gas diffuses 
through the surface, it reacts immediately at the 
platinum surface to form OH” ions. The OH ions 
diffuse through the film of electrolyte behind the 
membrane and react at the Ag surface to form Ag,0, 
which appear as a dark coating. Thus the eventual 
fate of the dissolved oxygen is to appear as Ag,0. 
Although the Ag is actually being consumed, in 
practice it will last semi-indefinitely. The cur- 
rent passing through the electrode is proportional 
to the rate of diffusion, and this in turn depends 
on the external tension (partial pressure). We thus 
have in essense a device for measuring the quantity 
of dissolved 0, since it varies directly with the 
tension (aeari ts law). 
The electrode assembly is shown in Figure 1A. 
A Ag-Ag.0 electrode is concentric around the center 
platinum electrode. A plastic film is stretched 
over this and traps a small amount of alkali behind 
it. The platinum is kept at -.7 volts. When there 
is no 0, the electrode current is very close to 
ZEX0. tt increases linearly with increasing 0 
tension in the external medium. A more pountece 
treatment of the electrode occurs in the original 
references cited. 
It should be kept in mind that the electrode is 
a tension-measuring device. It will read the same 
in fresh or salt water if they are equilibrated 
with the same gas mixture, although the 3-1/2 per 
cent salt content reduces the gas solubility by 
25 per cent. Thus its reading must be referred to 
the solubility of the liquid being measured. The 
extremes of salinity in the open sea, however, 
cause less than a 2 per cent error. It is worth 
noting that the electrode will work in any liquid 
which does not attack the plastic membrane, even 
gasoline. 
Such an electrode has been operated for as long 
as 6 weeks with less than 25% change in current. 
Since no abrupt changes occur, itis sufficient to 
calibrate only once per day to provide values good 
to a few per cent. If there is any doubt about the 
condition of the electrode it can be quickly remade 
with new plastic and KOH. Operation for a long 
Superior numbers refer to similarly numbered references at the end of this paper. 
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