Underway sequential measurements of temperature and the activities of pH, Na, Na plus K, Ca, total divalent cations, Cl,C03 (experimental) were taken for surface water of the central Pacific on the R/V Washington in June 1970. The sensing electrodes were in a flow-through manifold fed by the ship's uncontaminated sea water intake system. The electrode array was connected to a ten channel Digital scanner by which in sequence the output of each electrode-reference pair was sampled by a Wilde-Rodgers electrochemical meter for 10 seconds. The millivolt output of each pair, as shown on a digital voltmeter was photographed with a Super 8 movie camera on single frame mode. The. was triggered through the scanner once for each electrode pair one second before the next electrode pair was switched into the circuit or after a dwell time of 9 seconds. Data contained on each film frame besides the millivolt value are (1) time recorded by a digital clock, (2) electrode identity recorded by a binary light. code, and (3) month, day and year. Daily measurements in IAPO standard sea water plus two serial dilutions were used to define calibration curves for each electrode reference pair.

The equipment required attention only (1) once a day for calibration checks and (2) to change the film cartridge after 4000 measurements were made.

Such a system would be useful for inexpensive long term monitoring programs, for example in pollution studies, of chemical systems amenable to electrode measurements in marine bays and estuaries and the open ocean.


Major obstacles in obtaining sufficient serial environmental chemical data for estuarine and marine waters are (1) the long lag time between data collection and analysis if wet chemical methods are used and (2) the errors inherent in sampling and storage of samples before analysis. These problems are particularity acute on shipboard where there always is a lack of trained technicians and complete analytical facilities. Fortunately, the development of reliable and a wider selection of ion selective electrodes has suggested the feasibility of in situ continuous measurements by electrodes operative in the marine environment. In sea water because of its high ionic strength and the number of different ions in solution, not all electrodes have the ideal ion selective response which they would have in relatively dilute solutions with no interfering ions. 3 At present seven commercial electrodes apparently can be used in sea water: Cl t Na, pH, K, Na + K, Ca, and total divalent cation.

Furthermore commercially available electrochemical meters (in general the pH meter) usually can monitor only one electrode or at best five electrodes by switching without clumsy manual changing of the input terminal of the instrument.

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