In situ determination of pH and oxygen

The initial motivation to development of in situ determinations as opposed to discrete sampling and subsequent analysis has been the difficulty to obtain samples from sites such as sediments, biota or small scale water layers and interfaces. 

Today the much higher spatial and temporal resolution of measurements is a powerful incentive for in situ determinations rather than time-consuming sampling, sample transfer and batch determinations in the laboratory. 

While laboratory determinations are performed under constant or controlled conditions, in situ determinations have to eliminate or compensate for wide ranges of environmental changes. The rule of matching calibration and measuring conditions cannot be obeyed during in situ determinations which, consequently, very often suffer from reduced accuracies. The choice of either laboratory or in situ determination should be carefully considered. The effort required to achieve acceptable accuracies of in situ determinations may well outweigh the benefits of their seemingly fast and simple application. 

In situ determinations of pH and oxygen in seawater involve two different types of registration registration of temporal variations at fixed locations, e.g., monitoring applications, and registration of spatial variations, i.e., by vertically and/or horizontally moving probes. 

For the stationary registration, long-term stability of the sensor or analytical unit is the dominant criterion, while moving sensors have to be optimized for minimum response times. For example, the fast response time of membrane covered amperometric oxygen sensors and long-term stability are mutually exclusive. The sensors can only be optimized for one or the other criterion. 

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