Developing an optical sensor for the visualisation of pH gradients in marine sediments

Many important biogeochemical processes that occur in marine sediments are intimately linked to pH dependent processes, but surprisingly little is known about the details of the pH profiles generated. This is mainly due to the difficulty of obtaining samples and measuring representative profiles. Such profiles have previously been determined using glass-membrane micro-electrodes, but this is slow and electrodes easily break in harsh sediments (insertion is “blind”). Profiles have also been measured using fluorescent dye-based sensors, but their handling needs special light sources and filters and sensor membrane preparation is quite complex, which has restricted its use to laboratory studies. A simple optical pH sensor has been developed for this application by immobilising bromothymol blue in a PVC membrane. This device responds in a pH range suitable for marine sediments. The sensor is simple to use, the dye does not leach and it gives a visible colour readout so when photographed, the pictures can be used to extract Red Green Blue values and converting them to colour index values representing the sensor colour. This can be related to pH via a colour index v pH calibration using standard buffers or seawater by bubbling CO2. The sensor responds reasonably quickly and can be used for multiple cycles. Sensor strips have been attached to transparent plastic probes with a Nytran diffusion membrane added to provide a white background to stop interference from the colour of sediments. Preliminary tests have been carried out by inserting these probes in sediment cores (in the lab and aboard the RVs Endeavour, Discovery and Prince Madog), and also in the field at Stiffkey salt marsh, in order to measure vertical pH profiles. This has provided promising results. Seasonal profiles at Stiffkey have been taken, recording pH values ranging from 5-8. The sensors have been adapted for use with a Sediment Profile Imagery (SPI) system, to map the pH profile of marine sediments rapidly in-situ using various survey approaches.

Name dc:type.qualificationname

phd

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of East Anglia

Year dc:date.issued

2015

Author dc:creator

• Tariq, Sonia

Language dc:language

en