Abstracts 87

CO2, H2S, As, B and Hg. As the latter three species occur in both the water and steam phases and they can accumulate in soils, geothermal deposits and aquatic sediments to levels far in excess of their dissolved concentrations. Remobilisation of species in such sinks would release an elevated, potentially toxic, flush of the species into the environment emphasise this time-accumulation-remobilisation factor, the term "pollution time-sink" is used to describe these sedimentary elemental reservoirs.

To illustrate the environmental impact of

geothermal waters, the pathways and sinks of arsenic, boron and mercury from New Zealand geothermal systems are examined. All three elements form time-sinks in soils, sediments and geothermal deposits.

R e f e r e n c e

Nicholson, K. 1993. Geothermal Fluids: Chemistry and Exploration Techniques. Springer-Verlag. 263pp.

Determination of boron in environmental

samples by ion-selective electrode: an

evaluation

John Wood and Keith Nicholson

Environment Division School of Applied Sciences, The Robert Gordon University, Aberdeen ABI IHG, Scotland, UK

Boron is an essential trace nutrient but aqueous or soil concentrations in excess of about 4 mg kg q can be toxic to certain plants and crops. As boron is usually present in solution as the neutral species H3BO3, it is not generally removed by standard water treatment processes and with multiple reuse of waters can accumulate in rivers to unacceptable levels. This work is directed to developing a routine method for boron determination that is rapid, cheap and simple, and one that can be adapted to an automated, on-hne monitoring system.

The f luorobora te-se lec t ive electrode was chosen as the method which may best fulfil the above requirements. Although developed in the early 1970s, this technique has received little attention in the literature. Prior to application of the i n s t r u m e n t to e n v i r o n m e n t a l s amp le s and field-based systems, it was therefore necessary to first evaluate the analytical performance of the electrode and it is the results of this evaluation that are reported here.

The time of the electrode to attain equilibrium

emf was evaluated at boron concentrations from 0.05 to 5.0 mg L q. The response time increased with lower boron concentration. Minimum times to equilibrium emf ranged from one minute at 5.0 mg

1 1 L- B, to 5 minutes at 0.05 mg L- B. These represent minimum values and have been incorporated into the analytical procedure being developed.

The electrode calibration departs from linear Nemstian response at 0.35 mg L -I B, but despite curvature in the graph, this remains reproducible to 0.01 mg L 1 B using a 10 minute equilibration time. However, as curvature of the calibration graph increases with decreasing boron concentration, sensitivity of the electrode method is significantly

l reduced at concentrations below 0.03 mg L- B. Reproducibility of the results at 0.1 mg L-I B

and 1.0 mg L -1 B were examined by determinations of at least 20 al iquots of a batch solution. Performance was excellent with reproducibility's within 0.5% at both concentrations using a 10 minute equilibration time.