The use of on-line UV photoreduction in the flow analysis determination of dissolved reactive phosphate in natural waters

• A flow analysis system determines trace dissolved reactive phosphate (DRP).
• It is based on UV photoreduction of phosphomolybdate in the presence of ethanol.
• The use of unstable reducing agents (e.g. SnCl2 or ascorbic acid) is not required.
• The system can measure DRP in the concentration range 5–5000 μg L−1 P.
• The flow system was successfully applied to a number of natural water samples.

A highly sensitive flow analysis manifold for rapid determination of dissolved reactive phosphate was developed which uses ethanol and UV light to reduce phosphomolybdic acid, instead of the reactive and short-lived chemical reductants typically employed in molybdenum blue chemistry. This reaction is impractical to perform reproducibly in batch mode, yet is very simple to handle in a flow analysis system and uses a single, very long-lived reagent solution. Interference from common inorganic anions and organic phosphorus species was minimal, and good spike recoveries for a range of sample matrices were obtained. The proposed flow analysis system is characterised by a limit of detection of 1.3 μg L−1 P, linear range of 5–1000 μg L−1 P, dynamic range of 5–5000 μg L−1 P, repeatability of 0.8% (1000 μg L−1 P, n=10) and 5.6% (10 μg L−1 P, n=10), and sample throughput of 57 h−1. It is expected that this method will improve the feasibility of autonomous long-term environmental monitoring of dissolved reactive phosphate using inexpensive apparatus.

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