A SIMPLE METHOD FOR THE DETECTION AND THE A-POSTERIORI CORRECTION OF THE INTERFERENCE OF SULFIDE ON PHOSPHORUS MEASUREMENTS

We report on the interference that hydrogen sulfide can generate on th e measurement of phosphorus in seawater with the Murphy & Riley method . Free sulfide can be found in a variety of saline systems, from parti ally closed fjords to eutrophic estuaries, river saline wedges, saline lagoons, etc. in concentrations ranging from 0.02 up to 10 mM and, th us, may be present in water samples without the analysts suspicion. We tested the effects of sulfide concentrations ranging from 0.005 up to 0.7 mM on the measurement of phosphorus concentrations from 10 mu gP to 1.5 mgP-PO4 l(-1). We characterized spectrophotometrically, the pro duct and the dynamics of the interference and we provide a simple meth od to detect the interference ''a posteriori'' by computing the ratio between the absorbances at 350 and 690 nm. If sulfide has been measure d in parallel, the absorbances, even those that have had high interfer ence, can be corrected to values without the interference with the equ ations that we provide. We propose the routine use of this method when ever dealing with samples that could have some sulfide in solution.