Trace Level Total Phosphorus Determination Using Pre-Concentration of A Malachite Green-Phosphomolybdenum Complex.

The Florida Class III water quality criterion for total phosphorus in Everglades will likely be at or near 10 ppb, when it is adopted in 2003, as required by the Everglades Forever Act of 1994. However, current total phosphorus methods may not have the sensitivity to precisely and accurately measure water concentration at this low level. A new method that utilizes solid phase extraction/pre-concentration of a malachite green-phosphomolybdenum complex has been developed for the spectrophotometric determination of total phosphorus in water. The water sample was digested using a persulfate/sulfuric acid digestion (0.8% ammonium persulfate and 0.288 N sulfuric acid at 120°C for 1 hour) in a microprocessor controlled (time and temperature) block digester inside of a clean (Class 10) laminar-flow hood. Phosphorus in 10 ml of the digested water sample was then collected as a malachite green-phosphomolybdenum complex on a cellulose nitrate membrane filter. The filter was subsequently dissolved into 1 ml of methyl cellosolve, and the absorbance was measured at 620 nm. This method was found to have a much higher molar absorptivity, and a lower method detection limit than the standard ascorbic acid method. Near 100% total phosphorus recovery was obtained from a prepared 10 ppb phosphorous reference sample (pure organic phosphates and certified reference sediment). The linear calibration range was from 0.5 to 20 ppb of phosphorous with a linear regression coefficient of close 0.9999 and near zero intercept. The interference from silicate and arsenate was insignificant even when their concentrations in water were near the high end of those found in Everglades' water. The overall results of total phosphorus concentration ranged from 3 to 13 ppb for twenty Everglades' water samples determined using proposed method and using RFA method were very similar.