Determination of phosphorus in turbid waters using alkaline potassium peroxodisulphate digestion

The evaluation of the use of alkaline peroxodisulphate digestion with autoclaving or microwave heating for the determination of total phosphorus in turbid lake and river water is described. Procedures were evaluated by the analysis of suspensions (20, 50 and 100 μg P/l) of two standard reference materials, National Institute of Environmental Science, Japan No. 3 Chlorella and No. 2 Pond Sediment. Suspensions were prepared by adding these materials to distilled deionized water (pH 6) and homogenisation using shaking, sonification and stirring. Best recoveries of phosphorus were found when the final solution was 0.045 M in potassium peroxodisulphate and 0.04 M in sodium hydroxide and solutions digested in an autoclave at 120 °C for 60 min. or heated in a microwave oven at 450 W for 10 min. Complete recoveries of phosphorus (99–103%) from 20, 50 and 100 μg P/l Chlorella suspensions were obtained using both autoclave and microwave heating. For the Pond Sediment complete recoveries of phosphorus (99–107%) from the 20 and 50 μg P/l suspensions were obtained using both heating methods. Higher recoveries from the 100 μg P/l Pond Sediment suspensions were obtained using microwave heating (96 ± 1%) than autoclaving (88 ± 5%). Recoveries of phosphorus compounds (phosphates, and phosphonates) added to distilled deionized water and turbid lake water were near quantitative (91–117%) for both digestion procedures. Further analysis of Pond Sediment suspensions showed that complete recovery of phosphorus (98 ± 1%) from 60 μg/l suspensions was achieved with incomplete recoveries (92.3 ± 0.7%, 91 ± 2% and 91 ± 1%) from 70 μg P/l, 80 μg P/l and 90 μg P/l suspensions respectively. Comparison with the APHA-AWWA WPCF, nitric-sulphuric acid digestion method showed no difference in phosphorus measurements for the microwave procedure but that the autoclave procedure gave significantly lower recoveries of phosphorus (p < 0.01), however, differences were only 2–8%.