Determination of phosphorus in turbid freshwaters using alkaline peroxodisulphate digestion

Methods for determining phosphorus in turbid lake and river water using heating with an autoclave or a microwave and employing alkaline peroxodisulphate digestion have been investigated. Suspensions (up to 100 ugP/L) of two standard reference materials (NIES No. 3 Chlorella and NEES No. 2 Pond Sediment) were used to optimised procedures. Quantitative recoveries of phosphorus were achieved when the final solution to be digested contained 0.045 M potassium peroxodisulphate and 0.04 M sodium hydroxide and solutions were autoclaved at 120C for 60 min. or microwaved at 450 Watts for 5-10 min. Complete recoveries of phosphorus (99- 103%) from 20 ugP/U 50 ugP/L and 100 ugP/L Chlorella suspensions were obtained using autoclave and microwave heating. For the Pond Sediment suspensions complete recoveries of phosphorus (99-104%) from the 20 ugP/L and 50 ugP/L were obtained using both heating methods. Higher recoveries from the 100 u.gP/L Pond Sediment suspensions were obtained using microwave heating (961%) than autoclaving (885%). Further analysis of Pond Sediment suspensions using the autoclave heating showed that complete recovery of phosphorus (98l%) from 60 ngP/L suspensions was achieved with incomplete recoveries (92.30.7%, 91?2% and 91?1%) from 70 ugP/L, 80 ugP/L and 90 ug P/L suspensions respectively. Recoveries of phosphorus compounds (orthophosphate and phosphonates) added to distilled water and turbid lake water were near quantitative (91-117%) for both digestion methods. A range of turbid lake and river water (TP = 57-106 ugP/L; Turbidity = 16-200 NTU) were analysed for total phosphorus (TP) using the optimised alkaline peroxodisulphate digestion procedures and the APHA AWWA WPCF, sulphuric acid - nitric acid digestion procedure. No difference in total phosphorus measurements were found between the microwave digestion procedure and the APHA AWWA WPCF, nitric acid - sulphuric acid procedure. The autoclave procedure gave significantly lower recoveries of phosphorus (p<0.01), however, differences were only 2-8%. The effect of freezing (-20?C) water samples without or with the addition of 1% hydrochloric acid before determination of total phosphorus (TP) and total dissolved phosphorus (TDP) was also investigated. No significant change in total phosphorus occurred when samples were stored frozen without the addition of 1% hydrochloric acid in high and low density polyethylene bottles for up to 20 weeks and 2 weeks respectively after collection. Significant changes were found in total dissolved phosphorus when samples were stored frozen without the addition of 1% hydrochloric acid in high and low density polyethylene bottles after 1 day and 2 weeks respectively.