Comparison of effectiveness of different ashing auxiliaries for determination of phosphorus in natural waters, aquatic organisms and sediments by ignition method

An improved method for the determination of phosphorus in natural waters, aquatic organisms and sediments by ignition method is proposed. The recoveries of phosphorus (P) from selected inorganic and organic P-containing compound standards after ignition with different auxiliaries, such as MgSO4, Mg(NO3)2, MgCl2, Mg(Ac)2 and CaCl2, were compared. We found that the phosphorus from most compound standards could not be completely recovered when these compounds were ignited (450–500°C) with the MgSO4 as auxiliary and the baked residue was extracted with 0.2 mol l1 HCl for 30 min at 80°C or at room temperature. P recoveries, for example, were poor, less than 85%, if pyrophosphate and metaphosphate were ignited with the addition of MgSO4 prior to the extraction of the baked residue with 0.2 mol l1 HCl at 80°C for 30 min. In contrast, MgCl2, Mg(Ac)2 and CaCl2, as well as Mg(NO3)2, could all yield complete P recoveries at routine ashing temperatures (450–500°C). The results demonstrate that MgCl2 is a more effective auxiliary agent for the determination of phosphorus in natural waters, aquatic organisms and sediments by ignition method than MgSO4 which is commonly used.