Abstract
Heating is frequently applied to enhance the reaction rate for the determination of phosphate but it may cause unexpected errors. Experiments show that the spectrum of the blue color produced by the Murphy and Riley method is subject to change at different temperatures. The variation of the molar extinction coefficient at 880 nm was found to be -48 M-1cm-1°C-1, or a relative trend of -0.22 % °C-1 w.r.t. to the absorbance at 30°C. When the color formation reaction took place at >35°C, the absorbance started to climb. At temperatures >60°C, the results were erratic and the change becomes irreversible. These findings suggest that the measurement should be made at room temperature and any heating process above 35°C should be avoided. A thermostated cuvette holder set at 30°C is highly recommended for both manual and flow type operation.
Highlights
The molybdate-ascorbic acid method developed by Murphy and Riley [1] for the determination of phosphate in natural waters has been widely adopted in many scientific disciplines [2,3,4]
When the manual procedure is converted to an automated operation, heating is frequently applied to enhance the rate of color formation reaction for a fast throughput
The negative aspect of heating on the Murphy and Riley method for the determination of phosphate has long been overlooked by scientists but evidences found in this study reveals that the uncertainty induced by the temperature difference should not be ignored
Summary
The molybdate-ascorbic acid method developed by Murphy and Riley [1] for the determination of phosphate in natural waters has been widely adopted in many scientific disciplines [2,3,4]. When the manual procedure is converted to an automated operation, heating is frequently applied to enhance the rate of color formation reaction for a fast throughput. Benson et al [10] have reported a phenomenon where the peak signals decreased significantly when the bath temperature was raised up to 60°C; Drummond and Maher [11] and Zhang et al [12] have both observed noticeable silicate interferences at elevated temperatures. They have suggested that the determination should be preferably performed at room temperature
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