Jornal de Química Analítica Ambiental

Effect of Heating on the Color Formation Reaction in the Murphy and Riley Method for the Determination of Phosphate in Natural Waters

Abstract

Su-Cheng Pai, Tzu-Yuan Wang, Tien-Hsi Fang and Kuo-Tung Jiann

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.