Influence of phosphate concentration and reaction temperature when using the molybdenum blue method for determination of phosphate in water

The formation rate in water of the blue phosphoantimonylmolybdenum complex declines both with decreasing phosphate concentration (studied range 5–500 μg PO4-P l−1) and decreasing reaction temperature (studied range + 1 to + 21°C). At + 15°C, only solutions containing ≥ 50 μg PO4-P l−1 reached full colour expression within 5 min, and at + 10°C all tested concentrations needed a longer reaction time. At temperatures below + 15°C, not even a reaction time of 10 min is always sufficient for complete colour development. For instance, at + 7°C, maximum absorbance level is reached in 10 min only at concentrations ≥ 250 μg PO4-P l−1, whereas a solution of 5 μg PO4-P l−1 requires about 30 min. Slow formation rate of the coloured complex is a possible, insidious source of under-estimation, particularly when determining low concentrations of phosphate.