Molybdenum blue

The total orthophosphate (molybdenum blue) method is a lab test used to measure the amount of orthophosphate in a water sample. Orthophosphate is a form of phosphorus that’s directly available to plants and algae.

To do the test, a special chemical called molybdate is added to the water sample. It reacts with the orthophosphate to form a blue-coloured compound. Ascorbic acid is often added to help develop the blue colour. The darker the blue, the more orthophosphate there is. A device called a spectrophotometer measures the exact shade of blue and tells scientists how much orthophosphate is present.