*3.8.1.1.1 Molybdate blue method*

This colorimetric method involves reacting phosphate ions with molybdate reagents to form a blue complex. The intensity of the blue color is proportional to the concentration of DRP and can be measured spectrophotometrically.

*Major procedure involved include:*

	- *Collect a representative water sample in a clean and acid-washed container to prevent contamination.*
	- *If necessary, filter the sample to remove any particles that could interfere with the colorimetric reaction.*
	- *Prepare the molybdate reagent by mixing ammonium molybdate and a reducing agent (ascorbic acid) in an acidic solution.*
	- *The reagents are usually available as a kit or can be prepared according to standard protocols.*
	- *In a cuvette or test tube, add a measured volume of the sample and an appropriate volume of the molybdate reagent.*
	- *Using a spectrophotometer, measure the absorbance of the blue-colored solution at a specific wavelength, typically around 880 nm.*
	- *The absorbance value is proportional to the concentration of DRP in the sample.*
	- *Create a calibration curve using known concentrations of phosphate standards treated with the same reagents and procedure.*
	- *Use the calibration curve to determine the concentration of DRP in the sample based on its absorbance value.*
	- *Include blank samples (water without phosphate) and replicate measurements to ensure accuracy.*
	- *Regularly calibrate the spectrophotometer using standard solutions.*
	- *Express the results in units of phosphorus concentration (e.g., mg/L or <sup>μ</sup>g/L).*

Phosphate reacts with ascorbic acid in the presence of ammonium molybdate to form a blue color. The color intensity is measured using a spectrophotometer. *Major procedure involved include:*

	- *Collect a representative water sample and ensure it is properly preserved and stored to prevent changes in phosphorus concentrations.*
	- *If needed, filter the sample to remove any particulate matter.*
	- *Prepare the molybdate reagent by dissolving ammonium molybdate in an acidic solution.*
	- *Prepare a solution of ascorbic acid in deionized water.*
	- *In a cuvette or test tube, add a measured volume of the sample, the molybdate reagent, and the ascorbic acid solution.*

◦ *Mix the contents thoroughly and allow the reaction to occur for a specified period.*
