Question

What is the exact procedure for MOLYBDENUM BLUE METHOD using Spectrophotometer to determin phosphorus concenrration.. Pleas provide with details how to make standard and blank solutions.

Answer 1

The principle of this method involves the formation of molybdophosphoric acid, which is reduced to the intensely colored complex, molybdenum blue. This analytical method is extremely sensitive and is reliable down to concentrations of 0.1mg of phosphorus per liter of water.

Apparatus:

The Spectronic 20 will be employed in the measurement of color intensity of the blue solutions. A wavelength of 650nm will be used for the analysis.

Reagents:

a. Ammonium molybdate reagent

b. Stannous chloride reagent

c. 20mg/L stock phosphate solution

Procedure:

Note: Glassware should be washed thoroughly with hot water followed by rinsing with distilled water. Do not use phosphate containing detergents to clean equipment for this experiment.

Preparation of Standard Solutions: Prepare a 5 point calibration curve with concentrations ranging from 0 – 5mg/L Phosphate (including blank) from the provided stock standard solution.

The standard solutions and the blank will now be treated according to the following “color development” procedure. After measuring the absorbances of these solutions , make a plot of absorbance versus concentration.

Color Development in Sample

Follow this procedure for your standard solutions, your blank, and your water samples.

Place in an Erlenmeyer flask 25mL (measured with a volumetric pipet) of the water sample to be analyzed. Put 1.0mL (with a pipet) of ammonium molybdate solution into a flask and swirl to mix. Before proceeding make sure you are familiar with the use of the Spec 20. Next, to the flask add two drops of stannous chloride solution and mix by swirling. If phosphate is present, a blue color will develop in a maximum of five minutes.

Note: The time period is somewhat critical. Measurements should be taken anywhere from 5 to 15 minutes after addition of stannous chloride.

While you are waiting for the blue color to develop, set the wavelength to 650nm. Use the blank to set the zero absorbance. Using the 650nm wavelength, measure the absorbance of the blue sample.

Let me know if any doubts.