Phosphate-AscorbicAcid

Ammonium molybdate and potassium antimonyl tartrate react in acid medium with orthophosphate to form a heteropoly acid - phosphomolybdic acid - that is reduced to intensely colored molybdenum blue by ascorbic acid. This method for measuring phosphate is based on APHA Standard Method 4500-P E

Reagents and Equipment

  • Assembled colorimeter

  • Cuvettes

  • 5 mL screw capped tubes

  • Sulfuric acid, (14% v/v), Labchem, Cat # LC256002

  • Potassium antimonyl tartrate solution, Labchem, Cat # LC187201

  • Ammonium molybdate solution (4%), Labchem, Cat # LC112101

  • Ascorbic acid, Labchem, Cat # LC115309

  • 1mL adjustable micropipette

  • Distilled water. Optional, for diluting water sample

Colorimeter set-up

  • Wavelength: 625 nm. Use Red led (colorimeter RGB board).

  • Phosphate standard data: download a data file or prepare your own phosphate standard data

  • Before measuring samples, calibrate the colorimeter against water or a distilled water sample developed with ascorbic acid reagent as described below

Method

Step 1: Prepare fresh ascorbic acid reagent

  • In a beaker dissolve 0.88 g of ascorbic acid in 50 mL distilled water;

  • In a separate beaker add the reagents in the following order:

    1. 25 mL sulfuric acid solution;

    2. 2.5 mL potassium antimonyl tartrate solution;

    3. 7.5 mL ammonium molybdate solution;

    4. 15 mL ascorbic acid solution (prepared in step 1);

  • Mix the ascorbic acid reagent. The reagent is stable for 4 hours.

Step 2: Sample testing

  • Fill an empty sample bottle to the 5 mL line with your water sample

  • Add 800 µL of test reagent solution, cap and invert several times to mix

  • Note the time since adding the reagent to your water sample. When it has been at least 5-10 minutes, pour some of your sample into a cuvette, place in the colorimeter and click measure.The program will return the phosphate concentration in ppm.