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:
25 mL sulfuric acid solution;
2.5 mL potassium antimonyl tartrate solution;
7.5 mL ammonium molybdate solution;
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.