Phosphate
Introduction
Phosphate is measured colorimetrically using the common ascorbic acid method (APHA Standard Method 4500-P E).
Reagents
API phosphate test kit
1,000 ppm PO4 (330 ppm P) Phosphate standard, Labchem Cat # LC18570-1
Distilled water
Equipment
List of vendors and prices: https://sites.google.com/iorodeo.com/colorimeter/equipment
Colorimeter with RGB board (use red for measurement)
Cuvettes
1 x 100 mL volumetric flask
9 x 10 mL capped tubes
Pippete pump
5 mL pipettes
Prepare Calibration Solutions
Prepare a 37.5 ppm PO4 working stock: 3.75 mL of the 1,000 ppm standard in 100 mL distilled water in volumetric flask. Fill to the line with distilled water.
Dilute the 37.5 ppm working stock as outlined in the Table below
Method
IMPORTANT: This is a time-sensitive assay. Before starting make sure you read through the instructions below. You will need to use a timer to measure your sample 5-10 mins after adding Bottle # 2. After approximately 10 mins the solution becomes cloudy and you will not get an accurate reading.
Fill an empty sample bottle to the 5 mL line with your water sample.
Add 6 drops of Bottle # 1 to the sample, cap and invert several times to mix
Add 6 drops of Bottle # 2 and start timer.
Note the time since adding Bottle # 2. When it has been between 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.
In the colorimeter plotting program, select the Red (625 nm) LED on the standard RGB board and ppm as units. Calibrate the colorimeter with the 0 ppm phosphate solution and measure absorbance of all of the solutions in duplicate.
Sample calibration data
To view/use: Download the file and open in the colorimeter plotting software as described here.
In addition to the API test kit, we have also tested three other phosphate tests:
Hach PhosVer3 test: Phosphate-Hach-PhosVer3 . Measures low range 0 - 5 ppm nitrate.
Ascorbic acid test: Phosphate-AscorbicAcid. Measures 0 - 30 ppm phosphate. Uses off-the-shelf reagents (not a commercial kit).
Vanadate-molybdate test: Phosphate-Vanadate-Molybdate. Uses off-the-shelf reagents (not a commercial kit). Measures high range 0 - 90 ppm phosphate.