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

Standard Curves v1

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: