There are some interferences to consider:

  • Arsenate and silicates above 20 ppm will cause a false reaction. These will form a yellow color.
  • Fluoride, chlorides, and nitrites above 50 ppm will depress the phosphate reaction on the strips.
  • Heavy metals will depress the result.
  • Large quantities of oxidizing compounds will also depress the result.
  • Lastly, pH and temperature will affect the results. The pH of the sample should be in the range of 6-9. For temperature, the best results are obtained in the range of 18-30 C. Above or below this the reaction on the strip can be depressed.