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.
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