There are several things to note with the Phosphate test strip. First, the strip is actually detecting orthophosphates. Second, the pH range should generally be pH 6-9. Lastly, the test strip is designed for a defined testing range (0-100ppm Phosphate P). Anything above this range should not be used without diluting the sample first. Color response in concentrations above 100 ppm goes sharply down and somewhere around 500 ppm, it stays completely white without any color development.

Generally, no color reaction does not equate to no analyte present at all. Try diluting the sample first and then test again. The negative reaction might be an overloading of the reaction system on the test pad.

When Na3PO4 is used, the dosing should not go above 100 ppm (as P).