Simply stated, Litmus Papers cannot be used to determine the pH of clean water or exceptionally pure waters. Chemicals on the litmus strip actually alter the pH as the water is being tested. The TDS (total dissolved solids) of a tested liquid must be above 150 to 200 ppm (parts per million) to initiate a sufficient reaction on pH paper.
Litmus paper works reasonably well on bodily fluids such as urine, saliva or blood and on foods such as milk because they are very high in TDS (often above 5,000 ppm). The paper works poorly on many bottled drinking waters, even some municipal drinking waters, because of their low levels of dissolved solids.