Paper towels are not designed to remove the micro-particles that alter your ph, and will not itself alter the ph. (unless the paper towels have been bleached by the manufacturer) Better than nothing...but not by much.
And as you know...any tinting skews color-coded results.

What was your before and after results, and was the liquid honestly clear? If so...what kind of paper towels are you using? (they might be bleaching the water, lol.)

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Rusty Trichome Reviewed by Rusty Trichome on . Any soil growers with a ph pen? Need some help... My question is this: Does the ph of your run-off water change if you run it through a paper towel/coffee filter/kitchen towel? I ask this because as soil growers know, the run-off is usually tinted, which can throw off the results if you are using the liquid drops/colored chart ph test. I ran my run-off through a paper towel and it came out clear, after which I did the ph test. But, I wanted to make sure that the ph did not change because of the paper towel/coffee filter, etc... Rating: 5