Litmus paper works reasonably well for an order of magnitude accuracy and the colors that result from use match very well with the color indicator label in the plastic holder. You have to protect it very strongly from moisture, otherwise your readings will be distorted. I like to photograph used litmus paper in front of a colored indicator label for the second data entry (the first is the actual pH recorded on the notepad); However, the plastic case is so reflective that if you use it in a place dark enough for your camera to fire the flash, the flash will bounce off the case and blur the color image if you don't pay attention to the angle of incidence - very, very carefully.
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