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Letter from abroad with a lot of acidity (3) p.i.

I have little doubt that the answer is phi, which is a Greek letter (from abroad) and ph is a measure of acidity, but I cannot quite string the whole thing together. Any help much appreciated. Roger

PH 1 is the most acidic on the PH scale

pH1 is quite acidic, but not the most possible. pH0 is quite common, and negative pHs are possible. it's not a scale as such - it's -log hydrogen ion molarity