Comment: No, lemon juice is acidic, not alkaline/basic

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In reply to comment: I don't think so. (see in situ)

No, lemon juice is acidic, not alkaline/basic

Get some pH papers and do a simple test.

Below neutral = acid/acidic - like citrus juice
Above neutral = base/alkaline - like mineral water

A lot of people mix them up.

Maybe the rind is alkaline, but I've never consumed much of the rind in juicing. A little bit would remain if I was being lazy.

Whomever this information is coming from is misled and should be corrected.

Don't trust the internets, trust yourself, go to a pool store and get some pH papers and prove them wrong.

I don't know about watermelon though, probably near neutral.