While this whole "eat this to alkalize your body, eat that to acidify
your body" is a load of (politely) pseudoscience -- your body is very
good at keeping its pH just where it should be, since very minor variances
result is serious disorder -- it's true that what you eat affects the pH
of your *urine*.  Unintuitively, citrus fruits result in a more alkaline
urine.  This is mainly of interest to people who are prone to certain
types of stones in the urinary tract.

So, yes, drinking a little lemon juice may have a little effect on the
pH of one of your body fluids, your urine.  Whether this has any more
than a placebo effect on you is most unlikely.  If you really could
affect your blood pH this way, humans would have gone extinct millions
of years ago -- or rather, vertebrates would have long before, since
all use the same basic mechanisms.