What exactly is the function of Epsom salts in recipes for "old fasioned lemonade"?
Epsom salt is a compound containing magnesium and sulphur and originally came from boiling down water that came from mineral springs close to the English town of Epsom in Surrey. Epsom salts are listed as an ingredient in some very old lemonade recipes but there does not appear to be any particular culinary reason for the salts to be included in the lemonade.
However, Epsom salts have a laxative effect and we suspect that they were originlly added to lemonade as an old-fashioned "tonic", with the lemonade base hiding the taste of the salts and making them more palatable.
For a delicious lemonade without any additives we would suggest trying Nigella's recipe Cloudy Lemonade for a Sunny Day in Nigella Express (p278). Also we would not recommend ingesting Epson salts without first consulting your doctor.