Hmm, I happened to look at the authors web site and saw he featured these salts issues in a recent presentation at the Belgian Bar Show: http://www.artofdrink.com/soda-fount...ountain-to-bar
... interesting to flip thru his brief slides there. He shows how essential mineral salts were to the early history of soda and is promoting return to that with his "acid phosphate" formula instead of today's ubiquitous citric acid, as a mineraly and less acid tart agent.
Not that I am promoting his book... it is a quite unpolished self-published thing with grammar and spelling mistakes. But great meaty content, although weirdly organized. I find it more helpful than the standards "Homemade Soda" or "Artisan Soda", which mainly tell you to mix some hugely expensive fru-fru ingredients together with a big pile of dishes to clean up... for just a resulting 3 seconds of a nice swallow.
Not that this Darcy O'Neil can't overcomplicate... his original soda fountain version of coca cola (before Coke dumbed it down for bottling) shows this pitfall of labor-intensive complication: