Phosphoric Acid and Acid Phosphate
Acid phosphate is an acid/citrus substitute used in sodas and cocktails. It was a popular ingredient in soda fountains before the 1950s and has been produced again in recent years.
According to the Extinct Chemical Company, a producer of acid phosphate [link], "Acid Phosphate is actually partially neutralized phosphoric acid. The salts of calcium, magnesium and potassium (notice no sodium) are mixed with the phosphoric acid, which reduces the acidity and buffers the pH of the liquid. The process for making it is very similar to that childhood favourite combination of baking soda and vinegar. Once the reactions have finished, the end product has a pH between 2.2 and 2.4, where freshly squeezed lime juice has a pH of 1.9."
The FDA lists phosphoric acid [link] as "generally recognized as safe," both "when used in accordance with good manufacturing practice."
We are not aware of safety concerns with food-safe acid phosphate.
"Phosphate With a Twist" TheAtlantic.com [link]