Phosphate was a general term used by soda fountains for drinks incorporating phosphoric acid and phosphate mineral salts. These had longer shelf life than natural acids from citrus fruit, and provided an interesting acid bite that improved the flavor of carbonated drinks.
Phosphoric acid is still a major ingredient in cola drinks, but most other modern carbonated soft drinks use citric acid.
Recipes for soda fountain-era beverages will frequently refer to premixed phosphate syrups, or will incorporate acid phosphate solution directly.
The original "acid phosphate" was manufactured by the Horsford Chemical company from bone ash (calcium phosphate), and formulated to a liquid mixture with a
pH
or around 2 or 3.