Phosphoric acid has three arms where protons can attach. There may be one, two or three protons on each molecule, where the likelihood depends on pH. (The higher the pH, more OH- are available and they try to take away the H+ from the phosphate. It's the same as how hydroquinone, ascorbic acid etc are activated at higher pH, though they are losing H from their -OH to make O-. When these dissociation accompanies different optical properties, they are used as pH indicator dyes.) When you say sodium phosphate you should figure out which version you are talking about. Na2HPO4 is often said "dibasic form." Whichever form you get, as long as it's something-PO4, you can adjust to a desired pH by adding NaOH solution while monitoring the pH. Dibasic one requires least NaOH if you are looking at pH near or above 12.(posted 8179 days ago)I wish there was a book combining chemistry, physics and practical accumulation of empirical data like Troop's book.