thanks for your response. I have seen that is typical for many descaling products, but my understanding is that the lower the pH the more each of these acids exists in protonated form, making them less efficient as sequestrants. I think phytic acid sounds like a good option, just wanted to see if there were any others or additional things to consider.
@Abdullah Keep in mind that at low pH, cations (specially small ones) compete for binding sites with H+. That’s why you see that behavior shown in the graph. That shouldn’t be different if you had Phytic acid (I found a paper mentioning that no significant binding from Phytic acid to Ca2+ was found at pH lower than 5. The binding of group d metal ions (like Fe2+ and Cu2+) has the advantage of both relative cation size and back donation, which make complexes more stable.