Fig. 3: Speciation of CaP ion pairs from pH 2 to 12 calculated using PHREEQC. | Nature Communications

Fig. 3: Speciation of CaP ion pairs from pH 2 to 12 calculated using PHREEQC.

From: Redefined ion association constants have consequences for calcium phosphate nucleation and biomineralization

Fig. 3: Speciation of CaP ion pairs from pH 2 to 12 calculated using PHREEQC.

The curve for [CaHPO4]0 is black in the middle, while [CaH2PO4]+ and [CaPO4]− are grey on the left and right, respectively96,97. The top figure uses the ion association constants from Chughtai et al. while the bottom figure uses the constants from this work45. The effect of the large literature third ion association constant is immediately obvious; the [CaPO4]− ion pair is dominant over a much larger pH range. The crossover to other calcium-phosphate ion pairs moves down about one pH unit and up by about two units at low and high pH values, respectively. Note that the mole fraction of each species is based on the total amount of ion pairs in solution; especially at low pH a large mole fraction does not imply a high concentration, only that a large fraction of the total ion pairs exist as a given species. See Supplementary Figs. 13, 14 for the percentage of bound calcium across the same pH range, and how this correlates with the speciation of phosphate ions.

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