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Figures 2a, b and c.Determination of the critical point of precipitation in the different experiments.

In these figures the calcium and phosphate concentrations between additions is stable which indicates no precipitation occurs until the slow drop in pH (critical point of precipitation shown as the dashed line). The calcium to phosphate molar ratio slightly increases during the stepwise addition process but was found not to increase over 1.8 (on average). At the critical point of precipitation the automatic addition of the titrants commences.

Figures 2a, b and c also show that that the calcium and phosphate concentrations remain constant during precipitation. The addition of the titrants had to continue for up to 10 minutes, in order to recover enough precipitate. In table 2 the critical concentrations are given upon which precipitation took place. These concentrations are the average of three experiments and the range in concentration is also shown.

Table 2.Critical calcium and phosphate concentrations in the experiments.

Experiment

Calcium (10-3 mol l-1)

Phosphate (10-3 mol l-1)

Control (i.e. no organic ligand)

4.868 0.306

2.644 0.078

Acetate

4.877 0.451

2.683 0.341

Citrate

6.883 0.184

3.874 0.065

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