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The titrant concentrations were provided at the stoichiometric ratio of hydroxylapatite (calcium to phosphate 1.67 0.1) for calcium and phosphate. Preliminary experiments showed that the pH was only controllable when using a higher (than stoichiometric) concentration of base. The molar ratio of calcium: phosphate: hydroxyl-ions used for the titrants was 5:3:4. At the end-point of the experiment the solution was rapidly filtered through a 0.2 m syringe filter and then dried at 40C for identification.

Analyses

During the experiments 1.5 ml samples were taken to determine calcium and phosphate concentration of the solution. 0.8 to 1 ml of sample was diluted 10 times and acidified with 2% HNO3.  Calcium and phosphorus were measured using Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) with an ARL 3410. The precipitate was identified using X-ray diffraction (Enraf Nonius).

RESULTS AND DISCUSSION

Table 1 lists the initial concentrations, which show that the experiments started at the stoichiometric ratio for HAP of 1.67 0.08. The experiments were controlled at pH 7.03 with an accuracy of 0.05 and experiments were found to be reproducible.

Table 1.Initial calcium and phosphate concentrations of the experiments.

Experiment

Calcium (10-4 mol l-1)

Phosphate (10-4 mol l-1)

Molar ratio (Ca:PO4)

Control (no organic ligand)

3.37

1.96

1.714

Acetate

3.48

1.99

1.746

Citrate

3.29

2.00

1.644

Experimental results are shown in figures 2a, b and c. The figures display one of each of the triplicate result graphs for the three different experimental conditions studied here (control, acetate, citrate). These graphs show the stepwise increase of calcium and phosphate over time until precipitation takes place.

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