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Log f = -Az2I/(1+I-0.3I)(vi)

f =activity

z =valency

I =Ionic strength

A is 0.5 for water at 25C  (19).

Solution preparation

The solutions were made from stock solutions of 0.1 M organic acid. All chemicals used were of Analar® purity or better. Acetic acid was made up from glacial acetic acid solution and citric acid was made up using citric acid salt. The stock solutions were standardised with 0.1 M NaOH and were found not to deviate more than 7%. The NaOH solution was standardised with potassium hydrogen phthalate (COOHC6H4COOK) solutions. These titrations were carried out with a Mettler DL55 automatic titrator. Stock solutions of 0.1 M calcium and 0.06 M phosphate were made from calcium chloride and sodium phosphate salts; their concentrations were confirmed by ICP-AES analyses. Working solutions of 10-3 M organic acid were made in NaCl with a total ionic strength of 0.1 M. The initial calcium to phosphate concentrations were determined by calculation of concentrations of calcium and phosphate at a low supersaturation degree of 2. For these calculations the computer program PHREEQC was used (18). The solubility constant used in the calculations was determined in pH-drift experiments at 25C, ionic strength 0.1 M and final pH 7 ( 0.1) and was found to be log K= -57.4 (mol l-1)9 (17). The calcium and phosphate solutions added subsequently in order to slowly increase supersaturation, and the titrants were made at the molar ratio stoichiometric to hydroxylapatite (calcium to phosphate: 1.67  0.1) from the stock solutions, and adjusted to the required pH before making up the volume, in order to maintain exact concentrations. All solutions were made up in 0.1 M NaCl in order to maintain constant ionic strength during the experiment. The pH of the solutions was adjusted by addition of 0.1 M NaOH and equilibrated for 24 hours in air. All solutions used in these experiments were filtered through a 0.2 m Millipore® filter to minimise the possibility of introducing particles (e.g. dust) which could interfere with nucleation.

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