Apatite crystals solubility and dimension changes after Anchovy solution application

Anchovy fish (Stolephorus sp) contains high fluoride content in CaF2 form and is commonly consumed in Indonesia. Objectives: To evaluate the dimension changes of apatite crystals and analyze the acid solubility reduction of the crystals after applying anchovy solution. Methods: 0.09 mol (NH4)HPO4 added to 0.152 mol Ca(NO3)2.4H2O in 375ml distilled water, stirred and added with 125ml NH4OH, heated and dried to obtain apatite crystals. The obtained crystals were divided into 6 equal portions and sintered in 900°C for 1 hour to form crystal slabs. Dried anchovy fish from the market was heated and powdered. 5g of this anchovy powder was then diluted in 100ml deionized distilled water to prepare anchovy solution. The anchovy solution was painted onto 4 crystal slabs for 10min while 2 slabs will be left untreated as control. The length of the a and c axes of the crystal was measured using X-ray diffraction (XRD) and ICDD 1998. After XRD, the treated and control slabs were immersed in 10% phosphoric acid for 30sec and the solubility was then determined using Calcium Ion Selective electrode (ISE). Paired t-test was used to compare the changes between the treated and control slabs. Results: XRD results showed a statistical significant reduction in the length of the a axis from 9.4207±0.0017Å to 9.4023±0.0011Å after anchovy solution application, and to 9.4012±0.0008Å after acid immersion. The corresponding changes in the length of the c axis were from 6.8683±0.0029Å to 6.8898±0.0017Å and 6.8877±0.0013Å, respectively but statistically not significant. ISE analyses showed statistical significant differences in solubility of treated slabs (64.2±0.4ppm Ca) when compared to control (82.7±1.6ppm Ca). Conclusion: Anchovy solution application strengthen the apatite crystals by reducing the length of the a axis and their solubility in acid. Further studies are needed as only 6 samples were used in the current study.