**Author details**

Petr Ptáček

2 3 2

158 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

5 Ca 3 PO F ,OH 7 H 5 Ca 3 H PO

+ -- - + + - + + +® +

**1. Etch pit formation**: the process of etch pit formation describes the dissolution of apatite crystals containing structural defects (dislocations and inclusions). The presence of dislocations accelerates the dissolution because they give rise to continuous steps on the surface and the strain energy, which they cause in crystals, favors the etch pit formation. The pits appear at the dislocation outlets; they are 0.1–10 pm in size (the dimensions depend on the dissolution kinetics and dissolution time: they increase when the dissolu‐ tion progresses) and usually have a hexagonal shape according to the crystal symmetry

**2. Ion exchange model**: based on a supposition about the adsorption of protons and anions of acid (e.g. citrate anions) from a solution onto the surface of apatite and removing of

**3. Hydrogen catalytic model**: based on a reasonable suggestion about the adsorption of protons onto negatively charged oxygen ions of phosphate groups of apatite. The sorption

Chemical methods are used for the evaluation of reactivity of different phosphate rocks from which the fertilizers are manufactured for their possible direct application as fertilizers via

ammonium citrate are used as solvents for the extraction of P2O5. The latter is used mainly for the evaluation of calcined aluminum phosphates. Most of these reagents were not originally intended to evaluate the reactivity of phosphate rocks. For instance, neutral and alkaline ammonium citrate solutions were originally intended to separate chemical reaction products in superphosphate and other fertilizers from unreacted rock on the assumption that unreact‐ ed rock was insoluble in these reagents. The citric acid extraction was developed to evaluate basic slag, a popular fertilizer material in European countries. The formic acid extraction was

Nearly all extraction methods use the ratio of sample weight to extraction volume 1 g:100 ml.

 The extraction time usually ranges from 30 min to 1 hour. The temperature and the agita‐ tion during extraction test may be specified. For example, the AOAC method uses neutral

extraction time of 30 min at 65°C. The Wagner method uses 2% solution of citric acid, the

38 Neutral ammonium citrate is prepared by dissolving required amount of citric acid and neutralizing it with ammonium

<sup>39</sup> The amount of used solution is also expressed in the name of the method, e.g. 100 ml method or 150 ml method [222]. <sup>40</sup> The neutral ammonium citrate test was used as the official method in the United States, and the test by acidic acid was

of specified concentration (1 g of sample and 100 ml of solution) with the

empirical solubility test. Citric acid, formic acid, neutral ammonium citrate,38

3− groups into HPO4

2− and catalyz‐

and alkaline

2

**P63/M** of pure apatite [187],[215],[216],[217],[218],[219].

of protons results in the transformation of surface PO4

**3.4.3 Methods for the evaluation of reactivity of phosphate rocks**

es the dissolution process [187].

developed specifically for phosphate rocks [220].

hydroxide. The pH of the reagent is adjusted to neutral [221].

39

ammonium citrate40

developed for the comparison [222].

ions of calcium and phosphate into the solution instead [187].

HF, H O

4 2 4

<sup>+</sup> (50)

Brno University of Technology, Czech Republic

<sup>41</sup> It is believed that weak citric acid solution imitates the condition near the plant roots [222].
