Partial reaction displacement deposition

We described one example of this type of electrochemical deposition in Section 5.7 when we considered processes on a strip of Zn placed in a solution of CUSO4 (Fig. 5.11). In Chapter 5 we stated that there are two partial reactions in that system, as in an electroless system. In displacement deposition of Cu on Zn, electrons are supplied in the oxidation reaction of Zn ... 

Figure 9.1. Relationship between partial reactions in displacement deposition.

This is obtained via combination of the two partial electrode reactions, oxidation and reduction, reaction (9.5) and (9.6), respectively. Thus, in the displacement deposition of Cu on a Zn substrate, a layer of metallic Cu is deposited on the zinc while Zn dissolves into solution (Fig. 5.11). We stated that this reaction is possible since the Zn/Zn system has an electrode potential lower than that of the Cu/Cu system (Table 5.1 and Fig. 5.10). The overall displacement deposition reaction according to Eq. (9.7) can be considered as the reaction of the electrochemical cell... 

Tooth enamel, the hard, protective outer layer of the tooth, is 98% hydroxyapatite (Ca5(P04)30H). Although quite insoluble K p = 6.8 x 10 in water), demineralization, which is the dissolving of hydroxyapatite, does occur especially when the saliva contains acids. The reverse reaction, remineralization, also occurs. Remineralization is the re-depositing of tooth enamel. When hydroxyapatite is in solution with fluoride ions, a double-replacement reaction can occur. Fluoride ion replaces the hydroxide ion to form fluoroapatite (Ca5(P04)3p, /Cjp = 1 X 10 °). Fluoroapatite remineralizes the tooth enamel, thus partially displacing hydroxyapatite. Because fluoroapatite is less soluble than hydroxyapatite, destructive demineralization is reduced. 

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