Ga E-pH Diagram

The eight steps described in the previous section will now be applied to the construction of a Ga E-pH diagram. 

Step (1). Species identification. Perusal of descriptive inorganic chemistry texts will lead to the discovery of the Ga-, 0-, and H-containing species which persist in water. These species consist of the solids Ga and Ga(OH)3 and the soluble ions Ga+ and Ga(OH)4. It should be noted that Ga(OH)3 should occur in the basic region and that Ga will sit low on the E-pH diagram because it is the most highly reduced species. 

Step (2). Species placement. In accordance with the procedure, the most reduced species Ga (oxidation number 0) is placed in the lower left-hand corner. There are no other species with a 0 oxidation number, and therefore Ga is to be assumed to extend all across the bottom of the diagram. The other 

Step (4). Writing equations. Each line on the diagram symbolizes a species transformation. The following equations represent these reactions. 

Step (5). Free energy calculations. Consultation of standard free energy value tables gives the following AG° values in kj/mole for the four species being considered Ga (0.0), Ga+ ( — 159.4), Ga(OH)3 (—831.4), and Ga(OH)4 (—982.4). These values are attached to the above equations and AG° (reaction) values have been calculated using Equation (6). The AG° values for the electron and for H+ are both 0.00 kj/mole and that for HOH is —237.2 kj/mole. 

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By using the above procedures, Ga E-pH diagrams at soluble species concentrations of 10 M, 10 M, and 10 M can be made. Individual diagrams for the three concentrations are presented in Figures 2.4-2.6. These three figures have been combined to give Figure 2.7. Notice that E transition... 

Figure 2.1 E-pH diagram for Ga species. Soluble species and most solids are hydrated. No agents producing complexes or insoluble compounds are present other than HOH and OH . Species placement approximation.

C. Preparation. As indicated by the E-pH diagram, the ammonium ion NH4+ can be prepared by acidifying a basic solution of NH4OH (actually hydrated NH3) until a pH of 9.2 or below is attained. The counterion (anion) associated with the NH4 is determined by the anion of the acid used in the acidification. The precursor of NH4 , namely NH3, is produced by the reaction of N2 with H2 at an elevated temperature in the presence of a catalyst. The H2 for this reaction is generated from various substances including coke and natural gas (CH4). 

The line labeled (b)in Fig. 4.10 represents the behavior of E vs. pH for Eq. (4.41). The chemical behavior of water across all possible values of potential and pH as shown in Fig. 4.10 is divided into three regions. In the upper region, water can be oxidized to produce oxygen while in the lower region it can be reduced to form hydrogen gas. Water is therefore only thermodynamically stable between lines (a) and (b). It is common practice to superimpose these two lines (a) and (b) on all E-pH diagrams to mark the water stability boundaries. 

Figure 1 Schematic diagram of the arrangement of a tissue-based membrane biosensor (A) slice of tissue (B) bovine serum albumin conjugate layer (C) carbon dioxide gas permeable membrane (D) 0-ring (E) internal electrolyte solution (F) pH-sensing glass membrane (G) plastic electrode body. (Reproduced with permission from Kurlyama S and Rechnitz GA (1981) Analytica Chimica Acta 131 91.)...

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