Acid doping equilibrium

The DMAc-cast membrane was doped with various concentrations (30, 40, 50, and 60wt%) of aqueous H3PO4 (PA) solution or 14(M) PA solution or 9(M) PA solution. Equilibrium was reached after 3 days at room temperature. The acid doping levels were expressed as moles of PA per mole of PBI repeat unit. Cross-linked PBI membranes require either covalently or ionically higher acid concentrations [97]. 

The equilibrium hydration level depends both on temperature and on steam partial pressure. The water generated at the cathode and the extent to which it equilibrates in the membrane may vary between the different polymer electrolytes, then-acid doping level, and the preparation method of the electrolyte, e.g., the solvent used for the... 

Two processes take place. Water is absorbed due to the hydrolysis of the pyrophosphoric acid according to the reverse of (5.1), up to the point that A = 0, showing that the dimerization of phosphoric acid is a fully reversible process. As expected, the equilibrium constant of phosphoric acid dimerization increases with temperature. Further increase in Pn o resulted in solubility of free water in the HsPOVpolymer system. There is a similar dependence on PE2O for samples used sharp increase at low / h 0 and thereafter linear variation of A. An increase of the acid doping level of the polymers results in enhanced ability to absorb water. 

The study of various doping materials in a FAB matrix can, in fact, lead to the determination of chemical equilibrium constants as demonstrated by Caprioli (31) who was able to measure the pKa values of a series of acids. 

Polyaniline differs from other CPs in that the doping process may be associated with protonation of the N atoms in the chain (i.e., to a base <- salt equilibrium in the presence of acid, in addition to the usual redox reactions). This is discussed further in Chapter 13. The related processes are therefore more complex. On the other hand, the low cost and proces-sibility of PAni makes it a prime candidate for several applications. 

For the KCl-LiCl eutectic, some of our investigations give an excellent illustration of the levelling of acidic properties by high-acidic melt cations. For this purpose, we now consider Fig. 1.3.3, where the results of Pb2+ and Cd2+ titration with KOH are presented. The behaviour of the KCl-LiCl eutectic melt doped with a 0.05 mol kg-1 addition of these cations does not differ from that of the pure melt. That is, the changes of the oxoacidic properties of the solutions, as compared with the pure eutectic, cannot be detected. This is because the acidic properties of Pb2+ and Cd2+ cations are not stronger than those of Li+, and the following equilibrium ... 

This relationship is often called the law of mass actiony i en applied to semiconductor doping statisticsT The situation is again conceptually identical to that for the equilibrium constant relationship for aqueous acid/base dissociation. The relationship = [H+(aq)][OH (aq)] holds not only for the neutral liquid (i.e. the intrinsic , pH = 7 sample), but also for the proton and hydroxide concentrations in the presence of externally added sources of H+ (aq) or OH (aq) (i.e. the extrinsic or doped liquid). In the doped semiconductor, we are merely adding electrons or holes to control the carrier concentrations in the same way that the pH of water can be manipulated through the addition of acid or base. 

Basically, PBI-based membranes show high proton conductivities when properly doped with strong acids such as H3PO4 (PA) [5,8]. PBI is, in fact, a basic polymer, which chemically dissociates the PA with an equilibrium constant has been calculated to be 1.17 x 10 [9],... 

Pervaporation was next carried out to test the selectivity of polyaniline membranes toward acetic acid-water mixtures. Different feed ratios of acetic acid and water were pervaporated through both undoped and doped polyaniline, as shown in Fig. 33.18, where the feed water content is plotted versus the permeant water content. For comparison the vapor-liquid equilibrium curve for acetic acid-water [78] is plotted just above the line of no separation. From Fig. 33.18 it is clear that undoped polyaniline has a small preference for permeating water over acetic acid at essentially any composition. However, this small preference at an average water permeability of about 0.5 g mm/(m--h) is too low to have any utility. More interesting is fully HCl-doped polyaniline, which permeates water over acetic acid in a much more selective fashion. In fact, even with a mixture of 859f acetic acid-15% water, at least 93 wt % of the permeant was water. It should be pointed out that when undoped membranes are used in the presence of acids, they will partially dope the polyaniline, the extent depending on the pH of the acid used. However, when a... 

Fig. 33.18 Pervaporation of acetic acid-water mixtures through doped and undoped polyaniline membranes. All polyaniline experiments were carried out at room temperature. The vapor-liquid equilibrium curve is plotted based on data points from Ref. 78 at 110-115.3°C and 760 mm Hg.

The processes that take place during electrochemical transfer doping have been described by Chakrapani et al, [92] and are listed below. We assume that equilibrium is established and that the acidity arises from dissolved CO2. 

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