Reaction Mechanisms in Alkaline Solutions

The mechanisms of the electrochemical reactions of silicon electrodes in alkaline solutions at OCP have been investigated in many smdies due to their importance in the etching processes in micromachining. An important issue involving the reaction mechanisms has been whether the etching process at OCP is of chemical or electrochemical nature, that is, whether charge transfer processes associated with silicon dissolution and hydrogen evolution involve the carriers in the electrode. 

The experimental results in support of the chemical mechanism are (1) there is little difference between the etch rates of n-Si and p-Si at OCP and (2) the etch rate is essentially independent of the carrier concentration up to about lO Vcm 207,259,403 the other hand, the experimental results supporting the electrochemical mechanism are (1) the etch rate varies with electrode potential with the maximum near OCP, and the etch rates of n and p types are only similar near OCP, differing significantly at anodic and cathodic potentials as shown in Fig. 7.15 and (2) the i-V curves for n-Si and p-Si, although identical in terms of the chemical nature, are different in terms of carrier involvement, and the values of the characteristic potentials such as OCP and Vp are different for n-Si and p-Si and for different doping levels. 

According to Seidel et al. the dissolution at OCP is an electrochemical process with concurrent anodic dissolution of sihcon and reduction of hydrogen ions. The oxidation of silicon gives out electrons which are consumed for the reduction of hydrogen. Both OH and H2O are the active species in that OH is involved in silicon dissolution and H2O in hydrogen evolution  

The reaction scheme is supported by the fourth power dependence of the etch rate on OH and H2O concentrations observed experimentally (see Chapter 7 on etching of silicon). 

On the other hand, Palik er a/. suggested that silicon etching in KOH solu- 

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In contrast, the reaction mechanism in alkaline solution was shown to occur by nucleophilic attack by the anion of the peracid on the sulphoxide group53. Thus two different mechanisms seem to operate for the oxidation of sulphoxides to sulphones with peracids. At pH < 7 the sulphoxide acts as the nucleophile whilst at pH > 10 the peracid anion is the nucleophilic species. Presumably at intermediate pH values both mechanisms are operable. 

Our discussion of the reaction mechanism in alkaline solution has assumed that the bimolecular reaction of Co(CN)5OH 3 and N3" does not provide an important path for formation of Co(CN)5N3"3. 

Ortiz J, Gautier JL. Oxygen reduction on copper chromium manganites. Effect of oxide composition on the reaction mechanism in alkaline solution. J Elecfroanal Chem 1995 391 111-18. 

Depolymerization of starch in alkaline solution proceeds more slowly than in acid and produces isosaccharinic acid derivatives rather than D-glucose as a major product. The mechanism involves a -elimination-type reaction (48). 

Sodium hydrosulfite or sodium dithionate, Na2S204, under alkaline conditions are powerful reducing agents the oxidation potential is +1.12 V. The reduction of -phenylazobenzenesulfonic acid with sodium hydrosulfite in alkaline solutions is first order with respect to -phenylazobenzenesulfonate ion concentration and one-half order with respect to dithionate ion concentration (135). The SO 2 radical ion is a reaction intermediate for the reduction mechanisms. The reaction equation for this reduction is... 

Another hydroxylation reaction is the Elbs reaction In this method, phenols can be oxidized to p-diphenols with K2S20g in alkaline solution. Primary, secondary, or tertiary aromatic amines give predominant or exclusive ortho substitution unless both ortho positions are blocked, in which case para substitution is found. The reaction with amines is called the Boyland-Sims oxidation. Yields are low with either phenols or amines, generally under 50%. The mechanisms are not clear, but for the Boyland-Sims oxidation there is evidence that the S20 ion attacks at the ipso position, and then a migration follows. ... 

Pure parathion is a pale yellow, practically odorless oil, which crystallizes in long white needles melting at 6.0° C. (17). It is soluble in organic solvents, except kerosenes of low aromatic content, and is only slightly soluble in water (15 to 20 p.p.m. at 20° to 25° C.). Peck (35) measured its rate of hydrolysis to diethyl thiophosphate and nitro-phenate ions in alkaline solutions. He found that the reaction kinetics are first order with respect to the ester and to hydroxyl ion. In normal sulfuric acid the rate of hydrolysis was the same as in distilled water. Peck concluded that hydrolysis takes place by two mechanisms—a reaction catalyzed by hydroxyl ions and an independent uncatalyzed reaction with water. He calculated that at a pH below 10 the time for 50% hydrolysis at 25° C. is 120 days in the presence of saturated lime water the time is 8 hours. The over-all velocity constant at 25° C. is k = 0.047 [OH-] + 4 X 10-6 min.-1... 

Pell and Armor found entirely different products in alkaline solution. Above pH 8.3, the sole ruthenium product of the reaction of Ru(NH3)g+ with NO was the dinitrogen complex Ru(NH3)5(N2)2+. Under these conditions the rate law proved to be first-order in [Ru(NH3)g+], [NO] and [OH-]. A likely mechanism is the reversible reaction of Ru(NH3)3+ with OH- to give the intermediate Ru(NH3)5(NH2)2+, followed by electrophilic NO attack at the amide ligand and release of water. However, the kinetic evidence does not exclude other sequences. 

Chromiain(ii) Complexes.—The oxidation of chromium(ii) in alkaline solution has been studied polarographically and the reaction shown to be irreversible with = — 1.65 V vs. S.C.E. In the presence of nitrilotriacetic acid, salicylate, ethylenediamine, and edta the values were determined as —1.075, —1.33, — 1.38, and —1.48 V, respectively. The production of [Cr(edta)NO] from [Cr (edta)H20] and NO, NOJ, or NO2 suggests that this complex is able to react via an inner-sphere mechanism in its redox reactions. ... 

Anodic oxidation of malonate esters in alkaline solution gives the dehydrodimerization product by carbon-carbon coupling. The reaction mechanism has been... 

The shape of this wave and the variation with pH are both consistent with fast equ-librium reactions In the pH region lower than the value of pK, for the hydroxyl radical, the reactions of this hydroxyl radical dominate the electrochemical process. Controlled potential reduction at the potential of this first wave indicates a IF process and the principal products are dimers of the hydroxyl radical. The second wave in this acidic region is due to addition of an electron and a proton to the neutral radical. This process competes with dimerization in the mid-pH range where the two polarographic waves merge. Over the pH range 7-9, monohydric alcohol is the principal product. At pH <3 or >12, pinacols are the main products. Unsymmet-rical carbonyl compounds afford mixtures of ( )- and meso-pinacols. Data (Table 10.3) for the ( ) / meso isomer ratio for pinacols from acetophenone and propio-phenone indicate different dimerization mechanisms in acid and in alkaline solutions. 

Figure 7.9 shows the activity for H2 evolution of three samples of Ni [27]. Smooth and sandblasted Ni exhibit the same reaction mechanism (same Tafel slope, b), but a higher current for the latter. This is clearly due to the rougher surface of sandblasted Ni, that is, to purely geometric effects. The third sample is Raney Ni. This is obtained from an alloy of Ni with Zn or A1 that are then leached away in alkaline solution [47-49]. This leaves a very porous solid with intrinsically very small particle size. The figure shows that Raney Ni, in addition to a much lower overpotential for H 2 evolution, also exhibits a lower Tafel slope. This is clear evidence for the occurrence of electronic eflects (diflerent mechanism) together presumably with important geometric effects. 

The interconversion of aldoses and the respective 2-ketoses in alkaline solution may be somewhat more complex than originally supposed, as it has been reported that a partial transfer of hydrogen from C-2 of the aldose to C-l of the corresponding ketose occurs during the reaction.29 This observation is not inconsistent with isomerizations that involve 1,2-enediol intermediates. The transfer could occur as a result of a rapid conversion in which some of the protons originally at C-2 of the aldose molecules are retained by the solvent cage that surrounds the intermediate 1,2-enediol, and are, therefore, available for addition to C-l of the resulting ketose. It should be noted that other interpretations, such as hydride-transfer mechanisms, are also possible. 

In another set of experiments in alkaline solution it seems that with three hydroxides zinc reacts rather rapidly with porphyrins, but with four hydroxides it doesn t react nearly as rapidly. And if you replace the hydroxide by cyanides, you can stop the reaction altogether. I think this goes back to the remarks I made this morning, that each system may have its own particular mechanism. 

The ion-exchange mechanism of exfracfion does nof occur only for amino acids. We observed if also for cafecholamines [26]. These compounds are efficiently extracted into ILs in the cationic form, af pH 1-8. Af fhese pH, the primary (dopamine) or secondary (adrenaline and dobutamine) amino groups are protonated (catecholamines are oxidized in alkaline solutions at pH > 8). By analogy with amino acids, extraction may be described by the cation-exchange reaction ... 

Unfortunately, it is nontrivial to distinguish reliably between the complex-decomposition and sulphide-formation mechanisms. For example, in the study of PbS (as a precipitate) formation from thiourea [47], the two main results used to support complex decomposition were (a) very little sulphide was formed in alkaline solutions of thiourea and (b) addition of PbS powder catalyzed the reaction, seen by the disappearance of the induction time for precipitation and more rapid PbS formation when PbS was added at the start of the reaction. However, these results would also be obtained in a free-anion mechanism, for the following reasons ... 

Formic acid does not react at all in alkaline solutions carbon monoxide reacts better in alkaline than in acid solutions. Most surprisingly, no reaction at all can be produced with hydrogen, which under other conditions is readily oxidized. Hydrazine, which is present in adds almost exclusively as the corresponding hydrazonium salt, is very easily oxidized. This suggests that hydrazine reacts directly and not by the mechanism of previous separation of hydrogen. 

In alkaline solutions D-glucose forms 3-deoxy-D-en/f/iro-hexosulose and 4-deoxy-D-gft/cero-2,3-hexodiulose which yield saccharinic acids. Machell and Richards (57) have shown that 3-deoxy-D-en/fhro-hexosulose (14) is oxidized by 30% hydrogen peroxide to formic acid and 2-deoxy-D-erythro-pentonic acid (15). Recently Rowell and Green (58) found that 14 in the presence of oxygen also forms 15 in addition to the saccharinic acids. They inferred that the reactions with oxygen and hydrogen peroxide are very similar, but they did not present reaction mechanisms. 

The presence of such enolic ions in alkaline solutions of sugars was shown later by Isbell et al. (67). The mechanism of the alkaline -elimination reaction of substituted serine and threonine glycosidases based on this concept is shown in Figure 9. 

In alkaline solution (0.1 M KOH) and in the presence of N 0, the formation of formaldehyde and acetic acid is observed.137,138 The mechanism of these reactions still awaits clarification. 

From what is described in this section, it can be concluded that the kinetics of the oxidation reaction of sulphite and dithionite at a platinum electrode in alkaline solution are strongly affected by the nature of the platinum surface. This is important when a platinum electrode is used for a quantitative investigation of the kinetics of the oxidation of sodium dithionite and/or sulphite or as electrode material in the development of a sensor for the measurement and/or control of dithionite and/or sulphite concentrations. However, for sodium dithionite, it has no serious consequences because the limiting-current at 0.45 V vs. SCE does not change as a function of scan number. However, this oxidation is still irreversible (no return peak observed) which means that, in the onset of the voltammetric wave, the current is controlled by charge-transfer kinetics. Therefore, it is possible to investigate and obtain the mechanism of the oxidation of sodium dithionite, which is explained in the next section. 

Evidence that this reaction occurs in homogeneous acidic solution was found in the literature42. However, the present study was carried out in alkaline solution, and therefore it is assumed that this dissociation reaction occurs only at the surface of the electrode. If that is the case, the initial step of the mechanism, equation 6.8 is replaced by ... 

The second reaction is the rate determining step. This mechanism predicts the observed Tafel slope of 40 mV the fact that the same slope extends to high current densities indicates that the reaction proceeds with low Had coverage. The increase in Tafel slope at even higher current densities may indicate that the first step becomes rate determining. The same mechanism apparently holds also in alkaline solutions. 

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