Acidity electrochemical estimation

Seb cic Acid. Sebacic acid [111-20-6] C QH gO, is an important intermediate in the manufacture of polyamide resins (see Polyamides). It has an estimated demand worldwide of approximately 20,000 t/yr. The alkaline hydrolysis of castor oil (qv), which historically has shown some wide fluctuations in price, is the conventional method of preparation. Because of these price fluctuations, there have been years of considerable interest in an electrochemical route to sebacic acid based on adipic acid [124-04-9] (qv) as the starting material. The electrochemical step involves the Kolbn-type or Brown-Walker reaction where anodic coupling of the monomethyl ester of adipic acid forms dimethyl sebacate [106-79-6]. The three steps in the reaction sequence from adipic acid to sebacic acid are as follows ... 

The relative stability of the anions derived from cyclopropene and cyclopentadiene by deprotonation is just the reverse of the situation for the cations. Cyclopentadiene is one of the most acidic hydrocarbons known, with a of 16.0. The plCs of triphenylcyclo-propene and trimethylcyclopropene have been estimated as 50 and 62, respectively, from electrochemical cycles. The unsubstituted compound would be expected to fall somewhere in between and thus must be about 40 powers of 10 less acidic than cyclopentadiene. MP2/6-31(d,p) and B3LYP calculations indicate a small destabilization, relative to the cyclopropyl anion. Thus, the six-7c-electron cyclopentadienide ion is enormously stabilized relative to the four-7c-electron cyclopropenide ion, in agreement with the Hixckel rule. 

Having established the theoretical reference electrode, we are now able to estimate electrochemical phase diagrams. The starting point of the electrochemical phase diagram is the relative stabihty of the different species on the surface. If we consider a metal electrode in an acid electrolyte, we obtain... 

The electrolysis in aqueous sulfuric acid with methanol as a cosolvent was perfomed in a filterpress membrane cell stack developed at Reilly and Tar Chemicals. Because of the low current density of the process, a cathode based on a bed of lead shot was used. A planar PbOa anode was used. The organic yield was 93% with approximately 1% of a dimer. The costs of the electrochemical conversion were estimated as one-half of the catalytic hydrogenation on a similar scale. 

I. 4-methoxyacetophenone (30 //moles) was added as an internal standard. The reaction was stopped after 2 hours by partitioning the mixture between methylene chloride and saturated sodium bicarbonate solution. The aqueous layer was twice extracted with methylene chloride and the extracts combined. The products were analyzed by GC after acetylation with excess 1 1 acetic anhydride/pyridine for 24 hours at room temperature. The oxidations of anisyl alcohol, in the presence of veratryl alcohol or 1,4-dimethoxybenzene, were performed as indicated in Table III and IV in 6 ml of phosphate buffer (pH 3.0). Other conditions were the same as for the oxidation of veratryl alcohol described above. TDCSPPFeCl remaining after the reaction was estimated from its Soret band absorption before and after the reaction. For the decolorization of Poly B-411 (IV) by TDCSPPFeCl and mCPBA, 25 //moles of mCPBA were added to 25 ml 0.05% Poly B-411 containing 0.01 //moles TDCSPPFeCl, 25 //moles of manganese sulfate and 1.5 mmoles of lactic acid buffered at pH 4.5. The decolorization of Poly B-411 was followed by the decrease in absorption at 596 nm. For the electrochemical decolorization of Poly B-411 in the presence of veratryl alcohol, a two-compartment cell was used. A glassy carbon plate was used as the anode, a platinum plate as the auxiliary electrode, and a silver wire as the reference electrode. The potential was controlled at 0.900 V. Poly B-411 (50 ml, 0.005%) in pH 3 buffer was added to the anode compartment and pH 3 buffer was added to the cathode compartment to the same level. The decolorization of Poly B-411 was followed by the change in absorbance at 596 nm and the simultaneous oxidation of veratryl alcohol was followed at 310 nm. The same electrochemical apparatus was used for the decolorization of Poly B-411 adsorbed onto filter paper. Tetrabutylammonium perchlorate (TBAP) was used as supporting electrolyte when methylene chloride was the solvent. 

In electrochemical treatment of extracted groundwater, the operating costs for electrode consumption, power, and acid for the electrochemical unit are estimated at approximately 10 cents per 1000 gal of groundwater treated. At an anticipated flow rate of 20 gal/min (gpm), the operating costs are approximately 1000 annually. Labor and waste disposal costs for the electrochemical treatment process are estimated to be approximately 50 per day (D168869, p. 7-14). 

The electrochemical behavior of Re04 in aqueous solution is irreversible and not well-characterized. The estimated one-electron potential for the Re04 / couple in aqueous alkaline solution [3] (Table 1) is slightly more negative than that for Tc04 7 (Table 1 of Sect. 16.2). In aqueous acid media, reduction is thought to proceed by a two-electron transfer from Re(VII) to Re(V) [11], followed by disproportionation and formation of Re02 [12]. 

An internal electrochemical mechanism was proposed long ago for deposition on certain metal substrates, since the rate of deposition sometimes depended on the nature of the substrate [11].) The standard potential of Reaction (5.3) is -l- 0.08 V, considerably more positive than the rednction potential of S to (-0.45 V). Free sulphide, if formed, would be in a very low concentration, since it will be removed continually by precipitation of PbS this will move the S rednction potential strongly positive according to the Nemst equation [Eq. (1.32)]. This positive shift will be even greater than normal because of the non-Nemstian behavior of the S /S couple when [S] > [S ] (at least in alkaline solntion) [12]. In opposition to this, the solubility of S in the (slightly acidic) aqneons solntions is very low, which will move the potential in the opposite direction. Add to this the very small concentration of S in acid solution [Eq. (1.15)], and it becomes clear that it is not trivial to estimate the feasibility of the formation of PbS by free snlphide. The non-Nemstian behavior of the sulphur-rich S /S couple and the lack of knowledge of the solnbility of free S in the deposition solntion are the two factors that complicate what would have been a tractable thermodynamic calcnlation. 

Cyclic voltammetry has been also used for estimation of the rate constants for oxidation of water-soluble ferrocenes in the presence of HRP (131). There is a perfect match between the data obtained spectrophotometrically and electrochemically (Table IV), which proves that the cyclic voltammetry reveals information on the oxidation of ferrocenes by Compound II. It is interesting to note that an enzyme similar to HRP, viz. cytochrome c peroxidase, which catalyzes the reduction of H202 to water using two equivalents of ferrocytochrome c (133-136), is ca. 100 times more reactive than HRP (131,137). The second-order rate constant equals 1.4 x 106 M-1 s 1 for HOOCFc at pH 6.5 (131). There is no such rate difference in oxidation of [Fe(CN)e]4- by cytochrome c peroxidase and HRP (8). These comparisons should not however create an impression that the enzymatic oxidation of ferrocenes is always fast. The active-R2 subunit of Escherichia coli ribonucleotide reductase, which has dinuclear nonheme iron center in the active site, oxidizes ferrocene carboxylic acid and other water-soluble ferrocenes with a rate constant of... 

By far the largest sector of the battery industry worldwide is based on the lead-acid aqueous cell whose dominance is due to a combination of low cost, versatility and the excellent reversibility of the electrochemical system, Lead-acid cells have extensive use both as portable power sources for vehicle service and traction, and in stationary applications ranging from small emergency supplies to load levelling systems. In terms of sales, the lead-acid battery occupies over 50% of the entire primary and secondary market, with an estimated value of 100 billion per annum before retail mark-up. 

The electrochemical behavior of dimers containing the MoJ unit has been reported for several derivatives. Oxidation of Mo2(S04)J in acid solution formed Mo2(S04)4" (E1/2 = 0.22 V versus SCE), but rapid decomposition of the oxidized product ensued with a rate constant estimated from cyclic voltammetry to be in the range of 10-3 —10-1 sec-1 (75). Reproducibility of the electrochemical behavior depended on acid concentration, electrode history, and sweep rate and was generally indicative of complex processes not amenable to rigorous interpretation. 

Nafion membrane materials were originally developed for electrochemical applications.155 The acid strength (—Ho) is estimated to be between 11 and 13156 that is, the acidity of Nafion is comparable to that of concentrated sulfuric acid. In these systems, the sulfonic acid group is attached to a CF2 or CF group in a perfluorinated backbone. 

Several methods are available for estimating acidity in solution for the homogeneous superacids spectroscopy (UV, NMR), electrochemical methods, chemical kinetics, and heats of protonation of weak bases (9). Due to the heterogeneity of solid superacids, accurate acidity measurements are difficult to carry out and to interpret. The most simple and useful way to estimate the acidity of a solid catalyst is to test its catalytic activity in well-known acid-catalyzed reactions we usually compare the activity with that of Si02-Al20v... 

An improved method of preparation of cotarnine and opianic acid by the electrochemical oxidation of narcotine at a graphite anode in the presence of potassium dichromate has been described in a patent.3 In a study of the pharmacological effects of mescaline, the binding of the alkaloid with rat brain tissue has been examined, using 14C-labelled material.4 A technique for the separation, identification, and estimation of tyramine, methoxytyramine, and related phenethylamines by g.l.c. of their trimethylsilyl derivatives has been described in detail.5... 

Also, HPLC methods with electrochemical or fluorescent detection are used (H19, M3). In proteins, dityrosine can be estimated by immunochemical methods employing dityrosine-specific antibodies (K5). Measurements of o,o -dityrosine and o-tyrosine levels in rat urine express dityrosine contents in skeletal muscle proteins, and have been proposed as the noninvasive oxidative stress test in vivo. One should be aware, however, that A-formylkynurenine, also formed in protein oxidation, has similar fluorescence properties as dityrosine (excitation 325 nm, emission at 400-450 nm) (G29). Also, oxidation of mellitin when excited at 325 nm produces an increase in fluorescence at 400—450 nm, despite the fact that mellitin does not contain tyrosine. Oxidation of noncontaining Trp residues ribonuclease A and bovine pancreatic trypsin inhibitor with "OH produces loss of tyrosine residues with no increase in fluorescence at 410 nm (S51). There are also methods measuring the increased hydrophobicity of oxidized proteins. Assays are carried out measuring protein binding of a fluorescent probe, 8-anilino-l-naphthalene-sulfonic acid (ANS). Increase in probe binding reflects increased surface hydrophobicity (C7). 

---