Methanol-water electrode

Electrodes and Galvanic Cells. The Silver-Silver Chloride Electrode. The Hydrogen Electrode. Half-cells Containing an Amalgam, Electrode. Two Cells Placed Back to Back. Cells Containing Equimolal Solutions. The Alkali Chlorides as Solutes. HC1 in Methanol or Ethanol Containing a Trace of Water. The Alkali Chlorides in Methanol-Water Mixtures. The Heal of Solution of HC1. Proton Transfer Equilibrium from Measurements of E.M.F. 

In the cells discussed in Sec. 57 the solvent in every case was water. But in this chapter we shall discuss cells placed back to back, where one solution contains a solute dissolved in water, while the other contains the same solute dissolved in ethanol, or in methanol, or in a methanol-water mixture. When, for example, a hydrogen electrode containing IIC1 dissolved in ethanol is coupled to a Ag/AgCl electrode, also containing HC1 dissolved in ethanol, the cell may be written... 

The Alkali Chlorides as Solutes. In order to make a similar study of the transference of KC1, NaCl, and LiCl between water and methanol-water mixtures, the hydrogen electrode was replaced by an amalgam electrode, as described in Sec. 111. The arrangement when two cells having potassium amalgam electrodes are placed back to back may be written... 

Avdeef, A. Comer, J. E. A. Thomson, S. J., pH-metric logP. 3. Glass electrode calibration in methanol-water, applied to pKa determination of water-insoluble substances, Anal. Chem. 65, 42-49 (1993). 

Various other reducing methods are employed for the conversion of (3-nitro alcohols to amino alcohols, namely, electrochemical reduction.107 The selective electrohydrogenation of ni-troaliphatic and nitroaromatic groups in molecules containing other groups that are easy to hydrogenate (triple bond, nitrile, C-I) are carried out in methanol-water solutions at Devarda copper and Raney cobalt electrodes (Eq. 6.55).107... 

Fig. 3.68. Analytical HPLC chromatograms with detection of diode array of 4.7 x 10"5mol/l of R3R dye curve (1) before and curve (2) after 180 min of photoelectrocatalysis on the Ti02 thin-film electrode biased at +1.0 V in NajSCT, 0.025 mol/l. Curve (4) before and curve (3) after photoelectrocatalysis in NaCl 0.022 mol/l and curve (5) after bleaching of 4.7 X 10-5 mol/l of R3R dye submitted to a chemical treatment by active chlorine addition. The mobile phase was methanol-water 80 20 per cent with a flow rate of 1 ml/min and controlled temperature at 30°C. The column was a Shimpack (Shimadzu) CLC-ODS, 5 /an (250 mm X 4.6 mm). Reprinted with permission from P. A. Cameiro el al. [138].

Fig. 3.87. Chromatograms of the batch solutions before (dotted lines) and after hydrolysis (continuous lines) of three dyes on a diamond electrode. Hexane extract of SLB (org.) and aqueous extract of SLY (aq.) (right axis). Column Octyl, flow rate 0.8 ml/min, temperature 29°C, detection wavelength 220 nm, mobile phase aqueous phosphate buffer (pH 5)-methanol (50 50, v/v) (SLY, SNO) and linear gradient methanol-water (40 60, v/v) to 50 50 (SLB). Reprinted with permission from M. M. Davila et al. [149].

Although the accuracy of this explanation will be discussed later, it is easily understood that the behavior of the electrode is greatly influenced not only by the instantaneous potential of the electrode (potential dependence ) but also by the history of the electrode (time dependence). As this example shows, the electrochemical oxidation of methanol is a series of reactions in which methanol, water, intermediates and surface adsorbates are interacting with each other in various ways, and are yet to be fully understood. 

S. Mine, J. Jastrzebska, Rocz. Chem. 1954, 28, 519-520. In reporting the E20 value for copper in various methanol-water mixtures, these authors did not specify whether percent methanol referred to weight, volume, or mole per cent. Although the potential values are claimed to be referenced to aqueous SHE, the actual internal reference electrode used was not specified and no mention was made of a correction for the liquid junction potential. Since the actual liquid junction potential for aqueous reference electrodes in methanol-water mixtures was not made until a later date [ j —0.152 V for 80% methanol (w/w) M. Alfenaar, C. L. deLigny, Reel. Trav. Chim. Pays-Bas... 

Fig. 15 HPLC of vitamins A, D3, and E in the unsaponifiable fraction of milk. Column, 5 /nm Spheri-5 RP-18 (220 X 4.6-mm ID) mobile phase, methanol/water (99 1) containing aqueous 0.1 M lithium perchlorate, 1 ml/min amperometric detection (oxidative mode), glassy carbon electrode, +1.05 V, vs silver-silver chloride reference electrode. Peaks (1) retinol (2) vitamin D3 (3) a-tocopherol. (Reprinted from Ref. 143 with the kind permission of Elsevier Science-NL, Sara Burgerhartstraat 25,1055 KV Amsterdam, The Netherlands.)...

Fig. 11.12. Schematic diagram of a column-switching HPLC system. V-l, V-2, and V-3 switching valves. The position of °—c means position 1, and ° ° is position 2. P-1 and P-2 pumping system at 0.2 mL min-1 flow rate. Detector electrochemical detector with diamond electrodes. Column-1 and Column-2 Inertsil ODS-3. Loop 500 pL. A Mobile phase of 60% methanol-water containing 0.5% phosphoric acid.

The work reported by Ralph etal. (2003) is a well-rounded, self-contained essay on the DMFC. (See DMFC flow sheet in Figure 6.6.) Moreover, because Ballard/Johnson Matthey did not contribute on fuel cells at the Palm Springs Fuel Cell Seminar in 2002 (see below), Ralph etal. (2003) is the current information source, additional to the patents in the list of references. Note that the methanol-water mixture presents to the fuel electrode its associated methanol vapour pressure. The DMFC does not have an incompressible fuel. The cell needs circulators. It is incomplete. 

For 50% methanol-water mixtures three series of buffers involving acetate, succinate, and phosphate have been established in the temperature range from 10 to 40°C by emf data obtained without liquid junction. With 50% methanol solutions an aqueous saturated calomel electrode is used as reference electrode, since liquid-junction potentials are adequately reproducible for operational pH values. 

All mass spectra were obtained on a VG Quattro triple-quadrupole mass spectrometer (Micromass Inc., Altrincham, U.K.). Peptides were ionized with electrospray ionization under the following conditions mobile phase, methanol/water (50/50 v/v) needle voltage, 2.8 kV high voltage lens (counter electrode), 0.05 kV and skimmer potential, -12 V. The flow rate of the mobile phase for the spectra of the whole library was 100 pl/min rather than 10 pl/min, which was used for the samples after binding. All samples were dissolved in methanol. Either 5 or 10 pi of sample was injected. Two injections were performed for each sample. Data were acquired in Multichannel Analysis... 

The electrochemical reactions proceed in ethanol-water or methanol-water mixtures. Alternatively, gas diffusion electrodes can be used. 

The apparent acid dissociation constants (p s)Ka) of two water-insoluble drugs, ibuprofen and quinine, were determined pH-metrically in ACN water, dimethyl-formamide water, DMSO water, 1,4-dioxane-water, ethanol water, ethylene glycol-water, methanol water, and tetrahydrofuran water mixtures. A glass electrode calibration procedure based on a four-parameter equation (pH = alpha-i- SpcH -i-jH[H+] -i-jOH[OH ]) was used to obtain pH readings based on the concentration scale (pcH). We have called this four-parameter method the Four-Plus technique. The Yasuda Shedlovsky extrapolation p s)K a + log [H2O] = A/epsllon -1- B) was used to derive acid dissociation constants in aqueous solution (pKa). It has been demonstrated that the pK a values extrapolated from such solvent-water mixtures are consistent with each other and with previously reported measurements. The suggested method has also been applied with success to determine the pKa values of two pyridine derivatives of pharmaceutical Interest. Spectrometric, ultraviolet (UV) ... 

The other method used for liquid surfaces is the flow method of Kenrick (14) in which a jet of one solution is passed down the center of a tube whose walls carry a flowing layer of a second solution. The potentials between the flowing liquids are monitored with a quadrant or other electrometer. This method has been used with good results by Randles (15) and Parsons (16). Case and Parsons (17) compared the Kenrick and radioactive electrode methods for methanol-water mixtures. They found good agreement except at elevated methanol concentrations where methanol adsorption at the air electrode probably occurs. Measurement of the null current (compensation) potential in the Kenrick method is suitable for determining the surface potentials of solutions where rapid surface equilibrium occurs, but it is not convenient for spread monolayers or adsorbed films that have slow time effects. 

In a buffered methanol/water solution the electrochemical reduction of3,5-dichloro-/V,/V-diiso-propyl-1 A4,2,4,6-thiatriazin-l-amine shows a strong dependence on the measured half-wave potential ( — 0.86 to —1.30 V) from pH 1.87 5.72 of the electrolyte.80 This indicates protonation of the thiatriazinc, ring opening and secondary reactions of the intermediate. All isolated and identified reduction products are identical with those of the catalytic hydrogenation. The electrode reactions are not reversible. 

A series of semi-aqueous solutions of the samples, containing 3-60% (w/w) methanol were titrated. From these titrations, the ps a values (the apparent ionization constants in methanol-water solvent) were obtained, and the Yasuda-Shedlovsky procedure was applied to estimate the aqueous pKg values (Avdeef, et ah, Anal. Chem., 65, 42-49 (1993)). The four-parameter procedure was used for electrode standardization in both aqueous and semiaqueous solutions."... 

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