Sulfate electrode

In the experiment discussed (n-Si/0.6 M NH4F), the flatband potential (0.8 V vs. a saturated Hg-sulfate electrode) would have been immediately recognizable as the pronounced minimum between PMC and the photocurrent curve (Fig. 29). 

Fig. 5.6 (Left) Comparison of band energy levels for different II-VI compounds. Note the high-energy levels of ZnSe. Representation is made here for electrodes in contact with 1 M HQO4. The reference is a saturated mercury-mercurous sulfate electrode, denoted as esm (0 V/esm = +0.65 V vs. SHE). (Right) Anodic and cathodic decomposition reactions for ZnSe at their respective potentials (fidp, Fdn) and water redox levels in the electrolytic medium of pH 0. (Adapted from [121])...

Lead styphnate, 10 729 Lead sulfate electrodes, standard potential, 3 413t... 

Next we discuss four types of reference electrodes hydrogen, calomel, silver-silver chloric, and mercury-mercurous sulfate electrodes. 

Mercury-Mercurous Sulfate Electrode. In this reference electrode the metal is mercury, the sparingly soluble compound is mercurous sulfate (Hg2S04), and the source of S04 anions is sulfuric acid or potassium sulfate. The electrode is made in the same way as a calomel electrode, and it is represented as... 

Nikolic et al. reported the preparation and coulometric determination of quaternary ammonium iodides of procaine and of other local anesthetics [63]. After extraction from 0.33 M NaOH, the quaternary iodide salts were prepared by precipitation with methyl iodide in ethyl ether. The quaternary iodides were then coulometrically determined with the use of a Radiometer titrator. The method used a silver cathode and anode (in electrolytes of 2 M and 0.4 M H2SO4, respectively), and a reference mercurous sulfate electrode. For drug determinations in the range of 0.12 to 0. 96 mg, the standard deviations were typically found to be 4 to 8 pg. 

MetaUInsoluble Salt/Ion Electrodes. Electrode potentials are usually reported relative to normal hydrogen electrode (NHE a(H+) = 1, p(H2) = 1), but they are actually measured with respect to a secondary reference electrode. Frequently used secondary reference electrodes are calomel, silver-silver chloride, and mercury-mercurous sulfate electrodes. These secondary reference electrodes consist of a metal M covered by a layer of its sparingly soluble salt MA immersed in a solution having the same anion Az as the sparingly soluble MA. The generalized reference electrode of this type may be represented as M MA AZ and may be considered to be composed of two interfaces one between the metal electrode M and the metal ions Mz+ in the salt MA... 

Fig. 11.7 Parallel voltammetric screening of 64 Pt thin film catalysts for the electroreduction of oxygen in acidic solution. In the kinetically controlled region, the activity of all 64 catalysts shows good reproducibility. Conditions 0.5 M H2S04, oxygen saturated 20 mV s 1 anodic scan rate, 60°C, electrolyte stirring, potentials are plotted on the mercury/mercury sulfate electrode scale. Inset voltammogram of 64 Pt thin film catalysts in oxygen-free sulfuric acid, 20 mV s-1, prior to oxygen screening.

The mercury-mercurous sulfate electrode. Several commercial suppliers offer the mercury-mercurous sulfate electrode with a saturated potassium sulfate electrolyte. The potential (E° + E ) of this electrode system is 0.658 V on the hydrogen scale at 22°C.34 The electrode constitutes one-half of the Weston standard cell,35 an international secondary voltage standard, and is outstanding in reproducibility,36 in spite of the slight tendency of mercurous sulfate to hydrolyze and its rather high solubility. 

Data adapted from Advanced Course Text, Chapter 6, Appalachian Underground Corrosion Short Course, West Virginia University, Morgantown, WV, 1993, 1988 bCopper/copper sulfate electrode... 

Sodium Chloride Accurately weigh about 5 g of the sample, transfer into a 250-mL beaker, add 50 mL of water and 5 mL of 30% hydrogen peroxide, and heat on a steam bath for 20 min, stirring occasionally to ensure complete dissolution. Cool, and using silver and mercurous sulfate-potassium sulfate electrodes and stirring constantly, add 100 mL of water and 10 mL of nitric acid, and titrate with 0.05 N silver nitrate to a potentiometric endpoint. Calculate the percentage of sodium chloride in the sample by the formula... 

A mercury-sulfate electrode served as a reference electrode. All electrode potentials are referred to the potential of the reversible hydrogen electrode in the same electrolyte and at the same temperature as the test electrode. Adsorption measurements were performed in the 0.5 M H2SO4 solution prepared using special purity B-5 sulfuric acid and water doubly-distilled. To remove oxygen dissolved in the electrolyte, pure helium or argon was bubbled through acid solution. 

If chloride ions must be avoided, a mercury mercurous sulfate electrode [Hg/ Hg2S04(s), K2S04(s) E = 0.621 V versus NHE] may be employed. In alkaline solution a mercury mercuric oxide electrode (E = 0.098 versus NHE) may be useful. 

The function of the central electrode is to facilitate the exchange of sulfate ions between the two solutions. The equilibrium associated with a single mercury mercury sulfate electrode is... 

SECM SG/TC experiments were carried out to prove that the product of the initial two-electron oxidation process diffused into the solution, where it would react homogeneously and irreversibly. For these measurements, a 10 /xm diameter Au tip UME was stationed 1 /xm above a 100 /xm diameter Au substrate electrode. With the tip held at a potential of —1.3 V versus saturated mercurous sulfate electrode (SMSE), to collect substrategenerated species by reduction, the substrate electrode was scanned through the range of potentials to effect the oxidation of borohydride. The substrate and tip electrode responses for this experiment are shown in Figure 16. The fact that a cathodic current flowed at the tip, when the substrate was at a potential where borohydride oxidation occurred, proved that the intermediate formed in the initial two-electron transfer process (presumed to be mono-borane), diffused into the solution. An upper limit of 500 s 1 was estimated for the rate constant describing the reaction of this species (with water or OH ), based on the diffusion time in the experimental configuration. This was consistent with the results of the cyclic voltammetry experiments (11). 

Reagent grade chemicals were utilized. Electrodeposition was carried out in a conventional three-electrode cell under potentiostatic control with a mercury sulfate electrode as reference (in the following, all potentials are quoted versus this reference) and a platinum wire as counter electrode. Solutions were stirred and deoxygenated by bubbling nitrogen. Backside ohmic contacts of Si samples were achieved with an InGa eutectic. The i-U characteristics of the solid state junctions were measured in air. 

Only when this chemical reaction is brought into equilibrium, a clear dependency of the galvanic potential difference at sulfate electrodes on the SO2 concentration can be expected. For the equilibrium constant of the reverse reaction of Equation (25-77),... 

The Mercury-Mercurous Sulfate Electrode. A pool of mercury covered with a paste of mercurous sulfate and a solution containing sulfate. 

Fig. 6.—The Dependence on pH, and Curves Obtained with the Kalousek Switch, of 2,3-Dimethylquinoxaline in Acetate-Phosphate Buffers. (500 fiM 2,3-Dimethyl-quinoxaline. The pH values of the medium are given on the curves. The auxiliary potentials were 1 and 2, —l.OV 3 and 4, —1,1V and 5, —1.6 V. The frequency used was 6.25 Hz. The curve registrations were 1, 2, and 3, —0.2 V 4, —0.4 V and 5, —0.8 V versus saturated mercurous sulfate electrode.)...

Infrared absorption spectroscopy Isophthalic acid Low energy electron diffraction Lowest imoccupied molecular orbital Mechanically controlled break-junction Mercury-sulfate electrode Potential of zero charge q = 0 Quasireference electrode Real hydrogen electrode Reference electrode Alkanedithiols HS(CH2)nSH Self-assembled monolayer(s)... 

The half-cell generally used for cathodic protection in the field is the copper sulfate electrode. This consists of an electrode of electrolytic copper in a saturated solution of copper sulfate. The electrode can easily be made to have a large current capacity and will carry current better when it is acting as an anode than as a cathode. In the field, the cell is easily recharged and commercially pure copper sulfate solutions give potentials consistent to within 5 mV. 

There are several issues related to the reference and counterelectrodes. Reliable reference electrodes are commercially available. The saturated calomel electrode (SCE) is extremely robust and is commonly used for studies in chloride solutions. For studies in which chloride is to be avoided, the mercurous sulfate electrode (MSB) is suitable. The location of the reference electrode is critical in cells in which large ohmic potential drops exist. In these cases, a Luggin capillary should be used to bring the sensing location of the reference electrode close to the working electrode... 

The book edited by Ives and Janz [1 ] and more recently that by Bard, Parsons, and Jordan [2] contain both theoretical and practical aspects related to reference electrodes. Preparation, application and limitations of various types of reference electrodes such as the hydrogen electrode, the calomel and other mercury-mercurous salt electrodes, the silver-silver halide electrodes, and sulfide and sulfate electrodes are covered and general reference to these excellent critical reviews is recommended. 

---