Electrode alternative standard electrodes

The pH meter is standardized (calibrated) with the use of buffer solutions. Usually, two buffer solutions are used for maximum accuracy. The pH values for these solutions should bracket the pH value expected for the sample. For example, if the pH of a sample to be measured is expected to be 9.0, buffers of pH = 7.0 and pH = 10.0 should be used. Buffers with pH values of 4.0,7.0, and 10.0 are available commercially specifically for pH meter standardization. Alternatively, of course, homemade buffer solutions (see Chapter 5) may be used. In either case, when the pH electrode and reference electrode are immersed in the buffer solution being measured and the electrode leads are connected to the pH meter, the meter reading is electronically adjusted (refer to manufacturer s literature for specifics) to read the pH of this soluiton. The electrodes can then be immersed into the solution being tested and the pH directly determined. 

Elemental composition Na 58.93%, N 35.90%, H 5.17%. The compound may he decomposed cautiously with water (reaction is violent) under cooling to yield sodium hydroxide and ammonia. (Or it may he decomposed with anhyrous alcohol to form ammonia and sodium alcoholate. The alcoholate then may he treated with water to form sodium hydroxide). Ammonia liberated is dissolved in water and the solution is measured using an ammonia-selective electrode. Alternatively, ammonia is collected over horic acid solution containing a small quantity of methyl red indicator. The solution is titrated with a standard solution of sulfuric acid. Sodium hydroxide is measured hy titration with a standard solution of hydrochloric or sulfuric acid. 

Elemental composition Na 39.34%, Cl 60.66%. Aqueous solution may be analyzed for sodium by various instrumental methods (see Sodium) and for chloride ion by ion chromatography or chloride-ion selective electrode. Alternatively, the chloride ion may be measured by titration with a standard solution of silver nitrate using potassium chromate as indicator. Also, the salt can be identified by its physical properties. 

Because (like AG) refers to a difference in a state property, it can be evaluated in additive fashion along many alternative pathways. For this purpose, it is convenient to assign conventional ° values to each half-cell reaction [e.g., standard oxidation potentials as compiled in W. M. Latimer. Oxidation Potentials, 2nd edn (Prentice-Hall, New York, 1952)], such that the algebraic sum of the two half-reaction potentials equals the overall cell °. Such half-reaction ° values can in turn be obtained by choosing some standard electrode reaction as the conventional zero of the scale [such as the standard hydrogen electrode (SHE) for the l/2H(g) —> H+ aq) + e oxidation reaction, with she = 0]. Sidebar 8.2 illustrates a simple example of this procedure. 

Hydrogen electrode — A gas electrode where purified hydrogen gas is dissolved, usually in an aqueous solution, in which an inert electrode, preferably a - platinized platinum (- platinum black, -> electrode materials) electrode is inserted. The hydrogen electrode is of exceptional importance in electrochemistry because the -> standard hydrogen electrode (SHE) provides by convention a reference potential for all half reactions and thus the thermodynamic reference point for all energy calculations. An alternative form is the -> dynamic hydrogen electrode (DHE). 

For example, we showed in earlier chapters that the calcium ion concentration of an aqueous solution is readily determined by titration with a standard EDTA solution or by potential measurements with a specific-ion electrode. Alternatively, the calcium content of a solution can be determined either from atomic absorption or atomic emission measurements or by the precipitation of calcium oxalate followed by weighing or titrating with a standard solution of potassium permanganate. 

Standard electrode potentials of alkyl halides and nucleophiles have been used to pi edict die rates of SKT alternatives to Ss2 substitution mechanisms 134.49.50.08.601. A reaction much faster than predicted on the basis of Marcus theory is a likelv S 2 process, while similarity between prediction and rate, favors SET. Despite some disagreement as to the proper treatment rtf a dissociative electron transfer 48.7(). this approach has proven useful in helping to classify reactions as probably SN2 or SET in mechanism (most successfully with large, delocalized nucleophiles). 

Such emf values, under standard conditions, are known as standard electrode potentials or standard reduction potentials, and are given the symbol Alternatively, the standard hydrogen electrode may be placed on the right-hand side the potential so obtained is known as the standard oxidation potential The latter potentials are the standard electrode potentials with the signs reversed, the only difference being that the cells have been turned around. 

Although standard electrode reaction rate constants k are in principle obtainable from peak-to-peak separations in CVs or from extrapolations of Tafel plots to zero overpotential, the precision needed for measurement of AFei demands alternating eurrent voltammetry (ACV), in which a small AC potential of angular frequency lj is superimposed upon a DC potential ramp and the phase angle (p is extracted from measurements of the maximum in-phase and 90° out-of-phase currents.if the diffusion coefficients of the oxidized and redueed speeies are taken to be the same (D),... 

Calculation of Cell emfs Using Standard Reduction Potentials Note that in the case of the cadmium-silver cell above the emf of the cell equals the standard electrode potential of the cathode minus the standard electrode potential of the anode. So, alternatively, you can calculate any cell emf using the equation... 

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