Measurement Electrodes

Example 13 The following data were recorded for the potential E of an electrode, measured against the saturated calomel electrode, as a function of concentration C (moles liter ). 

Instmmental methods are useful for the determination of the total silver ia a sample, but such methods do not differentiate the various species of silver that may be present. A silver ion-selective electrode measures the activity of the silver ions present ia a solution. These activity values can be related to the concentration of the free silver ion ia the solution. Commercially available silver ion-selective electrodes measure Ag+ down to 10 flg/L, and special silver ion electrodes can measure free silver ion at 1 ng/L (27) (see Electro analytical techniques). 

Fluoride. A fluoride concentration of ca 1 mg/L is helpful in preventing dental caries. Eluoride is deterrnined potentiometrically with an ion-selective electrode. A buffer solution of high total ionic strength is added to the solution to eliminate variations in sample ionic strength and to maintain the sample at pH 5—8, the optimum range for measurement. (Cyclohexylenedinitrilo)tetraacetic acid (CDTA) is usually added to the buffer solution to complex aluminum and thereby prevent its interference. If fluoroborate ion is present, the sample should be distilled from a concentrated sulfuric acid solution to hydrolyze the fluoroborate to free fluoride prior to the electrode measurement (26,27). 

Since the Wenner formula [Eq. (24-41)] was deduced for hemispherical electrodes, measuring errors appear for spike electrodes. To avoid errors in excess of 5%, the depth of penetration must be less than a 5. Soil resistivity increases greatly under frost conditions. While electrodes can be driven through thin layers of frost, soil resistivity measurements deeper than 20 cm in frozen ground are virtually impossible. 

With four-electrode measurements effected from the surface, an average soil resistivity over a larger area is obtained. The resistivity of a relatively localized layer of earth or pocket of clay can only be accurately measured by using a spike electrode. Figure 3-18 gives dimensions and shape factors, Fg, for various electrodes. 

Direct analysis with the fluoride lon-selective electrode requires addition of total ionic strength adjustor buffer solution (TISAB) to the standard and to unknown samples Some advantages of this addition are that it provides a constant background ion strength, ties up interfenng cations such as aluminum or iron, which form a complex with fluoride ions, and maintains the pH between 5 0 and 5 5 According to the manufacturer s claim, reproducibility of direct electrode measurement IS 2 0%, and the accuracy for fluonde ion measurement is 0 2% [27]... 

For any type of nonaqueous electrolyte (nonaqueous solutions, melts, solid electrolytes) we can select suitable reference electrodes, measure the potentials of other electrodes, and set up tables of electrode potentials. The order of the reactions (electrodes) as a rule does not strongly differ between the different media. A strong reducing agent such as lithium will have a more negative potential than a weaker reducing agent such as copper, both in water and in other media. 

Ion probes. Determining the level of ions in solution also helps to control corrosion. An increase in concentration of specific ions can contribute to scale formation, which can lead to a corrosion-related failure. Ion-selective electrode measurements can be included, just as pH measurements can, along with other more typical corrosion measurements. Especially in a complete monitoring system, this can add information about the effect of these ions on the material of interest at the process plant conditions. 

Figure 8.17 Activities of Pt(l 1 l)-wML Pd electrodes from rotating disk electrode measurements, with corresponding ball models (a) electro-oxidation of formic acid in 0.1 M HCIO4 ...

Schmidt TJ, Gasteiger HA, Behm RJ. 1999b. Rotating disk electrode measurements on a high-surface area Pt/Vulcan carbon fuel cell catalyst. J Electrochem Soc 146 1296-1304. 

To ensure that the detector electrode used in MEMED is a noninvasive probe of the concentration boundary layer that develops adjacent to the droplet, it is usually necessary to employ a small-sized UME (less than 2 /rm diameter). This is essential for amperometric detection protocols, although larger electrodes, up to 50/rm across, can be employed in potentiometric detection mode [73]. A key strength of the technique is that the electrode measures directly the concentration profile of a target species involved in the reaction at the interface, i.e., the spatial distribution of a product or reactant, on the receptor phase side. The shape of this concentration profile is sensitive to the mass transport characteristics for the growing drop, and to the interfacial reaction kinetics. A schematic of the apparatus for MEMED is shown in Fig. 14. 

According to the foregoing theory, any halide electrode can measure Ag+, but not a halide whose solubility product is lower than that of the halide in the membrane so, although an Agl electrode measures I, Br and Cl, an AgCl electrode measures only Cl". 

Averaged from electrical conductivity and ionselective electrode measurements as described in the text... 

FIGURE 4.4 Schematic diagram of a membrane electrode measuring circuit and cell assembly. 

P. D Orazlo and G.A. Rechnitz, Ion electrode measurements of complement and antibody levels using marker-loaded sheep red blood cell ghosts. Anal. Chem. 49, 2083-2086 (1977). 

C. Rundle, A Beginner s Guide to Ion-Selective Electrode Measurements, http //www.nico2000.net/ Book/Guidel.html. (2005). 

The potential of an electrode measured relative to a standard, usually the SHE. It is a measure of the driving force of the electrode reaction and is temperature and activity dependent (p. 230). By convention, the half-cell reaction must be written as a reduction and the potential designated positive if the reduction proceeds spontaneously with respect to the SHE, otherwise it is negative. If the sign of the potential is reversed, it must be referred to as an oxidation potential. 

Supersaturation of up to nearly 4 orders of magnitude is indicated relative to a log K= 4.9 which reflects freshly precipitated HFO. When elimination of all data points which are below the detection limits for Fe(lll) and for electrode measurements, values of Eh measured agree with Eh calculated from Fe(ll/lll) determinations and speciation calculations and the revised ferrihydrite saturation index diagram looks like fig. 3. 

Fig. 3. Same as fig. 1 after removing all points below detection limits of Fe(lll) and of redox electrode measurements.

FIGURE 3.8 Impedance curves of pure LSM, 0.8 mgcm-2 GDC impregnated LSM and 5.8 mgcm-2 GDC impregnated LSM electrodes measured at 700°C in air. The impedance curves were measured after cathodic current treatment at 200 mAcm-2 for 120 min at the same temperature. 

The quadrupole splitting of the heat treated FePc/XC-72 electrode measured ex situ, prior to the electrochemical experiments, was larger than that found in situ. Smaller values for A have been reported for certain ferric hydroxide gels and for small particles of FeOOH (Table II), and thus the effect associated with the immersion of the specimen in the electrolyte is most probably related to the incorporation of water into the oxide structure. For this reason, the material observed in situ at this potential will be referred to hereafter as FeOOH(hydrated), without implying any specific stoichiometry. 

Apart from the convenience of the logarithmically compressed scale, the concept of pH remains popular because one of the most of measuring the acidity of an aqueous solution is the glass electrode measurement of which is directly proportional to pH, rather than to... 

We require an inert electrode when both parts of a redox couple reside in solution, or do not conduct the electrode measures the energy of the couple. 

Describe the basic design of a pH electrode. What kinds of protons in soil can a pH electrode measure ... 

Phene CJ. Oxygen electrode measurement. In Klute A (ed), Methods of Soil Analysis Part 1 Physical and Mineralogical Methods, 2nd ed. Madison, WI Soil Science Society of America and American Society of Agronomy 1986, pp. 1137-1160. 

Because electrode measurements of O2 uptake can detect intra- and extracellular oxidase activity, this assay can be used to measure the respiratory burst elicited by soluble and particulate stimuli. What is somewhat surprising is that, during stimulation of neutrophils with agonists such as fMet-Leu-Phe, the activated O2 uptake profile is biphasic (Fig. 5.11c). A rapid burst of O2 uptake (which coincides with measurements of cytochrome c reduction) is followed by a more sustained activity of lower magnitude. 

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