Commercial electrode gels

Commercial electrode gels usually use relatively high concentrations of potassium or sodium chlorides at a neutral pH. Since these concentration levels can irritate the skin, there are different types of gels are on the market offering trade-offs of low resistance versus gentleness to the skin. 

Figure 7.6 Skin surface electrode geometry and its equivalent electric model. Right Typical values at 10 Hz for a commercial wet gel ECG electrode. Note that before contact electrolyte penetration is >99% of the total impedance.

Specially designed electrodes in which the electrode tip is covered in a gel containing the appropriate oxidase are commercially available. A semipermeable membrane retaining the gel, and permitting oxygen diffusion, completes the assembly. 

The electrodes described in this section are commercially available. The body of these electrodes may be either glass or epoxy plastic, as we have discussed. Epoxy plastic electrodes are unbreakable. Some electrodes are gel-filled electrodes and are sealed. This means that the KC1 solution has a gelatin mixed with it. There is no vent hole, they cannot be refilled with saturated KC1, and solid KC1 cannot be added. 

Synthetically, the question of which product is formed from a given carbonyl compound, the pinacol or the alcohol, is obviously of great interest. We have already noted the critical importance of the proton-donating ability of the medium in this respect. Attention has turned to the effect of other variables on the pinacol/alcohol ratio. Formaldehyde can be reduced cleanly to ethylene glycol at an electrode composed of a special type of carbon, in a process of potential commercial significance50. Nonaka and coworkers have found that reduction of aldehydes proceeds more selectively to aldehydes at nickel-Teflon-silica gel composite electrodes51. Nonaka and coworkers have also shown that for aromatic... 

Fig. 8.5. Cyclic voltammograms performed at 50 mV s-1 with a 2 mm diameter platinum electrode in a commercial product (depilatory cream Klorane) and in two home-made formulas containing no antioxidant species (gel 1 and emulsion 1).

Commercial gel-filled pH probes were inserted into the soil column at 3 cm intervals, located between the anode and cathode (Figure 1). These were inserted through butyl rubber septa placed into holes drilled into the sides of the column. The pH probes were calibrated using standard buffer solutions before insertion into the soil column. Readings were taken daily for the duration of the experiment, by placing a reference electrode into a well at the top of the column, and connecting the reference and in-situ pH probes to an Orion Model EA 940 Expandable Ion Analyzer. All pH measurements were temperature compensated to 25 °C. 

Before using the pH electrode, it should be calibrated using two (or more) buffers of known pH. Many standard buffers are commercially available, with an accuracy of 0.01 pH unit. Calibration must be performed at the same temperature at which the measurement will be made care must be taken to match the temperature of samples and standards. The exact procedure depends on the model of pH meter used. Modern pH meters, such as the one shown in Figure 5.8, are microcomputer-controlled, and allow double-point calibration, slope calculation, temperature adjustment, and accuracy to +0.001 pH unit, all with few basic steps. The electrode must be stored in an aqueous solution when not in use, so that the hydrated gel layer of the glass does not dry out. A highly stable response can thus be obtained over long time periods. As with other ion-selective electrodes, the operator should consult the manufacturer s instructions for proper use. Commercial glass electrodes are remarkably... 

Figure 6-6. Commercially available chambers for acylamide gel electrophoresis of short and long gels. Arrows in the left apparatus point to the platinum electrode wires. (Courtesy of Savant Instruments, Inc., Hicksville, N.Y.)...

Blotdng apparatus (commercial or otherwise). This normally consists of a plasdc tank and lid, with pladnum electrodes wired on either side of the tank, a plasdc holder for the gel/membrane sandwich, and a set of nylon pads. 

The recent advent of semidry electrophoretic transfer (seevol. 3, Chapter 29) allows the rapid transfer of several gels simultaneously, in a stack of up to 6 gels/membrane sandwiches," each sandwich being separ rated from the others by a layer of dialyns membrane to prevent transfer of polypeptides from one sandwich to the next The technique uses relatively low power and small amounts of transfer buffer, and the electrodes consist of graphite plates several types of apparatus are now commercially available. However, in our experience, it is often difficult to exclude all air bubbles from the gel/membrane sandwich and to maintain even electrode contact throughout the transfer. 

Table 21-2 lists some liquid-membrane electrodes available from commercial sources. The anion-sensitive electrodes shown make use of a solution containing an anion-exchange resin in an organic solvent. Liquid-membrane electrodes in which the exchange liquid is held in a polyvinyl chloride gel have been developed for Ca-, K", NOj, and BF4. These have the appearance of crystalline electrodes, which are considered in the following section. A homemade liquid-membrane ion-selective electrode is described in Feature 21-1. 

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