Reference probes

In order to use the pH electrode described above, two half-cells (probes) are needed—the pH electrode itself and a reference electrode, either the SCE or the silver-silver chloride electrode—and two connections are made to the pH meter. An alternative is combination pH electrode. This electrode incorporates both the reference probe and pH probe into a single probe and is usually made of epoxy plastic. It is by far the most popular electrode today for measuring pH. The reference portion is a silver-silver chloride reference. A drawing and a photograph of the combination pH electrode is given in Figure 14.7. 

There is a potential drop V across the solution between the layer around the working electrode and the tip of the reference probe. This is related to the separation distance d by Equation 1.3 where i is the current flowing through the cell and K is the specific conductivity of the electrolyte. Tire reference electrode probe is... 

The sample and reference probes are reversed for positive readout. 

An equivalent circuit of the three-electrode cell discussed in Chapters 6 and 7 is illustrated in Figure 9.1. In this simple model, Rr is the resistance of the reference electrode (including the resistance of a reference electrode probe, i.e., salt bridge), Rc is the resistance between the reference probe tip and the auxiliary electrode (which is compensated for by the potentiostat), Ru is the uncompensated resistance between the reference probe and the working-electrode interphase (Rt is the total cell resistance between the auxiliary and working electrodes and is equal to the sum of Rc and Ru), Cdl is the double-layer capacitance of the working-electrode interface, and Zf is the faradaic impedance of the electrode reaction. 

Often the chemistry of an experiment dictates that the cell have special features. For example, products generated at the auxiliary electrode or materials leaking from the reference probe will occasionally interfere with the reac-... 

The auxiliary electrode shown in Figure 9.3A and C is a platinum wire loop concentric with the working electrode. A platinum wire or foil, a mercury pool, and carbon can also be used for this purpose. The graphite rods used for emission spectroscopy are useful as auxiliary electrodes. For optimum performance at short times, the position of the reference probe might have to be changed from that shown when using different auxiliary electrodes. 

The optimal composition of a FISH probe mixture for detection of changes in gene copy number in solid tumors includes two labeled probes. One probe directed towards the gene of interest and one probe directed towards a reference sequence for the chromosome on which the target gene is located. Traditionally, the reference probes are directed towards the non-coding centromere sequences. 

The acid or base strength is defined by comparing the interaction between the sites and reference (probe) molecules. For example, the acid strength is the ability of the site to convert an adsorbed reference neutral base into its conjugate acid. This process may be monitored by following the colour changes of the indicators or by such techniques as IR or NMR spectroscopies. Several criteria must be considered in the selection of these probes their pK value must be adapted to the acidity or basicity of the surface under study the size of the probe molecules should ensure that all sites are readily accessible finally, the probe should be selected so that its interaction with the surface is easily studied by the available experimental techniques. Catalytic test reactions are sometimes used... 

The primary standard betaine dye (44) is only sparingly soluble in water and less polar solvents it is insoluble in nonpolar solvents such as aliphatic hydrocarbons. In order to overcome the solubility problems in nonpolar solvents, the more lipophilic penta-t-butyl-substituted betaine dye (45) has additionally been used as a secondary reference probe [174]. The excellent Hnear correlation between the Ej values of the two dyes allows the calculation of t(30) values for solvents in which the solvatochromic indicator dye (44) is not soluble. Introduction of electron-withdrawing substituents e.g. Cl [323], F, CF3, C6F13 [324]) in the betaine molecule reduces the basicity of its phenolate moiety, which allows the direct determination of x(30) values for somewhat more acidic solvents. Moreover, the Hpophilic and fluorophilic penta(trifluoromethyl)-substituted betaine dye (46) is more soluble in nonpolar solvents e.g. hexafluoro-benzene) than the standard dye (44) [324]. Conversely, the solubility in aqueous media can be improved through replacement of some of the peripheral hydrophobic phenyl groups in (44) by more hydrophilic pyridyl groups, to yield the more water-soluble betaine dye (47) [325]. The Ej values of these new secondary standard betaine dyes correlate linearly with the x(30) values of (44), which allows the calculation of x(30) values for solvents in which only betaine dyes (45)-(47) are sufficiently stable and soluble for the UV/Vis spectroscopic measurements [324, 325]. 

Reference probes, usually employed as external standards, are VOCI3 (neat) in the case of NMR, and [Et4N][NbCl6] ([Et4N]Cl + freshly sublimed NbCls, saturated in ultrapure acetonitrile and sealed under N2 or Ar) in the case of Nb NMR. All data presented in this text are referenced against these standards. Since there are distinct temperature, counterion and solvent isotope effects upon shielding (cf. Section 2.1.3), these should be taken into account in order to provide reproducible data. 

The columns were attached to the end of the plastic tubes by which they are inserted into the calorimeter. Columns could therefore be readily changed with a minimum disturbance of the temperature equilibrium. Inside the plastic tube were the effluent tubing and the leads to the thermistor were fastened to a short piece of gold capillary with heat-conducting, electrically insulating epoxy resin. Veco Type A 395 thermistors (16 kQ at 25 °C, temperature coefficient 3.9%/°C) were used. These are very small, dual-bead isotherm thermistors with 1 % accuracy as such, they were interconvertible, comparatively well matched, and follow the same temperature response curve (within 1 %). An identical thermistor was also mounted in the reference probe. 

Fig. 1. Quantitative restriction analysis for the evaluation of class switch recombination in polyclonally activated B cells. The rearrangement of the Syl region is reflected by the disappearance of the S7l germline restriction fragment (). This is measured by determination of the ratio of hybridisation intensities of Sr, and a reference probe. The loss of Sy, is calculated from the comparison of intensity ratios in lane A (liver DNA) and B (activated B cells). Further details are given in Section 4.3.1. of this chapter.

Fossett and Prosser, using a 1.52 m-diameter tank, injected a small amount of tracer solution (usually as a very long pulse) into the recirculation loop. They defined the mixing time as the time from the start of injection to the time when there was no difference between the concentration measured by a probe in the tank and the concentration measured by a reference probe in a solution made up to the final mean bulk concentration. For single-nozzle jets they correlated their data by... 

The principle of microcalorimetry is iUustraled with Fig. A.10.7 (pTA of TA Instruments, Inc.). The tip of an atomic force microscope, AFM, is replaced by a Pt-wire that can be heated and modulated, as is illustrated in detail with Fig. 3.96. A typical resolution is about 1.0 pm with heating rates up to 1,000 K min. A temperature precision of +3 K and a modulation frequency up to 100 kHz has been reached. The figure shows the control circuit for localized thermal analysis. In this case the probe contacts the surface at a fixed location with a programmed force, controlled by the piezoelectric feedback of the AFM. A reference probe is attached next to the sample probe with its tip not contacting the sample, allowing for... 

Microcapsules with cationic character, prepared by the company Lipotec, have been used for application on cotton fabrics (Style 400, Test Fabrics, Inc). In a preliminary study, a sun filter (OMC) was used as a reference probe during the encapsulation process. 

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