Ac measurements

Comparative methods may be effectively used for measurements of partial surface areas, Ac, of components in porous composites, for example for active surface area in supported catalysts. The traditional methods of Ac measurements are based on chemisorption of H2, 02, CO, NOr. and some other gases that chemisorb on an active component, and have negligible adsorption on a support [5,54], The calculation of Ac is fulfilled by an equation similar to Equation 9.18 assuming some values of w and atomic stoichiometry of chemisorption [54]. But, unfortunately chemisorption is extremely sensitive to insignificant variations of chemical composition and structure of surface, which alters the results of the measurements. 

U. Weimar and W. Gopel. AC measurements on tin oxide sensors to improve selectivities and sensitivities , Sensors and Actuators B26-27 (1995), 13-18. 

The effective tunneling barrier may be quantified by measuring the distance dependence of the tunnel conductance or tunneling current [Eq. (1) or (2), respectively]. Experimentally, the decay constant, k, may be derived from dc or ac measurements. The more accurate ac modulation... 

Unfortunately, impedance has amassed quite a few different names. For example, it is common now to see the acronym EIS , meaning electrochemical impedance spectroscopy. Strictly speaking, impedance is not a form of spectroscopy because the sample is not absorbing photons of any kind. A further linguistic problem is that AC impedance is tautologous since the word impedance itself implies an AC measurement. In this present discussion, we will simply say impedance . 

Although the literature on electrodeposited electroactive and passivating polymers is vast, surprisingly few studies exist on the solid-state electrical properties of such films, with a focus on systems derived from phenolic monomers, - and apparently none exist on the use of such films as solid polymer electrolytes. To characterize the nature of ultrathin electrodeposited polymers as dielectrics and electrolytes, solid-state electrical measurements are made by electrodeposition of pofy(phenylene oxide) and related polymers onto planar ITO or Au substrates and then using a two-electrode configuration with a soft ohmic contact as the top electrode (see Figure 27). Both dc and ac measurements are taken to determine the electrical and ionic conductivities and the breakdown voltage of the film. 

Resistivity measurements are also routinely made with an ac four probe technique. The wiring would follow according to Figure 1 and the measuring currents used would be in the range 0.1 - 10 mA with frequencies of 100 Hz (9). For flux creep now known to modify susceptibility and critical current measurements care must taken with ac measurements of resistivity although for the low current densities involved the effect will not likely be observed except very close to Tc (10) or in a magnetic field. 

Anomalous effects are often observed due to solution entering the capillary at the instant the mercury drop falls. In ac measurements, this phenomenon leads to anomalous frequency dispersion [12], In addition, the drop time becomes irreproducible. These effects may be diminished to a large extent by coating the internal wall of the capillary with a film of silicone [13]. A tip made of hydro-phobic (solvophobic) material may also be attached to the glass capillary. For example, a polyethylene tip was used [14] to discriminate against the attack of fluorides on glass in the study of double-layer structure in the presence of fluorides. In another study, capillary tips were modified with commercial narrow-bore PTFE tubes to determine arsenic in basic solution [15]. This procedure is also used for the hanging mercury drop electrode discussed in Section III. 

Recently, Lenhard (43) has used phase-selective second-harmonic ac voltammetry to obtain more nearly reversible redox potentials for a group of cyanine dyes. The main advantage of this approach is the short time scale of the ac measurements. 

For a DC measurement, what type of interface would that have to be, ideally polarized or nonpolarized Would the same restriction apply in the case of an AC measurement ... 

Figure 6. CFT feedback circuit and instrumentation for AC measurements. Reproduced with permission from reference 17. Copyright 1982 Institute of Electrical and Electronics Engineers.

The results illustrated above show that the CFT method is suitable for making chemical-sensor measurements using both bulk polymers and, in particular, thin film materials that are intrinsically weak conductors. Therefore, the CFT looks premising for such materials as poly(phenylacetylene) derivatives 24., for which carefully shielded electrometer measurements have been required in the past because of current levels at the threshold of detectability. Furthermore, the fact that the CFT always makes AC measurements reduces the problem of DC polarization of electrodes. In addition, the CFT approach should be suitable for other "chemiresistor" applications, such as the metal-substituted phthalocyanines proposed by Jarvis et. al. 2 and for Langmuir—Blodgett films 26. which, because they are so thin, may prove impossible to use in parallel-plate form, but which can be routinely used with the high-sensitivity interdigi-tated-electrode approach provided by the CFT. 

Solutions of many proteins, synthetic polypeptides, and nucleic acids show large increases in permittivity c (u>) over that of solvent, normally aqueous, at sufficiently low frequencies f = w/2ir of steady state AC measurements, but with dispersion and absorption processes which may lie anywhere from subaudio to megahertz frequencies. Although our interest here is primarily in counterion fluctuation effects as the origin of polarization of aqueous DNA solutions, we first summarize some relevant results of other models for biopolymers. 

For AC measurements of current and voltage (impossible for a galvanometer) one uses a dynamometer The static magnetic field of the galvanometer is replaced by a small electromagnet that is powered by a known current of the same frequency as the current under study that moves through the rotatable coils. 

Immittance — In alternating current (AC) measurements, the term immittance denotes the electric -> impedance and/or the electric admittance of any network of passive and active elements such as the resistors, capacitors, inductors, constant phase elements, transistors, etc. In electrochemical impedance spectroscopy, which utilizes equivalent electrical circuits to simulate the frequency dependence of a given elec-trodic process or electrical double-layer charging, the immittance analysis is applied. 

Reactance — In alternating current (AC) measurements, reactance is the imaginary part of impedance given by ... 

Ps oxidation. Only very strong oxidizers react effectively with Ps [2]. The nature of the reaction can be characterized by coupling PALS and DB (or AC) measurements. The equations that have been commonly used in case of Ps oxidation, for the o-Ps PALS and DB parameters, are as follows ... 

DB or AC measurements are of course very valuable wherever spin interactions can be involved. A typical example is for an iron complex which suffers a change from high to low spin state when cooling below about 100 K. As Ps suffers a spin conversion reaction with the former state, measuring the narrow component (p-Ps) intensity of AC spectra gives direct information on the temperature at which the spin cross-over occurs [131]. 

The calculations of Table 15 reCa to the gas state, for which as yet th is a lack of Ac measurements. Saturation measurements in gases are extremely difScult owing to the smallness of the non-linearity, since densities are low, and dielectric breakdown in gases imposes a low limit on the externally applied electric fidd strength. The accuracy of avaUable measuring techniques has restricted the measurement of such small values of Ac as 10 in gases or 10 in vapours for strongly dipolar substances. 

It should be noted that the conductivity in ionic solids involves ion transport mechanism between coordinating sites (site-to-site hopping) and local structural relaxation. In general, the conductivity measured by ac measurements, can be expressed as a function of the conductivity from dc measurements, and the frequency (Bruce et al., 1983a,b Skinner and Munnings, 2002 Girdauskaite et al., 2006 Khrokunov et al., 2006 Berenov et al., 2007) ... 

O Neil AJ, Jee RD, Moffat AC. Measurement of the cumulative particle size distribution of microcrystalline cellulose using near infrared reflectance spectroscopy. Analyst 1999 Jan 124(l) 33-36. 

---