Drift method

It was noted that the content of functional groups on the surface of studied A1,03 was 0,92-10 mol/g of acid character for (I), FOS-IO mol/g of basic character for (II). The total content of the groups of both types was 1,70-lO mol/g for (III). The absence of appreciable point deviations from a flat area of titration curves in all cases proves simultaneously charges neutralization character on the same adsoi ption centers and non-depending on their density. The isoelectric points of oxide surfaces have been detenuined from titration curves and have been confirmed by drift method. 

In this contribution, the steady-state isotopic transient kinetic analysis-diffuse reflectance Fourier transform spectroscopy (SSITKA-DRIFTS) method provides further support to the conclusion that not only are infrared active formates likely intermediates in the water-gas shift (WGS) reaction, in agreement with the mechanism proposed by Shido and Iwasawa for Rh/ceria, but designing catalysts based on formate C-H bond weakening can lead to significantly higher... 

Jacobs, G., and Davis, B.H. 2007. Low temperature water-gas shift Applications of a modified SSITKA-DRIFTS method under conditions of H2 co-feeding over metal/ ceria and related oxides. Appl. Catal. A Gen. 333 192-201. 

The flow rate caused a large effect on peak areas with a 25% decrease causing a 17% increase in observed peak areas. This indicates that flow repeatability of an instrument must be better than 1.5% and ideally around 1% to achieve acceptable results from this assay procedure. Results also show that solvent evaporation and temperature changes need to be minimised to reduce the effects of drifting method conditions. The system suitability criteria for this method had to be derived excluding the results achieved for the non-rugged Techsphere column and are shown in Table 5.24. A fuller description of the results for this study can be found in reference [26]. 

We have studied the dissolution kinetics of calcite in stirred CO2 water systems at CO2 partial pressures between 0.0003 and 0.97 atm and between 5° and 60°C, using pH-stat and free drift methods (J ) Our results suggest a mechanistic model for reactions at the calcite-aqueous solution interface that has broad implications to the controls on calcite dissolution and precipitation under diverse chemical and hydrodynamic conditions. 

Experimental. We studied the dissolution of semi-optical grade crystals of Iceland spar (44.5 cm g and 96.5 cm g ) in dilute solutions as a function of pH, PCO2 and temperature. The pH-stat method was used to identify forward reactions far from equilibrium (in the near absence of backward reaction). The "free drift" method was used to study the reaction near equilibrium where both forward and backward rates must be considered. Details of the experimental procedures are given elsewhere ( ). 

There are many different methods of sample preparation. Most of the sample holders contain a cavity into which the powder sample is filled, usually from the top. However, the side-loading (also referred to as side-drift) method is considered the best packing method. Unfortunately, many commercial sample holders do not permit sample filling by this method. As a result, holders have been specially fabricated for this purpose. Since organic compounds predominantly consist of atoms with low atomic numbers, there will be significant penetration of X-rays. This can result in peak displacement as well as broadening. Detailed information on sample preparation is available in the literature. ... 

The term lEP has been used even quite recently for a zero point determined by drift method [18]. The principle of the method is as follows. A series of buffer solutions of equal volume and different pH (in this instance chloroacetic acid-sodium chloroacetate for acidic range and NH3-NH4NO3 for basic range) is prepared. The same amount of powder is added to each solution and the pH of the slurry is measured. The instant change in pH (negative or positive) induced by addition of powder is plotted as the function of initial pH. The pH at which this change equals zero is taken as the zero point. This method is in fact a modified potentiometric titration without correction. Consequently such results are referred to as pH in Table 3.1. Moreover, weak acids often adsorb specifically and this affects the obtained zero point, thus pristine value can be only obtained in case of fortuitous coincidence using this method. 

The terms batch equilibration [653], pH drift method [654], addition method [552], solid addition method [655], powder addition method (cited in [656] after [654]), potentiometric titration [234] ( sic —in the present book, the term potentiometric titration is reserved for a different method, described in Section 2.5), and salt addition [573] ( sic —in the present book, the term salt addition is reserved for a different method, described later in this section) refer to the same method, which is now described. A series of solutions of different pHs is prepared and their pHs are recorded. Then, the powder is added and the final pH is recorded. The addition of a solid induces a shift in the pH in the direction of the PZC. The pH at which the addition of powder does not induce a pH shift is taken to be the PZC. Alternatively, the PZC is determined as the plateau in the pHfln, (pH ,.,., .j) curve. The method assumes that the powder is absolutely pure (free of acid, base, or any other surface-active substance), which is seldom the case. Even with very pure powders, the above method is not recommended for materials that have a PZC at a nearly neutral pH. Namely, the method requires accurate values of the initial pH, which is the pH of an unbuffered solution. The display of a pH meter in unbuffered solutions in the nearly neutral pH range is very unstable, and the readings are not particularly reliable. The problem with pH measurements of solutions is less significant at strongly acidic or strongly basic pHs (see Section 1.10.3). The above method (under different names) became quite popular, and the results are referred to as pH in the Method columns in the tables in Chapter 3. The experimental conditions in the above method (solid-to-liquid ratio, time of equilibration, and nature and concentration of electrolyte) can vary, but little attention has been paid to the possible effects of the experimental conditions on the apparent PZC. The plateau in the pH, , (pH, ,, ) curve for apatite shifted by 2 pH units as the solid-to-liquid ratio increased from 1 500 to 1 100 [653]. Thus, the apparent PZC is a function of the solid-to-liquid ratio. 

The potentiometric mass titration method [657,658] produces results equivalent to those of the drift method described above. The same amount of base is added to three dispersions with different solid-to-liquid ratios and a constant ionic strength. The dispersions are titrated with acid, and the pH is recorded as a function of the amount of acid added. The intersection point of the obtained curves is taken as the PZC. In other words, the PZC is identified with the pH at which solid addition does not induce a change in pH. The drift method and mass titration are based on the same principle, the difference being that in potentiometric mass titration, the reagents are added in a different order. Potentiometric mass titration is affected by the acid or base associated with the powder in the same way as in the drift method and mass titration. The advantage of potentiometric mass titration over the drift method is that in the former the pH is measured only in buffered systems. 

In the mass titration method, the PZC is determined as the natnral pH of a concentrated dispersion. A detailed description of the experimental procedure can be found in [667], Mass titration become popular in the late 1980s [668,669], but the same method was already known in the 1960s as the pH drift method [183], Usually, a series of natural pH values of dispersions with increasing solid loads is reported, but only the natural pH of the most concentrated dispersion is actually used. The only role of the data points obtained at lower solid loads is to confirm that a plateau was reached in pH as a function of solid load that is, a further increase in the solid load is unlikely to bring about a change in pH. The mass titration method is based on the assumption that the solid does not contain acid, base, or other surface-active impurities. This is seldom the case, thus mass titration often produces erroneous PZCs. In this respect mass titration is similar to the potentiometric titration without correction illustrated in Figure 2.7, only the solid-to-liquid ratio is different. The experimental conditions in mass titration (solid-to-liquid ratio, time of equilibration, nature and concentration of electrolyte, and initial pH) can vary, but little attention has been paid to the possible effects of experimental conditions on the apparent PZC. The effect of an acid or base associated with solid particles on the course of mass titration was studied in [670], To this end, a series of artificially contaminated samples was prepared by the addition of an acid or base to a commercial powder. The apparent PZC of silicon nitride obtained in [671] by mass titration varied from 4.2 (extrapolated to zero time of equilibration) to 8.2 for time of equilibration longer than 20 days. The method termed mass titration was used in [672], but it was different from the method discussed above. 

A few PZCs reported in [1] (and quoted after [1]) were obtained by methods other than titration or electrokinetic methods. The principles of these methods were not described in detail, but names such as suspension effect, by adsorption of Zn and Co, and minimum gelation rate suggest that these PZCs do not meet the standards recommended in the present book, and such results are ignored in Chapter 3. On the other hand, a few PZCs obtained by means of the drift method cited after [1] are cited as such in Chapter 3. 

From drift method. Titration in 0.01 M NaCl starting at natural pH produced similar PZC. 

This paper presents examples of the application of diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy to the study of acid site chemistry and catalysis by product-selective molecular sieve materials. The most attractive features of the DRIFT method are that (i) uncompacted, high-surface-area powders can be studied in controlled chemical environments and over a wide range of temperatures (ii) reactants can be passed through a bed of the powdered sample and infrared spectra recorded simultaneously at temperatures in excess of 500 C with high species detection sensitivity and (iii) the sample can be repeatedly regenerated and exposed to reactants without being removed from the diffuse reflectance (DR) cell assembly. 

The DRIFT method has proven to be extremely useful for studies of the 0-H, 0-D, C-H, and C-D stretching region (nominally 2000 to 4000 cm" ) of molecular sieve materials (1-3). Kazansky and coworkers (4-6) have employed the DRIFT technique to investigate the combination and overtone structure of the 0-H and 0-D band systems... 

The DRIFTS method was used for studying the surface of polymer fibers, including wood fibers [717-720] and powdered cellulose [721, 722], A nanometer-range sensitivity was demonstrated in the detection of organic... 

The PZC values have been shown to change systematically with the extent of oxidation for example, the more oxidized the carbon, the lower its PZC [12] value. Other ways to determine the PZC are the pH drift method used by Lopez-Ramon et al. [72], as well as the reverse mass titration, a modification of the Noh and Schwarz method [63,68]. 

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