Selective titration

Photoemission of adsorbed xenon, abbreviated as PAX, is a site-selective titration technique, in which the UPS spectrum of physisorbed Xe reveals the nature of the Xe adsorption site [52, 53]. Since we are dealing with a weakly physisorbing atom, these experiments have to be done at cryogenic temperatures on the order of 50-60 K. We first explain the theory behind the PAX method and then illustrate the technique with an example. 

Selective titration by using suitable solvent and titrant of acidic/basic components of physiologically active moiety of a salt,... 

Thus, questions remain the large differences in acidity reported here might be attributed both to the nature of the catalysts (affected by the method of preparation, the loadings of the different elements, and whether the catalysts are in the oxide or sulfided states) and to the method of acidity measurement (total acidity determination or selective titration of acid sites with a specific strength). Furthermore, the protocols for measurement (e.g., the temperature and duration of outgassing before acidity determination) may also affect the results. Therefore, it can be inferred that some standardization of measurement conditions is required to reach consensus regarding the influence of phosphorus on the acidity of hydrotreating catalysts. 

Sodium methoxide dissolved in benzene-methanol mixture was introduced as a titrant by Fritz and Lisicki. Various applications have since been made with a number of basic solvents, notably ethylenediamine, butylamine, and dimethyl-formamide. Other titrants that have found use in basic solvents are potassium methoxide in benzene-methanol, potassium hydroxide in ethanol or isopropyl alcohol, and tetrabutylammonium hydroxide in ethanol, isopropyl alcohol, or benzene-methanol mixtures. Benzene-methanol has been recommended especially for selective titrations of mixtures of acids in pyridine as the solvent. 

Although there are reports involving the use of hydrogen alone,161 the surface composition of supported Pt-Ru bimetallic catalysts are more commonly measured using a selective titration method.162-164 The titration stoichiometry of the reaction between chemisorbed oxygen and gaseous CO is different for the two metals the ratio of surface metal/02/C0/C02 is 1/0.5/2/1 for Pt and 1/1/1/0.3 on Ru.164 These ratios are independent of surface composition and the concentration of Ru and Pt in the surface can be calculated from the equations ... 

There are a number of other useful reagents for complexoraetric titrations. A notable example is ethyleneglycol bis(jS-aminoethyl ether)-N,N,N, N -tetraacetic acid (EGTA). This is an ether analog of EDTA that will selectively titrate calcium in the presence of magnesium ... 

Metrohm markets microprocessor-controlled Instruments for routine coulo-metric (KF 652 Processor) and amperometrlc-voltammetrlc (EP/KF 678 Processor) analyses. These Instruments are frequently used for research purposes. Thus, the Mettler Memotltrator and the Metrohm Tltroprocessor 636/Rod Stirrer 622 were used by Johansson et al. [62] to demonstrate that potentiometric two-phase titrations can be carried out in an automatic fashion. The typical background noise from the electrodes can be reduced by Introducing hydrophobic anions or cations in the aqueous phase. These ions also affect the acid-base equilibrium by extracting the sample ions as ion-pairs into the organic phase, which allows the conditional acidity constant (apparent K ) to be manipulated to make selective titrations possible. 

The analysis of the content of surfactant actives of betaine solutions is rather complicated. No direct titration method for the determination of the betaine concentration was available in the past, since all methods known have lead to incorrect results due to the presence of protonable by-products such as glycolic acid or free amidoamine (38). A modified titration method has been developed in recent years which gives more accurate results due to the choice of a special solvent mixture (39). The use of methanol and ethylene glycol monomethyl ether allows the almost selective titration of a betaine in its complex solution. 

The present studies show that in spite of the fact that all four hemes have close individual a-peaks the usage of spectral decomposition method gives the opportunity to carry out the selective titration of each heme and to determined their midpoint potentials. Thus, our data indepen-... 

Friess, S.D. and Zenobi, R. (2001) Protein structure information from mass spectrometry. selective titration of arginine residues by sulfonates. /. Am. Soc. Mass Spectrom., 12, 810-818. 

Figure 13-6 shows pH ranges in which various metal ion indicators (discussed in the next section) are useful for finding end points. The chart also provides a strategy for selective titration of one ion in the presence of another. For example, a solution containing both Fe and Ca could be titrated with EDTA at pH 4. At this pH, Fe is titrated without interference from Ca. ... 

Selective titration of hydroperoxides in the presence of ferrous salts... 

The C-terminal amino acid is then separated from the amino acid hydrazides, e. g., by a cation exchange resin, and identified. It is possible to mark the C-terminal amino acid through selective titration via oxazolinone ... 

Because UPS delivers information from several top atomic layers, it is difldcult to characterize trace amounts of adsorbates. This limitation can be overcome e.g. by photoemission of adsorbed xenon (PAX) [90]. This technique is a site-selective titration technique, in which Xe adsorption sites are revealed by means of UPS it has been used effectively to characterize catalytic systems [91, 92]. An alternative surface sensitive technique capable of determining trace amounts of adsorbates is metastable impact electron spectroscopy (MIES). 

After masking, demasking is possible in some cases. Demasking offers supplementary possibilities of selective titrations. 

With calcon, the color passes from red wine to blue. The reaction is somewhat less sensitive than the preceding one. With this indicator, however, the selective titration of Ca " " in the presence of Mg " " is possible since the pH value is sufficiently high for Mg + to be quantitatively precipitated as hydroxide Mg(OH)2. EDTA does not react with the precipitated magnesium until free Ca + together with that fixed to the indicator are not yet themselves complexed by EDTA. 

Ca " " can also be directly titrated with EGTA in the presence of Mg + with zincon as the indicator. This selective titration is possible because the Ca +-EGTA stability constant is much higher than that of the Mg +-EGTA complex, respectively, 10 and 10. Hence, this titration is well adapted for the titration of weak quantities of Ca in the presence of Mg " ". The final point is detected with zincon. This is an indirect detection and, for it to be effective, a weak quantity of the Zn +-EGTA complex must be added to the reaction vessel. In such a way, and because of the respective eomplexation constants. 

General methods for the estimation of bismuth present in quantity include direct weighing as sulphide precipitation as sulphide, conversion to carbonate and ignition to oxide and precipitation as phosphate. Recently the use of EDTA as a titrant for bismuth has considerably simplified the determination of this metal in pharmaceutical mixtures. Bismuth forms a complex at pH 1 to 2 and at this degree of acidity few other metals likely to be encountered (with the exception of iron) interfere bismuth may therefore be selectively titrated in the presence of, say, aluminium, magnesium, or calcium. Various methods available have been discussed by Brookes and Johnson and the following general procedure is recommended ... 

Selective titration may be complemented by temperature -programmed desorption (TPD), the origin of which is to assume that, upon heat treatment of the carbon, the oxygen surface groups evolve at different temperatures either as CO (carbonyl or guinonlc structure), CO2 (carboxyl or lactonic structure) or CO plus CO2 (carboxyl anhydrides structure) [5]. 

A given oxidation treatment will modify the pore structure and the chemical nature of the surface of an activated carbon> the extent of the modification being a function of both the structure of the carbon and the oxidation treatment. The analysis of these two aspects and the comparison of selective titration and TPD methods of analysis of oxygen surface groups are the two main objectives of this work. 

Fig. 5. Relation between NaHC03 (meq/g) and NaOH (meq/g) consumption by selective titration...

The data for the oxidized carbons of series B show a large discrepancy between the two techniques, the low values obtained by titration are due to the narrow microporosity of the samples which makes difficult the access of the alkali to the surface groups (as shown in Fig. 2, N2 adsorption is restricted in this samples at the low temperature of 77K). These results may be taken to indicate that selective titration of oxygen surface groups can be applied only in carbons with relatively wide porosity as those os series M, A and R. 

There is a linear relationship between the groups that decompose to CO2 in TPD and the groups titrated by NaOH, but such relationship has not been found with CO-evolving groups. The results described in this study show that TPD is a more comprehensive technique than selective titration to study oxygen surface groups in carbon on the other hand, titration cannot be used in carbons with narrow micropores. 

The classical titration methods for determination of amines may be applied to ethoxylates. Primary amines are reacted with salicylaldehyde, allowing titration of secondary and tertiary amines, as the first break of the curve, then of the primary amines, to the second potentiometric break. In a second titration, primary and secondary amines are reacted with acetic anhydride, allowing selective titration of tertiary amines. 

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