Acidity, exchangeable measurement

Anion exchange resins have been used to determine extremely low concentrations of nitrite down to nanomoles in seawater. Wada and Hattori [148] formed an azo dye from the nitrite using N-1 naphthylethylene diamine dihydrochloride and then adsorbed the dye in an anion exchange resin. The dye is then eluted from the column with 60% acetic acid and measured spec-trophotometrically at 550 nm. 

Tartaric Acid. Quantitative measures of total tartrate are useful in determining the amount of acid reduction required for high acid musts and in predicting the tartrate stability of finished wines. Three procedures may be used. Precipitation as calcium racemate is accurate (85), but the cost and unavailability of L-tartaric acid are prohibitive. Precipitation of tartaric acid as potassium bitartrate is the oldest procedure but is somewhat empirical because of the appreciable solubility of potassium bi-tartrate. Nevertheless, it is still an official AO AC method (3). The colorimetric metavanadate procedure is widely used (4, 6, 86, 87). Tanner and Sandoz (88) reported good correlation between their bitartrate procedure and Rebeleins rapid colorimetric method (87). Potentiometric titration in Me2CO after ion exchange was specific for tartaric acid (89). 

Carboxyl content was determined by the magnesium-ion-exchange method (5). For chlorous acid-treated pulp, the carboxyl group concentration was found to be 14.8 meq/100 g for sheets reduced with SBH, the carboxyl group concentration was 3.4 meq/100 g. Neither had any significant aldehyde group content the copper number (6) of the chlorous acid sheets measured 0.9 meq/100 g that of the SBH sheets measured 0.3 meq/100 g. 

The proposition of a cube law for acid dependence could be disregarded with more confidence it is were not for some isotopic exchange measurements. Two studies of iodine exchange between I2 and iodate agree on the rate law... 

Radical enzymes are, like radical reactions in general, usually characterised by high turnover for the catalytic processes. It is hence rather difficult to study these reactions experimentally in order to gain direct insight into the mechanisms. In addition, several of the enzymes are membrane bound or anaerobic, why the determination of the crystal structures of many of these has been a formidable task. Most of the mechanistic proposals have thus been based on mutagenesis experiments (site specific exchange of an amino acid), kinetic measurements and isotope effects, and studies of inhibitor by-products . 

The following sequence of overall acidity (density of Bronsted and Lewis acid sites) measured via t.p.d. of ammonia was found for dealuminated Y zeolites exchanged with alkali-metal ions H-D-Y Li-D-Y > Na-D-Y > Cs-D-Y > K-D-Y the strength of the acid sites was particularly low in the case of K-D-Y. The lower the overall acidity of the support, the lower was the observed average oxidation number of Mo. The zeolites with reduced overall acidity exhibited a higher ability to decompose ammonia. 

The released alkali is titrated with standard acid to measure the exchangeable aluminium. 

An electron-transfer reorganization energy, X, of 0.94 eV (21) was also calculated from Marcus theory (22). This value is very close to that of the Fe(III)-Fe(II) system in acidic medium (23) and to cytochrome c (0.7 eV) and cytochrome b5 (1.2 eV) from measurements of self-exchange (24). As far as we are aware, this is the first example of a consistent X value for a heme protein measured electrochemically and by self-exchange measurements. 

Cubane has had an interesting place in the discussion of the correlation between C-H acidity and carbon hybridization. Its acidity was measured by the H exchange NMR technique and found to be about 6.6 x 10 as reactive as benzene. An experimental gas phase measurement of the proton affinity (PA) as 404kcal/mol is available. (See Tables 3.14 and 3.38 for comparable data on other hydrocarbons.) Both of these values indicate that cubane is somewhat more acidic than expected on the basis of the carbon hydridization. There appears to be unusual hybridization of the anion in this case. An AIM analysis suggests that the C—C bond paths in the anion are less than 90°, suggesting that the bonds bend inward toward the center of the ring. Sauers also noted an increase in s character on going from the hydrocarbon to the anion. Of the 28 deprotonations he examined, only cyclopropane and bicyclo[l.l.l]pentane also showed increased s character in the anion. 

Figure 16.23. Ion exchange in a poly(pyrroleperchlorate) film of 20 micron thickness after treatment with sulphuric acid as measured by EDX A, initial sample, B, sample treated with 10% H2SO4 for 240 min. Adapted from Polymer 21, 899 (1986), with pennission of Elsevier Science Ltd., Kidlington.

Glycosylated haemoglobins have been isolated following ion-exchange chromatography of haemolysed erythrocytes. Bound hexoses, after release from the protein by hydrolysis with oxalic acid, were measured as 5-hydroxy-... 

The kinetics of the protophilic dedeuteriation of thieno[2,3-6]-[2- H]- and -[3- H]-thiophen, thieno[3,2-6]-[2- H]- and -[3- H]-thiophen, and benzo[fc]-[2- H]- and -[3- H]-thiophen have been measured. The activation parameters for the deuterium-exchange reactions of the isomeric thienothiophens with t-butyl alcohol catalysed by potassium t-butoxide were determined. The relative deuterium-exchange rate constants for the 2-position in thiophen, the isomeric thienothiophens, and benzo[/>]thiophen were found to be 1 10 9 4, whereas the corresponding relative values for the 3-positions are 1 94 10 65. These partial rate factors were compared with those obtained upon acidic exchange and interpretation was attempted. ... 

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