Weak acids strength

With Ga-Beta it was found that, when the Si/Ga ratio increased from 10 to 40, the number of strong sites decreased drastically for Si/Ga between 10 and 25 and then reached a plateau above Si/Ga = 25 [53], The strength and density of acid sites in H(Ga, La)-Y were also found to be lower than those in HY crystals of the type used in FCC preparation (LZY-82) [250], Similar catalytic selectivities were obtained for both Ga-ZSM5 and A1-ZSM5 in Prins condensation of isobutylene with formaldehyde. Catalytic tests coupled with microcalorimetric measurements have shown that medium to weak acid strength sites favor the selectivity to isoprene [254],... 

Matsuda and his co-workers also showed that extensive isomerization of 4,4 -DIPB on a HM(20) catalyst was observed at high conversion of biphenyl in the presence of limited amounts of propylene (initial propylene/biphenyl ratio=1.0).44 However, the selectivity was almost constant during the reaction although the selectivity for 4,4 -DIPB was less than that of HM(10) catalyst. They suggest that the low isomerization activity of HM(10) is related to its weak acid strength. 

That it is not the esterification as such which causes the increase of the acid strength but the addition of the oxygen atom with its lone pair to the boron atom with a sextet is seen from the very weak acid strength of the compound of one molecule of diol with one molecule of boric acid (P. H. Hermans). ... 

Arsenic speciation by 1C can be simple and reproducible. One procedure, conductivity detection of As(III) and As(V), was reported by McCrory-Joy [17]. The procedure was sensitive and there was no interference by ions such as NO,c. HP04, and S04 , that are present in the sediment extract. However, detection of As(IIl) is not possible by suppressed conductivity because of its weak acid strength. 

Together with the well-accepted threshold DPE value for superacidity (250 kcal/mol) [18], it is noteworthy that only marginal variation in was observed for the TMPH" " adduct as DPE decreases from ca. 320 (weak acidic strength) to 250 kcal/mol (super strong acidic strength) [18]. As such, it is obvious that the P-TMP NMR approach is inferior for differentiating Bronsted acidic strengths in soHd acid catalysts. 

Methods Used to Measure Weak Acid Strength... 

Catalytic tests of Prins condensation of isobutylene with formaldehyde over iron-substituted MFI catalysts have shown that medium to weak acid strength sites favor the selectivity to isoprene [241]. [Ga]-ZSM5 was also tested as a catalyst for this condensation, but its catalytic selectivity was rather close to that of [A1]-ZSM5 [241]. 

The strength of an acid is measured by the value of its dissociation constant, strong acids, e.g. HCl, HNO3. being substantially fully ionized in solution and weak acids predominately unionized. 

Since the hydrogen-element bond energy decreases from sulphur to tellurium they are stronger acids than hydrogen sulphide in aqueous solution but are still classified as weak acids—similar change in acid strength is observed for Group Vll hydrides. 

In dilute aqueous solution hydrogen fluoride is a weak acid but the acid strength increases with the concentration of hydrogen fluoride. 

The strength of a weak acid is measured by its acid dissociation constant, which IS the equilibrium constant for its ionization m aqueous solution... 

Note that the concentration of H2O is omitted from the expression because its value is so large that it is unaffected by the dissociation reaction.The magnitude of provides information about the relative strength of a weak acid, with a smaller corresponding to a weaker acid. The ammonium ion, for example, with a Ka of 5.70 X 10 °, is a weaker acid than acetic acid. 

All other things being equal, the strength of a weak acid increases if it is placed in a solvent that is more basic than water, whereas the strength of a weak base increases if it is placed in a solvent that is more acidic than water. In some cases, however, the opposite effect is observed. For example, the pKb for ammonia is 4.76 in water and 6.40 in the more acidic glacial acetic acid. In contradiction to our expectations, ammonia is a weaker base in the more acidic solvent. A full description of the solvent s effect on a weak acid s piQ or on the pKb of a weak base is beyond the scope of this text. You should be aware, however, that titrations that are not feasible in water may be feasible in a different solvent. 

Directions are provided in this experiment for determining the dissociation constant for a weak acid. Potentiometric titration data are analyzed by a modified Gran plot. The experiment is carried out at a variety of ionic strengths and the thermodynamic dissociation constant determined by extrapolating to zero ionic strength. 

Because they are weak acids or bases, the iadicators may affect the pH of the sample, especially ia the case of a poorly buffered solution. Variations in the ionic strength or solvent composition, or both, also can produce large uncertainties in pH measurements, presumably caused by changes in the equihbria of the indicator species. Specific chemical reactions also may occur between solutes in the sample and the indicator species to produce appreciable pH errors. Examples of such interferences include binding of the indicator forms by proteins and colloidal substances and direct reaction with sample components, eg, oxidising agents and heavy-metal ions. 

The bottoms from the stripper (40—60 wt % acid) are sent to an acid reconcentration unit for upgrading to the proper acid strength and recycling to the reactor. Because of the associated high energy requirements, reconcentration of the diluted sulfuric acid is a cosdy operation. However, a propylene gas stripping process, which utilizes only a small amount of added water for hydrolysis, has been described (63). In this modification, the equiUbrium quantity of isopropyl alcohol is stripped so that acid is recycled without reconcentration. Kquilibrium is attained rapidly at 50°C and isopropyl alcohol is removed from the hydrolysis mixture. Similarly, the weak sulfuric acid process minimizes the reconcentration of the acid and its associated corrosion and pollution problems. 

Effect on Oxide—Water Interfaces. The adsorption (qv) of ions at clay mineral and rock surfaces is an important step in natural and industrial processes. SiUcates are adsorbed on oxides to a far greater extent than would be predicted from their concentrations (66). This adsorption maximum at a given pH value is independent of ionic strength, and maximum adsorption occurs at a pH value near the piC of orthosiUcate. The pH values of maximum adsorption of weak acid anions and the piC values of their conjugate acids are correlated. This indicates that the presence of both the acid and its conjugate base is required for adsorption. The adsorption of sihcate species is far greater at lower pH than simple acid—base equihbria would predict. 

The characteristics of soluble sihcates relevant to various uses include the pH behavior of solutions, the rate of water loss from films, and dried film strength. The pH values of sihcate solutions are a function of composition and concentration. These solutions are alkaline, being composed of a salt of a strong base and a weak acid. The solutions exhibit up to twice the buffering action of other alkaline chemicals, eg, phosphate. An approximately linear empirical relationship exists between the modulus of sodium sihcate and the maximum solution pH for ratios of 2.0 to 4.0. 

Kinetic mles of oxidation of MDASA and TPASA by periodate ions in the weak-acidic medium at the presence of mthenium (VI), iridium (IV), rhodium (III) and their mixtures are investigated by spectrophotometric method. The influence of high temperature treatment with mineral acids of catalysts, concentration of reactants, interfering ions, temperature and ionic strength of solutions on the rate of reactions was investigated. Optimal conditions of indicator reactions, rate constants and energy of activation for arylamine oxidation reactions at the presence of individual catalysts are determined. 

Lateral interactions between the adsorbed molecules can affect dramatically the strength of surface sites. Coadsorption of weak acids with basic test molecules reveal the effect of induced Bronsted acidity, when in the presence of SO, or NO, protonation of such bases as NH, pyridine or 2,6-dimethylpyridine occurs on silanol groups that never manifest any Bronsted acidity. This suggests explanation of promotive action of gaseous acids in the reactions catalyzed by Bronsted sites. Just the same, presence of adsorbed bases leads to the increase of surface basicity, which can be detected by adsorption of CHF. ... 

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