Presence Of Alkali Cations Research Articles

Kinetics and mechanism of the reduction of dodecatungstocobaltate(III) by ?-hydroxyacids in aqueous acid solution. Evidence for alkali metal ion catalysis

The kinetics of the redox reaction of dodecatungstocobaltate(III) [abbreviated as CoIIIW] with three α-hydroxy-carboxylic acids:e.g., mandelic, glycolic and lactic acid, were investigated in the 0–5.4 pH range where the complex remains stable and does not decompose. All reactions are first-order both in oxidant and reductant over the experimental pH range. A specific metal ion effect has been observed. Oxidations of the molecular form of the three acids and the anionic form of mandelic acid are catalysed by alkali metal ions and follow a general rate law: $$ - d[Co^{III} W]/dt = 2(k_s + k_c [M^ + ])[R][Co^{III} W]$$ where ks and kc account for the spontaneous and catalysed paths respectively, M+ is either an alkali metal ion (Li+, Na+ or K+) and [R], the reductant concentration. The oxidation of lactate and glycolate ions occurs only in the presence of alkali cations. The cation catalytic order: K+>Na+>Li+ observed in this study is in accord with the polarizability of the cations. An outer-sphere alkali cation-bridged mechanism is suggested for the electron transfer step.

Synthesis and characterization of silica-alumina prepared from tetraalkylammonium hydroxides

The synthesis and textural properties of amorphous silica-alumina prepared from tetraalkylammonium solutions in presence of alkali cations (sodium and potassium) have been studied. Alkali cations are needed to polymerize silicon and aluminium from the solution and they are preferentially incorporated into the silica-alumina. All the aluminium in the solidn is present in tetrahedral coordination. The samples are dealuminated and dehydroxylated by calcination in air, leaving materials with surface areas close to 300 m 2g −1, and very narrow pore distribution, centered at ca. 20 Å of pore radii. Steaming leads to deep dealumination, while the surface area strongly decreases and the microporous structure collapses.

Structure and stability of tetrapropyl-ammonium (TPA) ions in various alkali—aluminosilicate hydrogel precursors to crystalline (M)ZSM—5 zeolites

The stability of TPA species interacting with various (Al,Si) hydrogel environments in the presence of alkali cations has been evaluated by thermal analysis (combined t.g.-d.t.a.-d.t.g.). Three different TPA—gel associations have been characterized by d.t.a. endotherms : crystalline (TPAX) n clusters (X = OH −,Br −) which are also detected by XRD, monomeric (TPAX) species in direct association with the (Al,Si) gel, and labile entities, [TPA +…(H 2O) x] bulky water clathrates, that interact very weakly with the remaining negative charges available after neutralization of the gel by small (Li,Na,K) alkali cations. The formation, relative concentration and stability of these species essentially depend on the relative amounts of Al 2O 3 and M 2O present in the gel, as well as on the nature (structure forming/breaking) of the alkali cations. Monomeric as well as water clathrated species lead to crystalline (M)ZSM—5 upon heating.

An 27Al-n.m.r. investigation of ZSM-5 type zeolite aluminosilicate precursor species

27Al-n.m.r. experiments were performed on Al-bearing Na-silicate solutions, with or without addition of tetrapropylammonium (TPA) and alkali cations. The pH was varied from 0 to 10.5 and it plays a major role in the formation and nature of the precursor aluminosilicate gel because of competitive interactions between Al, Si, alkali, and TPA ionic species. Evidence is obtained for an interaction between Al species and the organic base cations which is attenuated in the presence of alkali cations. Replacement of Na by Cs cations decreases the degree of ordering in the environment of Al. The possible impact of these findings on the understanding of the synthesis of ZSM-5 type zeolites is discussed.

Polarographic study of the cadmium-ethylenediamine-tetraacetate chelate at pH about 8

Summary The d.c. polarographic reduction of cadmium(II) in a solution of the sodium salt of ethylenediaminetetraacetic acid (EDTA) at pH ⋍ 8 was investigated. The Cd-EDTA complex species is irreversibly reduced. The electrode process is preceded by a chemical reaction in which the inactive form of CdY 2− is transformed to the electroactive one. The limiting current and the half-wave potential (about −1.2 to −1.3 V vs . SCE) are strongly dependent on the concentration, charge and nature of the supporting electrolyte cation (neutral cation). In the presence of alkali cations the increase in current follows the lyotropic order. The charge of the Cd-EDTA species has been evaluated by the Koutecky method taking into account ion-pair formation in the bulk of the solution with the neutral cations. The result z =−2 indicates that the reducible species CdYM (where M represents the neutral cation) is not formed in the bulk of the solution. The observed chemical reaction takes place in the vicinity of the mercury drop surface prior to the electrode process. The form CdYM was further proposed as the electroactive form by comparing the electrochemical behaviour of the CdYH − species (present in the bulk of the solution at 2≤pH≤5) to that present in concentrated solutions at higher pH.