Alkali silica gel

The mechanism of action by which silane and siloxanes reduce expansion has been attributed to water repellence and air entrainment. Phosphate addition or coatings may interfere with the dissolution of silica gel and the formation of gel. It is also possible that phosphate reduces the osmotic potential and the swelling pressure in the gel. The manner in which air entrainment reduced expansion was attributed to the accommodation of alkali-silica gel in the air void system. For example, it was found that air-entrained concrete with 4% air voids could reduce AAR expansion by 40% [23]. 

Several cases of distress of concrete railway ties have been reported. Examination of cracks in such concretes has revealed that they were filled with secondary products. Ettringite was a prominent constituent, accompanied by CH, CaCOj, and alkali-silica gel. DTA data indicated the presence of a substantial amount of secondary ettringite in many samples but petrographic and XRD did not reveal any ettringite.t ] Thermograms were used to quantitatively estimate the amounts of ettringite formed in failed samples. 

Other possibilities are the reduction of nitro groups by applying the sample solutions to adsorbent layers containing zinc dust and then exposing to hydrochloric acid vapors [110] 3,5-Dinitrobenzoates and 2,4-dinitrophenylhydrazones can also be reduced in the same way on tin-containing silica gel phases [111] Cellulose layers are also suitable for such reactions [112] Seiler and Rothweiler have described a method of trans-salting the alkali metal sulfates to alkali metal acetates [113]... 

TLC has been traditionally regarded as a simple, rapid, and inexpensive separation method, currently used mainly for preliminary examinations to give an indication of the number and variety of pigments present and help in the selection of suitable separation and purification procedures for further analysis. To avoid epoxy-furanoid rearrangements caused by inherent silica gel acidify, one pellet of a strong alkali such as KOH or NaOH should be added to the water used to make the thin layer, or in case of ready commercial plates, 0.1% triethylamine (TEA) should be added to the mobile phase. 

V0)2+ ion in a square pyramidal coordination. On alumina only the latter configuration is stable. Tanaka and Matsumoto IBS) attributed a poorly resolved spectrum on silica gel to the formation of small crystallites of a V02+ salt. Upon dilution with an alkali salt the resolution increased. They also observed much better resolution on alumina, indicating that the vanadyl ion is more uniformly dispersed on this surface. 

The mechanism of the inhibitive action of LiOH proposed by Stark et al. [7] is attributed to the formation of lithium silicate that dissolves at the surface of the aggregate without causing swelling [7], In the presence of KOH and NaOH the gel product incorporates Li ions and the amount of Li in this gel increases with its concentration. The threshold level of Na Li is 1 0.67 to 1 1 molar ratio at which expansion due to alkali-silica reaction is reduced to safe levels. Some workers [22] have found that when LiOH is added to mortar much more lithium is taken up by the cement hydration products than Na or K. This would indicate that small amounts of lithium are not very effective. It can therefore be concluded that a critical amount of lithium is needed to overcome the combined concentrations of KOH and NaOH to eliminate the expansive effect and that the product formed with Li is non-expansive. 

Both alumina and silica gel are more stable physically than the common polystyrene supports. The alumina-bound catalysts are particularly promising because of their higher activity and higher selectivity compared with the silica gel-bound catalysts. Alumina also is stable in alkali. The alumina-bound catalysts 32 and 33 worked well for reaction of 1-bromooctane with concentrated aqueous sodium cyanide 118). 

Thiazolylazo)resorcinol [2246-46-0] M 221.2, m 200-202 (dec), A-max 500 nm. Dissolved in alkali, extracted with ethyl ether, and re-ppted with dil HCl. The purity was checked by TLC on silica gel using pet ether/ethyl ether/EtOH (10 10 1) as the mobile phase. 

The effect of the basicity of aldol condensation catalysts on their activity was thoroughly investigated by Malinowski et al. [372—379]. The observed linear dependence of the rate coefficients of several condensation reactions on the amount of sodium hydroxide contained in silica gel (Figs. 12 and 13) supported the view that the basic properties of this type of catalyst were actually the cause of its catalytic activity, though the alkali-free catalyst was not completely inactive. The amphoteric nature of the catalysis by silica gel, which can act also as an acid catalyst, was demonstrated [380]. By a stepwise addition of sodium acetate to a HN03-pretreated silica gel catalyst the original activity for acetaldehyde self-condensation was decreased to a minimum (when an equivalent amount of the base was added) by further addition of sodium acetate, the activity increased again because of the transition to a base... 

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