Silica gel

“ The solution must be prepared fresh daily. Loss of formic acid was observed after 24 h. Pipetting below the liquid level of the standard will minimize loss of formic acid from evaporation. APPENDIX B: SAMPLING TUBE PREPARATION Silica gel cleaning procedure: Add 500 to 600 mL deionized water, slowly and with stirring, to ca. 200 mL volume of silica gel in 1-L beaker. When exothermal reaction has subsided, heat in boiling waterbath for ca. ”

“ This method has been evaluated only in dry air [1] . Silica gel may have a reduced capacity at high humidity. The methanol peak could interfere in low-level analyses.
OTHER METHODS: This revises and combines Methods S139 and S142 [2] . The methods for other aliphatic amines are similar [3,4,5,6] . To avoid possible sample instability on silica gel, OSHA developed a method (OSHA 34) which employs XAD-7 coa ted with 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole for sampling followed by HPLC of the derivative [7] . ”

“ Silicic acid is a colloid. The gelatinous silicon hydrate is its hydrogel, the soluble hydrate is the hydrosol (Chapter XII.) Both varieties may be easily obtained from the alkaline silicates and from water-glass. The very same substances—that is, aqueous solutions of soluble glass and acid—taken in the same proportion, may produce either the gelatinous or the soluble silica, according to the way these solutions are mixed together. If the acid be added little by little to the alkaline silicate, with continuous stirring, a moment arrives when the whole mass thickens to a jelly, hydrogel ”

“ Silicon in the pure state is a dark brown powder; if ignited at a very high temperature it assumes a chocolate colour, which is supposed to be the allotropic condition, because it no longer burns when heated moderately in oxygen or air, and is not attacked by hydrofluoric acid. [Pg 162] The most interesting combination of silicon is the teroxide called silicic acid, silica (SiO3) . Silicon is next to oxygen so far as regards its plentifulness, and is found in the state of silica in nearly every mineral, but especially in rock crystal, quartz, flint, sand, jasper, agate, and tripoli. ”

“ Its specific gravity is 2·66. It cannot be fused by the most intense heat of our furnaces, but at the flame of the oxy-hydrogen blowpipe it melts into a limpid colourless glass. By peculiar chemical methods, an aqueous solution of it may be made artificially, similar to what nature presents us with in many thermal springs, as in those of Reikum and of Geyser in Iceland, and of most mineral waters, in minute quantity. There is no acid except the fluoric which can directly dissolve dry or calcined silica. Silica is composed of 48·04 silicon, and 51·96 oxygen. ”

“ When required, the analysis is best made on the ignited residue, and separately reported as "analysis of the insoluble portion."
Silicon only occurs in nature in the oxidised state; but the oxide generally known as silica (SiO2) is common, being represented by the abundant minerals—quartz, flint, &c. Silica, combined with alumina, lime, oxide of iron, magnesia and the alkalies, forms a large number of rock-forming minerals. Most rock masses, other than limestones, contain over 50 per cent. of silica. The following are analyses of some of the commoner silicates ”

“ The principle of its production is very simple, although great skill and experience are necessary to ensure its excellence. Silica (commonly under the form of sand) is heated with carbonate of potassa or of soda, and slaked lime or oxide of lead, until the mixture fuses, and combination takes place. After a time the melted mass becomes perfectly limpid and free from air-bubbles, when it is allowed to cool until it assumes the peculiar tenacious condition proper for working. ”

“ The artificial forms of silica are all amorphous.
The test for a silicate consists in fusing the suspected body with sodic or potassic carbonate, heating the residue with acid, and evaporating to dryness. If the residue be then treated with hot water the silica remains undissolved in the form of a white powder, which will yield a colourless bead when fused with sodic carbonate upon a piece of platinum foil before the blowpipe flame. If silica be fused with borax it becomes slowly dissolved, forming a clear, colourless bead. ”

“ By heating a solution of sodium silicate in a glass vessel the glass is attacked (an acid silicate being formed) and silica separates at ordinary temperatures in a hydrated amorphous form, at higher temperatures but below 180° as tridymite, and above 180° as quartz.
Silicates. — These compounds are to be regarded as salts of silicic acid, or combinations of silicon dioxide and metallic basic oxides; they are of great importance since they constitute the commonest rock-forming and many other minerals, and occur in every petrographical species. ”

“ Amorphous silicon is produced, like aluminium, by decomposing the double fluoride of sodium and silicon (sodium silicofluoride) by means of sodium: Na2SiF6 + 4Na = 6NaF + Si. By treating the mass thus obtained with water the sodium fluoride may be extracted and the residue will consist of brown, powdery silicon. In order to free it from any silica which might be formed, it is treated with hydrofluoric acid. This silicon powder is not lustrous; when heated it easily ignites, but does not completely burn. It fuses when very strongly heated, and [102] has then the appearance of carbon. ”

“ Again, cane sugar, which is composed of charcoal, oxygen, and hydrogen, crystallizes in hard transparent four-sided and irregular six-sided prisms, but is not called a salt. Silica or sand is found crystallized most perfectly in nature in six-sided pyramids, but is not a salt; it is an acid termed silicic-acid. Sand has no acid taste, because it is insoluble in water, but when melted in a crucible with an alkali, such as potash, it forms a salt called silicate of potash. ”

“ The silicates are decomposed by this treatment and the solution is evaporated to dryness on the water-bath, and when dry slightly heated. The silicic acid (silica) is by this treatment rendered insoluble. It is collected on a filter, washed, ignited, and weighed. The resulting compound should be mixed with ammonium fluorid and sulfuric acid, and after the disappearance of the silica the residue should be dried and weighed. The loss in weight represents the true silica. The loss in weight should be corrected by calculating the sulfates of the alkalies back to oxids. ”

“ Silicon never occurs in the free state, but combined with oxygen it forms silica, which constitutes more than one-half of the earth’s crust. The oxid of silicon occurs in the very common form of quartz, and likewise, as silicate of alumina, lime or magnesia. Silicon forms an essential part of many minerals, such as the feldspars, amphiboles, pyroxenes, and the micas, besides being an essential ingredient of many other minerals. ”

“ A fine, white, tasteless, infusible powder, insoluble in all acids, after being heated, except the hydrofluoric; requires the heat of the oxyhydrogen blowpipe for its fusion; approaches the precious stones in hardness; soluble in strong alkaline solutions; its salts are called SILICATES. Sp. gr. 2·66. See Glass, Glass, Soluble, &c.
Silica, Hydrates of. By pouring a dilute solution of sodium silicate into a considerable excess of hydrochloric acid the whole of the silica is retained in solution, together with the chloride of sodium formed by the action of the hydrochloric acid upon the soda. ”

“ Silex, or SILICA, abounds in flint, sand, sandstone, agate, jasper, &c.; it forms the basis of many precious stones, and particularly of those which strike fire with steel. It is rough to the touch, scratches and wears away metals; it is acted upon by no acid but the fluoric, and is not soluble in water by any known process; but nature certainly dissolves it by means with which we are unacquainted, and thus produces a variety of siliceous crystals, and amongst these rock crystal, 52 which is the purest specimen of this earth. ”

“ The anhydrous acid is not known, since on evaporating the aqueous solution it gradually decomposes into silicon fluoride and hydrofluoric acid.
Silicon chloride, SiCl4, was prepared by J. J. Berzelius (Jahresb., 1825, 4, p. 91) by the action of chlorine on silicon, and is also obtained when an intimate mixture of silica and carbon is heated in a stream of chlorine and the products of reaction fractionated. It is a very stable colourless liquid which boils at 58° C. Oxygen only attacks it at very high temperatures. ”

“ Another method is to precipitate a solution of silicate of soda or potassa (soluble glass) with dilute hydrochloric acid, and to treat the precipitate as before.
Nearly pure silica may also be procured by heating colourless quartz to redness, and plunging it into cold water, by which treatment the quartz is rendered so friable as to be easily reducible to fine powder. Ordinary flints, subjected to this method, are found to yield silica in a condition approaching to purity. Amorphous silica is much more easily attacked by solvents than the crystalline variety. ”

“ Physical properties (such as specific gravity, refractive index and X-ray diffraction pattern) and toxicological properties depend on the chemical composition and the molecular structure. Silica occurs in both crystalline and non-crystalline forms. Crystalline forms are physical states in which the silicon dioxide molecules are arranged in a repetitive pattern that has unique spacing, lattice structure and angular relationship of the atoms. Crystalline silica forms (often referred to as polymorphs) include quartz, cristobalite, tridymite, keatite, coesite, stishovite and moganite [2,3,4] . ”

“ It constitutes the principal portion of most of the hard stones and minerals which compose the crust of the globe; occurring nearly pure in rock crystal, quartz, agate, calcedony, flint, &c. Silica or silicic acid may be obtained perfectly pure, and also in the finest state of comminution, by taking the precipitate formed by passing silicated fluoric gas through water, filtering, washing, and igniting it, to expel the last traces of the fluoride of silicon. ”

“ The nodules of silex, especially those found in the chalk, are not always homogeneous and solid. Sometimes there is remarked an organic form towards their centre, as a madrepore or a shell, which seems to have served as their nucleus; occasionally the centre is hollow, and its sides are studded over with crystals of quartz, carbonate of iron, pyrites, concretionary silex or calcedony, filled with pulverulent silica nearly pure, or silex mixed with sulphur; a very singular circumstance.
Flints are observed to be generally humid when broken immediately after being dug out of the ground ”

“ Glass is substantially a solid solution of silica with a variety of metallic oxides, chiefly those of sodium, potassium, calcium and lead, sometimes magnesium, boron, zinc, barium and others.
By itself silica is too refractory to work easily, though silica glass has some very valuable properties, and the alkaline oxides in particular serve as the fluxes in common use. Other oxides are added to obtain various desired properties, and some impurities may go with them. ”

“ Silica and Insoluble Silicates.—Weigh up another 2 grams of the dried sample, and place them in a platinum dish; moisten with water, and cover with 20 c.c. of sulphuric acid. Evaporate and heat gently to drive off the greater portion of the free acid. Allow to cool; and repeat the operation. Extract by boiling with dilute hydrochloric acid, filter, wash, dry, ignite, and weigh. The quantity of insoluble silicates is determined by dissolving out the separated silica with a strong boiling solution of sodium carbonate. The residue (washed, dried, and ignited) is weighed, and reported as "sand." ”

“ Silica is probably the most widely distributed substance in nature. Silicate of alumina is, for example, the basis of all clayey soils, and silica, in the pure form of quartz, is the chief constituent of the sand of the sea and of all those rocks which are known as sandstones. Rock-crystal, amethyst, agate, onyx, jasper, flint, etc., are all varieties of silica. Crystalline silica is hard enough to scratch glass—a fact utilised, as we shall see, in the sand-blast which is used for the purpose of engraving patterns on glass. ”

“ When these lower hydrates are ignited without access of air, they are decomposed into hydrogen, silicon, and silica—that is, it may be supposed that they form silicon hydride (which decomposes into silicon and hydrogen) and silica (just as phosphorous and hypophosphorous acids give phosphoric acid and phosphuretted hydrogen) . When ignited in air, they burn, forming silica. They are none of them acted on by acids, but when treated with alkalis they evolve hydrogen and give silicates; for example, leucone: SiH2O3 + 4KHO = 2SiK2O3 + H2O + 2H2. They have no acid properties. ”

“ It will not take fire if heated strongly in the air or oxygen, even if before the blowpipe flame. Its density is such that it sinks in strong sulphuric acid, and hydrofluoric acid fails to dissolve it, although it is soluble in a mixture of hydrofluoric and nitric acids. It does not become oxidised, even if fused with potassic nitrate or chlorate, unless a white heat is obtained, when it burns brilliantly, giving rise, on so doing, to the formation of silica.
In addition to the above, a graphitoid form of silicon, occurring in plates, has been described by Deville and Wöhler. ”

“ It is certain, however, that silica apparatus must never be too strongly heated in contact with basic substances. Silica is easily attacked by alkalis and by lime, less readily by copper oxide, and still less by iron oxide. ”