Silicic acid

“ Like all feeble acid oxides it is capable of forming, with other acids, saline compounds which are but slightly stable and are very easily decomposed in the presence of water. The chief peculiarity of the silicates consists in the number of their types. The salts formed with nitric or sulphuric acid exist in one, two, and three tolerably stable forms, but for acids like silicic acid the number of forms is very great, almost unlimited. ”

“ Silicotungstic acid is obtained as quadratic pyramids from its mercurous salt which is prepared from mercurous nitrate and the salt formed on boiling gelatinous silicic acid with a polytungstate of an alkali metal. ”

“ 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. ”

“ 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. ”

“ Now, when carbonic acid acts upon silicate of soda or silicate of potash, we have already seen that the silicic acid is precipitated in the hydrated form, and that the carbonic acid has united with the soda or potash to form carbonate of soda or carbonate of potash. These substances being left in the painting and penetrating to a certain depth beneath its surface, must find their way out, and in almost every instance have done so in the form of an efflorescent substance, which has caused the picture to have the appearance of being mildewed over its surface. ”

“ During the first absorption of silicon fluoride by water, silicic acid is separated in the form of a jelly, but a certain quantity of the silicon fluoride also remains in the liquid, because the hydrofluoric acid formed dissolves the other part of the silica [9] and forms the so-called hydrofluosilicic [106] acid: H2SiF6 = SiF4 + 2HF = SiH2O3 + 6HF - 3H2O. That is to say, a metasilicic acid, SiH2O3, in which O3 is replaced by F6. This view of the composition of hydrofluosilicic acid may be admitted, because it forms a whole series of crystallisable and well defined salts. ”

“ Doubtless, just in the same manner as we now (by means of well-known reagents which are common in nature) precipitate it in the laboratory. With regard to that found in quartz lodes finely disseminated through the gangue, the change was brought about by the same agency which caused the silicic acid to solidify and take the form in which we now see it in the quartz veins. Silica is soluble in solutions of alkaline carbonates, as shown in New Zealand geysers; the solvent action being increased by heat and pressure, so also would be the silicate or sulphide of gold. ”

“ 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. ”

“ O,SiO2, where TiO is basic, and the trivalent metals occur in the divalent group R ́ ́ ́2O2; this formula is also supported by Zambonini.
More recently the problem of the constitution has been attacked by Bruckmoser, using Tschermak’s method of determining the nature of the salts present in silicates. In this method, the mineral is digested with hydrochloric acid, at a temperature not greater than 60°, until decomposition is complete; the silicic acid formed is washed by decantation, and dried in air at a constant temperature; it is weighed at regular intervals until the weight is constant. ”

“ Silicon chloride is purified from free chlorine by distillation over metallic mercury. Free silicon forms the same substance when treated with dry chlorine. It is a volatile colourless liquid, which boils at 59° and has a specific gravity of 1·52. It fumes strongly in air, has a pungent smell, and in general has the characteristic properties of the acid chloranhydrides. It is completely decomposed by water, forming hydrochloric acid and silicic acid, according to the equation: SiCl4 + 4H2O = Si (OH) 4 + 4HCl. ”

“ 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. ”

“ After moistening with concentrated hydrochloric acid the mass is boiled with water, filtered, and the filtrate examined according to the ordinary methods for silicic acid, alumina, iron, lime, magnesia, and alkalies. The residue after 356treatment with concentrated sulfuric acid is dried, but not ignited, and boiled with a concentrated solution of sodium carbonate with the addition of a little caustic soda, filtered, heated, and the silicic acid separated from the solution. ”

“ Add an additional 20 mL diazomethane derivatizing reagent to jar. Cap securely, and using platform shaker rotate 5 to 10 RPM for a minimum of 1.5 hours. 3. Transfer a 10-mL sample aliquot into a screw-top test tube with a PTFE-lined cap. Add approximately 10 mg of silicic acid to the solution. Allow to stand an additional hour. 4. Filter an aliquot through a 0.45-μm PTFE filter into a 2-mL GC vial. Label. NOTE: It is important to filter the samples because the silicic acid fines can build-up in the chromatographic system and cause deterioration of analytical column performance. ”

“ The simple oxidation of silicon develops an enormous quantity of heat, very much more than the combustion of carbon; a heat which is sufficient to refine the metal in the retorts of iron and steel works without the addition of carbon. Silicon, which in nature has passed wholly into the state of silicic acid, or been burned, must, at the moment of its combination with oxygen, have been the cause of an intense heating both in our own globe and in the other stars, which are also composed of silicates. ”

“ Contrary to popular belief, the major portion of the iron of the human body is found, not in the blood but in the muscles, even after their contained blood has been removed. Silicic acid is found in very small quantities in bones, hair, and blood. It is supplied by many vegetable foods. Calcium fluoride is found in teeth, and to a slight extent in bone. Fortunately for us, these inorganic foods, whose withdrawal exercises deleterious influences on the economy, are, as a rule, present in great quantity in the actual foods in a mixed diet. ”

“ The liquid rapidly deposits free selenium, owing to the oxidation of the hydrogen selenide retained in solution. Light appears to decompose the triselenide into free selenium and the subselenide B4Se.
Silicon selenide, SiSe3, has likewise been obtained by M. Sabatier by heating crystalline silicon to redness in a current of hydrogen selenide. It presents the appearance of a fused hard metallic mass incapable of volatilization. Water reacts most vigorously with it, producing silicic acid, and liberating hydrogen selenide. ”

“ This is the dialyzing vessel, and it is into this that the mixture of silicate and hydrochloric acid must be put. The solution should not be more than an inch deep in the dialyzing vessel, which is then made to float upon distilled water in a larger vessel of suitable size. The distilled water should be changed every day, until no precipitate can be obtained in it with nitrate of silver, and when this point is arrived at, all the chloride of sodium will have passed through the vegetable parchment into the larger vessel of water, and nothing but silicic hydrate will remain behind in solution. ”

“ The fine earth is saturated with a concentrated aqueous solution of hydrofluoric acid to which a few drops of sulfuric acid are added. It is then digested until nearly dry. If any undecomposed particles remain, the treatment is continued until complete decomposition is secured. The silica is thus all volatilized as hydrofluosilicic acid and the bases pre-existing in the soil are left as sulfates. This method of treatment is especially recommended when it is desired to estimate the whole quantity of any of the soil constituents with the exception of silica. ”

“ It must always be examined quantitatively for the residue left on evaporation. It is used occasionally for the examination of silicates. It attacks silica, forming fluoride of silicon, which is a gas. When the introduction of another base will not interfere with the assay, the substance may be mixed in the platinum dish with fluoride of ammonium, [Pg 56] or of potassium, or of calcium, and hydrochloric acid, instead of treating it with the commercial acid. It is only required in special work. The fumes and acid are dangerous, and, of course, glass or porcelain vessels cannot be used with it. ”

“ Notwithstanding all precautions, the ignited precipitate of silica is rarely wholly pure. It is tested by volatilisation of the silica as silicon fluoride after solution in hydrofluoric acid, and, if the analysis has been properly conducted, the residue, after treatment with the acids and ignition, should not exceed 1 mg.
The acid produces ulceration if brought into contact with the skin, and its fumes are excessively harmful if inhaled. ”

“ He conceives that the fact which he had some years before noticed, of the analogy between the base of silica, and that of boracic acid, together with those observed by Berzelius, furnish sufficient reasons for classing silica 174 amongst the acids, and for rendering it probable, that the oxide of iron and silica undergo a real chemical combination in the warm water, and that they are separated from the latter in consequence of the reduction of its temperature, after it has issued from the mountain. ”

“ Silicon.—All soils contain silica in abundance. Though silica forms so large a part of the ash of plants and is especially abundant in the straw of cereals, there is no evidence that it is required in plant life. Popularly, it is believed to “stiffen” the stems of cereals and grasses, but plants grown without it will do perfectly well. It would, however, appear that soluble silica does play some part in enabling phosphoric acid to be better assimilated by the plant. Silicates, however, have not justified their use as direct fertilizers. ”

“ 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. ”

“ 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. ”

“ In the use of silicates of soda and potash, one precaution is very necessary, namely, that you really have a soluble silicate, not a mere mechanical mixture of ground flint and alkali. This is a very different thing, and one which, if it be not carefully guarded against, will lead to nothing but disappointment.
Again, the silicate of soda may be properly made, in the first instance, but in a long exposure to the atmosphere, the soda attracts carbonic acid, and is liberated from the silex, and this has disappointed my expectation more than once. ”

“ In many respects it resembles silicon; it does not fuse when heated, and even oxidises with difficulty, but liberates hydrogen when fused with potash. When fused with silica it liberates silicon. With carbon in the electrical furnace it forms ZrC2, with hydrogen it gives ZrH2 (like CaH2, Winkler, Vol. I., p. 621) ; hydrochloric and nitric acids act feebly on it, but aqua regia [148] easily dissolves it. It is distinguished from silicon by the fact that hydrofluoric acid acts on it with great facility, even in the cold and when diluted, whilst this acid does not act on silicon at all. ”