SODIUM SILICATES, COLLOIDAL and CRYSTALLINE* JAMES G. VAIL Philadelphia Quartz Company, Philadelphia, Pennsylvania
The most familiar soluble silicates are those which are primarily characterized by colloidal properties. However, crystalline hydrated silicates of definite composition are now known and are equally remarkable and interesting i n their properties. Some properlies of silicates and the induslrial apfdicalions based upon lhem are discussed.
bodies may be associated with any desired quantity of water. The ease with which the anhydrous products may be put into solution ranges from great difficulty and the need of special conditions in the case of Na20,4SiOa to quite ready solubility a t the alkaline end of the series of colloidal products. The variation of properties among these composi+ + + + + + tions is conveniently suggested by three specimens. OLUBLE glasses, by which we mean soluble A solution of Na20.4SiOz containing 40% solids is silicates and silicate solutions, are perennially stiff enough at atmospheric temperatures to exhibit a interesting because of their capacity to display conchoidal fracture when a lump is broken in the hand a wide range of physical and chemical properties. and fluid enough to be rolled into a ball which exhibits This has made them favorite materials for the experi- lively bouncing properties. This ball floats readily mental chemist and the development engineer not to in a thinly fluid silicate of composition Na20.2SiOz mention the amateur patentee. Although from the containing approximately 50y0 solids. The most manufacturer's point of view few of the new uses give alkaline of the three colloidal silicates chosen for illusprospective outlets for substantial tonnage, and in a tration is a solution containing 63% solids. It also low-cost commodity tonnage is the only hope of profit, can be rolled into a ball which, instead of bouncing yet new applications and properties adapted to new when dropped on a hard surface, acts more like a industrial uses have a strong attraction to those of lump of lead. Instead of breaking with a conchoidal us who are naturally curious and who take pleasure fracture it may be drawn out into long threads like molasses taffy. Because of its high concentration it in solving technical problems. sinks in the liquid which floated the bouncing ball. The most familiar soluble silicates are those which We are not here dealing with definite compounds, for are primarily characterized by colloidal properties. The solutions a t high concentration are sirupy, the all the intermediate compositions can be made. These amount of solids they can contain a t a given viscosity are colloidal systems permitting the preparation of is a function of the ratio between sodium oxide and products of graded properties from one extreme of silica which, among industrial products, ranges from composition to the other. Silicate solutions of this Na20,4Si02 up to Na20.1.5Si02. These glass-like type are the basis of acid-proof and refractory cements, quick- and slow-setting adhesives, grease-proof coatings, * Presented before the Division of Chemical Education at the parting films for rubber, detergents, deflocculators, eighty-fifth meeting of the A. C. S., Washington. D. C., March and endless compositions of matter to be found in the 28, 1933, as a contribution to the symposium on "Recent Deliterature and the arts. velopments in Various Chemical Industries."
S
The reason that colloidal silicates more alkaline than NazO.1.5SiOz are not offered lies in the tendency of the preparations to crystallize. This applies to both fused glasses and systems which contain water. Soluble silicate crystals have long been known, hut their preparation in dry, free-flowing form, free from uncrystallized or supercooled material is a development of recent years (1). Four crystalline hydrates of sodium metasilicate are known ( 3 ) : NazSiOa.5Hz0,melting point 72.Z0, triclinic Na2Si03.6H20,melting point 62.8', monoclinic NazSiOa.8Hz0,melting point 48.3O, monoclinic NazSi03.9Hz0,melting point 47.8', orthorhombic The 5 and 9 hydrates are available to industry. These crystals dissolve with the absorption of heat, though uncrystallized liquids of identical composition either raise or make much smaller changes in the temperature of the water used to dissolve them. It is interesting to note that the admixture of two apparently solid materials each containing approximately the same quantity of water can yield, in a few minutes, a thin fluid while the temperature is lowered in the process. For example, a jelly-like silicate of NazO, 4SiOz composition, containing about 57% of water, is sufficiently solid to be handled manually. When this is mixed with an equal weight of the ennahydrate of sodium metasilicate, the two promptly react to form a thinly fluid, cold solution. It is obvious that no change of concentration is involved. The different alkalinities of silicate solutions of the same Na10 content are readily shown by a special mixture of indicators prepared by dissolving 0.05 g. trinitrohenzene in 50 cc. of methyl alcohol (97 to 99%). adding 10 cc. of distilled water, thoroughly mixing, then dissolving in the trinitrobenzene solution 0.08 g. indigo carmine (indigo disnlfonate), 0.03 g. malachite green, and 0.02 g. phenolphthalein. Solutions are then made up from four silicates having ratios 1:4, 1:2.6, 1:2, and 1:1, respectively (commercial grades "S," "Star," "D," and "Metso" sodium metasilicate 5 hydrate), in such a way that each contains 2.5% Na20. By adding 1 cc. of the special indicator solution to each 100 cc. of the silicate solution there is obtained a range of colors corresponding to pH's approximately 11.6, 11.9, 12.3, and 13. It is advisable to add the indicator last to the silicate of 1:4 ratio and shake immediately to avoid the possibility of coagulating the silicate by local dehydration by the alcohol of the indicator solution. This experiment illustrates by its range of colors the qualitative difference between the silicates when used as alkalies and buffers. A further comparison results from the observation of the effect of titration with acid upon pH. As most washing operations involve some degree of neutralization of the alkali and as pH affects the corrosion of sensitive materials being cleansed, such as soft metals and delicate fabrics, i t will be seen that discrimination should be used in the choice of silicates.
Sodium metasilicate pentahydrate, though it contains hut 28.9% total NazO, has actually about twice as much alkali available above pH 10 as soda ash with 58% Na20, and three times as much as trisodium phosphate. Compared with caustic soda, however, its activity is very much milder-a fact which is welcomed by the dry-cleaning industry where solvent is clarified in an alkaline solution. A small quantity of caustic solution accidentally carried over in the.solvent can work great damage to the sensitive fabrics being dry-cleaned, whereas experience has shown the risk with metasilicate to be much less and its properties are such as to make it a very effective means of reclaiming the spent solvent (4), (5), (6), (10). The silicates, it should be remembered, are neutralized by very weak acids. Even a sodium bicarhonate solution will react with any of the silicate solutions to form carbonate and, if the concentration is sufficient, to precipitate silica. On the other hand, dilute solutions may be neutralized or made acid without immediate precipitation. The gel-forming characteristics are to a considerable extent a function of the concentration of silica so that while sodium bicarbonate easily causes the gelation of a solution of 1 :4 composition, it is much less likely to cause this effect in the stable solution of metasilicate. The reaction between silicate and bicarbonate solutions can be conveniently illustrated by dissolving 30 cc. of "En silicate (Naz0.3.2SiOz, 40° Baumk, clear) in 100 cc. of distilled water. Separately dissolve 5 g. of sodium bicarhonate in 60 cc. of distilled water and add 5 to 10 drops of phenolphthalein indicator solution and 5 drops of methyl orange. Only the alkaline color of the methyl orange appears. Now mix the two solutions well and allow to stand quietly. The mixture which takes on the brilliant red of the phenolphthalein a t the moment of mixing gradually develops an opalescence and sets to a stiff gel. Among the uses for crystalline sodium metasilicate is a wide range of cleaning operations. First, in laundering it has been shown to be an efficient builder for soap ( 2 ) . It increases the lather and detergent effect in soft water and is a useful wetting-out agent with or without soap in what the laundryman calls the break. This is the initial step of the washing procedure where albuminous materials are dispersed, soap-destroying compounds neutralized, and loosely attached dirt removed a t low temperature before the washing proper. Under conditions appropriate to efficient laundering, the metasilicate is easily and completely rinsed from the fabric and no deposition of silica in the fiber takes place. Ash determinations of cotton goods washed twenty times show no measurable increase in the non-cellulose material. References to the accumulation of ash in fabrics laundered in the presence of silicate solutions are usually based either on opinions unsupported by experiment, or on the use of silicates a t inappropriate concentrations, or in conjunction with added materials able to decompose them.
In the cleaning of metals preparatory to electroplating and finishing, a high degree of perfection in the removal of grease is required. The excellent wetting power of metasilicate, either alone or with an addition of about 1% of rosin, together with the low risk of injuring the metal surface, has been shown to be a substantial technical advantage both in those processes where still baths are used and when the cleaning action is accelerated by the liberation of gas on the surface of work made cathode or anode in an electric circuit. The ease with which the solution can be completely rinsed from the cleaned articles, usually with water followed by an acid dip, is another reason for the use of this new reagent (S), (9). In the cleaning of glassware, a lustrous surface is obtained by using metasilicate of low concentration, and accumulated scale is gradually removed from bottle-washing machinery. On account of the undesirability of completely removing deposited materials which have a lubricating effect from chains and hearing surfaces which operate in the hot wash liquors, i t is usual to use metasilicate combined with caustic soda a t some sacrifice of efficiency. It is to be hoped that this penalty will not always be imposed, for machines could be designed which do not depend on lubricated parts moving through wash liquors. The germicidal effect of sodium metasilicate is greater than that of other alkaline salts when used alone, though not as great as that of caustic soda. The germicidal effect of caustic soda is increased by the addition of sodium chloride and/or alkaline salts of which sodium metasilicate is the most potent (7). Severe corrosion of milk bottles washed in caustic solutions is a common experience but can be entirely controlled by the use of metasilicate either alone or combined with the caustic. The hydrates of sodium metasilicate are very soluble. The ~entahvdratemelts in its water of crvstallization a t 7 i . 2 ' ~ .'It is interesting to note t h a i the sirupy liquid thus formed is so easily supercooled that recrystallization of the melted mass does not ordinarily occur. When used for general cleaning of tile and cement surfaces, it has been repeatedly observed that subsequent cleaning operations are easier. The reason is found in the deposition in porous structures, such as portland cement, of a siliceous deposit which makes the surface less permeable. This is a variant of the familiar phenomenon which occurs when silicate solutions are used to preserve eggs by depositing gels in the pores of a calcareous shell or when the siliceous types of silicate solutions are used for hardening and oil-proofing concrete floors. Thus i t will be seen that the crystalline silicates differ from the colloidal forms in providing ready solubility in cold water, high pH sharply modified from the behavior of caustic solutions, greater adaptability to the exigencies of industrial cleaning and, a t the same time, show a close relationship in their behavior to the more familiar forms.
LITERATURE CITED
( 1 ) C. L. BAKER, U. S. Pat. 1,898,707 ( 2 / 2 1 / 3 3 ) . ( 2 ) C. L. BAKER,Ind. Eng. Chem., 23, 1025 (1931). presently to be published ( 3 ) BArcEn, PABST,AND WOODWARD, in The American Mineralogist. Cleaning and Dyeing World, 19, 29 (4) T. K. CLEVELAND, Inrr^ 101'), \--.., L*"I,.
Drycleaner, 9,17-9.24 (May, June, 1932). ( 5 ) W. K. COOLEY, (6) W.,nK. COOLEY AND C. B. FUNDALL, ihid.. 9 , 16,. 23 (July. .,", . , ' " Z L
(. 7.) S. . R . HALL,"Chemical sterilization with alkalies." Ph.D. dissertation. Iowa State College 1930. ( 8 ) W. L. PINNER,Phila. Quartz Co., Bulletin 465 (1931). ( 9 ) J. G . VAE, Trans. A m . Inst. Chem. E n g . . 2 5 , 1 2 3 4 2 (1930). Ldy. b Dry Cleaning J . (Canada), 13, (10) J. S. WARRINGTON, 9 (1933).
