Natural waters, silica content

Silica. The siUca content of natural waters is usually 10 to (5 x lO " ) M. Its presence is considered undesirable for some industrial purposes because of the formation of siUca and siUcate scales. The heteropoly-blue method is used for the measurement of siUca. The sample reacts with ammonium molybdate at pH 1.2, and oxaUc acid is added to reduce any molybdophosphoric acid produced. The yellow molybdosiUcic acid is then reduced with l-amino-2-naphthol-4-sulfoiiic acid and sodium sulfite to heteropoly blue. Color, turbidity, sulfide, and large amounts of iron are possible interferences. A digestion step involving NaHCO can be used to convert any molybdate-unreactive siUca to the reactive form. SiUca can also be deterrnined by atomic... 

Brombenztiazo (BBT) is known to be one of the best reagents for extraction-photometric determination of cadmium(II). The reagent also fonus complexes with Co(II), Cu(II), Fe(II), Ni(II), Zn(II). The aim of this work was to develop a solid-phase reagent on the base of BBT immobilized on silica gel for sorption-spectroscopic and visual test determination of Cadmium, and also for soi ption-atomic-adsoi ption determination of total heavy metals contents in natural waters. 

Detection.—Apart from naturally occurring ores of vanadium, vanadium steels, and ferrovanadium, the commonest compounds of vanadium are those which contain the element in the pentavalent state, viz. the pentoxide and the various vanadates. The analytical reactions usually employed are, therefore, those which apply to vanadates. Most vanadium ores can be prepared for the application of these reactions by digesting with mineral acids or by alkaline fusion with the addition of an oxidising agent. When the silica content is high, preliminary treatment with hydrofluoric acid is recommended. Vanadium steels and bronzes, and ferrovanadium, are decomposed by the methods used for other steels the drillings are, for instance, dissolved in sulphuric acid and any insoluble carbides then taken up in nitric acid, or they are filtered off and submitted to an alkaline fusion. Compounds of lower valency are readily converted into vanadates by oxidation with bromine water, sodium peroxide, or potassium permanganate. 

Ginzburg, I.l. and Kabanova, Ye.S., I960. Silica content in natural waters and forms of its occurrence. In Kora vyvetrivaniya (The Weathered Layer). Izd. Nauka, Moscow, 3 313-342 (in Russian). 

The highest content was found in formation waters at depths of 2-3 km, and in hydrothermal waters from 200-400 to 500-700 ppm. The content of silicic acid in some sodium carbonate-bicarbonate brines can be as high as 2 700 ppm at pH 10 (Jones et al. 1969). However, natural waters usually show a concentration of silicic acid considerably lower than the solubility limit of amorphous silica under the same conditions (Fournier and Rowe 1962 Table 3.5). Consequently, modern subsurface waters are undersaturated with respect to amorphous silica, while marine water is unsaturated also with respect to quartz (Fournier and Rowe 1977). 

The compositions of the invention contain 3-15 parts, per 100 parts of sand binder mixture by weight, of a binder system comprising a water soluble alkali metal silicate, amorphous colloidal silica and water. The key is to have very finely divided amorphous silica particles of colloidal size dispersed within the alkali metal silicate bond. It is inherent in the nature of water soluble alkali metal silicates having a molar ratio Si02/alkali metal oxide (M2O) above about 2.5, that colloidal silica is present. In the case of silicates having a ratio higher than 3.5, the colloidal silica content is such that they may be employed without adding more colloidal silica, but in the case of alkali metal silicates of lower sUica/alkali metal oxide ratio there is little or no amorphous colloidal silica present so that amorphous colloidal silica must be added in order to produce the cores and molds of the present invention. 

Some of the available quaternary ammonium silicates have been listed by Weldes and Lange (19), although the nature of the ion was not disclosed. Molar ratios of Si0j (NR4)20 range from 1,2 1 to 30 1 and silica contents of the solutions from 10 to 45%. Hydrated amorphous powders are also listed with ratios of 15.6 1 and 19.7 1. These dissolve rapidly in water and are remarkably soluble in mixtures of water and water-miscible organic solvents such as acetone and the lower alcohols. 

Natural zeolites such as Clinoptilolite (i.e., popularly known as Clino zeolites) and Chabazite have applications in various diversified fields such as water treatment, fertilizer application for soil amendment and plant growth by establishing better retention of nutrients. The Clinoptilolite has been broadly accepted for its usage in agriculture, soil amendment and feed additives because of its higher acid resistant silica content (viz., Si/Al = 1-5) [14]. However, such zeolites are contaminated by other minerals (e.g., Fe ", S04, Quartz, other zeolites, and amorphous glass) and hence they may not be suitable for several important commercial apphcafions where uniformity and purity are essential [8]. 

There appears to be, as noted for natural waters and also for soil minerals (Natarov and Betin [1962] Lerbekmo and Platt [1962]), a correlation between dissolved silica and the pH of the leaching solution. For instance, in saturated soil extracts at pH 6 and 9, the dissolved silica contents were 36 and 6 ppm, respectively (Raupach [1957]). An inverse relationship between the silica dissolved and the pH of the extracting solution was also obtained by McKeague and Cline [1963], who, however, made the additional important observation that there is no general relationship between the pH and the silica concentration of extracts from different soils. 

Ginzburg, 1.1., and E. S. Kabanova, 1960. Silica content and its form in natural waters. Kora Vyve trivaniya, Akad. Nauk SSSR, Inst. Geol. Rudnykh Mestorozhden., Petrog., Mineral, i Geokhim. 3 313. 

Here, we have taken advantage of the low silanol content in pure silica LTA and CHA type zeolites synthesized in fluoride medium to get better resolved 29Si and H solid state NMR spectra in the silanol region. This has allowed us to investigate the nature of the silanol groups in zeolites and their interaction with water molecules. 

Silica used as a filler for rubbers is silicon dioxide, with particle sizes in the range of 10-40 nm. The silica has a chemically bound water content of 25% with an additional level of 4-6% of adsorbed water. The surface of silica is strongly polar in nature, centring around the hydroxyl groups bound to the surface of the silica particles. In a similar fashion, other chemical groups can be adsorbed onto the filler surface. This adsorption strongly influences silica s behaviour within rubber compounds. The groups found on the surface of silicas are principally siloxanes, silanol and reaction products of the latter with various hydrous oxides. It is possible to modify the surface of the silica to improve its compatibility with a variety of rubbers. 

---