Fluorosilicic acid

Fluorosilicic acid also has the unique ability to differentiate effectively between TBDMS and TIPS ethers, as seen in the competitive deprotection experiment in eq 3. The presence of bulky cosolvents, such as t-butanol, which serve as ligands to silicon, further enhance selectivity at the expense of reaction rate. The reaction time can be reduced by increasing the amount of H2SiFe however, the accompanying increase in acid concentration precludes the use of acid-labile substrates. 

Form Supplied in 25% aq solution, clear and colorless. Upon dehydration, this reagent decomposes giving HF and SiF4. Handling, Storage, and Precautions fluorosilicic acid is toxic and very corrosive. Like HF, this compound attacks glass and therefore must be stored in plastic containers. Use the same precautions as for aq HF solutions. Use in a fume hood. 

For the deprotection of silyl ethers, aq H2Sip6 is superior to aq HF. Fluorosilicic acid is a more potent cleaving agent than HF, allowing its use in stoichiometric or even catalytic quantities (eqs 1 and 2). The lower acid concentrations result In milder reaction conditions that are compatible with several acid labile moieties (see below). 

The effects of increasing the steric bulk of the substrate at carbon were also probed. Conpounds (l)-(4) (eq 6) were paired and used in competitive deprotection reactions, providing the results reported in Table 1. Excellent selectivity was observed in the deprotection of primary TBDMS vs. tertiary TBDMS, and secondary TBDMS vs. tertiary TBDMS derivatives. On the other hand, selectivity was only fair for primary TBDMS vs. secondary TBDMS, and secondary TBDMS vs. secondary TIPS ethers. As observed before, t-butanol provided greater selectivity than t-BuOH-MeCN solvent mixtures. 

A Compd. B Solvent Temp. (°C) mmol H2S1F6 Time (h) % A ( 2%) %b ( 2%) Selectivity 

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Crude lead contains traces of a number of metals. The desilvering of lead is considered later under silver (Chapter 14). Other metallic impurities are removed by remelting under controlled conditions when arsenic and antimony form a scum of lead(II) arsenate and antimonate on the surface while copper forms an infusible alloy which also takes up any sulphur, and also appears on the surface. The removal of bismuth, a valuable by-product, from lead is accomplished by making the crude lead the anode in an electrolytic bath consisting of a solution of lead in fluorosilicic acid. Gelatin is added so that a smooth coherent deposit of lead is obtained on the pure lead cathode when the current is passed. The impurities here (i.e. all other metals) form a sludge in the electrolytic bath and are not deposited on the cathode. 

Acetic acid Acetic anhydride Acid mixtures Battery fluids Chloroacetic acid Chlorosulphonic acid Chromic acid Dichloroacetic acid Fluoroboric acid Fluorosilicic acid... 

Chemical Designations - Synonyms Fluorosilicic acid, Hexafluosilicic acid, Hydrofluosilicic acid. Hydrogen hexafluorosUicate, Sand acid, Silicofluoric acid Chemical Formula HjSiF -HjO. Observable Characteristics - Physical State (asshipped) Liquid Color Transparent, straw colored, colorless Odor Acrid, sharp. 

It is completely inert to 98% sulphuric acid to at least 160°C and to even higher temperatures at lower concentrations. Practically, it may be used to 200°C in all concentrations and to 225-250°C at concentrations between 80% and 90%. Fuming sulphuric acid containing sulphur trioxide attacks tantalum at room temperature as do hydrofluoric and fluorosilicic acids. 

A number of cement materials are used with brick. Standard are polymer resin, silicate, and sulfur-based materials. The most widely used resins are furane, vinyl ester, phenolic, polyester, and epoxies. Carbon-filled furanes and phenolics are good against nonoxidizing acids, salts, and solvents. Silicates and silica-filled resins should not be used in hydrofluoric or fluorosilicic acid applications. Sulfur-based cements are limited to 93°C (200°F), while resins can be used to about 180°C (350°F). Silicate-based cements are available for service temperatures up to 1000°C (1830°F). 

Recall that, because of the strong H-F bond, hydrofluoric acid is a weak acid with a small acid dissociation constant Ka of 6.8 x 10-4. In contrast, the other binary acids of the halogen family—HC1, HBr, and HI—are strong acids that completely dissociate in water.) The fluorosilicic acid produced, H2SiF6, is a water-soluble substance with a structure as in Fig. 6.3.1. 

Figure 6.3.1 The molecular structure of fluorosilicic acid, produced by the action of hydrofluoric acid (HF) on glass.

Depressants used in these experiments include oxalic acid and fluorosilicic acid. 

A number of different depressant combinations were examined during a laboratory development test programme. Oxalic acid, citric acid and fluorosilicic acid were among the... 

Figure 23.10 Effect of levels of fluorosilicic acid on individual mineral recoveries.

In a modified process, potassium substitutes for calcium to form BiTMgeCag which liquates to the top of the bath and is removed from the molten lead. The Betts process is based on electrolytic refining using a solution of lead flu-orosilicate and fluorosilicic acid. While lead is deposited on the cathode, bismuth goes to the anode where it is collected with other impurity metals. It is then filtered, dried, smelted, and further refined, depending on the purity desired. Impurities are removed by adding molten caustic and zinc, and finally by chlorination. 

As far as the addition of fluoride to drinking water is concerned, the key requirement is that the compound chosen should yield free fluoride ions readily on dissolution in water [123]. Sodium fluoride can be used as it is reasonably soluble and dissolves readily [124]. However, even by the 1950s it was proving relatively expensive, and so the use of alternative compounds was investigated [125]. This identified fluorosilicic acid and its disodium salt as possible alternatives (Table 4), and their use as the source of fluoride in drinking waters continues to this day. 

Hydrofluosilicic Acid. See Fluorosilicic Acid in Vol 6 of Encycl, p F141-R... 

Fertilizer Acid Filmerine Fish Oil Flaxseed Oil Flowers of Antimony Fluorine Fluoroethylene Fluorosilicic Acid Fluorosulfonic Acid Fluorosulfuric Acid Fluorspar Fluosilicic Acid Fluospar... 

FLUOSILICIC ACID Fluorosilicic Acid, HexaRuosllicic Acid, Hydrofluosilicic Add, Sand Add, Silicofluoric Acid Corrosive Liquid, 11 3 0 0... 

Introduction of Fluorine with Silicon Tetrafluoride and Salts of Fluorosilicic Acid... 

Silicon tetrafluoride, fluorosilicic acid, and barium and disodium hexafluorosilicate are poisonous. In mice, the oral LD50 value for disodium hexafluorosilicate is 61 mg kg-. 6... 

Strong inorganic acids such as sulfuric acid,31 fluorosulfonic acid,31 or fluorosilicic acid.63... 

Silicon(IV) fluoride can be scrubbed by passing the gas into a vessel containing water or aqueous ammonia. The silicon(IV) fluoride reacts with water to produce fluorosilicic acid [dihydrogen hexafluorosilicate (2-)] by the reaction shown by the equation... 

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