Lead hexafluorosilicate

Fluoride, hexafluorosilicate, and sulphate in the presence of each other the following differences in solubilities of the lead salts are utilized in this separation lead hexafluorosilicate is soluble in water lead fluoride is insoluble in water but soluble in dilute acetic acid lead sulphate is insoluble in water and in boiling dilute acetic acid, but soluble in concentrated ammonium acetate solution. (See Table 5.12.)... 

Purification is carried out by adding zinc, which removes silver (Parkes process), or by electrolysis in a solution of lead hexafluorosilicate and hex-afluorosilicic acid (Betts process). 

Silicon, germanium, tin and lead can make use of unfilled d orbitals to expand their covalency beyond four and each of these elements is able (but only with a few ligands) to increase its covalency to six. Hence silicon in oxidation state -f-4 forms the octahedral hexafluorosilicate complex ion [SiFg] (but not [SiCl] ). Tin and lead in oxidation state -1-4 form the hexahydroxo complex ions, hexahydroxostannate(IV). [Sn(OH) ] and hexahydroxoplum-bate(IV) respectively when excess alkali is added to an aqueous solution containing hydrated tin(IV) and lead(IV) ions. 

Dining maintenance work on casings of fans used to extract perchloric acid fumes, seven violent explosions occurred when flanges sealed with lead oxide-glycerol cement were disturbed. The explosions, attributed to formation of explosive compounds by interaction of the cement with perchloric acid, may have involved perchlorate esters and/or lead salts. Use of an alternative inorganic silicate-hexafluorosilicate cement is recommended. 

Wash well with cold water. Divide into two parts. (1) Smaller portion. Add excess 2m acetic acid and boil (this will dissolve any lead fluoride). White residue indicates sulphate. The residue is soluble in 6m ammonium acetate solution. (2) Larger portion. Treat cautiously with concentrated sulphuric acid and test with a moist glass rod. Milkiness of the water and etching of the tube indicate fluoride. Add Ba(N03)2 solution and warm gently. White crystalline precipitate indicates hexafluorosilicate. 

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