Sulfites, determination gravimetric

A gravimetric method was proposed for Ag and other precious metals, based on reduction with dipyrone (39)121. Determination of Ag(I) ions, strongly complexed with ammonia, sulfite, thiocyanate or thiosulphate, was carried out by potentiometric titration with sodium A.A-diethyldithiocarbamate (11). The titration end-point was better defined when using a sulfide ion-selective electrode (ISE)137. Polypyrrole-based ISEs are poisoned by Ag(I) cations leading to modified selectivities. Ag(I) ions can be implanted... 

Like cements, the elemental composition is determined by XRF or AAS techniques. The XRF bead is made using lithium tetraborate at 1050°C. Sulfide content cannot be determined by XRF. Sulfite, SO3 , and sulfate, S04 , are safely analyzed by XRF. Na2C03 -I- K2CO3 fusion is carried out for Ca, Mg, Fe, and A1 analysis by AAS. Lanthanum chloride is used as a sulfate interference suppressant. Gravimetric sulfate determinations are also carried out by precipitation as barium sulfate. The Leco Carbon-Sulfur Analyzer can also be used for quality control purposes. The fluoride is determined by XRF or a pyrohydrolysis method. The measurement of particle size distribution is carried out in a manner similar to that for cements and clays. 

The copper content in cupric compounds is usually determined by gravimetric analysis. In the conventional method, Cu is usually reduced to Cu by the addition of sulfurous acid, ammonium hydrogen sulfite, ascorbic acid, or ferrous sulfate. It is then precipitated as cuprous thiocyanate by the addition of ammonium thiocyanate and estimated gravimetrically. 

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