Calcium-thiocyanate method

The most important methods for the preparation of polyacrylonitrile are solution polymerization and suspension polymerization. The former method is particularly convenient, since when a solvent for the polymer is used, the resulting solution may be utilized directly for fibre spinning. Concentrated aqueous solutions of inorganic salts such as calcium thiocyanate, sodium perchlorate and zinc chloride make suitable solvents suitable organic solvents include dimethylacet amide, dime thy Iformamide and dimethylsulphoxide. Emulsion polymerization suffers from the disadvantage that the monomer has appreciable water-solubility and the formation of polymer in the aqueous phase can lead to coagulation of the latex. This tendency is reduced by the addition of ethylene dichloride to the system. 

Alkaline earth thiocyanates dissolved in nonaqueous solvents have received less attention [286-288]. Ionic pairs [M NCS ] and [M (NCS )2] usually cannot be distinguished by vibrational methods. However, dimeric species M2(NCS)4 are observed in calcium thiocyanate solutions in acetonitrile and in strontium and barium thiocyanates dissolved in a number of solvents (methyl acetate, tetrahydrofuran, or acetone). This association species seems to be absent in nonaqueous solutions of both beryllium [286] and magnesium [287,288] thiocyanates. The proposed structure of the barium thiocyanate dimer and the Raman v(CN) spectral region of a 1 M solution of Ba(SCN)2 dissolved in THF are shown in Fig. 20. 

Although cellulose is not soluble in common organic solvents, a series of special solvents have been found to be able to dissolve celMose. However, only some of them can be used to spin cellulose fibers via direet methods. So far, all these viable direct solvents consist of two components, e.g., N-methylmorpholine-N-oxide/ water (NMMO/HjO), lithium chloride/dimethylacetamide, trifluoroacetic acid/di-chloroethane, calcium thiocyanate/water, ammonia/ammonium thiocyanate, zinc chloride/water, and sodium hydroxide/water. 

The number and variety of ion-specific electrodes is rapidly increasing with no end in sight. At the present writing, it is possible to use such electrodes to determine, either by direct or indirect measurement, ionic concentrations of the following species ammonia, bromide, cadmium, calcium, chloride, cupric, cyanide, fluoride, fluoroborate, iodide, lead, nitrate, perchlorate, potassium, sulfide, sodium, sulfur dioxide, and thiocyanate, all by direct measurement, and by titration methods aluminum, boron, chromium, cobalt, magnesium, mercury, nickel, phosphate, silver, sulfate, and zinc. 

The Food and Agriculture Organization of the United Nations has recommended methods for the determination of mercury in pesticides. The method selected depends on the other constituents of the formulation and in the presence of copper the method of Brookes and Solomon (see p. 419) is most suitable. In the presence of most other constituents the selected method is one in which the sample is refluxed with concentrated sulphuric acid and potassium nitrate before determining the mercury volumetrically with thiocyanate. When large amounts of calcium carbonate or highly chlorinated compounds e,g, benzene hexachloride) are present, the mercury is isolated as a sulphide before conversion to the ionic form with strong acid. 

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