Adsorptives other than nitrogen

Water is another adsorptive which has often been used, but its complexity of behaviour renders it generally unsuitable for the evaluation of total surface area. Consideration of this important topic is deferred to Chapter 5. 

Argon is frequently used for the determination of surface area, usually at 77 K. Like the other noble gases, argon is of course chemically inert and is composed of spherically symmetrical monatomic molecules. Argon stands in 

In their pioneer work, Brunauer and Emmett adopted the value a (Ar) = 13-8 for the molecular area of argon, by insertion of the liquid density Pi in the standard equation (2.27). The same figure was recommended by McClellan and Harnsberger as a result of their comprehensive survey of the literature, already referred to. These workers noted that the recorded values of a (based on a (N2) = 16 2 A ) extended over the wide range 10-19 A, and concluded that the area occupied per molecule of argon in the completed monolayer varied from one adsorbent to another. 

The results obtained when the adsorbents are non-porous oxides are somewhat different. The values of a (Ar) referred to a,n(N2) = 16-2 A are 

Molecular area, a (Ar) of argon at 77 K on graphitized carbon blacks (Argon BET plots constructed with p (liquid) surface areas determined by BET-nitrogen, with a (Na) 

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As remarked on p. 214, the validity of the nonane pre-adsorption method when adsorptives other than nitrogen are employed for determination of the isotherms, has been examined by Tayyab. Two organic adsorptives, /i-hexane and carbon tetrachloride, which could be used at or near room temperature, were selected and the adsorbents were the ammonium salts of... 

It would clearly be of interest to discover how far the nonane method can be used with adsorbates other than nitrogen. A study along these lines has been carried out by Tayyab, but a discussion of his rather unexpected results is best deferred until the role of fine constrictions has been considered (p. 228). Meanwhile it may be noted that the applicability of the technique seems to be limited to adsorptives such as nitrogen or argon which have negligible solubility in solid or supercooled liquid n-nonane. 

If the adsorption isotherm is to be determined at some temperature other than room temperature-liquid nitrogen temperature, for example—the sample tube is placed in a suitable thermostat. This is indicated by the dotted line in Figure 9.3. In this case two sets of readings are made with the nonadsorbed gas, one at room temperature and one with the thermostat in place. In this way the partitioning of the dead space between the two temperature regions can be determined. Several additional considerations should be cited that are important in actual practice ... 

Wils et al. (25,26) previously reported an entirely different approach to TDG analysis. TDG in urine was converted back to sulfur mustard by treatment with concentrated HC1. The sample treatment is less straightforward than the methods described above, but analysis as sulfur mustard is facile. Urine, plus 2H8-TDG as internal standard, was cleaned up by elution through two C18 cartridges. Concentrated HC1 was added and the sample stirred and heated at 120 °C. Nitrogen was blown over the solution and sulfur mustard isolated from the headspace by adsorption onto Tenax-TA. The method was used to detect TDG in urine from casualties of CW attacks (see below). A disadvantage of this method is that it may convert metabolites other than TDG to sulfur mustard. This is supported by the detection of relatively high levels of analytes in urine from control subjects. Vycudilik (27) used a similar procedure, but recovered the mustard by steam distillation and extraction. 

Similarly to the concept of the biographical nonuniform surfaces mechanism (7.97) can be used to derive a rate expression in supposition of lateral interactions. Assuming that surface species other than chemisorbed nitrogen are present on the surface in inferior quantities, which is backed by experimental evidences showing that nitrogen adsorption on iron catalysts proceeds at a rate approximately equal to that of ammonia synthesis, the equilibrium constant of step 2 in eq. (7.97) can be expressed, following the general treatment, as... 

Adsorption by carbon, which is one of the oldest adsorption methods used, has been reviewed and evaluated for the preconcentration of trace metals (794). Many authors have discussed the preparation of activated charcoal and carbon from a wide variety of usually local sources. The applications to water treatment are far too numerous to mention other than a few. Jo (795) carbonized a resin and a gum and hydrated the residue above 600 C to produce an adsorbant selective for cadmium(II). Kuzin et al, 196) used deashed active carbon and oxidized carbon for the quantitative sorption of copper, lead, zinc, and nickel from nearly neutral solutions containing 1-2 M alkali-metal halide. Pearson and Siviour (797) converted the metal-ion species to amine complexes before adsorbing these onto carbonaceous materials such as brown charcoal char or cellulose. Mercury vapor can be removed from a solution by reduction followed by passage of a nitrogen stream and adsorption by activated charcoal (798). Activated carbon, which had been oxidized with nitric acid, has been used to extract several metals including divalent nickel, cadmium, cobalt, zinc, manganese, and mercury from fresh water, brine, and seawater (799, 200). 

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