O medicamento como mercadoria simbólica

Abstract

3). The powder loses 22% mass as water gradually on heating to 350°C (calculated water content = 22.1 %) with marginal retention of structure. Further heating to 700°C results in conversion to monoclinic beryllonite, NaBeP0 4 , with no weight loss. If the initial synthesis mixture is heated for too long at 70°C (or more rapidly at 100°C), the product converts to Harvey and Meier's beryllophosphate G, a gismondine derivative. Another zeolite X analogue (DPZ-1B) is available in the zincophosphate system, using sodium and tetramethyl ammonium ions as templates. A clear solution, containing 32 mmol NaOH, l34 mmol tetramethyl ammonium hydroxide and 64 mmol H 3 P0 4 , was prepared in 75 cm 3 of water and cooled to about 4°C. To this was added a pre-cooled solution of 48 mmol Zn(N0 3)2 to give a gel that converts to a milk on shaking. The milk settles rapidly and the solid may be recovered after standing for 5 h at 4°C. The X-ray powder diffraction pattern is indexed as face-centred cubic, a = 25.226 A. The product seems to be fairly sensitive to the ratio of sodium to zinc in the synthesis mixture: if less than 2/3, hopeite (Zn3(P04h·4H20) is formed, and if greater, a variety of hydrated sodium zinc phosphates (including the sodalite phase described above) contaminate the product. It is tempting to speculate that the sodium ion acts as a template for 'sodalite' cubes in solution which then condense around the large alkyl ammonium ions to form the X structure. Zeolite Li-A(BW) analogues. Four new materials with the lithium aluminosilicate structure (Li4Ai4Si4016' 4H 2 0 (orthorhombic Pna2 1)13 have been made in which Be or Zn completely replace AI, and P or As replace Si. (1) Li4Be4P4016' 4H z O (DPZ-4A). This analogue often begins to appear if the RHO synthesis described above is carried out at higher temperature (100°C), and may be the impurity phase noticed by Harvey and Meier9. The synthesis is optimized by using 12 mmol Be(N0 3)2 and 14 mmol H 3 P0 4 in 20 cm 3 of water with subsequent addition of 37 mmol LiOH in 9 cm 3 of water (pH 2.5). After soaking overnight at 100°C, all RHO lines have disappeared from the diffraction pattern, leaving the pure Li-A(BW) phase (orthorhombic, Table 4). The phase is unstable to water loss (13% by mass, occurring in three steps on heating to 350°C; calculated water content = l3.96%) with loss of the initial structure. (2) Li4Be4As4016' 4H z O (DPZ-4B). Carrying out the equivalent berylloarsenate RHO synthesis at lower pH (37 mmol LiOH, pH = 3) and higher temperature (100 0c) results in rapid (overnight) disappearance of the initially formed RHO phase, and conversion to the Li-A(BW) material (orthorhombic, Table 4). Water loss is complete at 260°C (two steps, 10% by mass; calculated value = 10.4%). (3) Li4Zn4P4016' 4H z O (DPZ-4C) and (4) Li4Zn4As4016. 4H 2 0 (DPZ-4D). These Li-A(BW) analogues are easily prepared in a similar way to the Be counterparts, but lower temperatures suffice (phosphate: 36 hat 70°C, pH 7; arsenate: 48 h at 25°C, pH 7). The arsenate in particular recrystallizes readily to the phenakite analogue 14 LiZnAs0 4 if the synthesis is carried out at 100°C. The powder diffraction patterns for all of these phases are detailed in Table 4. The pH, water concentration, heating time, ionic ratios and solution homogeneity before gel formation all play a critical role in the synthesis of these phases. For example, we have obtained seven different hydrated sodium zinc phosphates by simple variation of these synthesis conditions. Further chemical and structural characterization of these and other materials is in process and will be published in more detail elsewhere