The acid radical preset in magnesium nitride is
Answers
Explanation:
Magnesium silicate is prepared from stock solutions of sodium metasilicate or magnesium nitrate. The solution volumes required to achieve the selected Mg/Si target ratios (0.5, 0.6, 0.75, 1.0, and 1.5 represent gel numbers 1–5) are cooled to about 0 °C and mixed with stirring in a flask kept immersed in an ice-water bath. The sodium silicate solution is added first, followed by the slow addition of the magnesium nitrate solution with stirring. The samples are allowed to warm to ambient temperature. The precipitates are collected, washed with water, and dried over solid CaCl2 at ambient temperature for 5–7 days. The magnesium silicate samples obtained are kept for 24 h at 25 °C (fresh gels). A second series of reaction products having specific target Mg/Si ratios (0.5, 1.0, 1.5, and 2.0, represent gel numbers 6–9) are prepared at 20–25 °C and subsequently stored in double-distilled, deionised water for 6 months at 85 °C to examine the effects of ageing (aged gels). The gels obtained are tested by energy-dispersive X-ray (EDX), XRPD, FT-IR, and solid-state 29Si NMR. In order to establish the actual Mg/Si ratios in the prepared solid gels, EDX analysis is performed and the results are presented in Table 7.10. In this study, the target ratios were selected either to match or closely match known magnesium silicate minerals: sepiolite, Mg4[Si2O5]3(OH)2·4H2O (Mg/Si = 0.67); talc, Mg3[Si2O5]2(OH)2 (Mg/Si = 0.75); and serpentine, Mg3[Si2O5](OH)4 (Mg/Si = 1.0). They are included deliberately to serve as benchmarks during the characterization stage. Amongst fresh gels, the data scatter is largest for gels 1–3; possibly also gel 5 if the outlier is considered but scatter is generally much reduced when the gels are kept for 6 months at 85 °C. Thus, studies with EDX demonstrate improved homogeneity in the aged gels.
Table 7.10. Energy-dispersive X-ray (EDX) data for the synthetic magnesium silicate gels
Gel No. Target Mg/Si ratio Measured Mg/Si ratio
Mean Range
Gels aged for 24 h at 25 °C
1 0.50 0.66 (20) 0.49–0.83
2 0.60 0.75 (25) 0.35–1.03
3 0.75 0.80 (26) 0.36–0.93
4 1.00 0.90 (24) 0.82–1.03
5 1.50 0.96 (20) 0.78–1.53
Gels aged for 6 months at 85 °C
6 0.50 0.82 (21) 0.73–0.87
7 1.00 0.86 (23) 0.72–1.02
8 1.50 0.89 (21) 0.79–0.95
9 2.00 0.94 (22) 0.80–1.12
The XRPD patterns of two fresh gels (gels 1 and gel 5) kept at 20–25 °C are identical (Fig. 7.17A). The results prove that the precipitates are gel-like single phases of low crystallinity and all gels in both storage conditions show almost the same XRPD patterns. The results also indicate that brucite, Mg(OH)2, did not coprecipitate in quantities sufficient to be detected by XRPD. Although the broad peak at 3.6–3.0 Å compares reasonably well with that of amorphous silica, its coprecipitation with gels is very unlikely. This finding is also supported by the FT-IR and 29Si NMR analysis, as shown later. Figure 7.17B displays the XRPD patterns of gels 5 and 9 for comparison. Their appearance suggests that the internal structure of the aged gels does not significantly differ from those of fresh gels.