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7.1 Describe how you would determine the following:
(a) The temperature at which a porous metal-organic framework (MOF)
(i) loses solvent molecules from its pores
(ii) undergoes a collapse of the framework to form a dense solid.
(i) Thermogravimetric analysis and/or combined thermogravimetric/DSC/DTA. The loss of solvent molecules will involve a weight loss and is also likely to be endothermic in a DSC/DTA trace.
(ii) The framework collapse will not involve a weight loss but is likely to be strongly exothermic and the material re-crystallises with a new denser structure. Therefore DTA or DSC would be used to determine the temperature at which this process happens.
(b) The amount of the antibiotic drug cefazolin (C14H14N8O4S3) that has been incorporated between the layers of the Zn(OH)2 in the hybrid inorganic-organic composite Zn(OH)2:C14H14N8O4S3.
CHNS analysis would give the proportion of cefazolin relative to Zn(OH)2 (inert in the CHNS analyser). Note that combined TGA/DTA with mass spectrometry could also be used to determine the decomposition temperature at which the organic molecule is released from the zinc hydroxide host. ICP could be used to determine the zinc content of the composite
(c) The liquid product of the reaction of I2(s) with Cl2(g).
Potential products are the interhalogen compounds ICl, ICl3 and ICl5. These could be readily distinguished by mass spectrometry from the parent molecular ion peak. As the product is a liquid and likely to be volatile standard methods of introducing and ionizing the compound, such as electron impact ionization, EI, would be applicable.
(d) The composition of the solid product from the reaction of CH3NH3Cl with PdCl2 in aqueous 2 M HCl.
This is a recent example from the literature (Chem. Eur. J. 2016, 22, 2146 – 2152) where the product was characterized by energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), carbon, hydrogen, nitrogen, and sulfur (CHNS) elemental analysis, ion chromatography (IC), and inductively coupled plasma optical emission spectrometry (ICPOES).
(e) Accurate values for the zinc and copper content of a phase in the solid solution Cu2−xZnxSnS4 with x~0.95.
EDX is unlikely to produce very accurate values for Cu and Zn as they are neighbouring elements in the periodic table and the X-ray emission spectra will overlap. More accurate would be dissolution (in acid) followed by AAS.
(f) The nitrogen to oxygen ratio at the surface (to a depth of 5 nm) of a series of silicon oxynitride thin films heated to various temperatures.
Depth profiling SIMS can be used. This will provide high sensitivity, at parts per billion (ppb), concentrations of elements, Si, O and N as the surface is gradually eroded by the incident ion beam.
7.2 The mass spectrum of a sample of P4S10 contaminated with a small level of oxygen showed a peak at ~332 mu. (D.W. Muenow and J. L. Margrave. J. Inorg. Nucl. Chem. 34, 89 (1972)). Which molecular phosphorus oxysulfide species, PnSmOp, could give rise to this peak and how could the actual species producing this peak be identified using mass spectrometry?
The possible RMM values for PnSmOp are 31n+32m+16p. Likely reaction products will derive from oxidation of P4S10 to give of P4SxOy whose RMM will be 124 + 32x + 16y. Equating this to the observed332 mu. peak gives 32x + 16y = 208 and possible compositions are P4S6O+, P4S5O3+, P4S4O5+ and P4S3O7+ for the parent ion giving rise to this peak. All these are reasonable possibilities as phosphorus forms oxides and sulphides of the composition P4Xn with n = 6, 7, 8, 9, 10. Very high resolution mass spectrometry could be used to distinguish these species using the mass defect with 2 x 16O and 1 x 32S corresponding to 31.9988 and 32.066 mu respectively. Measuring the exact mu of the peak at ~332 mu. will allow the exact ratio of sulfur to oxygen to be determined. So that, for example P4S6O+ will be at 4×P (30.97376) + 6×32.066 +1×15.9994 mu = 332.290 mu while P4S3O7+ will be at 4×P + 3×32.066 +7×15.9994 mu = 332.089 mu.
7.3 How can SIMS be used to study the rate of nitrogen diffusion in silicon nitride at 1000 °C?
An isotope tracer technique can be used. A sample of isotopically pure Si315N4 can be synthesised and then heated at 1000°C in 14N2. 14N will diffuse into the sample displacing 15N. The sample is then analysed using depth-profiling SIMS and the level of 14N and 15N determined. The rate of diffusion of nitrogen can be determined from the depth profile.
7.4 A standard solution for analysis of iron using AAS was prepared by dissolving 0.1563g of iron wire in acid and diluting to 250 cm3. After a further 100 times dilution the solution had a measured absorbance in the atomic absorbance spectrometer at 248.3 nm of 0.843. Calculate the concentration of iron in a natural water sample having an absorbance at 248.3 nm of 0.543.
The concentration of iron in the standard sample made up to 250 cm3 was (0.1563 g /55.84 gmol-1) × 4 mol.dm−3 = 0.0112 M. After 100 times further dilution the standard concentration becomes 1.12×10−4 M. From the Beer-Lambert law the absorbance is directly proportional to concentration so
Csample/1.12×10−4 = Asample / Astandard = 0.543/0.843
giving the concentration of iron in the natural water sample as 7.21 ×10−5 M.
7.5 Heating a 20 mg sample of black CrO2 on a thermogravimetric balance to 300 °C in H2(g) led to the formation of a green product with a 1.26 % weight gain. Cooling the sample to room temperature and reheating to 300 °C in oxygen gas yielded 20 mg of pure CrO2. Heating a 20 mg sample of black CrO2 on a thermogravimetric balance to 1000 °C in H2(g) led to a 35.00% weight loss. Interpret these observations.
The initial weight increase on heating under hydrogen implies hydrogen uptake so the reaction occurring is the reductive insertion of hydrogen to give CrOOH(s). The weight increase associated with this is 1.008/(52+32) = 1.20% in good agreement with the experimental value. The reaction is reversed on heating in an oxidising environment to yield the initial mass of CrO2 and H2O(g) which is lost. At high temperatures the reduction of CrO2 involves loss of oxygen as water so
CrO2 → CrOx + (2−x) H2O
(52+16x)/84 = 65/100
Giving x = 0.16 or very close to zero. So under these condition CrO2 is reduced to chromium metal
7.6 The thermal decomposition of metal borohydrides, MBH4 can proceed to give two different end products. The two possible reactions are
MBH4 (s) → M(s) + B(s) + 2 H2 (g)
or
MBH4 → MH + B + 3/2 H2 (g)
(i) Heating LiBH4 to 800°C on a TGA apparatus results in a 13.9 % weight loss. Determine the reaction pathway.
(ii) Thermal analysis data for KBH4 and KH are shown in Figure 7.12: Interpret these data.
Figure 7.12 Thermal analysis data for KBH4 and KH (Adapted from J. A. Dilts and E. C. Ashby Inorg. Chem. 11, 1230 (1992)).
(a) The weight loss with M = Li for the two possible decomposition pathways are respectively
LiBH4 (s) → Li(s) + B(s) + 2 H2 (g)
Weight loss is (1.008 × 4)/(6.94+10.81+(1.008×4) ×100% = 4.032/21.782 ×100%= 18.5%
And for
LiBH4 → LiH + B + 3/2 H2 (g)
It is ¾ of that (as 3/2 H2 is lost rather than 2H2) at 13.88 %. Therefore the second route is that occurring with the experimental weight loss observed at 13.9%
(b) From the KH DTA trace an exothermic process occurs at around 350 °C. This is mirrored in the DTA trace from KBH4 and is associated with a weight loss from 94.4 to 92.5 % of the initial weight. These data can be interpreted in terms of the decomposition pathways
(i) KBH4 (s) → KH (s) + B (s) + 3/2 H2 (g)
with a weight loss of 5.6% (1.008 ×3/(39.1+10.81+4.032) × 100) which occurs between 250 and 300°C. Note that the DTA trace shows some structure which implies that some other intermediate hydride phase may be involved in the decomposition. Followed by
(ii) KH (s) → K(s) + ½H2(g)
with a further weight loss of (1.008)/53.94 × 100 % of the initial mass of 1.87 %. This corresponds to the weight loss from 94.4 to 92.5%.