In the Laboratory edited by
Cost-Effective Teacher
Harold H. Harris
Variable Temperature Equipment for a Commercial Magnetic Susceptibility Balance
University of Missouri—St. Louis St. Louis, MO 63121
Albert Lötz Department Chemie, Universität München, D-81377 München, Germany;
[email protected] The magnetic susceptibility balance MSB-MK11 of Sherwood Scientific, Ltd. is attractive for educational purposes because of its compact design and especially its relatively low price. This is also mirrored in the Web pages of a number of chemistry departments that use this instrument in their courses (see also ref 1). The manufacturer does not sell variable temperature equipment for this balance. The method of simply inserting hot or cold samples is doubtful for several reasons discussed in Sherwood’s Web pages (2). Although many interesting chemical problems can be studied by measuring just room temperature susceptibilities, data obtained at different temperatures are clearly more desirable. Low temperature measurements (77 K and lower) appear impossible to perform on this instrument, yet data in the temperature range of 20–80 °C can be provided, as is described in this paper.
sample tube 3
5 4
6 7
insulation
plastic T-connector
air entrance
2 lid
1
bridge
Modification of the Commercial Balance The balance works according to the Gouy method modified by Evans (3), in which the sample is stationary and the magnet attached to the beam of a balance. The MSB-MK1 balance uses two disk-shaped permanent magnets of 15 mm diameter that form a gap of 7 mm. The sample glass tube has a 4 mm o.d. and a 0.4 mm wall thickness that fits in a brass tube of 6 mm o.d. and 5 mm i.d., which guides the glass tube in the gap between the magnets and protects the mechanical and electronic components in case of glass breakage. Initial tests aimed at essentially retaining this system for variable temperature work failed, because the balance drifted too much when water of different temperatures was conducted through the brass tube. It was therefore replaced by a common laboratory glass tube (alkaline resistant glass) of the same inner and outer diameters. This entailed changing from water to air as a thermostating medium, because breakage of the glass tube would flood the electronics. For the details of the design see Figures 1 and 2. Hazards If the air pressure is not appropriately set at the valve and the line is jammed anywhere—for example by a broken sample tube—dangerous pressure may result and damage ensue. A safety valve is recommended. Operation of the Variable Temperature Equipment The air pressure is adjusted for a flow of 60 SCFH (standard cubic feet∙h; 1 SCFH = 28.3 L∙h). This flow rate proved sufficient for an acceptable temperature decrease near the sample. With the water temperature at 55 °C a decrease of 2 °C from the
glass tube
sample
Figure 1. Cross-section of the sample insert. Key: magnet
1. Silicone hose fixing the glass tube in pre-existing holes 2. Silicone hose connecting glass tube and T-connector
insulation 8 thermoelement 9
3. Pyrex NMR tubes2 of the same o.d. and i.d. as those of the Sherwood sample tubes 4. Polyethylene hose that fits snugly on each sample tube at exactly the same height above its lower end 5. Self-adhesive tape for securing the position of 4 6. Silicone hose overlapping 2 mm with 4 for sealing the upper section of the T-connector
1
bottom plate
7. Polyethylene hose enclosed by 6, stop for 4, for reproducible vertical positioning of the sample tubes with regard to the magnet 8. Foam elastomer tubing3
air exit
9. Type K (NiCr–Ni) thermoelement,4 in the form of two teflon-insulated twisted wires attached to a battery-operated, digital indicating device of 0.1 °C resolution. Tip of probe placed 3.5 cm below the center of the magnet to avoid disturbance of the magnetic field.
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 1 January 2008 • Journal of Chemical Education
107
In the Laboratory
10 bar house supply reduced to 0.2 bar
flow meter RMB-54 20–200 SCFH Dwyer Instruments Michigan City, IN
recirculating thermostat F6/B5 Haake, Karlsruhe
heat exchanger helix of 9 windings of 7 cm diameter
of 20 °C. Simultaneously, a constant thermoelement and balance reading with an empty sample vial is obtained. After zeroing the balance, the empty vial is removed with running air stream, and the standard and samples are inserted. Changing the sample tubes with running air stream requires only a few seconds. The samples reach their final temperature within 1–2 minutes. Small deviations from the temperature of the empty vial of less than 1 °C can be corrected with an adjustment of the air flow by ±5 SCFH. After several samples have been measured at the same temperature, a check for a reading of zero with the empty vial shows a maximum shift of one least significant digit when all balance readings are made in the 10× range of the instrument.
water 15 mm silicone hose
silicone hose
8 mm
air
copper tubing
plastic T-connector Figure 2. Top: Block diagram of air processing. Bottom: One of the two terminal parts of the heat exchanger.
Table 1. Performance Comparison with Magnetic Standards Temperature/°C; First row: Waterbath; Second row: Air at sample 23.0
40.0
60.0
80.0
95.0
80.0
60.0
40.0
22.4
23.0
36.8
52.8
68.8
81.0
68.8
52.9
37.0
23.0
χg/(10¯6
cm3/g)
Gram susceptibility 1st row: χg = C × 10¯6 cm3 g–1/(T/°C + θ) ; 2nd row: Experimental HgCo(SCN)4, C = 4981, θ = 283 (4), used as standard 16.28 15.58 14.83 14.16 13.68 14.16 14.83 15.57 16.28 Ni(en)3S2O3, C = 2759, θ =230 (5) 10.91 10.34 9.76
9.23
8.87
9.23
9.75 10.33 10.91
10.81 10.10 9.62
9.13
8.88
9.38
9.68 10.16 10.81
(NH4)2Fe(SO4)2 ⋅ 6H2O, C = 9500, θ = 274 (6) 31.99 30.57 29.07 27.71 26.76 27.71 29.06 30.55 31.99 32.59 30.54 28.91 27.64 26.75 27.64 29.10 30.58 32.46 CuSO4 ⋅ 5H2O, C = 1758, θ = 273 (7) 5.94 5.67 5.40 5.14 4.97 5.14 5.39 5.67 5.94 6.12 5.73 5.47 5.20 5.02 5.20 5.50 5.67 6.03
T-connector to the bottom plate of the balance was measured, with a decrease of 0.1 °C over the sample length and a mean temperature within this length of 50 °C. At a water temperature of 95 °C and a mean temperature of 83 °C the temperature decreased 1.2 °C over the sample length. The water bath of the thermostat needs 10–15 min with running air stream to reach the final temperature after a change 108
Performance Check and Application Example Performance was checked with four magnetic standards at five different temperatures in the range of 23–81 °C (see Table 1). The susceptibilities from the literature (Curie–Weiss Law) were reproduced within 1–2%. The quality of the measurements at higher temperatures was not worse than those performed at room temperature; the relative errors are of the same magnitude as those quoted in Sherwood’s balance manual. An example performed in our physical chemistry practical course (fourth semester) is the determination of the magnetic moment and the Weiss constant of MnF2 and Mn(phen)2Cl2 (phen = o-phenanthroline). The quite different internal magnetic interactions in these compounds are evident from their Weiss constants; they can easily be understood from their structures. Notes 1. Sherwood Scientific, Magnetic Susceptibility Balances Page. http:// www.sherwood-scientific.com/msb/shmagway.html (accessed Nov 2007). 2. Wilmad Labglass Home Page (part 405-PS). http://www. wilmad-labglass.com/group/2029 (accessed Nov 2007). 3. Armacell Home Page (AP/Armaflex tubes [AF-1-010]). http://www.armacell.com/ (accessed Nov 2007). 4. Greisinger Electronic, GmbH (GTH1170; probe GTF300). http://www.greisinger.de/index.php?task=2&wg=153; http://www.greisinger.de/files/upload/de/produkte/kat/89.pdf (accessed Nov 2007).
Literature Cited 1. Teweldemedhin, Z. S.; Fuller, R. L.; Greenblatt, M. J. Chem. Educ. 1996, 73, 906–909. 2. Sherwood Scientific Ltd. http://www.sherwood-scientific.com/msb/ msbfaq.html (accessed Nov 2007). 3. Evans, D. F. J. Phys. E: Sci. Instrum. 1974, 7, 247. 4. Figgis, B. N.; Nyholm, R. S. J. Chem. Soc. 1958, 4190–4191. 5. Curtis, N. F. J. Chem. Soc. 1961, 3147–3148. 6. Jackson, L. C. Phil. Trans. Roy. Soc. (London) 1923, A224, 1, cited in Selwood, P. W. Magnetochemistry, 2nd ed.; Interscience Publishers: New York, 1956; p 26. 7. Magnetic Susceptibility Balance Instruction Manual; Sherwood Scientific, Ltd.: Cambridge, UK, 1995; p 22.
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Journal of Chemical Education • Vol. 85 No. 1 January 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
