the microscale laboratory Literature Cited 1. Flash, P, Galle, F.; Radil, M. J. Cham.Edw. IWS,66,958. 2. Hel&,G. K; Johnaan,H.W., Jr.Sel&edEqwimntain C7~anicChomislry,3rd ed.; Freeman:New York. 1983;pp 96100. 3. Mayo, D. W : Pike, R. M.; Butcher, S. S. M i w m d e Organic Ldomlory, 2nd ed.: Wfiey: New Y d , 1989:pp 15&155. 4. Roberts, R. M.; Oilberf J. C.; Rodewald, L. 8.;Wi.sOi.so,A. S. M d e m Ewwimontol oganv chamlsfry,4th ed.: Saunders: NewYork, 1985,pp337-145. 6. Sayed, Y ; Ahlmar*, C. A,; Mad", N. H. J Cham. Educ. 1988,66,174. 6. Pavia, D.L.;Lampman, G . M.; &, G. S.;Engel. R.G. InlmducflonLo OrwicLab. omtory 'Ikhniqus;Sauldrrs: Philadelphia, 1990:p 149.
Microscale Techniques For Determination of Magnetic Susceptibility John ~ o o l c o c k 'and Abdullah Zafar Indiana University of Pennsylvania Indiana, PA 15705 There are several traditional techniques for determining the magnetic susceptibility of a transition metal complex: the Gouy method, the Faraday method, and the NMR method (13). The table compares the Farady and NMR methods as well as a modified Gouy device previously described in this Journal (4).
It does not require a separate magnet or power supply. It plugs into a standard 115-Voutlet. It has a digital readout that provides quick and accurate readings with a sensitivity comparable to that of traditional methods. This device has been advertised as measuring the mass susceptibility of solid samples as small as 100 mg and determining the magnetic susceptibility of liquids and solutions. Since the features of the MSB-1 seem to make it ideal for use by students, we have examined the minimum sample size required for accurate measurements of magnetic moments with this balance. We have compared this technique to the NMR method and to the modified Gouy balance (4). Theory and Operation of the MSB-1 Balance The MSB-1 has the same basic equipment configuration as the Gouy method. However, instead of measuring the force exerted by the magnet on the sample, it measures the equal and opposite force exerted by the sample on a pair of suspended permanent magnets (5). The following general expression in cgs units of an3g-' for the mass susceptibility X, can be derived for the MSB-1 balance in much the same way as the relationship used in the Gouy method is obtained.
Comparison of the Maln Techniques Used To Determine Magnetic Susceptibility Modified Gouy Types of Solids Only Samples Used
Faraday
NMR
MSB-1
Solutions Solids. Solids, Liquids, Liquids, and Only and Solutions Solutions 10-300 0.1-50 1 4 0 mg mg mg
175-500 mg Solid Sample Size Solution Not Applicable 0.20 mL Sample Size Cost of $100 $17,000 Instrument (ref 5) (Cahn Instruments) (Source)
0.0250.50 mL
0.070.30 mL
$56,000 $3,000 (Varian (Johnson EM-360) Matthey)
Although the Gouy method is the most common of these techniques, it often requires the largest sample size. The Faraday and NMR method each use delicate or relatively expensive equipment, and the NMR method can be used only with solution samples. More recently a new type of magnetic susceptibility balance, the MSB-1, has been developed by D. F. Evans of Imperial College, London (marketed by Johnson Matthey) (5). This balance has several convenient features and is easily portable. It is compact (11.8 in. long x 8.7 in. wide x 5.3 in. tall) and lightweight (7.3lh). 'Author to whom correspondence should be addressed. 2Presented in part at the 200th ACS National Meeting. Washington, DC, August 1990,and the 11th Biennial Conference on Chemical Education, Atlanta, GA, August 1990. A176
where L is the sample length in an;m is the sample mass in grams; C is the balance calibration constant; R is the digital display reading when the filled sample tube is in place; R, is the digital display reading for the empty sample tube; x', is the volume susceptibility of air (0.029 x lod cgs); and A is the cross sectional area of the sample. The second term,x'B, is usually ignored with solid samples. Although the correction in these cases is small, it must be included to determine mass susceptibilities of liquids (x,) and solutions (x.) For these samples eq 1can be modified to
Journal of Chemical Education
where d, is the density. If the density is not known, i t can be calculated h m the mass and volume of a solution placed in the sample tube using a microliter syringe. The cross-sectional area we use in this equation is calculated from the inside diameter (the sample diameter) provided by the manufacturer. For the standard 3.23-mm-i.d. MSB-1 tube, the cross-sectional area is 0.0819 em2. The solution susceptibility X, in cgs units is then converted to solute mass susceptibility xgusing the Weidmann additivity relationship.
where m lis the mass of the sample in grams, m. is the mass of the solvent in grams, and X, is the mass susceptibility of the solvent. The mass of solute and solvent should be measured as the solution is being prepared. Once the mass susceptibility of the sample has been found by one of the above methods, the molar susceptibility, XM, and the effective magnetic moment in Bohr magne-
tons (BM) are then calculated in the usual manner. All the calculations described above have been automated using a computer spreadsheet template. Thus, the students need only enter their data into labelled cells on the sheet from which the program will automatically calculate x,, &, XM, and ua. ~l;'followin~ representative student data were obtained for solid HelcotSCN~~l usinn the 2.00-mm-i.d. tube. This compound c i a common magnetic susceptibility standard. R, = -70 R=42l length = 2.81 em temperature = 25.3 'C C = 1.30 mass = 0.1139 g
.. .
This data yields a mass susceptibility value of 1.61 x lo5 cgs (0.45% error) and a kffof 4.39 BM. Errors in the calculated values of mass susceptibility using the MSB-I with solid or solution samples range from about 1% to 2%when compared to literaturi values~Allkfi values lie within the expected ranges for a particular transition metal ion. Negative values for R immediately indicate that the sample is diamagnetic. Limits for Solid and Solution Sample Sizes using the MSB-1 Balance
The two sizes of sample tubes commonly used with the MSB-1 have inside diameters of 2.00 and 3.23 mm, respectively. These are advertised as using a minimum of 100 and 250 mg of solid sample, respectively. We used 1.00mm4.d. tubes (donated by JohnsonMatthey) in anattempt to further decrease the minimum sample mass. By placing increasing amounts of Hg[Co(SCN)41 i n each of these tubes, we found that the calculated value of kffincreased to the literature value of 4.4 BM as the mass of the sample, the sample length, and the value of R increased. This is shown in the figure for the 1.00-mm tube. From these results we have determined that the minimum sample length required for each tube is about 0.9-1.0 cm, which corresponds to the point a t which the sample completely fills the volume in the tube between the two disk magnets. Since adding more solid will place the sample outside the field between the magnets, it is not surprisine that R and the effective mametic moment are unaffe&d by increasing the sample l>hpast this point (also seen in the fiewe). This minimum s a m* ~ l elene-th wrresponds to a mikimum sample mass of 10 mg for the 1.00-mm-i.d. tube 40 mg for the 2.00-mm-i.d. tube 100 mg for the 3.23-mm-i.d. tube
As indicated above, the minimum sample size is also the maximum sample size required by this device. In addition to using the 1.00-mm4.d. tube to decrease the amount of solid required, this can also be achieved by measuring the mass susceptibility of a paramagnetic compound in solution. Since both concentration and sample volume determine the minimum mass of solute reauired, we examined the effect of each of these quantities on p.m. Usinr! aaueous solutions of c o ~ p e d l lsulfate ) pentahvdrate in t h i 3123-mm4.d. sample tube with a constant sample volume of 250 pL, accurate values of bewere obtained only for concentrations of 0.2 M (32 mg/mL) or greater. As with
The effect of increasing sample mass on the calculated value of for H~[CO(SCN)~] in the MSB-1 balance using the 1.00-mm-1.d.sample tube. solid samples, the value ofR increased with concentration until 0.2 M was reached. After this, R remained constant. The effect of sample volume on the calculated value of kff was also determined by measuringR while 10-mL aliquots ofthe 0.2 M CuS04solution were added to the 3.23-mm4.d. tube. As the volume was increased, R and kffdecreased until a volume of 70 pL was reached. At this point these quantities become constant with an expected ktf of 1.98 BM. Avolume of 70 pL corresponds to the minimal sample volume observed for solids (0.9 cm x 0.0819 cm2). These results show that the minimum amount of solute needed to obtain accurate values of kffis about 2.3 mg. Thus, to give a comfortable margin, it should be possible to use about 5 mg of any solute to prepare a 100-pL solution for use with the MSB-1. The values of both R and decrease with increasing sample volume because an empty 3.23-mm4.d. tube has an R value that is less negative (about -31) than a tube filled with water (R = -80). Since the diamagnetism of the solvent increases faster with increasing volume than the paramagnetism of the solute, the value ofR should become more and more negative until the diamagnetism of the full sample tube is measured. Thus, when the volume of solvent is less than 70 pL,the R value for the solution is higher than it should be. Under these conditions eq 3 will compute a solute paramagnetism that is too large. We have found that only manganese(I1) sulfate solutions, with five unpaired electrons for each metal ion and with concentrations of 0.2 M or larger, give accurate results with the 2.00-mm-id. tube. Also, no solutions give ac(Continued on page A178)
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t h i microscale laboratory with the 1.00-mm4.d.tube. Thus, the curate values of 3.23-mm-i.d. tube is best for measuring solution susceptibilities. Other Methods for Determining Magnetic Susceptibility
As noted earlier there are other methods that have been used to determine mass susceptibility, including a modified Gouy method previously described in this Journal (4). We were interested in determining the minimum sample size for this device because it has a significant cost advantage over the MSB-1 and othermagnetic susceptibility systems. We constructed our apparatus as described by the authors using disk magnets (Edmund Scientific; catalog no. as the standard to exam530,962). We used H~[CO(SCN)~] ine the effect of increasing sample mass on the value of h~ for copper(I1) sulfate pentahydrate. Our results indicate that the minimum sample size for this device is approximately 175 mg. Although this is much higher than the MSB-1, it is still within the range of many micmscale pmcedures. We also tried to measure the magnetic susceptibility of 1M solutions of CuSOa.We were unsuccessful because the change in weight of the magnets and the yoke must be at least10 mg toobtain an accurate value of ia.Only concentrated (1 M) manganese(I1) sulfate solutions changed the weight enough to give an accurate value f o r k .
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Another method commonly used to determined the mass susceptibility of solutions a t the microscale level is the NMR method developed by Evans (6). According to Lijlinger and Scheffold, a concentration of 2 x lo3 M and a volume of 25 pL is all that is needed for microscale NMR measurements (7).The lowest concentration that we could measure conveniently was 4.2 mgImL (2.63 x 10" M) with Av = 5.8 Hz when we used an 80.13-MHz Bruker NRBOAF NMR spectrometer and solutions of copper(I1) sulfate in DzO of decreasing concentration. (The indicator species was 1 4 % t-butyl alcohol.) With a sample volume of 25 pL placed in the capillary insert, as little as 0.1 mg of solute is required with this method. No references for the NMR method discuss the use of FTNMR instruments. We have found that locking an FTNMR spectrometer requires the following replacements for the liquids that are typically used in both tubes of the coaxial cell: DzO for water, CDC13 for chloroform, and CDBCOCDB for acetone (7,8). However both t-butyl alcohol and TMS can still be used as the indicator species. The tube should be spun as fast as possible, and the sweep width on the FT-NMR should be expanded fmm about -10 to +10 ppm so that there will be little or no "fold over" (9) of the spinning sidebands that can make determination of Av difficult. There seem to be no literature values for the mass susceptibility of deuterated solvents in common references
(1,2, 10). Using the MSB-1, we have determined the diamagnetic susceptibility of deuterium oxide and deuterated and -4.97 + 0.05 x 10-7cgs chloroform: 4 . 3 7 f 0.13 x respectively, compared to -7.21 x 10" and -4.97 x 10" cgs for water and chloroform. Also, high-field spectrometers with superconducting magnets require a special variation of the oripinal eauation piven bv Evans (lZ).Finallv. it is importantto measure the temp&ature of the ~ ~ l t ' p r o b e at each session. As discussed in this Journal (11,. chanees in solution density with temperature can lead to sign%cant errors in the value of kff. Conclusions The MSB-1 balance can easily perform magnetic susceptibility measurements on both solid and solution samples with awiderange of sample sizes. As little as 10mgof solid sample is needed for measurements of ireff,and as little as 3-5 mg of solid is required for a solution. As an alternative. the NMR method and the use of the modified Gouy device can complement each other well: The marmetic suscevtibilitv of solutions can be measured with thcformer, andsolidscan be measured with the latter. As little as 0.1 me of solute is needed for the \ 3 l R method. while the modked Gouy method requires about 175 mg of solid. Using either of these approaches, students can easliy measure both the solid and solution magnetic susceptibilities of microscale samples and with a spreadsheet tem-
plate quickly calculate hff.If the lab instructor selects transition metal compounds that have different magnetic properties in the solid and solution state, students will learn that these measurements can provide important structural information as well as determine the electron configuration of the transition metal (13). Acknowledgment We would like to thank Jeff Lucht of Johnson Matthey and Dr. Steven Bogdanski of Sherwood Scientific Ltd. for providing additional technical information regarding the MSB-1 device and for suppling the l-mm-i.d. sample tubes. Literature Cited 1. e g i s , B.N.; Leuis, J. In Modrrn C m r d i ~ t i o nChamiahy; Lewia, J.;Willdna, R.G., Eda.; W h y : New York, 1960: p p 4004% 2. Selwood. P W. Mogmtoehmisfry, 2nd ed.: Jntersdence Publishera, Wiky: New Vnrk ... .., ,966 . ....
3. Jolly, W. L. SnythesiaandChomdrtiaafionofhorgonieCompounds;Renti-Hall: Engelwood C M s , NJ. 1970: p p 389-385. 4. Eeton, S. S.; Eaton, G. R.J. Chem. Edue. ISTS, 56, 170-171. 5. Mognotie Suseepfibility Bolanwlnstnrefion Monunl,JohnsonMatthey: Wsyne, PA, 3-0 A""".
6. Evans,V. F J. Chem Soe. 1968.2W3-ZW5. 7. Ldliger, J.:Seheffold,R. J Chem. Edue. 19?P,49, 646647. 8. Crawford, T. H.; Swanson, J. J Chem. Edue. 1811,48,332386. 9. Sehsffer, C; Yoder, C. Infrodvctlan l o Multinuclear NMR; BenjaminiCummings: Menlo Park, CA, 1987: pp 6 1 4 2 . 10. CRC H o n d h d of Ckmislry and Phyaicp, 49th ad.; We&, R. C., Ed.; CRC: Boea Raton. FL. 1968: p E-115E-132. 11. Ostfeld, V.; Cohen,I. A. J Chem Educ. 1872,49,829. 12. Schubert. E. M. J C k m . Educ 1892,69,62. 13. Szafran, 2.: Pike, R. L.; Smgh, M . M . Mieraseole Inorgonie Chemisrry: A Comp m k n s u e h b o m f o r y E I P I ~ ~ NWfiey: P : New Yo*, 1991; p p 231-235: 257-260.
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