Thermodynamics and Separation Efficiencies for Gas-Solid Chromatography with Modified Alumina Columns Garrard L. Hargrove and Donald T. Sawyer Department of Chemistry, Unicersity of California, Ricerside, Calif. 92502 The thermodynamics and separation efficiencies of gas-solid chromatography have been studied for Na2P04- and NazS04-modified alumina adsorbents. The results indicate that the magnitude of the nonspecific interactions are strongly dependent on the amount and type of modifier, whereas the specific interactions are essentially independent of these variables. For either the NaaP04-or the NanS04-modified adsorbent, the energetics of interaction and the entropy losses on being adsorbed are much greater for molecules having pi-electron systems than for systems having only sigma bonds. The data also establish that Na2P04reduces the magnitude of the nonspecific interactions with alumina to a much greater extent than does NanSOa. With gas-solid adsorbents the stationary phase contributions to band broadening are less than in gas-liquid chromatography, and have a magnitude that is no greater than the gas phase cont r i but ions.
SINCETHE ADVENT of gas chromatography both the thermodynamics and the kinetics of the gas-liquid partitioning process have been studied extensively. However, similar investigations relating to gas-solid chromatography (GSC) have been limited almost exclusively to the thermodynamic aspects of GSC (1-7). That work which has been concerned with band dispersion (8, 9) has neglected the contributions from gas phase nonequilibrium. Unfortunately, the latter can be the major contribution to band dispersion based on the present results. With the development of homogeneous, large surface area adsorbents by Scott and Phillips (6, 7) studies of the thermodynamics of adsorption are possible while remaining on the linear portion of the isotherm. Such linear behavior is advantageous because the rate equation for gas chromatographic efficiency then is applicable. This makes it possible to isolate the various contributions to band dispersion in GSC and to establish their functional dependence.
loglo VST= -Al?{/2.303
For elution GSC the corrected specific retention volume at the column temperature, VXT(ml per meter2 of surface area), for a particular sorbate-sorbent pair is equal to the distribution coefficient, K , which, for low sample sizes, also represents the initial slope of the adsorption isotherm (4). Thus, the integrated form of the Clausius-Clapeyron equation becomes ~
~~~
~~
~~
(1) B. T. Guran and L. B. Rogers, ANAL.CHEM., 39,632 (1967). (2) G . hi. Petov and K. D. Scherbakova, “Gas Chromatography 1966,” A. B. Littlewood, Ed., Elsevier, Amsterdam, 1967, p 50. (3) J. King, Jr., and S.W. Benson, ANAL. CHEM., 38,261 (1966). (4) R. L. Gale and R. A. Beebe, J . Phys. Chem., 68,555 (1964). (5) A. V. Kiselev, “Gas Chromatography 1964,” A. Goldup, Ed., Elsevier, Amsterdam, 1965, p 238. (6) C. G. Scott and C. S . G. Phillips, Ibid., p 266. (7) C. G. Scott, “Gas Chromatography 1962,” M. van Swaay, Ed., Butterworths, Washington, 1962, p 36. (8) P. E. Eberly, Jr., J . App/. Chem., 14,330 (1964). (9) H. W. Habgood and J. F. Hanlan, Can. J . Chem., 37, 843 (1959).
+ constant
(1)
where A n f is the isosteric heat of adsorption per mole of adsorbate at low surface coverage. Idealized standard states have been adopted because the objective of the present study is the comparison of relative, rather then absolute entropies of adsorption. Thus, the gas phase standard state of the adsorbate is defined as a partial pressure of 1 atmosphere with the adsorbate vapor behaving as an ideal gas. The adsorbed standard state is that suggested by De Boer and Kruyer (IO) which also has been used by Scott (7)-i.e., the mean distance between adsorbed molecules is defined to be the same as in the three-dimensional gas phase standard state. This leads to a standard state surface concentration of 4.1 X lO-$/T moles/cmz at the column temperature (7). Combining this with K (or VXT)the equilibrium partial pressure, p, of the sorbate vapor above the solid can be determined. This permits the differential molar free energy change, AGO,for the transfer of 1 mole of vapor at 1 atmosphere pressure to a state where its equilibrium vapor pressure is p to be evaluated
AGO = R T l n p
(2)
The described adsorbed standard state, which is within the Henry’s law region of concentration, implies that the experimentally measured isosteric heat of adsorption, ABt, is equal to the differential molar heat of adsorption, A g o (7). Hence, with means of evaluating AGO and ABo, the differential molar entropy of adsorption, ASo, can be determined ASo = (AR0 - AGo)/T
(3)
RATE THEORY
Band dispersion in linear, nonideal GCS can be expressed by the relation
HETP = A ADSORPTION THERMODYNAMICS
RT
+ Bol~e,+ ZCguf, + Cafue,
(4)
where HETP is the height equivalent to a theoretical plate; A , Bo/ue,and ZC,ui, represent the contributions to plate height due to “eddy diffusion,” longitudinal diffusion, and resistance to gas-phase mass transfer, respectively; C,fue, is the contribution due to resistance to mass transfer from the kinetics of the adsorption-desorption process; f is the James-Martin compressibility factor (11); and ue, and ufo are, respectively, the total carrier gas velocity and interstitial carrier gas velocity at the outlet pressure (12). EXPERIMENTAL
The modified adsorbents were prepared from type F-1 alumina (Analabs, Inc., Hamden, Conn.). This material was added to an aqueous solution of the modifying salt and the resulting slurry was evaporated to dryness. The dry (IO) J. H. de Boer and S . Kruyer, Proc. Acad. Sci. Amsterdam, 55B, 451 (1952). (11) A. T. James and A. J. P. Martin, Biochem. J . , 50,679 (1952). (12) G. L. Hargrove and D. T. Sawyer, ANAL. CHEM.,39, 945 (1967). VOL. 40, NO. 2, FEBRUARY 1968
409
adsorbents were then sieved to the desired mesh range and packed into pre-weighed 0.125-inch 0.d. stainless steel tubing, The packings were held in the columns by small plugs of glass wool. The column lengths varied from 36 to 91 cm for the preliminary experiments; however, for the majority of the experiments, the column lengths were 91.4 cm. The adsorbent was activated at 300" C for 2 hours with a constant flow of dry He. Its weight was then determined by subtracting the weight of the empty column from that of the packed column. The interstitial porosities were determined by using the procedure of Bohemen and Purnell(13). A technique similar to that employed earlier (12) was used to determine the specific permeabilities of the packings to gas flow, B,, at a temperature of 150" C. The specific areas of the various adsorbents were determined using the continuous flow method of Nelsen and Eggersten (14). An extensively modified Aerograph Model 600 oven and temperature controller and an Aerograph Model 500 D electrometer were used for the gas chromatographic measurements. The modified oven, which maintained the column temperature to 1 0 . 5 " C, provided minimum pre- and postcolumn dead volumes. Chromatograms were recorded with a Leeds and Northrup Speedomax H with O S - , 3-, and 15inches/minute chart speeds; the time of sample injection was marked electronically on the chart. The HETP for a particular sorbate was determined from the chromatographic trace using the expression HETP = L( ~ 1 1 2 '/5.545(tr) ) *
where L is the column length, wliZ the width of the peak at its half-height, and t , the retention time of the peak. The volume flow rate at ambient temperature and pressure were measured using calibrated soap-bubble flow meters. This quantity was corrected to column temperature by multiplying it by the ratio of the column temperature to ambient temperature. The inlet pressure was determined using a mercury manometer, the outlet pressure was 740 =t5 torr. The velocities in Equation 1 were calculated by the expressions (12) ue, = (F.R.),L/F, uto
= ue,/et
(6) (7)
where L is the column length, (F.R.), the carrier gas flow rate at the outlet pressure and column temperature, F, the total column free-gas volume, and e; the interstitial porosity of the packing. F, is the difference between the total system dead volume as measured with methane at 300" C, V,, and the pre- and post-column dead volumes, V,,,. The instrumentation was such that V P , ,did not contribute more than 6 to the total dead volume of the system. The specific retention volume VaT (ml per meter2), was determined by the relation
where A is the total surface area of the column. The surface area available to the sorbates has been assumed to be equal to that determined by the BET equation using low temperature nitrogen adsorption data. Scott (7) has shown that this represents a good approximation for modified aluminas. The capacity factors, k ' , were evaluated by k' = VSTA/Vd
(9)
The gas phase diffusion coefficient ratios, which were necessary for evaluation of the adsorption-desorption mass(13) J. Bohemen and J. H. Purnel1,J. Chem. SOC.,1961,360. (14) F. M. Nelsen and F. T. Eggersten, ANAL.CHEM., 30, 1387 (1 958).
410
0
ANALYTICAL CHEMISTRY
I
I
10
20
% LOADING,
J 30
Wf./Wf.
Figure 1. Specific surface area, A,, of F-1 alumina as a function of amount of inorganic coating Upper curve Lower curve
=
Na2S04
=
NaaPOa
transfer term, C,, were determined using the open-tube chromatographic technique (15). All test sorbates were reagent or chromatographic grade. For the preliminary investigations 1-pl vapor samples were used. Samples for the main experiments, injected with a 0.5-pl syringe, varied from 0.0025 to 0.0050 p1 of liquid. Both feed-volume and linear-isotherm requirements were met at these sample levels. This was established by the fact that 0.050-p1 samples yielded the same values for V,T and HETP, within the experimental error-i.e., increasing the sample size by 10 times did not alter the band width or the distribution coefficient, K . RESULTS AND DISCUSSION Effect of Coating on Surface Areas and Adsorption. The variation in specific surface area and energetics of adsorption with per cent modification has been determined for F-1 alumina (80-90 mesh) modified with 0, 5, 10, 20 and 30% by weight of Na2S04and of NaaPOa. The specific surface areas, A , , of the two types of modified adsorbents as a function of amount of inorganic coating are indicated by Figure 1. Both salts produce a monotonic decrease in A , with increased loading; however, Na3P04is much more effective than Naz SO4 at reducing the surface area. This probably is due to chemical reaction between the substrate alumina and the Na3PO4modifier. Two types of interactions can occur between adsorbate molecules and the ionic surfaces of the adsorbents produced by the modification procedure. The first of these is due to dispersion forces and can be termed a nonspecific interaction (5). Adsorbates possessing either spherically symmetrical electron shells or sigma bonds interact with the adsorbent by this process. The second type of interaction involves adsorbate molecules having isolated sites, individual bonds, or a system of bonds of high electron density. Molecules with pi-electron systems, lone electron pairs and related functional groups can interact specifically with the ionic surface (16). The variation of the energetics for nonspecific interactions with per cent modification can be evaluated by measuring log (15) G. L. Hargrove and D. T. Sawyer, ANAL.CHEM., 39, 244 (1967). (16) D. J. Brookrnan and D. T. Sawyer, ANAL.CHEM., 40, 106 (1968).
V,T for a molecule which is subject only to such interactions. Determining the functional dependence of specific interaction energies on surface modification is more difficult because any test molecule which interacts with a surface also interacts nonspecifically. However, this functional dependence can be determined by comparing the behavior of two molecules which are similar in structure and physical properties such that they have the same nonspecific interactions with the surface. If one of the molecules also is subject to specific interactions, then the difference in retention for the two molecules is a measure of these specific effects. Figure 2 illustrates the variation of both types of interactions with loading for Naz SO4 and Na3P04at a column temperature of 100" C. The lower set of curves, which represent the logarithm of the specific retention volume of pentane as a function of the amount of coating, illustrates the dependence of nonspecific interactions on the degree of modification. The upper curves present the values of log[ VsT(pentene-l)/VsT(pentane)] -log [V,,* (pentene-1) /VsoT(pentane)] as a function of the amount of loading. The data represent the variation of specific interaction energies with degree of modification relative to those for a column with zero loading. The assumption has been made that the logarithmic terms are functions only of the specific interaction of a pi-bond with the surfacei.e., that the nonspecific interactions of pentene-1 and pentane with the surface are the same and cancel in the expression. Figure 2 indicates that both specific and nonspecific interactions become essentially independent of amount of coating above 10% by weight. The relative change with amount of coating for the nonspecific interactions is much greater than that for the specific interactions. Thus, surface modification appears to block the high energy nonspecific sites (such as capillaries) rather than to alter the energetics of the specific sites present. Thermodynamics of Adsorption. For the experimental conditions used, VsTfor a particular sorbate-sorbent pair is equal to the distribution coefficient, K . Thus, the variation of VsTfrom sorbent to sorbent for the same sorbate molecule indicates differences in partition behavior due solely to changes in the adsorbent. In addition, comparison of the retention volumes of a series of molecules relative to that of a standard molecule as a function of adsorbent provides insight to the functional dependence of nonspecific and specific interactions on the adsorbent. Table I summarizes values of VsTat 200' C for all of the sorbate-sorbent pairs as well as of V r e l ,the retention volume of each sorbate on a particular sorbent relative to the retention volume of n-hexane on that sorbent. Data also are included from Kiselev's work on graphitized carbon, an adsorbent which only interacts nonspecifically with sorbate molecules (5). Comparison of the two aluminas establishes that the Na2S04-rnodified adsorbent gives substantially larger values of VsTfor all four sorbates. However, the relative retention volumes of the sorbates are essentially the same for the two forms of alumina with the exception that the Na3P04column has greater relative interaction with n-electron systems. The values of V,,[ for cyclohexane are the same for the Na3P04and the Na2SOa adsorbent, which indicates that the values of VsTfor n-hexane and cyclohexane increase proportionately on Na2S04. This implies that similar proportional increases also take place in the nonspecific interactions of benzene and cyclohexene such that the decrease in Vret for these molecules at the Na2S04 adsorbent (relative to the Na3P04adsorbents) must be associated with the decrease in specific interactions with the surface. The value of Vrelfor cyclohexane is less than unity for all three
0.51
LL
u
I
I
I
I
0
10
20
30
% LOADING, Wt./Wt.
Figure 2. Effect of surface coating on nonspecific (lower curves) and specific (upper curves) interactions Specific retention volumes for uncoated alumina; VTgo(pentane), 0.26 ml/meter2; VTso(pentene-l), 0.56 ml/meter* log [ ~ s / R o = ] log [ VTs(pentene--l)/VTs(pentane)I - log [VT,o(penbne-l)/ 0, Na2S04; 0, NaaP04.
VTso(pentsne)I
adsorbents. This implies that the magnitude of nonspecific interactions is strongly dependent on the closeness of approach of the individual carbon atoms of a molecule to the surface. Thus, while n-hexane can bring all six carbon atoms equally close to the surface, cyclohexane can bring only four carbon atoms to within the same distance from the surface. This functional dependence on closeness of approach of carbon atoms, or perhaps carbon-carbon sigma-bonds, is further substantiated by the value of V r efor ~ benzene on graphitized carbon. Benzene, being completely planar, allows all six carbon atoms to approach the surface when adsorbed horizontally. The fact that V,el is somewhat less than unity for this sorbate-sorbent pair indicates that the C-H sigma-bonds also contribute to nonspecific interactions. Because benzene has fewer C-H bonds than n-hexane, it interacts less strongly with the graphitized carbon surface. The heats of adsorption (150'-200" C) and entropies of adsorption (200" C) for the various sorbates on the two modified aluminas are summarized in Table 11. The heats have been determined by fitting retention volume data for the temperature range of 15O0-2OO0 C to Equation 5 using a least squares procedure. The residuals in all cases are less than 2z, which indicates that the heats of adsorption are temperature independent over this range. Also included in Table I1 are similar A g o and A& data for hexane, cyclohexane, and benzene on graphitized carbon (3, as well as values of AA& where
Table I. Specific Retention Volumes and Relative Retention for Four Sorbates on Three Adsorbents at 200' C 20 Na3P04- 20 NapSO4- Graphitized alumina alumina carbon V*T. V"T. VaT. Compound mllmeterz V,.I ml/meterZ V,,I mllmeiert Y,,r Hexane 0.0219 1.00 0.0345 1.00 0.180 1.00 Cyclohexane 0,0198 0.90 0.0313 0.90 0.070 0.39 Cyclohexene 0.0358 1.64 0.0520 1. 5 1 Benzene 0.0780 3.56 0.113 3.28 o:iio d.67
z
z
VOL 40, NO. 2, FEBRUARY 1968
411
Table 11. Heats and Entropies of Adsorption for Four Sorbates on Modified Alumina and Graphitized Carbon at 150"-200" C 20 NatP04-alumina 20 % NalS04-alumina Graphitized carbon Compound -AR0, -A90, AA,S,, -AI%, -90, AAR, --BO, -AS., A&%, kcal/mol e.u. e.u. kcal/mol e.u. e.u. kcal/mol e.u. e.u. Hexane 6.5 10.0 0.0 9.2 14.7 0.0 10.6 14.4 0.0 Cyclohexane 6.0 9.0 1.0 8.5 13.5 1.2 9.3 13.6 0.8 Cyclohexene 8.0 12.1 -2.1 10.3 16.3 -1.6 ... ... ... Benzene 10.2 15.3 -5.3 12.2 18.6 -3.9 9.8 13.6 0.8
Sorbate Hexane Cyclohexane Cyclohexene Benzene
Table 111. Summary of Column Parameters A. DIFFUSION COEFFICIENT RATIOS AT 150-200" C Dl.4He), 150" C DMWl Vapor D1,dAr) cmz/sec Hexane 3.58 0.513 Cyclohexane 3.62 0.529 Cyclohexene 3.58 0.540 Benzene 3.59 0.614 B. CAPACITY FACTORS (k') AND RATEEQUATION COEFFICIENTS FOR HELIUM AT 150" C 1. 20% wt/wt Na3P04-alumina(100-120 mesh); column surface area, 414 meter* k' A X 10*,cm E,, cm2/sec ZCJti X 103, sec
c, x IO',
6.8 1.4 f0 . 4 0.30 0.77 k 0.07 5.7 1.2 f 0.3 0.34 0.87 k 0.06 13.0 0 . 6 f0.1 0.39 0 . 8 6 f 0.03 38.2 1.0 f 0 . 2 0.41 0.84 f 0.04 2. 20% wt/wt Na2S04-alumina(100-120 mesh); column surface area, 672 meter*
Hexane Cyclohexane Cyclohexene Benzene
sec
1.0f0.2 0.7 f 0.1 1.1 f 0.2 1.7 f 0 . 3
24.5 20.6 42.8 117.0
1.9 f0 . 4 1.4 f 0.3 1.6 f 0 . 3 1.6 f.0 . 3
C. INTERSTITIAL (ti) AND TOTAL ( e t ) POROSITIES AND PERMEABILITIES (E,) AT 150" C ti et E, X lo8,cm* Column 20 % NarPOralumina 0.40 f 0.01 0.79 f 0.02 7.1 i0 . 2 0.44 f 0.02 GIass beads 0.44 f 0.02 9.1 f 0 . 3
Reference to Table I1 indicates that the sum of all the interactions of the sorbates with the sorbent surface increases going from the NazPOa- to the NazSOa-modified aluminas, but that the percentage change in ARo and A&, is greater for hexane and cyclohexane than for the molecules whose principal interaction with the modified surface is specific in nature. Hence, the entropy losses of cyclohexene and benzene relative to n-hexane for adsorption on the Na2SO4are less than on the Na,POa-modified surface. Finally, the heats and entropies of adsorption for cyclohexane and hexane on the Na~SO~-alumina adsorbent are about the same as for the graphitized carbon, whereas ABoand A& are much lower for these molecules on the NasP04-modifiedsurface. Rate Equation Studies. For temperatures and pressures employed in the present study, the corrected retention volumes for a particular sorbate-sorbent pair have been found to be independent of the carrier gas (17). This indicates that the distribution coefficient, K, and the adsorption kinetics also are independent of the carrier gas. Thus, the procedure developed by Perrett and Purnell (18) can be used for isolating the stationary phase mass transfer term, C,. Equation 4 can then be rearranged to yield
(HETP
- C e f ~ e . ) U e , = Bo + Aue, + ~ ( C g , / ~ t ) u e , * (11)
(17) D. J. Brookman, G. L. Hargrove, and D. T. Sawyer, ANAL. CHEM., 39, 1196 (1967). (18) R. H. Perrett and J. H. Purnell, ANAL.CHEM., 34, 1336 (1962).
412
ANALYTICAL CHEMISTRY
I
I
I
I IO
I 20
I
I
30
40
I
0.21
5 a IW
I 0.1(
(
u e o , cm Isec Figure 3. Column efficiency of a 20x wt/wt NalPO,alumina column (total surface area, 414 m*) for four sorbates as a function of carrier gas velocity at 150°C 0 , Benzene; 0, cyclohexene; 0, cyclohexane A,
n-hexane
where ui, has been expressed in terms of ue, (see Equation 7). Once the value of C, has been determined, data taken over a wide range of velocities can be fitted to Equation 11 using multiple regression procedures to yield the remaining rate equation coefficients. This procedure has been used to determine C,,A , Bo, and ZC,,/E~for four sorbates on a 207, wt/wt Na3P04-alumina column (100-120 mesh) at 150" C. Data for C, with a 20% wt/wt Na2S04 alumina column also are included. The He/Ar diffusion coefficient ratios for the sorbates, which have been used for the C, calculations, are summarized in section A of Table 111. A curve fitting procedure has been applied to the helium data using an IBM 7040 computer. The results of these calculations are summarized in section B of Table 111. The curves generated using these calculated rate equation coefficients duplicate the experimental results with a deviation of less than 4 over the complete range of velocities studied as illustrated by Figure 3 (the lines are the calculated curves). The relative magnitudes of the mass transfer coefficients in Table 111 indicate that the stationary phase resistance to mass transfer, C,, contributes to band dispersion in GSC to about the same extent as gas phase nonequilibrium, Z C J E ~ . At higher velocities (where f approaches 0.5) gas phase resis-
tance to mass transfer contributes about 50% of the total HETP, and kinetic nonequilibrium contributes about 40 of the HETP. The A terms in Table I have values with the expected magnitude for a nonporous packing; i.e., a particle diameter (12). The values for the Bo terms are lower than expected because of the low values of the obstruction factors, y. The latter can be evaluated by the relation 7 = BOW,,
which gives values for y that range from 0.30 for hexane to 0.35 for benzene. However, the specific permeability, B,, and the interstitial porosity, Et, for the Na3P04-alumina column are both lower than those for a glass bead column with the same mesh range and column geometry, Such low values of ei and Bo would result in less longitudinal diffusion than is normally observed and thus cause y to have low values. The interstitial porosities, total porosities, and specific permeabilities of the GSC and glass bead columns are summarized in section C of Table 111. RECEIVED for review July 27, 1967. Accepted December 13, 1967. Work supported by U. S. Atomic Energy Commission Contract No. AT(11-1)-34,Project No. 45.
Chromazurol S Spectrophotometric Determination of Alumina in Iron Ores, Sinters, and Open Hearth Slags O m P. Bhargava and W. Grant Hines The Steel Company of Canada, Ltd., Chemical and Metallurgical Laboratories, Wilcox Street, Hamilton, Ont, Canada
MUSTAFIN et a!. ( I ) indicated the possibilities of using Chromazurol S for the colorimetric determination of aluminum in steel. This reagent was employed by Brockmann and Keller ( 2 ) , but variable amounts of aluminum are lost during the hydroxide precipitation of iron, as reported by Werz and Neuberger ( 3 ) among others. Brockmann and Keller and Buck ( 4 ) indicated, however, that a separation is not necessary in control work, and we have successfully applied the reagent to the rapid determination of aluminum in steel (in the range 0.01-0.15 and in zinc (0.01-0.5 %) with excellent accuracy and precision. Extension of this to determinations in ores and slags seemed feasible, and the method described below proved to be rapid and accurate. With a slight modification, it was found applicable also to these materials even when containing up to 10 fluoride.
x)
x
EXPERIMENTAL
Reagents. Water used is distilled, then deionized. Chromazurol S reagent (Merck No. 2477) 0.050 gram is dissolved in 125 ml of 50% v/v ethanol. Ascorbic acid: 1 . 6 x aqueous solution is made fresh each day. Acetate buffer, sodium acetate trihydrate 7.5 aqueous solution, is stored in a polyethylene bottle. The standard iron solution is prepared by dissolving 1 gram of iron (Ferrovac E) in 20 ml of HC1 and a
few drops of HNOI. After boiling to approximately 2 ml, 15 ml of (1 1) HCI are added and the solution is diluted to 250 ml in a volumetric flask. This provides a solution in which 1 ml contains 4 mg of Fe. The standard aluminum solution is prepared from 8.946 grams of AlC13-6Hz0 by dissolving in water, adding 5 ml of HCI, and making up to 1 liter (1 ml = 1 mg of AI). Five milliliters of this solution in turn are pipetted into a 500-ml volumetric flask, 2 ml cf HCI are added, and the solution is diluted to the mark. This provides a working solution in which 1 ml = 10 pg of Al. Recommended Procedure. Using a polyethylene scoop, sodium peroxide (1-1.5 grams) is placed in a clean nickel crucible (preferably heavy gauge). The ore, sinter or slag sample is then added and mixed thoroughly. (Sample weights of 0.25 gram for alumina levels below 1 or 0.1-gram levels up to 5 % are convenient.) Contents are fused to a dull red heat over a Bunsen burner, swirling the crucible, until a homogeneous melt is obtained and no specks are visible. After cooling, the crucible is placed into a 2 5 0 4 beaker,
+
(1) J. S. Mustafin and E. A. Kashkovskia, Zauodsk. Lab., 24, 1189 (1958). ( 2 ) H. Brockmann and H. Keller, Arch. Eisenhuttenw, 35, 367 (1964). (3) W. Werz and A. Neuberger, Zbid., 26, 205 (1955). (4) L. Buck, Chimie Anal. (Paris),47, 10 (1965). VOL 40, NO. 2, FEBRUARY 1968
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