Vacuum Fusion Analysis Apparatus and Determination of Oxygen in Chromium WILLIAM S. HORTON AND JOSEPH BRADY Knolls Atomic Power Laboratory, General Electric C o . , Schenectady, X. 1..
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A
LTHOUGH the workers introducing the vacuum fusion
method for determining gases in metals ( 1 2 ) considered nonfemous metal samples, the early use was limited to iron and steel (28). Recently, however, a number of general articles have appeared discussing applications of this method to nonferrous metals. \Vork by Sloman (21-25) in England and by Mallett ( 1 4 ) in the United States should be mentioned. While a remarkable experimental and theoretical account of work covering a number of metals has been given by Sloman and coworkers (M), niany articles have appeared discussing single metals, especially for the determination of oxygen in t.itanium ( 7 , 29), in zirconium (26), and in copper (11). I t appears that t.he best results are obtained from each type of apparatus using procedures developed for that specific apparatus. There is ample indication (27) that' varying results among investigat,ors are influenced by the diffrreni experimental arrangements. For that reason the ap-
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tion of the form: R a clC cd? tlT teT* kCT, where a, cl, cp, ti, t z , and k are the fitted constants, R is the recovery in per cent, C is the weight per cent of chromium in the bath, and T i s the operating temperature. Much better results were obtained by using a molten bath of 759?0 iron and ~ F B ~ o tin by weight than by using one of pure iron. At 1600" C. the method with the alloy bath gave about 8570 recovery when no chromium was present. When consecutive samples had increased the chromium to about 5% of the bath, the oxygen recovery dropped to about 7070. The results agree with those obtained by modification of an acid-solution method. Oxygen may be determined in chromium by the vacuum fusion method with certain limitations and using occasional standards to check recovery.
This report describes the apparatus designed and constructed for research in vacuum fusion analysis. The work was undertaken to provide methods for the determination of oxygen, nitrogen, and hydrogen and nonferrous metals. This is exemplified by a n investigation for determining oxygen in chromium. Blank rates and results for National Bureau of Standards samples of cooperative steels compare favorably with those of other laboratories. Blank rates were indistinguishable, whether measured for 10 or 20 minutes. The presence of stopcock grease gave a measurably higher blank rate than when t h e grease was absent. The small increase should interfere only with obtaining extremely low blanks. The recovery of oxygen from mixed powders containing chromium and chromic oxide was fitted to an equa-
paratus is rather carefully described in this paper. This description of the apparatus and its characteristics and an experiment with stopcock grease is followed by a report of an investigation for determining oxygen in chromium TT-ith the described apparatus. APPARATUS, CHAR4CTERISTICS, AND EFFECT OF STOPCOCK GREASE
General Description. For the apparatus described it was desired to attain a low blank rate, to be able t o handle a wide range of oxygen composition among the samples, and to use no stopcocks. The first two goals were imposed by the expected nature of the analysis requests from other sections of the laboratory. The last requirement was imposed in order to be able to test the influence of stopcock grease on the blank. The apparatus is represented schematically in Figure 1. Except for certain sections of the furnace, F', the lines are constructed of borosilicate glass with an outside diameter of 1 inch from F' to the main loop and clockwise to the circulating pump, P , and an outside diameter of '/2 inch elsewhere. A 5-kx. General Electric Model 4HL2A5 electronic induction heater is used to heat the furnace. Temperatures in excess of 2500" C. have been attained with this heater, although this has been unnecessary in any work done to this date. Volumes I and 11 are calibrated by the expansion method (16) to within 1%. Closing valves D, E, and F forces the collected gas into the smaller volume of approximately 1 liter. Closing valves D, E, and G instead utilizes both volumes or a total of about 5 liters. The maximum collectable pressure is about 1.4 mm. of mercury, Since a sample of 0.5 gram is the normal size used in this apparatus, materials up to 1 2% oxygen can be handled. Homogeneous materials, for which small samples of the order of 20 mg. may be used, can be handled up to 30% oxygen. In practice 10-mg. samples of material containing about 30% oxygen have been analyzed. Components. The vacuum furnace is similar to that of Beach and Guldner (3) and is shown in detail in Figure 2.
Figure 1. Schematic Diagram of Apparatus 2. U-type cutoff I , I I , Calibrated volumes, 1 and 4 liters, respectively A . B . C. M . Glass-mercury solenoid valves ' F'. Vacuum furnhce GI, Gz. McLeod gages, 5OOp and 10 m m . , respectively 6 3 . Thermocouple gage I,'. Mercury lift for the introduction of samples P. Mercury diffusion circulation pump, also used for gas extraction Ti, Tz. Cold traps
The large standard-tapered ground joint used a t the bottom by them is replaced here by a butt-type joint which is sealed on the outside with Apiezon Wax IT. This greatly facilitates changing crucibles and avoids the occasional freezing of the tapered joint. The glass-enclosed iron slug used by Beach and Guldner as a shield for the optical window is omitted. The standard-tapered joint used for the Rindow was probably first employed by Walter (29). The connections to the system and the sample loader are made through ball joints to facilitate removal for cleaning, re1891
ANALYTICAL CHEMISTRY
1892 pair, or replacement. The ball joints are waxed on the outside. A Heinze, 3400-r.p.m., continuous-duty air blower is used to cool the outside of the furnace. The graphite crucible and funnel are made of AUC grade material purchased from the National Carbon Co. This assembly ia placed in a quartz tube 5 ' / 2 inches long by 2 inches in diameter and is surrounded by graphite powder, 100 to 200 mesh. The powder is made by filing or machining AUC stock. Sational Carbon Co. powder No. 38 may also be used. The quartz tube is then suspended from the hooks in the furnace by 0.050-inch platinum wire.
%
dBall
jolnt
Figure 3. and vacuum
T Part
Q
_cN
part
Schematic Diagram of Furnace
P. Borosilicate glass
Q.
Borosilicate glass t o quartz grade Quartz
ne mercury i i I t , usea 60 inrroauce samples ana wnicn seems t o have been suggested first by Derge and coworkers ( 8 ) , avoids being restricted to a given set of determinations or a given order as generally occurs with the usual loading tree. It also avoids lengthy storage under vacuum which tends to remove hydrogen from steels. Nonmagnetic samples may be wrapped with iron wire; the type used as standard chemical iron is satisfactory. Powders may be placed inside small iron or nickel capsules. To collect and circulate gases the mercury diffusion pump described by Xaughton and Uhlig (17') is used. After it was installed in the apparatus, the pumping speed was found to be about 12 liters per minute. The highest pressure that the pump would collect a t this speed was about 1.4 mm. of mercury. The main evacuation pump is a D P I three-stage glass, oil diffusion pump backed by a Welch Duo Seal mechanical pump. I n place of stopcocks and U-type mercury cutoffs, a glassmercury solenoid value was designed. Although sketches of these valves appear in the literature ( I S ) , dimensions and other design data are given in this report and in Figure 3, as they may prove useful to others. The coil, developed by the electrical products group of the General Electric Research Laboratory, has the following characteristics: Inductance
D.c. resistance Identification No.
T
Slug or Soft Iron Win wt. 6.5 grama
v
Q.G.
slug or soft iron wire
i ----t to analytlcol system
Figure 2.
So01 off g a b o v a top o f plunger when In UD position
1 . 2 5 henries, iron slug out 1 55 henries, iron slug in 286 ohms W.S.F. 3241580
With about inch of mercury, the valve provides a closure for pressure differentials up to about 2 mm. Greater differentials tend to blow the inverted cup opm or to hold it down against the
I t
IMake " o ' o 'ports
(I"O.0.)
about 6" long
Large JIercury-Glass Solenoid Valve
A small valve is also used and has 'Iz-inch ports instead of I-inch. The solenoid, not shown here, i s described in the text.
pull of the pickup coil. Since the installation of these glassmercury solenoid valves no accidents attributable to them have occurred. Previouslv the U-type cutoff had been involved in two or three accidents and had always presented a safety problem because of the slender glass tubes used for mercury risers. .4dditional advantages of the valves are ease and speed of operation. Assembly of the controlling switches with indicating lights on a panel provides a quick indication of the opened parts of the system. To make possible 24-hour continuous operation of the main evacuating and circulating pumps, water safety switches (Figure 4 ) were designed to shut off the heating current in case of water failure. The cooling water from the diffusion pump enters the pressure head tube. If the stopcock is partly open, the water in the tube will seek a level such that the pressure head causes a rate of flow out the bottom drainage tube which equals the rate of flow inward. Adjustment of the stopcock will bring this level to about the middle of the tube. Sufficient mercury is placed in the side tube to connect the contacts. When the water fails, the level in the pressure head tube drops because of drainage through the bottom tube. With loss of the pressure head, the mercury assumes an almost level position opening the contacts and shutting off the heater current. The top drainage tube provides an exit for water in case of sudden pressure surges in the water system. I n order to reduce these surges, a constant flow regulator (Fisher Scientific Co. catalog No. 15-529) is placed directly on the line supplying the cooling water. A General Electric electromagnetic relay, catalog No. CR-2790-ElOOA2 (115 volts, 60 cycles) is interposed between the contacts and the power to the heater to reduce sparking. Of course, an electronic relay would practically eliminate this trouble. Copper oxide is prepared by the method of Murdock, Brooks, and Zahn (16), resulting in a copper oxide containing 1% iron oxide. The caked precipitate is chopped with a spatula and the 6- to 30-mesh material is separated and used. For trap T,methanol is previously cooled to its freezing point by successive small additions of liquid nitrogen. The alcohol must not be entirely frozen for two reasons: Temperatures cold enough to trap carbon dioxide in the wrong trap may be attained and their is some danger of breaking the Dewar flask containing the alcohol because of expansion on freezing. The more usual dry ice-acetone mixture permits a water vapor pressure of 0.5 micron of mercury and some transfer to the carbon dioxide when the latter is collected. Temperatures are read with a Leeds and Northrup optical pyrometer (catalog S o . 8622-C) calibrated with a National Bureau of Standards tungsten lamp. Procedure. Prior to each analysis the crucible is degassed by heating to about 200" C. higher than the temperature to be used in the analysis. For iron and steel samples operation is a t about
V O L U M E 25, NO. 12, D E C E M B E R 1 9 5 3 1650' C. and degassing a t 1850" C. Degassing is continued until the blank rate a t the operating temperature drops to the desired value. This is determined by closing valves 111,D,E, and F and collecting gas for 10 minutes. When a satisfactory blank rate has been obtained, a cleaned and weighed sample is introduced through the mercury lift with the assistance of a magnet, and the small air bubble which generally accompanies it is pumped out. Valves ill.D,E, and F are closed and the magnet is used to bring the sample over the vertical tube of the furnace, where it is dropped into the crucible. The evolved gases are pumped clockwise (Figure 1) by P into the calibrated volume. After measurement of the pressure on G, or G?,E. J , and K are opened and B and C are closed: thus the gases are circulated over the trap containing the copper oxide which is heated to about 400"C. Traps TIand Tz are cooled with the cold methanol previously described and liquid nitrogen, respectively. .is the oxidation proceeds, water and carbon dioxide are condensed in T I and Tz,respectively, and the pressure decrease is conveniently followed on the thermocouple gage. When the rate of decrease is zero as checked on the McLeod gage, the residual gas is collected in the calibrated volume by closing E; the pressure is measured on the McLeod gage and recorded as nitrogen. I'alves J and K are closed and the nitrogen is discarded through E. Alf, and the vacuum lines. After evacuation, JI is closed and C opened; the liquid nitrogen is removed from 7'1, the re-evolved carbon dioxide is circulated around the main loop for a minute or two to remove any traces of water that may have distilled over from Ti, and is collected in the calibrated volume by closing E , and is measured and discarded. The methanol is removed from T , and the entire system evacuated in preparation for another analysis. The difference between the original total preqsure and the sum of nitrogen and carbon dioxide pressures is attributed t o hydrogen.
I_ from
pressure hood t u b e
d
1893 is about 3.0 liters per minute in the range 200 to 0.2 microns pressure. I n order to determine the pumping speed of the circulating pump, P (Figure I), about 300 liter-microns of nitrogen were admitted and collected in the large volume. With G and F closed the system was evacuated to a t least 0.1 micron. Valves B , D,E, F , G, J , K , and M were now kept closed and a t a recorded time G was opened. The pressure collected in the small volume was measured a t various time intervals. If P, is the pressure at time, t, in the small volume, the total loss of pressure in the large volume, APL,during time t - t o is given by V
APL = P, 2
(1)
VL
where to is the initial time of opening valve G, V , is the known size of the small volume, and VL is for the large volume The volume ratio is known from the calibration data or can be obtained from the initial pressure in the large volume and the final pressure in the small volume. At time t , therefore, Va PL = P L ~ APL = P L ~ P, VL
(2)
This pressure, P L , now approximately follows the exponential law, so that by choosing one time near the beginning and another near the end, an apparent average pumping speed may be calculated from S = [ 2 . 3 0 3 V ~log ( P ~ / P z ) ] / (tz ti) (3) The numerical suliscripts indicate the early and late values of P and t. In the apparatus described herein this determination was made for various amounts of heater power supplied to the circulating pump. The speed was found to be a maximum of 12 liters per minute a t about 265 watts. N o doubt this will vary as a function of the rate of cooling provided and the amount of insulation placed around the pump. The maximum pressure collectable by the pump was measuied bj- admitting various quantities of nitrogen and noting the masiilium value for which the thermocouple gage showed less than 0.1 micron remaining on the low pressure side. At 265 watts this pressure was found to be 1 4 mm. of mercury. By increasing the heater power to 300 watts 1.87 mm. of mercury could be collected, but a drop to 10 liters per minutewas noted for the pumpingspeed. The lower power is non' used in normal operation. A commonly achieved blank rate in this apparatus is 0.1 to 0.2 liter-micron per minute. This is compared with that of other hhoratories in Table I.
wmp-7
Table I.
Comparison of Blank Rates
Blank Rate, lIl.,"our(STP)
i
to drain
Figure 4.
Diagram of Water Safet) Switch
.kt the point marked 2 (Figure 1) the usual L--tvpe mercury cut-off is emplo? ed because another type of analysis, expected to be possible in the same apparatus, requires a valve capable of holding a larger pressure difference than the mercury-glass solenoid valves used elsewhere in the apparatus During evacuation of gases from the calibrated volumes or the entire apparatus, it was found best to close this cutoff. When the larger volume is required for the gases evolved from the sample the valves D,E, and G are closed and F and H are opened. After measuring the total pressure in this case, it is convenient to close F and to continue the anal sis with the smaller part of the sample using the small volume. $he excess gas is discarded a t the end during the general evacuation of the apparatus. System Characteristics. With the circulation pump operating the average pumping speed for evacuation of the entire system
Reference
0.60 0.046 f 0.01,i 0.006 0.004 0 . 9 i 0.1 0.75 i 0 . 2 6 1.0 1.9 0.55 i 0 11 0.012 i 0.004
The preliminary results of analyses performed with the a p paratus are summarized in Table I1 for oxygen and nitrogen determined in the eight cooperative steels prepared by the National Bureau of Standards and used in an international cooperative study ( 2 7 ) . Because of the small size of sample (0.5 gram) and the probable concentration gradient along a radius of the cylindrical bar of the steels, pie section-shaped pieces were used in order to obtain representative samples of the cross section. The arithmetic averages for the eight steels listed in Table I1 are slightly different than the so-called best values assigned by
ANALYTICAL CHEMISTRY
1894 Table 11. Results on Cooperative Steels Oxygen Nitrogen Found, Found, Steel no p.p.m. Cb sc s,d p.p.m. C s 80 33 17 26 181 35 4.5 4.5 1 3 154 18 11 29 33 31 39 118 138 2 3 28 40 133 135 24 24 161 168 3 3 6 2 15 17 50 5.6 23 25 26 4 7 8.9 14 22 25 26 40 93 92 5 25 16 19 44 l!O 11 56 1 0 6 3 48 50 12 17 69 28 1060 129 937 7 3 174 26 33 13 9 10 8 4 120 38 a n, Kumber of determinations. b C, Arithmetic average of t h e results reported b y all of t h e laboratories t h a t participated in the s t u d y for the given stsel. C 8 Estimated standard deviation for the present results. d SI., Estimated standard deviation for the results of the different C O O P erators.
.
t h e committee reviewing the results of the cooperative study. I n general, s is less than sE since s is for one laboratory and sc for 15 laboratories. This would be in agreement with the general feeling that variance within one laboratory ought to be less than variance between laboratories. However, the comparison is obscured by the fact that so is for the means of the 15 cooperators while s is for the individual results of one laboratory. Steel 5 has generally been used in this laboratory as a standard to detect unsatisfactory operation. T o test the effect of stopcock grease on the blank rate it was decided to compare alternate blank rates with and without the presence of the grease in the region near the furnace. I t also appeared desirable to determine the effect of length of time for blank rate measurements. I n order to test the two effects simultaneously, and with increased precision, a 22 factorial experiment (6) was chosen as a useful design. The treatments and levels were as follows: Treatments Grease Blank collection time
Levels Present absent IO mindtes, 20 minutes
Apiezon grease was placed in a small nickel cup and placed in the upper section of the mercury lift. Both treatments a t each level were tested in combination with one another. The data obtained are listed in Table 111.
given these results. It appears safe to conclude, therefore, that Apiezon M grease causes an increased blank rate. This increase -about 0.024 liter-micron per minute in this case-may not be of practical significance, however, unless extremely low blanks are being attempted. On the other hand, in the low pressure combustion method for carbon determination (16, SO), the effect might be greater because of combustion of the grease vapors and probably should be checked by users of that method. DETERMINATION O F OXYGEN IN CHROMIUM BY VACUUM FUSION METHOD
The approach to the problem in this laboratory differs somewhat from the excellent one that Sloman and coworkers (26) recently described. Their method for determining oxygen involves the following steps. The minimum operating temperature is determined for the oxide of the metal being investigated. An experiment is made to determine the maximum concentration of metal allowable in the iron bath or flux used in the graphite crucible. Then the gas content of the sample of metal used for the preceding step is determined accurately. Repeated determinations are made to check the procedure and to approach the maximum concentration determined earlier to discover a t what point low results become apparent. As a check the metal of known gas concentration is then used to repeat the first step. The following work done a t this laboratory on chromium (a metal not included in the recent Sloman experiments) involved the following steps: 1. A synthetic mixture of oxide and metal powder was analyzed for oxygen a t a variety of temperatures using an iron or other type of bath. Careful account was kept of all materials added to the graphite crucible-for example, samples, bath material, and wrappings or capsules for samples. 2. The chromium metal powder used in step 1 was analyzed for oxygen by an independent method. The known amount of oxygen in the synthetic chromium-chromic oxide mixtures could then be computed, and the recoveries determined in step 1 could also be computed according to
R=
Set
R u n Description= N-10 G-20 G-10 1-20
Blank R a t e , p/Min 0.177 0.182 0.177 0.157
111
N-10
0.201 0.236 0.227 0.190
G-20 G-10
h--20
N, no grease present; G, grease present; 10, 10-minute blank determinations: 20, 20-minute blank determinations. a
Set I (not shown) was invalid, since it was performed in a different manner and had resulted in grease being present when not desired. Set I1 was expected to be relatively insensitive to the effects looked for, since temperatures more erratic than usual were observed. Set I11 appeared to be satisfactory. The sets were performed on different days. Analysis of variance performed on set I11 alone indicates that time has no significant effect on the blank rate. The presence of stopcock grease, however, increases the blank rate, with the chances being only about 1 to 21 that the difference could have been obtained by chance alone. Sets I1 and I11 can be analyzed together with the effect of days (or sets) removed. This analysis also gives no significant time effect. The grease shows an increased blank rate again, this time with the odds being about 1 to 32 that chance could have
x
100
3. The values of R were next fitted by the method of least squares to an equation of the form
R =u Table 111. Effect of Stopcock Grease on Blank Rate
oxygen found oxygen expected
+ CiC + cL" + tiT + tzTz + kCT
(5)
where a,C I , CZ, t i , tz, and k are constants, C is the neight per cent of chromium in the molten bath into which a given sample is dropped, and T is the centigrade temperature at which the recovery was determined. This form of equation was chosen because it is simple and allows interacting and conflicting effects to be expressed. 4. Steps 1 and 3 were repeated for other bath materials in an attempt to improve recovery. Materials. CHROMIUM.Metal powder purchased from the Fishe; Scientific Co. was used. This was labeled "about 98% pure. The major impurities as determined in this laboratory were found to be 1.37, iron, about 0.67, silicon, and 0.2% carbon. According to this research, it contained about 0.57, oxygen. Other minor impurities amounted to about 0.37,. CHROMICOXIDE. Baker's analvzed powder containing 0.4% unidentified substances not precipitated by ammonium hydroxide was used. IRON.National Research Corp. High purity iron mas used a8 a flux material. TIN. Baker's analyzed tin (20-mesh granules) was used as an additive to the iron flux. ALUMIXCM.Foil as purchased from Fisher Scientific Co. in roll form was used as a wrapping for powder. WIRE. Standardizing iron n-ire from J. T. Baker was used to wind around aluminum-\
