lated for the total 70 determinations is =!= 10%. The dai,a used in calculating this prrcision wc'w obtained diiring a time period of one year by three different operators v sing three different apparatus. The relative standard deviation calculated for 15 determinations of oxygen in a coatrd uranium carbide sample containing only 75 p.p.m. of oxygen is =t12yo. Because the degree of homogeneity of the samples is unknown, the measure of precision includes the variance due t o heterogeneity and experimental errors in the method. At present, over 4,50 saniples having various oxygen contents ha1 e been analyzed in duplicate. and the two results obtained for each sample have not differed by more than 10% in the majority of the determinations. KOstandards are rivailable for oxygen in uranium carbide samples coated with pyrolytic graphite, so that a good estimate of the accu-acy is not possible. However, an attempt was made to determine the recov:rp of oxygen from a sample of pure uranium dioxide. Enclosing the oxide sample in a graphite pellet or capsule W M unsuccessful because of the tendencv of the graphite to be blown out of the crucible and cause arcing between the c-ucible and the concentrator core. A series of determinations was made by removing the crucible
from the furnace and adding a weighed quantity of oxide. Twelve samples weighing from 0.4 to 0.7 mg. gave an average oxygen recovery of 96%, with a relative standard deviation of &IO%. The errors involved in the change of procedure plus the weighing errors are included in the results. The slightly low recovery may be due to the tendency of these samples of high oxygen content to be blown from the crucible before intimate contact with the graphite is achieved. A sample of uranium carbide particles coated with pyrolytic graphite was analyzed for oxygen by this method, using the unground material, and also by an outside laboratory using a vacuum fusion method subsequent to grinding the coated particles in an inert atmosphere. The results obtained by the two methods did not differ by more than 10%. Agreement also was found with values obtained here with the usual inert-gas fusion method when the sample was ground in an inert atmosphere prior to analysis. It should be possible to apply this method to the determination of oxygen in other pyrolytic graphite-coated materials such as uranium-thorium carbide particles or other coated carbides, providing their oxides are reduced and a metal carbide-carbon eutectic mixture is formed below 2550' C. Under
optimum conditions, 10 analyses in duplicate can be made in one day. ACKNOWLEDGMENT
The authors thank C. F. Metz, under whose supervision this work was performed, for his valuable advice and assistance, and D. E. Hull and D. E. Carlson for advice in the installation of the induction furnaces with current concentrators. LITERATURE CITED
(1) Hull, D. E., Los Alamos Scientific Laboratory, Los Alamos, N. h'l., private communication, 1962. (2) Kempter, C. P., J. Less-Common Metals 4, 419 (1962); U. 6. At. Energy Comm. Rept. TID-15185(1962). (3) Krupka, 1LI. C., Los Alamos Scientific Laboratory Rept. LA2611 (1962). (4)Leitnaker, J. M., Ibid., LA-2402 (1960). (5) Leitnaker, J. M., Bowman, M. G., Gilles, P. W., J. Chem. Phys. 36, 350 (1962). (6) Litz, L. M., Ph.D. dissertation, Ohio
State University, Columbus, Ohio,
1948. (7) Potter, J. L., Murphy, J. E., Heady, H. H., ANAL.CHEM.34, 1635 (1962). (8) Smiley, W. G., Zbid., 27, 1098 (1955).
RECEIVEDfor review April 26, 1963. Accepted July 1, 1963. Loa Alamos Scientific Laboratory is operated under the auspices of the Atomic Energy Comrmssion.
Carbon Determination in Elemental Boron CHEN-WIN KUO, GARY T. BENDER, and JOE M. WALKER Department of Chemistry, Kansas State College of Pittsburg, Pittsburg, Kan. b A convenient, rapid, and highly precise method for the determination of the carbon content of elemental boron has been developed. Under controlled conditions, powdered boron, mixed with tin accelerator in a prefired steel crucible, is cornbusted in a highfrequency induction furnace. The carbon content is converted into carbon dioxide and detected electroconductometrically. National Bureau of Standards steel samples were used for standardization purposes. The method shows a standard deviation of fl.7 to f21 using standard steel Samples in which the carbon content ranged from 20 to 400 p.p.m. The results of boron analysis gave a standard deviation of f5.8 to =t25 in the range of 350 to 1000 p.p.rn. carbon. The prefiring of the crucibles was necessary when working with lowcarbon materials. The critical role of boron particle size regarding combustion was demonstrated, as well as errors introduced by mixing boron with steel. The time required for prefiring
the crucible, loading and combusting the sample, and data recording of a single run was approximately 25 minutes.
T
HERMODYNAMICALLY, boron is an excellent solid fuel for rockets because of its large heat of combustion which releases considerable energy per unit volume of fuel. Unfortunately, elemental boron does not oxidize easily. Confronted with the rapid growth of rocket propulsion, the combustion of boron is now being extensively investigated. Boron can also be used in many technical devices such as windows, filters, thermistors, and resistors, because of its electrical and optical properties. However, a t the Boron Conference (1) held in Asbury Park, N. J., in 1959 (the specific objective being the bringing together of scientists for the discussion and exchange of information related to solid state science), S. Benedict Levin pointed out that further progress in the use of boron has been
frustrated by the lack of very pure boron in any form adequate for research. Both governmental and industrial organizations have tried very hard to isolate high-purity boron in their laboratories and to develop an analytical method for the determination of the carbon content. Carbon represents the largest impurity in the high-purity boron thus far produced. Not a single article related to the determination of carbon in boron has appeared in the literature. This subject involves essentially two major problems. The first is the conversion of the carbon to carbon dioxide, and the second is that of the quantitative measurement of the evolved carbon dioxide. A method for oxidizing the boron sample and conversion of the carbon content into carbon dioxide was developed in this laboratory. The procedure consisted of firing a powdered boron sample in an oxygen atmosphere with tin accelerator under carefully controlled conditions in a high-frequency induction field. The electroconductoVOL 35, NO. 10, SEPTEMBER 1963
1505
mm(J
Ba(OH+ COMBUSTION SYSTEM
r l m
REFILLING SYSTEM
T o Atmosphere
I
.
141 OXYGEN PURIFICATION SYSTEM
l i
I
r
3
ABSORPTION SYSTEM
CircIiitd t o Detection System
-fkk DETECTION
15cm.
A.C.
Schematic diagram of entire system
Figure 1 . 1.
2. 3. 4. 5. 6.
7. 8.
9. 10. 11. 12. 13. 14. 15. 16. 17. 1 a. 1 9.
Figure
O x y g e n cylinder CuO tube Preheater Mg(CIO& scrubber Ascarite scrubber MnOz scrubber High-frequency induction furnace Two-stage pressure regulator Constanbtemperature water bath Absorption cell with electrodes Reference cell with electrodes Standard resistor Bo(0H)a solution 0.1 N " 0 3 Deionized water Soap-bubble flowmeter Oscilloscope Slide-wire Power supply box
metric method was chosen for the carbon dioxide detection because of its rapidity of operation, simplicity of apparatus, and high sensitivity. EXPERIMENTAL
Apparatus and Materials. A schematic diagram of t h e entire system is shown in in Figure 1. The system consisted of five individual functional units: Oxygen Purification Unit. A inch copper tube, filled with Mallinckrodt wire-form analytical reagent grade copper(ic) oxide, was heated by a Hevy Duty Electric Co. Multiple Unit tube furnace giving a temperature of 600" to 800" C. Linde hospitalgrade oxygen was run through the preheater, which oxidized any carbonaceous matter present in the oxygen stream. The oxygen was then passed through a copper tube, 24 inches long and 3 i 4 inch in diameter, which was packed with G. F. Smith Chemical Co. reagent grade magnesium perchlorate, Arthur H. Thomas Co. Ascarite (8- to ao-mesh), and Leco 501-60 specially prepared manganese dioxide. Any possible mois1506
I:o"v.
SYSTEM
ANALYTICAL CHEMISTRY
ture, carbon dioxide, and sulfur dioxide in the flowline was thereby removed. Combustion Unit. The purified oxygen was then introduced into the Leco Model 523 high-frequency inductioii furnace. To obtain more uniform combustion, a pressurized system was desired. Since the original, Leco-de-
Dimenrion,
Figure 2.
I"
cm.
Double O-ring seal
3. Absorption cell
signed, silicone rubber connection between the quartz combustion tube and the absorption unit could not hold a pressure in excess of 3 p.s.i., a speciallyconstructed brass double 0 ring seal, which is shown in Figure 2, was incorporated into the oxidation chambsr. Instead of the conventional Leco quartz combustion tube, a quartz tube with a length of 7l/2 inches, and of 13/8-inch 0. d. was used. All of the flowlines and connections were made of 1/4-inch copper tubing and fittings. The system has been operated and tested under a n oxygen pressure of 25 p.s.i. without failure. Leco 501-76 carbon-free tin accelerator (20- to 40-mesh), 528-35 crucibles, and 528-40 covers VI ere used in this study. Carbon Dioxide Absorption Unit. The absorption unit consisted of a Beckman Instrument Ltd. Model 9200 two-stage pressure regulator, the spiral type absorption cell, a plastic cylinder, a TJnity Pump Co. BL-131 matercirculating pump, and a Precision Scientific Co. Model 62690 constanttemperature water bath. Figures 3 and 4 show the structure of the absorption and reference cells. The combustion gas mixture coming from the induction furnace was first run through a manganese dioxide tube and then controlled by the pressure regulator to the desirable range and introduced into the spiral-type absorption cell where carbon dioxide reacted with the barium hydroxide solution. After the completion of carbon dioxide absorption, the gaseous niisture n as passed through anhydrous magnesium perchlorate and ascarite scrubbers which were to prevent the backing-up of carbon dioxide and moiqture from the atmosphere. Following these scrubbers, the gas passed through a 50-ml. soap-bubble flowmeter and into the atmosphere. To keep a constant temperature in both cells, a
-
-~ 0.1
lOcr
T n
I 35 cm.
\
1 70cm. ..~
I
35 cm.
Figure 4.
Reference cell
rubber hose was connected from the pump to the bottom of the plastic cylinder where the cells were housed. The water in the brtth was circulated during operation at a temperature of 3oo f: 0.l0 c. Detection Unit. The detection unit was essentially a modified Wheatstone bridge as shown in Figure 1. It consisted of a specially-constructed powersupply box, a 400-ohm standard resistor, a Du Mont Go. Type 241 oscilloscope, Leeds &- Sorthrup Co. S o . 4258 4-digit, 10-turn slide-wire, arid a pair of platinum electrodes (Figure 5) with cell constants of 0.270 and 0.280 cm.-l, respectively. The horizontal input of the scope was a 60-cycale,1.6-vo1tsJpeakto-peak potential. A 60-cycle, 6-volts signal from the poLver-supply box was fed into the vertical input of the scope. The P-attenuation IT as set a t 1:1. When the bridge aa:> balanced, a symmetrical loop m mould be seen on the scope. The decreas: of barium hydroxide concentratior in the absorption re11 due to the precipitation of barium ions by the carbon dicxide would cause a decrease of electrical conductivity, which in turn would cause t distortion of the loop on the scope. The degree of distortion was a function of the change in electricnl conductivity, and was determmed by rebalanci i g the slide-wire. The upper position oi the snitch on the pon er-supply box connected the 400ohm standard resistor and the reference cell as two opposing arms of the Wheatstone bridge, while the horizontal position gave a connection of the reference cell against the absorption cell. Barium Hydroxide Refilling Unit. A 0.0065M barium h vdroxide solution
WRS chosen because of the carbon concentration in the boron samples and the fixed cell constants of the electrodes. Barium hydroxide solution absorbs carbon dioxide from the air and causes a large change in conductivity when using such a dilute concentration. The process of refilling the cells was made in a closed system to give a minimum exposure of the barium hydroxide to the atmosphere. Deionized mater and 0.lN nitric acid were used for cleaning the cells. Procedure. The oscilloscope was first turned on and the circulating water system put into operation. Both the reference and absorption cells were washed twice with 0 . 1 s nitric acid, twice with deionized water, and twice with the 0.0065M barium hydroxide solution. The washing solution was vacuumed from the cells and 60 ml. of the barium hydroxide solution was added by means of an automatic buret into both cells. The oxygen supply was nom turned to 10 p.s.i. (or 2.5, 5, 20 P A . ) and regulated to a proper flow rate of 200 i d . per minute (or 50, 100 ml. per minute) which was read by the soap-bubble flowmeter and a stopclock. The system was then ready for operation. The carbon blank of the Leco 528-35 “carbon-free” crucible varies from 50 to as high as 120 p.p.m. This gave very poor results, especially in determining low-carbon materials. To eliminate the widely-varied carbon background, all the crucibles were prefired preceding the firing of boron or steel samples. This prefiring procedure was accomplished by neighing 0.4 gram of KBS steel and 1.0 gram of Leco tin accelerator in the fresh crucible, then firing the crucible in the induction furnace for 15 minutes. The unknown sample was then loaded in the prefired crucible while still hot t o prevent it from absorbing carbon dioxide from the atmosphere. Different kinds of TRS steel 11-ere used in the prefiring process but no deviation na. observed. Table I shows the difference in results taken by using prefired and fresh crucibles. Boron bars were cracked in a “diamond mortar” and sizetl-separated into speci100, 50, 20, and 10 mesh (AWl‘I\I fication) by means of a set of stainless steel sieves. Samples of boron, 0.25 =t0.001 gram, nere weighed on a 11ettler-Gram-Atic-Balance and mived very
Copper Wire 7
25cm.
7-
3
8cm.
i
Cylindrical
Pt € / e c t r o d e s
+I 3 c
Figure
5. Electrodes
carefully with 1.0 gram of Leco tin in the prefired crucible. After the sample crucible fias loaded, the filament switch of the high-frequency induction furnace was turned on, the nscillator allowed to stabilize, and the oxygen flow rate was checked and readjusted if necessary. By adjusting the position of the slidewire, the bridge was then balanced to get a symmetrical electron loop on the scope which indicated the null point. The initial position of the slide-wire (RA2,related to the resistance of the absorption cell before carbon dioxide absorption) was read followed by activation of the high frequency induction furnace. While the sample was burning, RAl (related to the resistance of the reference cell) nas taken. The furnace was turned off when the oxidation mas complete. The final reading, RA3, (related to the resistance of the absorption cell after carbon dioxide absorption) was taken 10 minutes after oscillator activation. To obtain better precision, only the middle range of the slide-tiire (400 to 600 units) was utilized. Khen the reading of the absorption cell became larger than 600 units after several runs, due to the decreasing barium hydroxide conductivity, the oxygen
Table I.
Comparison of Results Using Prefired and Fresh Crucibles Prefired crucibles Fresh crucibles Wt. Leco \Vt. 170a AG TVt. Leco Wt. 170a AG Sample tin, steel, (mho Sample tin, steel, (mho no. gram gram X 10-5) no. gram gram :x 1 0,9990 0.3994 12 0 11 1,0009 0.3996 18.3 2 1.0002 0.4002 12 4 12 1.0004 0.4010 18.0 3 0.9999 12 0 0.3999 13 0,9998 0.4006 15.2 4 1,0008 11 3 0 3998 1 . 0000 14 0.3999 15.8 a 1. 0000 12 2 0.4006 1.0006 15 0.4005 17.8 6 1,0002 0.4001 16 0.3990 11 9 1.0004 16.4 7 1.0001 17 12 0 1,0007 0,4005 0.4002 17.6 8 12 2 0.9995 0.4002 18 0.9996 0.3992 17.5 9 1 .0000 12.1 0.4006 19 15.3 0.9997 0.4000 10 1,0006 12 0 1,0006 20 0.4002 0,4001 15.3
VOL. 35, NO. 10, SEPTEMBER 1963
1507
was shut off and the absorption cell washed and refilled M ith fresh barium hydrosirle solution. The drift of the reference cell was very small and needed to be refilled only a t the beginning of each day. The preparation of a standard calibration curve was accomplished by oxidizing various mixtures consisting of fractional weights of N13S steel samples (0.2 t o 0.8 gram) and Leco tin (1.0 gram) utilizing the same procedure as above. Because of the overflow of molten slag under high pressure, which would contact and crack the quartz combustion tube, steel samples of more tlian 0.8 gram were not fired a t higher than 10 p.s.i. when prefired crucibles were used. Trace carbon was found in Leco "carbon-free" tin accelerator, ranging from 2 to 4 p.p.m. This was proved by firing plain tin in prefired crucibles, and comparing the 4 G values of 0.6 gram of N3S 55e steel +1.0 gram of tin against 0.6 gram of YBS 55e steel. It was also noticed that under 10 p s i . osygen pressure, steel samples of less than 0.6
Table
II.
Analysis of Carbon in NBS 1700 Steel
Wt. Carbon sample, No. AG content, gram runs (10-6 mho) p.p.ni. 0.2 5 5 . 3 f 0.089 1 0 4 f 1.7 0.4 13 10.6 f 0.324 208 f 6.4 0.6 7 16.0 f 0.582 312 f 11.3 0.8 5 2 1 . 2 f 1.073 416 f 21.1 0 2 pressure = 10 p.s.i. Flow rate = 200 ml. per minute Bath temp. = 30" C. Analysis of Carbon in NBS
Table 111.
55e Steel
wt.
Carbon sample, No. AG content, gram runs (10-6mho) p.p.m. 6 1 . 0 f 0.091 22 f 2 . 0 0.2 0.4 6 2.0 f 0.120 44 f 2.6 0.6 6 3 . 2 f 0.125 66 f 2.6 0.8 5 4.6 z!= 0.121 88 f 2.3 0, pressure = 10 p.5.i. Flow rate = 200 ml. per minute Bath temp. = 30' C.
~~~
gram could not be oxidized without tin accelerator using Model 523 induction furnace, while samples of more than 0.6 gram could. The carbon blank due to the tin was subtracted each time from the final AG value. TREATMENT
OF
DATA AND RESULTS
AG represents the conductance change of the barium hydroxide solution in the absorption cell before and after the oxidation of the sample, which is in turn proportional to the amount of carbon in the sample. I t was calculated from the four readings taken during each run: R,, resistance of the standard resistor = 400 ohms; Ral, reading of the slide-wire in checking the resistance of the reference cell against the standard resistor; R.42, reading of the slide-wire in checking the resistance of the absorption cell before carbon dioxide absorption; and R A ~reading , of the slide-wire in checking the resistance of the absorption cell after carbon dioxide absorption. Since the AG calculation is very t h e consuming (about 8 minutes per single 4G, using an electrical calculator), the data were treated by a Royal McBee LGP-30 digital computer. Results for NBS 170a and 55e steels are summarized in Tables I1 and 111. A straight line was obtained in the range of 104 to 416 p.p.m. of carbon for NBS 170a steel, which has a carbon content of 520 p.p.m. A line having some curvature was obtained in the range of 22 to 88 p.p.m. of carbon using NBS 55e steel, which has a carbon content of 110 p.p.m. Results of carbon analysis in different boron samples are tabulated in Table IV. The precision shows excellent agreement for samples having a carbon content lower than 0.2% using 0.0065Af barium hydroxide solution. For samples of comparatively higher carbon content, a more concentrated barium hydroxide solution should be used in order to obtain a similar accuracy of measurement.
~
Table IV.
Sample no. 1 3 3 4 5
6 7
8 9
IO 11 12
13
14 15
16
1508
Results of Carbon Determination in Boron b y Conductivity Method
Sample size 100-mesh 100-mesh 100-mesh 100-mesh 100-mesh 100-mesh mixed mixed mixed mixed
mixed mixed mixed
mixed unknown 100-mesh
ANALYTICAL CHEMISTRY
No.
runs 5 6
3
6
7 4 6 6
5 4 2 3 3 3 3 2
AG (10-5 mho) 9.15 f 0.215 11.99 f 0.318 8.29 8 . 3 f 0.255 4.42 0.071 6.47 f 0.124 11.20 f 0.235 10.7 f 0.294 13.4 f 0.637 6.8
106
74.4 31.6 10.1
45.4
59.5
Est. carbon content, p.p.m.
740 f 17.2 970 f 24.5 660 664 f 20.3 354 f 5.8 538 f 10 905 f 18.9 890 f 24.5 1,070 f 51.9
560
10,Ooo
6,000
2,550 815
3,500 4,800
DISCUSSION
For steel analysis, the control of various parameters is not too important because complete combustion is more easily obtained. As for boron analysis, some of the parameters such as sample size, sample weight, tin/boron weight ratio, etc., must be critically controlled so as to obtain optimum oxidation of sample, The following variables have undergone investigation and optimum conditions found. Osygen Pressure. Talley (d) studied the oxidation of boron rod (1 mm. in diameter) in an oxygen atmosphere by electrical resistance heating and found that the rate of reaction in Region 111 (1000" to 2500" K.) is controlled by the diffusion rate of gaseous boron oxide. The higher the total pressure (oxygen pressure), the lower the diffusion rate will be. Thus, the rate of combustion is inversely proportional to the oxygen pressure and exponentially related to the temperature of the oxide film. In RegionIV(temperature above 2500"K.), the reaction rate is limited by the gasphase diffusion of oxygen through boron oxide vapor. Talley's statement (2) was unimportant for oxidizing finely-powdered boron with tin in steel-prefired crucibles using a high-frequency induction heater in the pressure range of 2.5 to 20 p.s.i. Osygen pressures of 2.5, 5, 10, and 20 p.s.i. were investigated with no significant difference in rate and completion of combustion being observed. Sample Size. The size of boron particles is the most important factor relating complete combustion of sample. The proposed oxidation mechanism of powdered boron in a high frequency induction field is: Initiation Stage: (0 5 to 0 30 seconds). The tin accelerator would initiate the combustion, but a t the beginning only the surface layer, which is exposed to the oxygen atmosphere, is oxidized and forms a liquid boron oxide coating [temperature below 1200" K., Regions 1-11 according to Talley (a), which isolates the inside boron layers from the oxygen atmosphere. Propagating and Combustion State: The heat generated during the initiation step would cause vaporization of the liquid boron oxide layer, thereby destroying the liquid barrier. If the boron-tin mixture is well distributed, more boron would be oxidized as the temperature increases. The boron-tin mixture boils and the combustion is then carried to completion and the crucible stays hot for 3 to 10 minutes (temperature above 1200' K., Regions 111-IV). Final Stage: When all of the oxidizable matter in the crucible has under,mone combustion, the plate current is redwed and the crucible begins to cool. If the boron particles are packed too
+
+
i Influence of Tin/Boron Ratio in Combustion
Table V.
5.
Ave . AG
Figure 6. Effect of sample size on combustion of boron
c
Tin/ boron
Wt. tin, grams
wt. boron, grams
2
1.0 2.0 0.5 1.0 2.0
0.5 0.5 0.23 0.26 0.25
4 2 4 8
G
100
L
-
Oxygen pressure: 10 p.s.i. 20
50 SAMPLE
SIZE
IO
value, mho X 10-5
12 6 12 12.1
5
Size: mixed Flow rate: 200 ml. per minute Bath temp.: 30” C.
(mesh)
tightly, complete oxidation is difficult to obtain. If the amount of the boron sample is too small, nct enough heat will be generated to evaporate the liquid film and the combustion will cease after only a few layers of the particles adjacent to the surface have been oxidized. Boron samples, con2,isting of a variety of mesh sizes, were analyzed. More complete oxidation was obtained using finely-powdered boron samples (
