Determining Traces of Oxygen in Bismuth Metal E. S. FUNSTON AND SHERMAN A. REED NEP.4 Division, Fairchild Engine and Airplane Corporation, Oak Ridge, Tenn. A procedure for determining traces of oxygen in bismuth i s outlined. Oxygen values are calculated from the hydrogen consumed during reduotion of the oxides present. The method is rapid and the necessary apparatiir is inexpensive and can he easily fahrioated.
T .
HE measurement of traces of oxygen in hismuthmetal was necessitated by a recent study of corrosion effects of molten bismuth on several ferrous alloys. A literature survey of the availahle methods for determining micro m o u n t s of oxygen in metals indicated that interest had centered, in general, on high melting alloys. The vacuum fusion method, for example, first described by Walker and Patrick (10) and later modified by numerous investigators, has been extensively used for iron and steel analysis. In other procedures (t,4-9, 11) oxides of iron, copper, lead, and tin were reduced by hydrogen and the resulting water was collected in a suitable ahsorhent and weighed. Baker ( 1 ) quantitatively reduced the oxides.of tin, lead, and copper with hydrogen and measured the volume of gas consumed with a conventional gas buret. Similarly, Xlima (S) reduced mercuric oxide with hydrogen in a closed system and measured the hydrogen loss in a modified Victor Meyer apparatus. Because bismuth metal and its oxides are volatile a t tempers, tures usnally required in the vaouum fusion method, and because of the cost of such an elaborate system, a more simple means of analysis was desirable. Preliminary experiments in which water was collected during hydrogen reduction were unsuccessful, because it was impossible quantitatively to ahsorb the minute amounts of moisture produced from the reaction. The gas system as described here was therefore selected as the most suitable means of analysis. By measuring the hydrogen consumed during reduction as little as 2 p.p.m. of oxygen were determined with a desirable degree of accuracy. The lower limit of measurement depended solely on the accuracy with which one could read a buret.
PROCEDURE
The sample should be thoroughly cleaned and dried to remove any grease or foreign material which might tend to reduce the oxides present. Care should be taken if milling or grinding the sample is necessary, because hismuth oxidizes readily in air. One to 10 grams of metnl, drpmding on the approximrttp oxygen content. are 5,-eiehed dirratlv into a clean. dry uorcelain boat. After the boat & placed in'the reduction'ohLmher, the citp is lightly greased and clamped into place. The huret is next filled with meicur and closed off I,y the pinch clamp, M, and the balance of t l e wstem is evaeuatr:d for a t least 5 minutes and checked for leeks. With the t w e t s t i l l closed, the apparatus is alternately filled with hydrogen and evacuated st least five times to remove all extraneous gases. The evacuated apparatus is then filled with hydrogen until B slight positive pressure is attained and the thrre-v-ay stopcock i s t . u m d to doso off t,he gas inlet tube. The pinch clamp is then openwl :and the morcury level lowered to fill the buret with hydrogen. When the system has come to room tcmpersture, the height of thc rneroury leveling bulb is adjusted until atmospheric pressure is attained RS indicated by the manomet,er. The buret readinp, loom temperature, and haronirtric
DETAILS OF CONSTRUCTION
The apparatus shown in Figures 1 and 2 consist8 a i two essential parts: the reduction chamher and the gaas micro huret. Figure 2 schemtic$ly illustrates the details of construction of the unit. A 50-cc. mercury reservoir, A , to allow for expansion of the hydrogen during the heating period, is sealed to a commercial 5-oe. microburet, R, which is graduated in 0.01-ec. divisions. The huret is conneoted to the reduction unit by 8 capillary tube, E, 1 mm. in inside diameter, which leads to a three-way mercury seal stopcock, D,for evacuating and filling the apparatus with hydrogen. A smell meroury manometer, F,is sealed to the eapillary tube as shown. The reduction chamber, J , fabricated from a quarta-horosilicitte glass graded seal 19 mm. in inside diameter, is fitted with B 28/15 male hall joint, and a cap, L, made from a female joint, is held securely in plaoe by B spring elsmp. The ohamher is joined to the rest of the unit by a %mm. quartz tube which is bent a t a right angle and held in place by a short,length of Tygon tubing, G. The vertical section of the quarts tube, H , is filled with 30-mesh aihydrous magnesium perchlorate t o absorb traces of moisture from the reaction. The quarta end of the reaction chamber is heated by a small furnace made by winding a hollow Alundum core, I , with No. 20 Nichrome nrire. Temperatures are controlled by B Variao. The entire unit may he mounted on B plywood stand, 8 s shown in Figure 1.
Figure 1. Apparatus
V O L U M E 2 3 , N O . 1, J A N U A R Y 1 9 5 1 Table I. Trial So.
191
.4ccuracy Obtained by Hydrogen Reduction Method
Weight of Bis03 Added
Oxygen Equivalent 0.62 0.79 0.69 0.64 0.60 0.49 0.24 0.44 1.44
6.02 7.65 6.68 6.62 5.81 -1.71 2.32 4.27 11.10
NO.
la 2a 3a lb 2b
Weight of Bismuth Taken
(initial volume
Hydrogen
Loss,
S.T.P.
Oxygen Found
Error
0.869 1.103 0 964 0.897 0.838 0,679 0.238 0.615 1.602
0.60 0.78 0.69 0.64 0.59 0.49 0.23 0.44 1.44
-0.02 -0.01 0.00 0.00 -0.01 0.00 -0.01 0.00
I
.oo
Table 11. Reproducibility Obtained by Hydrogen Reduction Method Trial
temperature and pressure made. The oxygen in the sample may then be calculated by the following equation:
Deviation from Mean
Oxygen Found
Grams
%
70
10.06210 10,18608 10.16382 10.22759 10.11480
0.0203 0.0199 0.0201
+0.0002 -0,0002 0,0000
0.0238 0.0236
-0.0001 0.0000
D
-Hz I N L E T
T.O '
VAC
yc oxygen
- final volume) x
100
1.402 wt. of sample (mg.)
=
EXPERIMENTAL
To test the over-all accuracy and precision of the method, a quantity of bismuth metal was purified and freed of traces of oxide by agitating the molten metal in a hydrogen atmosphere for several hours. After milling to lOO-mesh, the metal was analyzed by the above procedure and no oxygen was detected. Ten-gram portions of the bismuth were then mixed with weighed amounts of bismuth trioxide and tested for oxygen content. Results of these tests are shown in Table I. Results of tests to determine the reproducibility of the method are shown in Table 11. Additional experiments were performed to determine the applicability of the method for measuring oxygen in iron and copper. Samples of both cupric and ferric oxides were run by the outlined procedure. -4s shown in Table 111, a positive error repeatedly occurred with copper, while a negative error resulted with iron samples. Positive errors with cupric oxide appeared to be caused by traces of carbon in the sample which formed carbon dioxide with part of the oxygen present. Calculating the carbon dioxide equivalent of the carbon, determined on separate samples, showed that the maximum error was less than 0.05 mg. Loaresults from iron were expected because the metal could not be melted with the furnace; however, this method appears to be feasible for measuring surface oxidation on iron specimens. Table 111. Determination of Oxygen in Iron and Copper Sample NO.
Oxide Taken
,Tf g .
Oxygen Equivalent Mg. Cupric Oxide
Oxygen Found
Error
Jig,
Mg.
14.29 19.51 16.72 23.46 32.03 12.28
-0.54 -0.32 -0.74 -0.77 -0.94 -0.53
-3 0 .
Ferric Oxide 1 2 3 4 5
4
6
49.35 65.99 58.08 80.62 109.71 42.63
14.83 19.83 17.46 24.23 32.97 12 81
CONCLUSION - 5
0
The hydrogen reduction procedure described offers a simple, inexpensive, and relatively rapid means of measuring traces of oxygen in bismuth metal. If 10-gram samples of metal are used, as little as 2 p.p.m. of oxygen can be determined with an average deviation of less than 1 p.p.m. Traces of carbon in the bismuth will result in a positive error which may be corrected by determining the carbon in a separate sample and subtracting the volume of carbon dioxide from the final buret reading. LITERATURE CITED
Figure 2.
Diagram of Apparatus
cooled to room temperature by a stream of air directed on the furnace. The leveling bulb is adjusted to bring the gas to atmospheric pressure and the buret reading, barometric pressure, and room temperature are again recorded.
Calculation. The volume of the apparatus must first be determined, because slight room temperature differences may occur betn-een the initial and final buret readings. Each buret reading must be subtrarted from this volume and conversions to standard
(1) Baker, W. A.,Metallurgia, 40, 188-9 (1949). (2) Bassett, W.H., and Bedworth, H. A., T r a m . Am. Inst. Mining Eng., 15263 (1926). (3) Klima, J., 2. p h y s i k . chem. Unterricht, 43,265-7 (1930). (4) Mchlillen, R. H., Met. Chem. Eng., 11, 86-7 (1913). (5) Oberhoffer, P., Iron Age, 102, 1573 1918). (6) Paterson, J. H., and Blair, H., J . Soc. Chem. I d . , 38, 328-30 (1919). (7) Rooney, T. E., J . Iron Steel Inst., 1924, 37-43 (September). (8) Schmita, F., Stahl u. Eisen, 38,541-2 (1918). (9) Seth, R. von, Jernkontorets Ann., 83,113-50 (1928). (10) Walker, W. H.. and Patrick, W. A., Orig.Com. 8thIntern. Congr. Applied Chen., 21, 139-48 (1912). (11) Worner, H. W., J . Inst. Metals, 66, 131-9 (1940). RECEIVED June 2, 1950. Based on work performed in part under Contract Number W33-038AC-14801 (16250) for the U. S. Air Force.
