Sample Temperature in the Carrier-Distillation Arc SOL WEXLER' Los Alamos Scientijic Laboratory, Los Alamos, N . M .
CRIBNER and Mullin ( 9 ) have described a carrier-distilla-
S tion procedure for the determination of trace impurities in uranium, in which 2y0 gallium oxide is added to the uranium oxide sample and the intimate mixture is excited in the anode of a direct arc. The gallium oxide "carrier" is stated to stabilize the arc and to sweep out the minute amounts of impurities volatilized by heat from the electrode, thus effecting a separation from the more refractory uranium oxide. The degree of volatilization of each impurity oxide should depend on its vapor pressure a t the charge temperature during the course of arcing. With the usual current of 10 amperes, distillation of even the more volatile oxides into the arc is not complete in one or in some cases several burnings. Impurity spectral lines are observed when the spent uranium oxide charges are mixed with fresh carrier (2 mg. of gallium oxide) and re-arced (1, 2 ) .
are compared with optical observations made on samples Lhrougk holcs in the sides of the burning craters. PROCEDURE
The hot junction of the 10% rhodium-platinum alloy-platinum thermocouple (Figure 1)was embedded in synthetic 100-mg. uranium oxide charges (containing about 30 elements in 10 p.p.m. concentrations). Each 20-mil wire was led through a small hole in the electrode wall and shielded from the graphite by a porcelain collar. The sample holder was the usual 0.25-inch (0.6-cm.) spectroscopically pure graphite with a crater 4 mm. across and 8 mm. deep. Opposite was a pointed 0.125-inch graphite rod, with an electrode gap of 4 mm. h Dietert-Applied Research Laboratory direct current supply energized the arc. Readings of electromotive force were taken with a Leeds & Northrup potentiometer indicator. Because the sample temperature increased very rapidly in the first several seconds of burning, it was found advisable to set the indicator a t progressively increasing values and then record the times when the galvanometer needle swept past zero. Temperature measurements were also taken with a Leeds & Northrup optical pyrometer focused on a eide of the sample through a 1.5-mm. hole in the crater wall. RESULTS AND DISCUSSION
k
GRAPHITE ANODE
The sample temperature of 2yO gallium oxide in uranium oxide mixtures exhibits a very rapid initial rise, followed by a leveling off (Figure 2). As one would expect, the rate of increase and the magnitude of the steady-state temperature vary directly with the arc current. The establishment of a fairly constant sample temperature speaks for a stable though steep temperature gradient down the electrode.
r-
I
PORCELAIN RING
I O % R h - P I V I Pt TMERMOCOUPLE
Figure 1. Hot-Junction Thermocouple Assemhly
4 00
200
A knowledge of the sample temperature would be helpful in understanding the incomplete volatilization of impurities RS well as other questions relating to the mechanism of the carrierdistillation method. Scribner and Mullin have estimated the charge temperature to be near 2000" C. when the current is 10 amperes. This value is based on their observation that the charge sinters to a fine cake but does not melt on arcing, which may indicate a temperature slightly below the melting point of uranium oxide. I n addition, they measured the temperature of the outer surface of a sample-bearing electrode with an optical pyrometer and found it to vary from 2300' C. near the bottom to 2800' C. near the top. I n this paper are described two methods of determining the temperature of 100-mg. uranium oxide synthetic samples, with and without carrier. Sample temperatures during the course of arcing are measured with a thermocouple embedded in the powdered uranium oxide. These 1
Present address, Argonne Xational Laboratory, Chicago, Ill.
20
40
60
80 100 TIME ( S e e . )
120
140
160
180
Figure 2. T e m p e r a t u r e Rise w i t h Time as F u n c t i o n of Arc C u r r e n t 2 % GaxOa
The carrier substance was changed and the current kept constant a t 10.5 amperes in a second set of experiments (Figure 3), in which 2% gallium oxide (boiling point about 1800' C.), 4y0silver chloride (1564'), 5% barium fluoride (2257"), and 6% cadmium oxide (1385O) were selected to give a reasonably wide range of carrier volatility. The concentration of the carrier was that necessary to give the same length of silent period for each charge. Included are two runs with uranium oxide to which no carrier was added. Evidently, the temperature during
1166
V O L U M E 2 3 , NO. 8, A U G U S T 1 9 5 1
l 1600
-
e
o
0
1167
e
1400
Table I. The 1200' to 1500' C. temperature range is in good agreement with that from the thermocouple measurements. However, the pyrometer procedure is believed to be less accurate by reason of difficulty in focusing the instrument exactly on the hole, a precaution necessitated by the steep temperature gradient down the electrode.
Table I. 1000
800
600
-
A-5% o-U308
Steady-State Temperatures Measured w i t h Optical Pyrometer
Carrier (in 100 Mg. UaOs 2% Gag08
Temgerature,
2.5% BaFl
1225 1220
2% AgCl
1335 1375 1355
2% CdO
1275 1340 1220
Vi308 alone
1400
Ea F,
ALONE
the usual exposure time (the first 35 to 45 seconds) is little dependent on the particular carrier substance. However, the presence of the carrier appears to raise the sample temperature some 200' to 300" over the approximately 1100" C. observed for uranium oxide alone. This suggests that the increased analytical sensitivity due to addition of carrier is in part the result of the higher sample temperature. Steady-state temperatures, with a current of 10.5 amperes, obtained earlier from the pyrometric method are assembled in
C.
1435 1595 1320 1450
The thermocouple measurements indicate that the sample is at its maximum temperature for less than half the usual exposure period. Furthermore, the steady-state temperature appears to be lower than the boiling points of most of the impurity oxides in uranium oxide. These observations may explain in part the lack of complete volatilization of the impurities in a single arcing. LITERATURE CITED
(1) Fred, M.,and Maishall, TT' , piivate communication. (2) Scribner, B. F., and Mullin, H. R , J . Research .Vatl. Eur. S'tundw d s , 37,379 (1946). RECEIVED September 5, 1950.
Determination of Parathion and p-Nitrophenol in Technical Grade Materials and in 'Dust Pieparations ICATIIRYN O'KEEFFE AND P. R . AVERELL Stantford Research Laboratories, .4merican Cyanamid Co., Stamford, Conn.
' l H E x i d e s p i ~ a duse of parathion (0,O-diethyl 0-p-nitrophenyl
I thiophosphate) as an agricult,ural insecticide necessitated the
development of procedures for the analysis of the technical product as well as dust and wettable powder preparations. To datc the only method reported for the analysis of technical parathion is the polarographic procedure of B o m n and Edxards (91, in which the nitro group is reduced at the dropping mercury electrode. For the estimation of small amounts of parathion, Averell and Sorris ( 2 ) have described a colorimetric met,hod involving the reduction of parathion to thr amiiio derivative, which 1-naphthyl)-ethylenediamon diazotization and coupling with S-( ine dihydrochloride gives a dye; this procedure has been modified by Gage (6). Although this procedure has been used extensively for the estiniat,ion of residual parathion in plant and animal tissue, the accuracy to be expected from a colorimetric procedure makes it. unsuitable for the analysis of technical parathion or concentrated dust preparations thereof. This paper describes a method which has been used in the authors' laboratory for over 3 years for the assay of technical grade parathion and of dust formulations. Parathion is separated from p-nitrophenol, the most likely impurity, by mild alkaline extraction of an ether solution; the p-nitrophenol in the aqueous layer is determined colorimetrically as sodium p-nitrophenoxide;
the parathion in the ether layer is reduced by zinc and hydrochloric acid; and the amino group so formed is titrated with standard sodium nitrite solution. This reduction-nitrite titration procedure is essentially that described by Callan and Henderson (4). ANALYSIS OF TECHNICAL PARATHION
Reagents. Ethyl ether. Sodium carbonate, 1%, 10 grams of sodium carbonate per liter of solution Sodium hydroxide, 1 S , 40 granis of sodium hydroxide per liter of solution Sodium hvdroxide, 0.1 S,1 volume of 1 N sodium hydroxide in 9 volumes of water Acetic acid-hydrochloric acid, 9 volumes of glacial acetic acid in 1 volume of concentrated hydrochloric acid Sodium nitrite qolution, 0.1 -\', 6.90 grams of sodium nitrite per liter of solution, standardized against anhydrous sulfanilic acid Sodium (or potassium) bromide, C.P. Zinc dust, iron-free Hydrochloric acid, concentrated Potassium iodide-starch paper Sulfanilic acid, Eastman Kodak Instrument. A photoelectric colorimeter equipped with a cell 1.5 cm. in diameter and a filter to give maximum transmittance between 400 and 450 nil*. A spectrophotometer may be used, although the filter photometer gives adequate precision,
