through the 0.3 and 0.8% hydrogen fluoride calibration points does not pass through the origin. This is normal: A blank current of about 0.1 ma. i s obtained from a solution containing no hydrofluoric acid. INTERFERENCES AND ACCURACY
Variations of nitrogen dioxide content in the range from 12 to 1670 have no effect on the fluoride determination. Limited data for the extremes, 0 to 20%, showed little or no effect. Various metals known or suspected to be impurities in red fuming nitric acid as a result of corrosion of aluminum or stainless steel containers mere studied as possible interferences. Aluminum, iron, nickel, chromium, manganese, and magnesium caused no
trouble when present in amounts equivalent to 1% in the original undiluted acid. Copper could he tolerated up t o 0.2%. Silicon, n-hen added as iYa,SiO,, prcduced no effect in amounts equivalent t o 0.1% Si in the original acid; 0.2% caused the current to decrease by about loyo. A solution of NalSiFa in nitric acid having the same fluoride content as a 0.6% solution of hydrofluoric acid in nitric acid gavc, when diluted, the same current. An absolute accuracy to +0.0570 is easily obtained by the procedure d e scribed. More careful Standardization allowing for the slight departure from linearity vould result in grcater accuracy. Although the procedure has been used only for determining fluoride in nitric acid, its freedom from interfer-
ences may lead t o general use as a rapid method of estimation of fluoride. ACKNOWLEDGMENT
The assistance of Frances P. Dean in performing the laboratory work is gratefully acknowledged. William J. Barrett gave useful advice and counsel. This vork was supported by the Wright Air Development Center of the U. 8. Air Force, F. S. Forbes, Project Engineer. LITERATURE CITED
B. B., Morrison, J. D., ANAL. CHEM.27, 1306 (1955). (2) Wayman, D. H., Zbid., 28, 865 (1956). (3) Willnrd, H. H., Winter, 0. B., IND. EXG. CHEM., ANAL.En. 5 , 7 (1933). (1) Baker,
RECEIVED for review September 5, 1957. .4ccepted February 11, 1958.
Evaluation of Inert Gas Fusion Method for Rapid Determination of Oxygen in Steel J O H N I. PETERSON’, FLORENCE A. MELNICK, and J O H N E. STEERS, Jr. Grohom Reseorrh loborofory, Jones & Ioughlin Steel Corp., Pittsburgh 30, Pa. b The inert gas fusion method has been improved and evaluated for use as a rapid method of determining oxygen in steel samples. The method has been tested b y comparison with the vacuum fusion method on a variety of steels. It permits a determination of oxygen in most steels in 7 minutes or less. Most of the advantages of vacuum fusion are retained in the method, yet a considerable simplification in apparatus and its operation is gained. The analyses ore in agreement with those obtained b y vacuum fusion.
FUSION is generally considered t o be the most suitable method for determining the oxygen content of steel. Unfortunately, it does not satisfy the need for a simple, inexpcnsive, easily operated, and rapid method for measuring the oxygen content of steel for process control purposes. Of the several methods available for the determination of oxygen in steel, the only one which appears promising i s the inert gas fusion method, which is based upon the reduction of oxides in a molten sample by carbon, hut does not involve a high vacnnm system. I n 1940 Singer (9) described an apparatus for the rapid determination of ACWN
’ Present address, E. I. du Pont de Nemours h Co., Waynesboro, V a 1086
ANALYTICAL CHEMISTRY
oxygen in steel by this method. It involved passing purified nitrogen over a graphite crucible containing the molten sample in a tin bath, oxidizing the carbon monoxide formed with copper oxide, and absorhing the carbon dioxide in a weighing bulb. He obtained satisfactory analyses of a variety of steels, hut a minimum of 20 minutes was required for an analysis of rimming steels and a much longer time for aluminum-killed samples. I n addition, the blank was relatively large and variable, representing about 10% of the gas measured, and the apparatus was complicated by ahsorbents and reagents for purification. Smiley (3) adapted this method, with considerable improvement of the apparatus, to the determination of oxygen in nuclear materials. His principal innovations were a more convenient furnace containing a platinum melt, the use of an indicating oxidation reagent which operates a t room temperature, and the measurement of the carbon dioxide in a capillary trap manometer. The capillary trap provides a rapid and simple means of volumetric measurement. A detcrmination could he made in 12 minutes. I n this method argon was used and passed through a purification furnace. I n the work described here the apparatus has been further simplified and its application t o rapid steel analysis evaluated.
APPARATUS
Figure 1 shows the experimental layout. The apparatus used was essentially that described by Smilev, but with the following modifications. The argon purification and pressure regulating systems were found to be unnecessary and undesirable. Commercial standard
Figure 1. Apparatus for inert gas fusion method A . Vacuum line B . Capillary trap and manometer C. Blower
D. Schutse’s reagent E. Smiley’s furnace
or welding grade argon is sufficiently pure to use without further treatment. I n fact, the blanks are as low as those reported by Smiley. The system may be conveniently maintained a t about 1 p.s.i. positive pressure by use of the argon tank regulator. The most important feature in niaintaining a pure supply of argon is to avoid any means of diffusion of air into the system previous to the furnace. This means that an all-metal system must be used. Use of any rubber tubing connections or of a tank regulator haling neoprene or similar diaphragms results in a high blank. Soldered, 1/4-incli copper tubing construction mas used throughout the system; any necessary connections to glass were sealed mith Apiezon 11- wax. Short rubber tubing connections niay be used to connect the furnace exhaust and the oxidation reagent tube to copper tubing. The use of Smiley's platinum melt in steel analysis is unnecessary, and this evperience showed that a tin melt serves little purpose. l l u c h better results were obtained by including several pieces of 20-mesh, high purity graphite in the crucible, and adding niore during the succeeding analyses to replenish them as they are dissolved. This allows the stable oxide particles in samples to come into much niore effective contact with carbon, a s they are not wetted by the carbon-saturated melt. A water absorbent follon-ing the furnace is also unnecessary in steel analysis. A few tiny drops of iTatPr can sometimes be detected in the capillary trap after warming, but this does not affect the determination. A blower is used to warni the capillary trap to room temperature rapidly; honw-er, a water bath niay be used. A leveling bulb on the capillary manometer is convenient. The apparatus, then, consists simply of a n argon tank and regulator, Smiley's furnace and an induction generator, a glass tube containing Schutze's oxidation reagent, the capillary manometer and blower, a small vacuum pump, and four metal valves . The valves used were Hoke. style 482. a bellows valve with comer tubing attached and a Kel-F sea{. Similay valves niay be obtained from several manufacturers. The valve used for flow control should not he left closed for long periods of time because relaxatioii of the plastic on release of p r e w r e makes flow control difficult. The advantage of these valres is that they can lie soldered to a mounting plate. The f l o ~rate of argon through the 3ystem is controlled by the valve a t the entrance to the capillary trap. The manometer will indicate flon. rate and may be calibrated by temporarily inserting a flownieter in the system in place of the oxidation reagent. The capillary trap volume was found by application of Boyle's law. -4 formula relating the manometer reading to oxygen content of the saniple was derived from the ideal gas lan-. Details of this may be found in Smiley's paper (8).
Table I.
Steel
Analysis of NBS Cooperative Oxygen Samples
so.
Type
1 2
Low carbon, rimming Med. carbon, high manganese,
3
1 3
!i
i
silicon killed Bessemer screw, rimming Low carbon, aluminum killed Low carbon, silicon killed Med. carbon, silicon killed Open hearth iron, rimming Low carbon, aluminum killed
The oxidation reagent was prepared zrccording to Smiley's directions (4). I t rras contained in a borosilicate tube, 5 nini. in inside diameter and 14 c1n. It m-a9 long, with glass ~ o o plugs. l connected to the copper tubing bJ- a Aort length of rubber tubing and sealed with silicone grease. The reagent tube also serves to catch graphite dust TI hich accumulates; however, the system must be cleaned of this every fenmonths. The furnace was heated by an induction coil of 1/4-inch copper tubing, eight turns close spaced with 3-inch inside diameter. -4450-kc. current from a 7.5kn-. generator vias used. Only about 1 kn-. of power 11-as dissipated by the crucible in operation; hov-ever, a larger generator n a s used because of the loose L oupling. The furnace plunger niay be conveniently operated by a solenoid taken irom an electrically operated v a h e , onnected to a variable transformer. (
Inert Gas Fusion Sample Oxygen, \)-eight, g. 70
Oxygen, Accepted Range, %
0 171
0.016
0.016-0.020
0 233 I) 193 0 203 0 170 0.192 0 2i3 0 186
0.016
n
0.018 0.004
0.010 0.006 0.102 0.017
012-0.018 . . ~ 0.014-0.020 0.001-0.004 0,007-0.011 0.005-0.008 0.100.110 0.015-0.019
-
checked with a n optical pyrometer. The crucible should be operated a t about 1700" C., or a pyrometer reading of about 1650" C. The mercury level in the manometer should previously have been adjusted so that a zero reading is obtained under vaccum. The sample is dropped into the furnace and argon flow continued for the desired reaction time. Five minutes is sufficient for most steels. At the end of this period the flow control valve on the manometer is closed, allowing the trap to evacuate. This requires 1 minute or less. Then the vacuum valve on the manometer is closed t o seal the manometer, the coolant removed, and the blower turned on. Within 1 minute, when the mercury level stops dropping, a reading may be taken and converted to oxygen content of the sample. The analysis time is 2 minutes greater than the fusion time used, ordinarily making a total of 7 minutes. Another sample can be run immediately.
PROCEDURE
The system must be flushed, nith tlie furnace hot, until the blank is reduced to a sufficient level (ordinarily 3 to 10 y of oxygen). This requires froni 15 minutes when a crucible has been changed, t o 3 hours m-lien the apparatus has been idle a considerable time. During the initial flushing the ovidation reagent is by-passed to avoid excessive depletion of the reagent. This is the only rea9on for providing a by-pass system. Graphite particles may be added a t any convenient time through the sample inlet or by teniporarily removing the crucible, as a maxim i n i of about 5 minutes is required to reduce the lilank afterrrard in either case. T o make a determination, argon is passed through the system a t 0.5 to 1 pound pressure a t the tank rcgulator and a flow rate of about 200 nil. per minute. The manometer valve leading to the exhaust punip is open, and flow rate is controlled by the other nianonieter valve. The flow rate should be adjusted before the trap is cooled, because cooling will change the calibration of the flow rate. Liquid nitrogen is placed on the capillary trap, and a sample is inserted through the furnace stopper. Less than 1 minute is required to clear the system of air which came in with the sample, during n-hich time tlie crucible temperature can lie
EXPERIMENTAL EVALUATION
The evaluation of this method rests on coniparisons with vacuum fusion analyses of various steels. Because no standards are available for the analysis of oxygen in metals, and precisely reproducible sampling of steel is a problem, the best approach to a test of the method appeared to be comparative analyses on samples cut from adjacent positions in the material. The inert gas fusion values shonn in the tables were not selected, but repre3ent the results as they were obtained, in order to retain the value of this niethod of presentation as a picture of the reliability of the method. In most cases only one analysis of each steel by the two methods was done. I n the inert gas fusion, the method of nieasurement allows good estimation only to thc nearest 0.001% oxygen. K h e n a discrepancy larger than this was observed, secondary analyses mere done. These discrepancies can be attributed to the sampling problem. Considering the small sample size used for the inert gas fusion, the occurrence of these errors is surprisingly infrequent. For all vacuum fusion values 01)taiiied in this laboratory a K'ational ReVOL. 30, NO. 6, JUNE 1958
* 1087
search Corp. Model 912-C apparatus was used. I n all analyses by the inert gas method a 5-minute fusion was used and the crucible contained 20-mesh graphite. The sample surfaces were cleaned by filing or abrasion with silicon carbide paper. Cuts were made with a jeweler's saw and were not further cleaned. Table I shows the results on the eight NBS cooperative samples (6). Excellent agreement with the accepted values was observed. Some difficulty with sampling these steels arises, but it was found suitable to use sectors cut from 0.030-inch thick slices of the bars. T o test the inert gas fusion method for general utility, a number of low-carbon,
low-alloy steels were analyzed. These results are shown in Table 11. Agreement between the two methods is good except in the cases denoted by a n asterisk in the difference column. Although the difference is not large, additional analyses did not resolve the discrepancy, as in the other cases. There are two such cases in Table 111, also. I n addition, analyses by both methods on some high-purity iron spectrographic electrode gave a n average of 0.049% oxygen for 12 determinations by inert gas fusion and an average of 0.057% oxygen for eight determinations by vacuum fusion. These were the only cases encountered of disagreement be-
Table It.
Sample NO. 1 2 3 4 5 6 7
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
Type Bessemer Bessemer Open hearth, duplex Open hearth, duplex Open hearth, rephosphorized Open hearth, rephosphorized Open hearth, low carbon Low carbon, fine grain Rimmed Low alloy, high strength Low alloy, high strength Low alloy, high strength Low alloy, high strength Low alloy, high strength Low alloy, high strength Low alloy, high strength Low alloy, high strength Low alloy, high strength Low alloy, high strength Low alloy, high strength Low alloy, high strength Low alloy, high strength Low alloy, high strength Low alloy, high strength Low alloy, high strength Ferrovac E Ordnance Ordnance Ordnance Ordnance Spectrographic standard Spectrographic standard Spectrographic standard Spectrographic standard Spectrographic standard Table 111.
NBS No.
Type
401 402 404 406 409 410 411 437 439 440 441 445 446 442 448 450
Basic open hearth, 0.4C Basic open hearth, 0.8C Basic electric Chrome-vanadium Nickel steel 2 Cr-1 hIo Cr-Mo (SAE X 4130) Tool steel Tool steel Tool steel Tool steel Stainless steel Stainless steel Stainless steel Stainless steel Stainless steel
1088
ANALYTICAL CHEMISTRY
tween the methods. The cause of this disparity could not be ascertained. Table 111shows the results of a similar comparison of these methods on NBS steel standards for spectrographic analysis. These were in the form of 0.25-inch diameter rods, mostly of highalloy type (I). The purpose of this comparison was to show the applicability of the inert gas fusion method to steels of more varied chemical composition. As before, the agreement is good except in two cases. CONCLUSIONS
The data show that the inert gas fusion method is generally suitable for the determination of oxygen in steel.
Analysis of Low-Carbon, Low-Alloy Steels
Inert Gas Fusion Sample wt., g. Oxygen, % 0.257,0.309 0.021,O. 014 0.296 0.017 0.326,0.257 0.017,0.015 0,306 0,019 0.344 0.014 0.268 0.015 0.237 0.013 0.248 0.0039 0.017 0,0174 0.262 0.0030 0.0046,O. 0043 0.195.0.252 0,441' 0.0034 0.322,O. 435 0.0053,O. 0029 0.343 0.0053 0.197,O. 300 0.0025,O. 0040 0.235 0.0034 0.220.0.550 0.015.0.016 0.499' 0.0034 0.0049 0.364 0.OOii 0.366 0.0035 0.345 0.0022 0.384 0.0165 0.356 0.019.0.023 0.399,O. 526 0.485 0.0043 0.254 0.0035 0.291,O. 0.0076,0.0087 381 0.201 0.011 0.0092 0.328 0.368 0.011 0.348 0.0043 0.339 0.0085,O. 0063 0.487 0.0082 0.381 0.0072 0.447 0.0063
Oxygen, %, Vacuum Fusion 0.0138 0.0167 0.0152 0,0199 0.0154 0.0165 0.0138 0.0037 0.0174 0.0032 0.0039 0.0037 0.0032 0.0045 0.0035 0.0038 0,0177, 0.0194 0.0038 0,0040 0.0055 0.0036 0.0028 0.0161 0.0229 0.0043
0.0048 0.0102,0.0116 0.011 0.0092 0,010 0.0040
0.0068,O. 0065 0.0084 0.0079 0.0050
Difference, % ' +0.007,0.000
0.000 +0.002,0.000 -0,001 -0,001 -0.0015 -0.001 0.000 -0.001 0.000 +0.001,0.000 0.000 +0.002,0.000 +0.001
-0,001,$0.0005 0.000
*
0.000 +0.001 -0.001 0.000
-0.001
0.000
-0.004,O.OOO 0.000 -0.001
*
0.000
0.000
+o. 001
0.000 +0.002,0.000 0.000 -0.001 +0.001
Analysis of NBS Steel Spectrographic Standards
Inert Gas Fusion Sample wt.,g. Oxygen, % 0.190,O. 227 0.0195,O. 0229 0.0047,O. 0029 0.167,O. 380 0.0074,0.0070 0.177,O. 302 0.293 0.0063 0.138 0.0056 0.0117 0.196 0.0070 0.100 0.0134 0.157 0.232 0.0054 0,247 0.0053 0.0051 0.215 0.183 0.0388 0.0113 0.168 0.0327,O. 0322 0.254,O. 236 0.188 0.0075 0.0080 0.238
Oxygen, Vacuum Fusion 0.0222 0.0029 0.0092,O. 0097 0.0071 0.0051 0,0119 0.0062 0.0126 0.0042 0.0045 0.0047 0.0374 0.0111 0,0364,O. 0353 0.0074 0,0088
Difference, % -0.003,+0.001 +0.002~0.000 -0,001
+o. 001
0.000 +0.001 +o. 001
+o ,001 +o. 001 0I000 +o. 001 o.ooo* 0.000 -0.001
Results corresponding with vacuum fusion analysis are obtained because the reaction is the same in both methods. In the inert gas fusion method the presence of additional graphite in the crucible has been found to be the key to satisfactory results. This may also be suggested as an aid to certain vacuum fusion difficulties. Although a 5-minute fusion time was used on the analyses reported in this work, even shorter times can be used on some steels with low contents of stable oxides. For regular analysis of low-
oxygen material, greater sensitivity of measurement can be obtained by making the manometer of smaller sized capillary. The advantages of inert gas fusion analysis are its rapidity and simplicity over any other available method. Agreement with vacuum fusion analyses should not be mistaken for proof of accuracy in measurement of the true oxygen content of samples. No satisfactory means of establishing the oxygen content of steel samples on an absolute basis has been found, and vacuum fusion
analysis has bccn accepted with rescrvations on this point. LITERATURE CITED
(1) Natl. Bur. Standards, Circ. 398 (1954). (2) Singer, L., IND.ENG.CHEM.,API'AL. ED. 12,127-30 (1940). (3) Smiley, W. G., ANAL. CHEM. 27, 1098-1102 11955). ( 4 ) Smiley, IT. G., Y'uclear Sci. Abstr. 3. 391-2 - - - - ilR491. ( 5 ) Thompson, J. G., Vacher, H. C., Bright, H. A., J . Research Satl. Bur. Standards 18, 259-93 (1937). RECEII-SDfor review July 30, 1957. .Iccepted January 10, 1958. - 2
\ -
Evaluating Concentrations of Spectrally Absorbing Vapors in Dynamic Systems Spectrophotometric Techniques and Equipment F. A. GUNTHER and R.
C. BLINN
Department of Entomology, University of California Citrus Experiment Station, Riverside, Calif.
M. J. KOLBEZEN Department of Plant Pathology, University of California Citrus Experiment Station, Riverside, Calif.
C. W. WILSON Research Department, Sunkisf Growers, Inc., Ontario, Calif,
R. A. CONKIN Monsanto Chemical Co., St. louis, Mo. ,Vapor-phase fumigants supplied by in-package sources are used to protect citrus fruit against decay during shipment and storage. These fumigants may be sorbed by both the fruit and the package, and rates of generation from the source, rates of disappearance within the package, and magnitudes of sorbed residues must be known to permit design of a satisfactory release system as the in-package source. Dynamic techniques were used to evaluate one of these fumigants, ammonia in air. They were directly applied to the determination of both sorption and desorption of ammonia by various fruits, vegetables, and packaging materials as well as to the evaluation of release patterns from candidate ammonia generators designed for in-package use. The techniques are based upon both a manually operated spectrophotometric apparatus and an automatic multisampling apparatus incorporating a ratio-recording spectrophotometer.
K
fumigants for preventing or suppressing the growth of the blue-green molds Penicillium digitatum NOWN
Saccardo and P . italicum Wehmer during shipment and storage of citrus fruits are most effective when continuously present in sufficient quantity in the vapor phase. Development and exploitation of these fumigants require information on concentrations of these vapors in the atmospheres within commercial packages and on depletion resulting from sorption by both fruit and packaging materials. Direct ultraviolet spectrophotometric measurement a t 204.3 mp (1) has been applied to ammonia in small concentrations in air. This procedure promises to be applicable to any fumigant or vapor selectively absorbing spectral energy in any portion of the available ultraviolet regions. AMMONIA AS A FUNGICIDE
I n its present application as a fumigant, ammonia is a fungicide (3, 6). Both as vapor and as aqueous solution, it will kill many microorganisms, including a number of fungi, if the concentration is sufficiently high ( 5 ) . However, ammonia vapor can damage many fresh fruits and vegetables (4). For optimum preservation of citrus fruit, careful control of duration of ex-
posure and concentration of ammonia is required, for they must be great enough to be effective but low enough to minimize rind damage ( 7 ) ; how these requirements are met by the present inpackage generators of ammonia is reviewed elsewhere ( 2 ) . To be practicable, the source or generator must establish and then maintain an optimum concentration of ammonia. This requires supplying the amounts sorbed by both fruit and fiberboard package or carton, in addition to that required for satisfactory fungicidal action. Study of the rate of depletion of continually renen-ed atmospheres of constant ammonia contents was selected to give results most easily translated into commercial practice. I t seemed best to treat the fruit and the packaging materials separately and then to apportion the results to individual package units in any combinations required. BASIC TECHNIQUE
To determine amounts of ammonia in an ammonia-air stream before and after it had been depleted by exposure to either fruit or fiberboard an ultraviolet spectrophotometric procedure was used VOL 30, NO. 6, JUNE 1958
1089
