isolation. The results again confirm that the deuterium is stably bound under the analytical conditions. In Table IV, the close agreement bettveen found and made up values on known mixtures leads to an over-all accuracy for the assay to about zt47,. After the completion of this work. a paper by Karanagh and Kuzel (3) came to the authors’ attention. Their method for fluorometrically determining gibberellic and gihberellenic acids was particu-
larly useful for assaying fermentation beers and samples taken from various stages of purification. LITERATURE CITED
(1) Fukushima, D. K., Gallagher, T. F., J . Biol. Chem. 198, 861 (1952). (2) Jones, S. L., Robinson, I. D., A4rison, B. H., Trenner, K. R., ANAL.CHEM. 28, 482 (1956). ( 3 ) Kavanagh, F., Kusel, N. R., J. Agr. Food Chem., in press. (4) Maneelli, M. A., Abstracts of Papers,
132nd Meeting, ACS, p. 35C, 1957. (5) Trenner, N. R., Example 1, U. S. Patent 2,483,251 (1949). (6) Trenner, N. R., Arison, B. H., Walker, R. A Y A L . &EM. 28, 530 (1956). (7) Trenner, N. R., Walker, R. W., Arison, B. H., Buhs, R. P., Ibid., 21, 285 (1949). (8) Trenner, N. R., Walker, R. W., Arison, B. H., Trurnbauer, C., Ibzd., 23, 487 (1951).
w.,
RECEIVED for review Kovember 14, 1957. Accepted February 13, 1958.
Rapid Estimation of Hydrofluoric Acid in Red Fuming Nitric Acid BERTSIL B. BAKER’ Southern Research Institute, Birmingham, Ala. ,This simple and rapid method for estimating hydrofluoric acid in red fuming nitric acid is based on measurement of the current from the spontaneous electrolysis of the diluted nitric acid between an aluminum anode and a platinum cathode. Reasonable amounts of expected impurities do not interfere. The method will determine 0.6% hydrofluoric acid in red fuming nitric acid in a few minutes with an absolute accuracy to at least +0.05%.
R
fuming nitric acid, used as a n oxidizer for certain rocket fuels, commonly has about 0.67, hydrofluoric acid added as a corrosion inhibitor. The acid, stored in aluminum or stainless steel, may contain aluminum, iron, and the alloying metals as impurities. I n the usual colorimetric methods for fluoride determination these metals would interfere, and therefore preliminary separation by the Willard-Winter steam distillation (3) would be necessary. The R a y m a n method for determining hydrofluoric acid in nitric acid ( 2 ) uses an ion exchange column for removal of these metals, followed by a thorium titration of the fluoride. If high accuracy is not necessary, the electrometric method described saves considerable time. ED
APPARATUS AND SOLUTIONS
The apparatus !vas essentially that used by Baker nnd Morrison ( I ) t o detect traces of fluoride in air or water. It consisted of an aluminum !Tire anode (0 125 inch in diameter, 99 99% pur:) and a platinum cathode (0 05 inch 111 diameter) connected to a milliammeter, Present address, Polychemicals Department, E. I. du Font de l;emours & Co , Kilmington, Del.
0- to 1-ma. range. The meter had an internal resistance of about 50 ohms. A small polyethylene beaker held the solution, which was magnetically stirred. The electrodes were clamped t o remain the same distance apart and were immersed 1 inch in the electrolyte. Standard solutions of known fluoride content were prepared by dissolving 0 99 and 2.64 grams of sodium fluoride in 100 ml. of red fuming nitric acid, corresponding t o 0 . 3 and 0 8 weightwei ht % hydrofluoric acid in nitric acicf These solutions were stored in bottles made of Kel-F trifluorochloroethylene polymer (Minnesota Mining and Manufacturing Co., Jersey City Chemical Division, Jersey City, S . J.), or for short periods in polyethylene. PROCEDURE
Dilute the two standard solutions and the red fuming nitric acid unknown 1 to 500 with distilled water. Add a measured volume of the diluted 0.3% standard to the cell, start the stirrer, and lower the electrodes into the solution to a fixed depth. After 60 seconds read the current. Repeat for the 0 8% standard. Plot the two points, and draw a straight line between them. Measure the current from the diluted unknown, and read the per cent by weight of hydrofluoric acid from the graph. DISCUSSION
A polyethylene beaker was used as the electrolysis cell t o avoid the etching of glass. However, the 1 to 500 dilution was made with ordinary volumetric glassware, and the solution was quickly transferred to polyethylene for storage. Slight etching of the pipet was observed after continued use, but it did not interfere with the determination. The prescribed weights of sodium fluoride for Preparation of the 0.3 and 0.8y0 standard solutions are based on
the composition of red fuming nitric acid commonly used as an oxidizer for rocket fuel: acid containing 12% nitrogen dioxide and having a density of 1.57. If Kel-F bottles are not available for storage, two aqueous solutions containing 99 and 264 mg. of sodium fluoride per liter may be prepared. Then 10 ml. of these solutions may be combined with the 1 ml. of red fuming nitric acid at the time of dilution to 500 ml. A dilution ratio of 1 to 500 was the most suitable. Less dilution (1 to 100) gives larger currents at the sacrifice of linearity, the response flattening somewhat a t about 0.5% hydrogen fluoride. The exact current obtained depends upon many factors, among them temperature, rate of stirring, geometry of electrodes, and the condition of the surface of the electrodes, such as the degree of etching or the presence of an oxide film. However, a 0- to 1-ma. milliammeter is usually in the correct range. As the current has a temperature coefficient of 4y0 per degree Centigrade, temperature differences should be avoided by diluting the sample and standard with the same water a t the same time. Calibration with the 0.3 and 0.8% standards should be performed a t the beginning of each series of analyses, and again after several unknown samples have been measured. A new pair of electrodes, or a pair not recently used, should be cleaned and conditioned before calibration is attempted. Cleaning is easily done by opening the circuit and immersing the electrodes in the undiluted 0.3% standard fluoride solution for several minutes. It is then necessary to perform several calibrations until reproducible values are obtained. An extension of the straight line VOL. 30, NO. 6, JUNE 1958
1085
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 a r e 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
