Critical Factors in Determination of Oxygen in Titanium by the Vacuum-Fusion Method W. M. ALBRECHT end M. W. MALLETT Battelle M e m o r i a l Institute, Columbus, Ohio ARIOUS methods for the determination of oxygen in titanium by the vacuum-fusion method have been reported, Sloman (4)uses a carbon-saturated iron bath contained in a graphite melt crucible fitted with a graphite cover, and completes the extraction in 20 minutes a t 1800' C. Derge ( 1 ) dissolves the sample in an iron bath in a coverless crucible. A few grams of tin are added to the bath a t 1400" C. before the sample is dropped. The extraction is completed a t 1800' C. in about 30 minutes, but extreme care must be taken in raising the temperature to 1800" C. to keep objectionable spattering to a minimum. TYalter ( 5 )uses a giaphite crucible containing graphite chips and fitted with a graphite cover. A piece of C.P. tin, weighing approximately twice the weight of the titanium sample, is added to the crucible a t 1200' C. simultaneously with the sample. The temperature is quickly raised to 1900' C , where the extraction is completed in 30 to 45 minutes. Sample weights reported in each method are small (0.2 to 1.5 grams). A number of laboratories throughout the country, including this laboratory, had made numerous attempts to duplicate the iesults of the above moiks by following the described techniques in what appeared to be reasonable detail. I n general, the results were low and rather erratic, and i t appeared that some subtlety of detail in the procedure was being overlooked. Consequently, the operating details of the various methods were studied systematically.
series of standard titanium-oxygen samples using this same technique (see Table I). The standards were prepared by drilling small holes in "as deposited" iodide titanium rods, inserting weighed additions of dried C.P. titanium dioxide, and plugging the holes with iodide titanium. These specimens were then arc melted in a water-cooled copper crucible in an argon atmosphere at 10-cm. absolute pressure using a tungsten-tipped electrode. The resulting ingot was turned over and remelted to ensure a uniform distribution of the oxygen content.
Table I.
Vacuum-Fusion Determination of Oxygen in Titanium
Sample Oxygen, Weight '?&a Weight, Added as Estimated Gram Ti02 total Exptl. 0,554 0.00 0.012b 0.431 0.012b ... 0.00 0.411 0.10 0.096 0.09 0,394 0.092 0.09 10-1 0.10 0.181 20-1 0.16 0.17 0.187 20-1 0.239 0.16 0.17 0.189 20-1 c 0.365 0.16 0.17 0.183 0.00 ... 0.487 0-2 0.016b 0,463 0.018b 0-2 0.00 ... 0.458 10-2 0.10 0.12 0.118 0.432 10-2 0.12 0.10 0.116 0.226 50-2 0.52 0.484 0.50 50-2; 0.52 0.50 0 499 0.168 0.52 0.461 0.149 50-2 0.50 a Average blank correction was equivalent t o 0.001 weight %. b Measure of base level oxygen contents of arc-melted standards, added t o value of oxygen added as Ti02 to obtain total estimated oxygen content. C Samples analyzed in iron-tin bath using graphite cover; all other analyses were run in dry graphite crucible, containing graphite chips and dropping tin with sample. Sample Identification 0- 1 0-1 10-1
iPPARATUS
Preliminary studies of the analytical method for oxygen in titanium were done with an apparatus ( 2 ) which had given excellent results for steels. However, as small samples appeared to be essential for the successful determination of oxygen in titanium, all later runs were made with a precise apparatus ( 3 ) developed for the analysis of molybdenum. Gas volumes can be determined in this apparatus with a sensitivit! equivalent to 0.001 weight % of oxygen in a 0.5-gram sample. The average blank correction for the apparatus is 0.001 Teight 70.
...
A few preliminary analyses using the iron-tin bath method of Derge ( 1 ) also were made. Little or no gas was recovered from the samples. Further analyses of the standards were made by this method, except that a graphite cover was used. Again, the use of a cover greatly increased the oxygen recovery. Sample weights for the analysis of the titanium-oxygen standards were in the range of 0.15 to 0.6 gram (see Table I ) . DISCUSSION OF R E S U L T S
EXPERIMENTAL PROCEDURE
The necessity for using small samples is closely linked with other aspects of the analytical techniques, particularly in the method of Walter. There seems to be a somewhat critical ratio of sample weight to amount of graphite chips, if optimum dispersion of the titanium sample is to be achieved. Apparently, the dispersion is brought about by the fluxing of the sample by tin. As the tin vaporizes, the graphite chips mix by blowing about in the crucible. The action spreads the sample as a thin coating on the chips. Without the crucible cover, the sample and chips are expelled from the crucible to varying degrees and correspondingly low results are obtained. If larger samples were used, more tin would be required to dissolve the sample and more graphite chips would be needed to produce the same degree of dispersion. As the amounts of tin and chips go up, the problem of retaining the chips in the crucible becomes more difficult and the results become lower and more erratic. In the iron-tin bath of Derge, one of the chief functions of the tin, besides acting as a fluxing agent, appears to be its sweeping action, which helps remove the carbon monoxide reaction product as the tin boils from the melt. The crucible cover pnevents the loss of sample from the crucible as titanium-coated graphite chips in the tin-flux method and as spattered metal in the iron-bath technique. The cover also
Examination of published procedures (1, 4 , 5 ) revealed three somewhat uncommon procedural details which appeared n orthy of further investigation. These were the use of small samples, weighing 0.5 gram and less, and the use of a tin flux and a crucible cover. Large samples, generally, are used in vacuum-fusion analj-sis in order to gain precision and to average out variables in composition of the material being analyzed. Therefore, it n-as not clear whether the use of small samples had significance other than the economy of getting several analyses from small experimental melts. Preliminary analyses were carried out on samples weighing 0.2 to 10 grams, using analytical techniques very similar to those of Walter (6) except that no cover was used on the crucible The recovery of the oxygen was low and erratic but increased with decreasing sample weights The best oxygen recovery was obtained using 0.2- to 0.5-gram samples. Other analyses made with and without tin additions to the crucible indicated that a greater oxygen recovery was obtained with the use of a tin flux which probably aids in a faster and better dispersion of the sample. The tin-flux procedure was repeated, this time using a crucible cover. Immediately, there was a remarkable increase in the amount of gay collected from a sample. With this encouragement, it was decided to analyze a
401
ANALYTICAL CHEMISTRY
402 minimizes vaporization of titanium and subsequent gettering of evolved gases, and seems to maintain more uniform temperature conditions within the working zone of the extraction crucible. Evidence of the effectiveness of the above-discussed techniques is given by the results of the vacuum-fusion analyses of titanium listed in Table I. Samples 0-1 and 0-2 were double arc-melted iodide titanium with no titanium dioxide addition. The nnalyses of these samples are the base oxygen levels for the arcmelted standards and are a measure of the original oxygen content of the metal plus the oxygen picked up during arc melting. The estimated total oxygen of each standard was the summation of this base value and the weighed addition of oxygen as titanium dioxide. It is general experience that the values obtained for a multiplicate series of vacuum-fusion analyses agree to about 10 relative yo. However, the analyses in the present work, using either
method of analysis, agree somewhat better than this. The experimental results also agree with the estimated oxygen contents within 10 relative 70with an over-all average of about 6 relative c/o. LITERATURE CITED
(1) Derge, G., J. Metals, 1, 31 (October 1949). W., ‘I. Trans. Am. Soc. MetaZs, 41, 870 (1949). (2) iMallett, > and Griffith, C. B., “Vacuum-Fusion Analysis of (3) Mallett, M. W., Molybdenum,” Trans. A m . SOC.Metats, in press. (4) Sloman, H. A,, and Harvey, C. A. (appendix by Kubaschewski, O.),J . Inst. Metals, 80, 391 (1951-52). (5) Walter, D. I., ANAL.CHEM.,22, 297-303 (1950). RECEIVED for review July 21, 1953. Bccepted September 21, 1953. The work described in this paper was part of a program sponsored a t Battelle Memorial Institute by Aeroneutical Research Laboratory, Wright Air Development Center, under Contract No. AF33(616)-103, Expenditure Order No. R-463-5BR-1.
Rapid Microcolorimetric Determination of Dissolved Oxygen W. F. LOOMIS
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The Loomis Laboratory, Greenwich, Conn.
T
HE Winkler method of determining dissolved oxygen requires samples of water as large as 250 ml. (1, 10-12’). hlthough several micro-Winkler modifications have been described for use with 1-to 10-ml. test samples, they all require the construction of specialized syringe-pipets and take 15 to 30 minutes per analysis (3,4, ?‘,9). The present paper describes a colorimetric method for use with 0.5-ml. samples, which may be completed in about 1 minute. No specialized equipment need be constructed, as a standard tuberculin syringe is adapted for direct use in a Beckman DU spectrophotometer, an instrument so sensitive that changes in concentration as small as 5 2 % saturation Kith air ( f 0 . 2 mg. of oxygen per liter) may be detected over a range that extends from fully anaerobic (0%) to fully aerobic (100%) conditions. The principle of the method lies in measuring the color change from yellow to red that occurs when a solution of reduced indigo carmine is partially oxidized by the oxygen in the test sample, a modification of the color change from yellow to blue that takes place on its complete oxidation as described by Efimoff (2’, 8). APPARATUS
Xitrogen tank, fitted with low pressure reducing valve and gage. A 1-ml, tuberculin syringe, graduated in 0.01 ml., is used. Provision is made for mixing the contents of the syringe on shaking by inserting a small steel ball (or nail head) in the barrel of th; syringe. To avoid exposing the sample to air during the spectrophotometric measurement, the syringe is placed directly (after its needle has been removed) in the borosilicate glass absorption cell (10-mm. light path) of a Beckman DU spectrophotometer. Provision is made for locking the syringe in place by cementing a small block of wood, through which a hole has been made just large enough to hold the neck of the syringe, to the bottom of the borosilicate glass cell; and winding a 5-mm. strip of tape around the middle of the syringe (just below the 0.5-ml. mark) until the barrel of the syringe fits snugly inside the absorption cell. The syringe may be made to lock into position, preferably one with as clear a light path as possible, by making this bushing of tape oval in cross section with vertically placed strips of tape applied to one side. The neck of the syringe and the steel ball are masked from the light path with a 1-em. piece of black tape applied across the front of the lower portion of the absorption cell. Beckman D U Spectrophotometer. As the height of the syringe precludes closing the sample compartment with its usual cover, a substitute is made by stapling 2 inches of black cloth to the edges of a 4 X 5 X 5 inch lightrtight cardboard box which is placed over the syringe during determinations.
Glass bottles are fitted with rubber caps through which No. 22 h podermic needles may be inserted repeatedly without leakage o f air. Sixteen-ounce narrow-mouthed acid bottles with rubber caps (Fisher Scientific Co. 2-922 and 3-225) are very satisfactory; 60-ml. serum bottles (3-220) may also be used. REAGENTS
Sufficient glucose and potassium carbonate are added to a 1.0% solution of indigo carmine (National) to make it 1% with respect to each substance. A serum bottle is half-filled with this solution, the rubber cap wired in place, and the remaining air space flushed with tank nitrogen for 10 minutes through two No. 22 hypodermic needles, after which it is left under 5-pound pressure of nitrogen. The reagent is reduced by incubating it for about an hour in a water bath a t 80” to 90°C. PROCEDURE
A blank absorbance value is obtained by filling the syringe exactly to the 0.50-ml. mark with reagent alone. The experimental absorbance value is determined by filling the remainder of the syringe to the 1.00-ml. mark with a sample of the water to be tested. 2.0
1.6 h
I
1
e
e
0
PO 40 60 80 % SATURATION WITH AIR
100
Figure 1. Calibration Curve Showing Essentially Linear Relationship of Absorbance at 580 MH to Per Cent Saturation with Air
