I n addition to the oxygen blank, factors limiting the sensitivity of the method are the available beam current, the beam area, and the irradiation time. The beam current used in these runs, 1 pa., could be increased to as high a value as 50 pa. provided that arrangements were made for water cooling the samples during irradiation. The beam area can be increased somewhat, although special instrumentation must be provided to eliminate errors due to nonuniforniity of the beam. The irradiation time can of course be increased to any desired value; homever, irradiation for one half-life will give half the activity that would be obtained a t saturation. Because of the activation of longer-lived impurities and because of the high cost of cyclotron time, irradiation for more than one half-life is not recommended. Therefore, measurement of films as thin as 1 A. is entirely feasible; however, in such a situation the blank would probably prove to be the limiting factor. The total amount of oxygen measured in such a situation would be 6 x pg. (1.2 x IO-Spg. of 0’s). The upper limit on film thickness is determined by the proton range and is about 1 mil or 250,000 A. for 4-m.e.v. protons. For samples as thick as this, the activity will not vary linearly with thickness and careful calibration with standards will be required. From Figure 2 a precision of approximately ilOyo is indicated. The various sources of error which could contribute to this include variations in film thickness, fluctuations in beam current, and counting precision. Of these the most significant is probably variations in film thickness introduced during anodizing. The anodizing voltage was accurate only to =1=1volt a t best, and variations of the order of magnitude observed could easily have been in-
troduced. The other factors were controlled to approximately =tl%as previously noted. In applying this method to other materials, it should be noted the quantity measured is atoms of oxygen per unit of area. Thus, some assumption must be made about the density of the film in order to obtain a value for the thickness. CONCLUSIONS
Proton activation of naturally occurring oxygen-18 according to the reaction 01* (p,n) FI8 has been successfully applied to the measurement of oxide film thickness on tantalum. The films were prepared by anodization and ranged from 500 to 2000 A. in thickness. With a proton energy of 4 m.e.v., the lower limit of the method is about pg. of oxygen-18 in an area of 0.38 sq. cm. This corresponds to a thickness of about 1 A. of TazOs. This limit can be attained only in the absence of dissolved oxygen in the metal substrate. The upper limit of the method is determined by the proton range and is about IO6 A. for 4-m.e.v. protons. For 4-m.e.v. protons, the elements causing interferences in the oxygen measurement are copper, nickel, zinc, and titanium. Titanium can be eliminated by choice of a proton energy below 3.8 m.e.v. While this work has been confined to a study of oxide film thickness, the method is equally applicable to determination of the oxygen content of almost any type of thin film and, with proper standardization, to the determination of trace amounts of oxygen in metals. ACKNOWLEDGMENT
The author thanks W. H. Bauer of Rensselaer Polytechnic Institute for
helpful advice and criticism. The assistance of Charles Baker and the operations crew of the Brookhaven cyclotron in performing the irradiations is also acknowledged with thanks. Thanks are also due to H. L. Finston, of Brookhaven National Laboratory, who kindly supplied the 100-channel analyzer used for the radiation measurements, and to V. IT. Perry, of the General Electric Co., who assisted with the radiation measurements. LITERATURE CITED
(1) Basile, R., Compt. rend. 239, 422
(1954). (2) Beard, D. B., Johnson, R. G., Bradshaw, W. G., Nucleonics 17, KO. 7 , 90 (1959). (3) Blaser, J. P., Boehm, F., Narmier, P., Peaslee, D. C., Helv. Phys. Acta 24, 3 (1951). (4) Ibid., p. 441. (5) Ibid., p. 465. (6) Coleman, R. F., Perkin, J. L., Analyst 84,233 (1959). (7) Fogelstrom-Fineman, I., HolmHansen, O., Tolbert, B. M.,Calvin, P.I., Intern. J. Appl. Radiation and Isotopes 2, 280 (1957). (8) Osmund, R. G., Smales, A. B . , Anal. Chim. Acta 10, 117 (1954). (9) Overman, R. T., Clark, H. RI., “Radioisotope Techniques,” pp. 392-5, McGrav-Hill, New York, 1960. (10) Strominger, D., Hollander, H. M., Seaborg, G. T., Revs. Modern Phys. 30, 585 (1958). (11) Tanaka, S., Furukawa, M., J . Phys. SOC.Japan 14, 1269 (1959). (12) Vermilyea, D. A., General Electric Co., Schenectady, N. Y., private communication, 1960. (13) Vermilyea, D. A,, J . Electrochem. SOC.102, 655 (1955). RECEIVED for review September 28, 1960. Accepted December 8, 1960. Based on a thesis submitted in partial fulfillment of the requirements for the degree of Master of Science, Rensselaer Polytechnic Institute. Division of Physical Chemistry, 138th Meeting, ACS, New York, N. Y., September 1960.
Quantitative Determination of Metallic Iron in the Presence of Iron Oxides in Treated Ores and Slags M. G. HABASHY 25 Sulfan Hussein Sfreef, Alexandria, Egypt, U. A. R.
b Metallic iron in the presence of iron oxides is determined in uncrushable Samples such as sponge iron, Slags,
copper obtained is multiplied by atomic weight of iron atomic weight of copper to Obtain
treated Ores, The is shaken with copper sulfate and water and also with mercury which catalyzes the reaction. After complete displacement of the iron by copper, the copper equivalent is filtered, redissolved in nitric acid, and determined electrolytically. The per cent of
the per cent of iron. Results show a mean toleranceof &0.1%. The inter-
586
ANALYTICAL CHEMISTRY
ference Of e’ements which have a higher oxidation Potential than copper can be eliminated. Data are given which show the effect of particle size of the sample on the determination.
T
done so far on determining metallic iron in presence of iron oxides does not completely solve the problem for this determination, especially in samples of coarse particles which cannot be crushed. The principal methods for the determination of metallic iron in samples containing the metal in the presence of its oxides are the mercuric chloride method of Wilner and Merck (8), the HE WORK
ferric chloride method of Christensen ( I ) , the copper sulfate method of Frerichs (2) and the cupric potassium chloride method of Riott (6). In these methods the sample must be crushed to a fine powder before analysis and this process results in the loss of some of the ferrous content (3). On the other hand, the mercuric chloride, ferric chloride, copper sulfate, and cupric potassium chloride solutions do not react completely with all of the metallic iron present when dealing with a coarse sample. I n the author's method, copper is used as the displacement element and the presence of a catalyst brings the reaction to completion. Also particle size is not as critical. EXPERIMENTAL
Iron was determined using the five methods. Samples with different particle sizes were chosen by crushing the samples and passing them through a series of sieves of varying mesh sizes. Mercuric Chloride Procedure. About 0.5 gram of ferrum reductum (reduced iron) in the form of a fine powder (a coarse powder is not decomposed quantitatively) is placed in a 100-ml. graduated flask from which the air had been replaced by carbon dioxide. Three grams of solid mercuric chloride are added and 50 ml. of water. The flask is heated over a small flame and the liquid boiled for 1 minute. Boiled water is added to the mark and after cooling to 15' C. the mixture is again diluted to volume. The flask is shaken and then allowed to stand until the precipitate settles. The liquid is poured through a dry filter and the filtrate is caught in a flask filled with carbon dioxide. Twenty milliliters of the fitrate are acidified with 20 ml. of HBO,, and titrated with 0.1N potassium permanganate solution (6)* Ferric Chloride Procedure. To about 0.5 gram of ferrum reductum in a 100-ml. flask (previously filled with carbon dioxide) are added 50 ml. of ferric chloride solution (1 gram of anhydrous ferric chloride in 200 ml. of water). The flask is stoppered and the contents shaken frequently during the next 15 or 20 minutes. The solution is diluted to volume with cold boiled water, mixed, the flask stoppered, and allowed to stand overnight. Twenty milliliters of the clear supernatant liquid are removed by pipet and titrated as above with 0.1N potassium permanganate solution. One third of the iron thus found corresponds to the weight of metallic iron present in the sample. Old Copper Sulfate Procedure. Two tenths gram of sample is treated with 30 ml. of 10% copper sulfate solution and the resulting ferrous sulfate is treated with a standard potassium permanganate solution (O.lN), after the metallic copper and excess of copper sulfate have been removed by displacement with 2
grams of aluminum shavings in 10 ml. of 1:1sulfuric acid solution. Cupric Potassium Chloride Procedure. A sample which should not contain more than 0.1000 gram of metallic iron is weighed out and transferred to a 125-ml. Erlenmeyer flask t h a t has been swept free of air by COz. Thirty milliliters of the cupric potassium chloride reagentprepared according to the original paper on this method (5)-are poured into a glass graduate, 5 drops of glacial acetic acid are added from a pipet, and a stream of COz is bubbled throu h the solution for a few minutes. T f e reagent is then decanted into the Erlenmeyer flask and agitated a t room temperature for 30 minutes with a steady stream of COz bubbling through the solution. The flask should be swirled occasionally to prevent caking of the sample since this retards solution of the iron. The contents of the flask are then filtered a t once through a No. 40 Whatman filter paper, or its equivalent into a 30-ml. Erlenmeyer flask under light suction using a platinum cone, and the whole is washed thoroughly with cool distilled water, keeping the volume from 125 to 150 ml. About 1 gram of aluminum shavings (iron-free) and 6 ml. of 1 :1 HC1 are added, and the solution is boiled gently under an atmosphere of C 0 2 until it is colorless and all the copper is displaced. While still under COz, the Aask with its contents is cooled in ice water. The colorless solution is filtered through a No. 42 Whatman filter paper, or its equivalent, into a 500-ml. Erlenmeyer flask under light suction, using a platinum cone and the residue is washed thoroughly with cold distilled water. Next, the solution is titrated with standard potassium permanganate to a faint pink end point after 10 ml. of Zimmermann-Reinhardt reagent are added. The blank is determined by proceeding through all the steps of the method using the same amounts of reagent and solution, but bubbling ( 2 0 2 through the solution for only a few minutes instead of for 0.5 hour. New Copper Sulfate Procedure. An accurately weighed sample, 0.2 to 1.0 gram is mixed with 20 ml. of distilled water in a conical flask. Ten grams of crystalline copper sulfate and 1 ml. of purified mercury are added, and the mixture brought to boiling and then cooled to room temperature out of contact with air (stoppered). The flask is shaken on 8 mechanical shaker (300 r.p.m.) for 1 hour. With coarser samples more shaking time is necessary with a maximum of 4 hours. The contents of the flask are then filtered through regular filter paper and washed with distilled water. Excess copper sulfate is filtered from the copper amalgam which is contaminated with copper oxide and copper hydroxide, together with the other oxides and insoluble compounds originally present in the sample. The final washings should be checked for copper by any of the known methods. In this case the procedure used is the electrolysis of 200 ml. of the final washings
until less than 0.0003 gram of copper is deposited on a platinum cathode. The filter paper and its contents are transferred to a beaker containing about 100 ml. of 5% nitric acid where the copper goes into solution as cupric nitrate. The copper content of the nitric extract is determined electrolytically (7). The percentage weight of metallic iron is calculated by
Experiments were carried out on six samples and results are compared in Table I. These data show that poor results are obtained when either the mercuric chloride, ferric chloride, old copper sulfate, or cupric potassium chloride methods are applied to samples which consist of large particles. However, with finely powdered slag samples, the five methods agree well, showing that the problem is largely one of particle size. INTERFERENCES
The presence of other metallic ions is shown by the addition of ammonium hydroxide to the copper nitrate solution. The hydroxide is precipitated and can be removed by filtration. If the presence of any other element in the metallic state is suspected and its oxidation potential is higher than that of copper, the following modification should be added to the procedure. A given weight of sample is placed on a filter paper (funnel) and washed with air-free water until the washings show no indication of soluble salts. The filter paper and washed sample are placed in a conical flask and the procedure continued as above. The interfering elements are determined in the main copper sulfate filtrate. Their values are converted to their equivalent of copper and subtracting this latter from the total equivalent of copper (obtained from electrolysis) gives the net copper equivalent replaced by metallic iron alone. BRANCH APPLICATION
I n the well -known potassium dichromate method ( 4 , 6 , 7 ) ,for the analysis of a sample for its ferrous and ferric contents, it should be treated with hydrochloric acid in an oxygen-free atmosphere-e.g., COZ. The ferrous content can then be determined by direct titration with a standard potassium dichromate solution. When the total iron is determined by reduction with the stannous chloride method, the ferric content can be deducted by difference. VOL 33, NO. 4, APRIL 1961
587
The method cannot be applied t o determine the total ferrous radical in the ferrous slags, reduced ores, or even in ores containing some metallic forms.
Table 1.
This is due t o the fact that any metallic form present in the sample-e.g., iron, will react during dissolution with hydrochloric acid liberating hydrogen which
Comparative Results
Metallic Iron, yo Old CuS04 KCuCla Llrthod Method
CuSO1/Hg
Sample No.
HgCh Method
1
8.50 13.36 14.95
Sponge iron” 10.33 25.38 11.98 30.16 18.15 36.98
20.30 36.22 35.70
62.21 62.35 62.48
1
16.52 12.20 20.13
Sponge ironb 28.20 13.66 37.68 28.50 35.25
38.18 32.35 48.10
58.40 58.20 58.33
20.08 22.15 25.26
Sponge irone 27.78 35.39 20.13 42.28 33.23 46.76
44.14 46.50 51.68
58.36 58.28 58.19
11.11 10.55 17.50
Sponge iron0 13.26 26.46 15.25 32.97 16.35 36.21
35.10 31.05 30.17
67.05 67.10 67.23
16.28 20.22 31.30
Sponge ironb 15.78 40.18 12.13 35.00 17.27 32.20
34.40 30.22 38.30
62.11 61.98 62.09
28.18 35.72 22.16
Sponge ironC 13.29 35.34 28.71 44.45 25.95 46.81
50.85 45.40 54.11
61.81 62.10 61.96
13.24 11.53 18.12
Sponge irona 16.26 40.00 19.28 26.34 10.22 32.31
49.77 42.20 53.41
70.50 70.35 70.65
15.11 14.13 18.89
Sponge ironb 19.23 29.94 20.64 38.18 22.18 42.14
38.34 46.44 53.16
68.22 68.31 68.18
29.19 25.28 41.16
Sponge ironc 34.14 36.42 23.16 44.19 38.98 45.25
58.95 66.44 62.00
68.43 68.26 68.38
3
3
FeC& Method
18.11
LIcthod
86.10 86.28 86.22
Slag sampled 6.89 6.65 6.98 6.71 7.08 6.77
6.72 6.79 6.69
6.65 6.72 6.63
British Standard iron sample’ 99.75 69.75 33.65 99.88 48.13 28.22 99. i o 75.35 63.18 Particle size of sampleB >2.5 mm. and 1.0 mm. and 0.5 mm. and 0.25 mm. and
