Determination of Divalent Chromium in Slags James W. Robison, Jr. Special Metals Corp., New Hartford, N. Y . Robert D. Pehlke
Department of Materials and Metallurgical Engineering, The University of Michigan, A n n Arbor, Mich. 48104
Over the past four decades numerous researchers have attempted to describe the behavior of chromium oxides in steelmaking slags. The lack of a suitable analytical technique to differentiate the valence states of the chromium in solution in such slags has limited most investigators to models based on an assumed predominant valence. Lingane and Pecsok ( 1 ) describe the chromous ion as "the most powerful reductant used in the form of a standard solution in volumetric analysis." In consequence, it is most difficult to dissolve siliceous slags without oxidizing the chromous ion. To circumvent this difficulty, Frohberg and Richter @) reacted simple synthetic slags containing Cr2+ with a platinum boat. The Cr2+ decomposed to produce Cr3+ and metallic chromium. The metallic chromium dissolved in the platinum boat, and was found gravimetrically. McCoy and Philbrook (3) dissolved iron-free slag samples in sulfuric acid solutions containing an excess of [VO3-], and titrated the excess. Similarly, Frohberg, Richter, and Bradatsch ( 4 ) employed iron chloride (FeC13) as an oxidizing agent present in excess, titrating the resultant ferrous ion with calcium permanganate. All of these methods are inapplicable to slags with iron oxide contents of the same magnitude as the chromium oxide content. However, much current interest centers on strongly deoxidized high chromium (>lo wt YO Cr) metal melts, such as stainless steels. For melts held under strongly reducing conditions, the ratio of ferric to ferrous ion in the slag would be less than 0.1 (5). Moreover, the total iron content of such a slag will be less than the total chromium content of the slag. The analytical technique which follows was developed for use with such slags.
EXPERIMENTAL Tandon and Mehrotra (6-8) found t h a t a wide range of ions, including vanadium (as VOa-), could be successfully titrated with chromous ion solutions. A standard solution of chromous ion in dilute sulfuric acid was prepared by t h e technique of Lingane and Pecsok ( 1 ) (reduction by amalgamated zinc under a n atmosphere of deoxygenated hydrogen). This solution of Cr2- was standardized against 0.1000N Cu2- (Figure 1). T h e standardized Cr2+ solution was titrated potentiometrically with a 0.087,V solution of [VO3-]. This titration was reproducible t o within 1 part in 1000, and limited by t h e precision of t h e volumetric equipment. The end point was detectable to a fraction of a drop even in 0.02N solutions. All titrations involving the chro. Lingane and R L Pecsok, Anal. Chem., 20,425-8 (1948) M . G. Frohberg and K . Richter, Arch. Eisenhuttenw., 39, 799-802
J. J.
(1968). C. W . McCoy and W . 0. Philbrook. "The Physicai Chemistry of Steelmaking," John Wiley and Sons, New York. N . Y . . 1958, pp 93-8. M . G. Frohberg, K . Richter, and R . Bradatsch, Arch. Eisenhuttenw.. 38, 773-4 (1967). E T. Turkdogan and P. M. Bills, Arner. Ceram. SOC.Buil., 39, 682-7 (1960) J . P. Tandorn and R . C. Mehrotra, 2. Anal. Chern., 162, 31 (1958). J P Tandon and R . C. Mehrotra. 2. Anal. Chem., 164, 314-9
(1958) J P. Tandon and R . C Mehrotra, (1962).
2.
ANALYTICAL CHEMISTRY, VOL
Ana/. Chern..
4 6 , NO
187, 410-5
7, JUNE 1 9 7 4
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,
I
' = -
Twenty ml 0.1000N Cu2+ titrated with Crz+ solution (prepared by the method.of Lingane and Pecsok ( 7 ) ) . Cr2+ prepared as 0.100N: titrates as O.lO02N. Titration in 100 rnl HCI under deoxygenated argon Figure 1.
Table I. Reproducibility of Analytical Technique Analysis No.
% Fe
?& cr-2
1 2 3 4 5 6
0.382 0.402 0.422 0.390 0.410 0.377
0.582 0.622 0.564 0.559 0.630 0.591
mous ion were carried out under an atmosphere of flowing deoxygenated nitrogen. T h e [vas-] solutions were in t u r n titrated with 0 . l N and 0.02N solutions of ferrous sulfate. T h e titration was performed in 6M HzS04 using Ferroin (1,lO-o-phenanthroline ferrous sulfate) a s a visual indicator as recommended by Edmonds (9). With a ferrous ion concentration of 0.01M in t h e Ferroin, four days of the indicator required a blank value of approximately 0.02 ml. This blank value was found by comparison of visual and potentiometric end points. Precision was again within 1part in 1OOO. The precision of t h e individual steps having been established, known quantities of the standard 0.100N Cr2+ solution were added under argon t o solutions of 6M HzS04, 0.05M H F and an excess of [VO3-]. T h e excess [VOs-] was titrated with ferrous sulfate. Visual and potentiometric end points agreed closely, and the end points reflected t h e precision of the individual steps described above. Adapting this technique t o analytical use, synthetic slags were crushed t o -70 mesh. Half-gram samples of the crushed slag (base composition: 45 wt 70CaO, 35 w t 70 SiOz, 10 wt 70A1203, 10 wt la MgO: plus chromium oxides) were digested for 20 minutes in 30 ml of hot 0.6M H & 0 4 with 3 ml of H F and a known quantity of [vas 1. Experiments revealed no difference in results when digestion was accomplished under air or argon (so long as a n excess of oxidizing agent, [V03-1, is present), so routine digestions were done in air. Platinum vessels were used. In hot 6M H z S 0 4 , the titer of the [VOs-] was reduced approximately 1% per hour. so the digestion was carried out in dilute sulfuric acid as rapidly as possible. Although the reduction of [Yo3-] occurring in 20 minutes in the blank solutions was m i t e small. the titer of the blank solutions was used for subsequent calculations. (9) S. M . Edmonds. P h . D . Dissertation, Columbia University. New York, N . Y . . 1933.
After diluting t o 200 ml with HzO, a 10-ml aliquot was drawn. The remaining 190 ml of solution was acidified to 6M with H2S04, cooled in a n ice bath, and titrated with 0.02N ferrous sulfate solution. This titration established the total number of moles of Cr2+ and Fez+ in the original sample. The end point was noted potentiometrically and visually, using 1,lO-ferrous orthophenanthroline as an indicator. The 10-ml aliquot previously drawn was analyzed for iron (and total chromium) by atomic absorption spectrometry, and the Cr2- content of the slag determined by difference.
100% Fe2+, a correction may be applied to the Cr2+ determined by titration by assuming only 95% of the iron in the slag is divalent. The uncertainty in slag iron valence is then approximately &5Y' of the total slag iron content. In the example cited above, this would be *0.02 wt Yo Cr2+. Extended discussion of these procedures and results, and of the procedures used for total chromium and total iron analyses in conjunction with determination of divalent chromium in slags is presented elsewhere ( 1 0 ) .
PRECISION OF ANALYTICAL TECHNIQUE To ascertain the precision of this technique, multiple analyses were performed on a crushed slag sample. The results of these analyses are presented in Table I. The mean iron analysis was 0.397 wt YO,with a standard deviation of 0.0173 wt % Fe. The maximum deviations from the mean were +0.025 wt % and -0.020 wt %. The mean Cr2+ concentration was 0.591 wt %; the standard deviation was 0.029, with maximum deviations of +0.039 and -0.031, as weight per cent Cr2+. In addition to these analytical uncertainties, an additional uncertainity is present from the earlier assumption that all the iron in the slag was present as divalent iron. As the iron in the slag may be expected to be from 90% to
CONCLUSION In the application of this technique, several advantages are noted. A single 0.5-gram slag sample is sufficient for determination of Crzf, total iron, and total chromium (by AAS of the 10-ml aliquot). The laboratory procedure is relatively straightforward requiring a minimum of time and effort. Accuracy is sufficient for most metallurgical investigations. Received for review August 24, 1973. Accepted February 1, 1974. This work was supported in part by the American Iron and Steel Institute. (10) J W Robison Jr Arbor, Mich 1973
Ph D Thesis. The University of Michigan, Ann
Conversational Mass Spectral Search System Display and Plotting of Spectra and Dissimilarity Comparison Stephen R. Hellerl and Deena A. Koniver Heuristics Laboratory, Division of Computer Research and Technology, National lnstitutes of Health. Bethesda. Md. 20074
Henry M. Fales and G. W. A. Milne Laboratory of Chemistry, National Heart and Lung Institute. National lnstitutes of Health, Public Health Service, Bethesda. Md. 20074
An interactive, conversational mass spectral retrieval search system, available over ordinary telephone lines from a centrgl PDP-10 time sharing computer in the Division of Computer Research and Technology a t the National Institutes of Health (NIH) has been used by over 200 researchers since September 1971. This system has now been made available internationally over the GE computer network to scientists on four continents. This report describes the dissimilarity index comparison option, the microfiche option in the system, and the use of a graphics terminal for the display and plotting of spectral data. The overall purpose of this work is to explore and present a variety of ideas and methods for mass spectral data library searching and presentation of results. The system uses the basic technique suggested by Biemann ( I ) of an "abbreviated spectrum" file consisting of the two most intense peaks in every 14 amu interval, beginning at m / e = 6. In addition, molecular weight and molecular formula about each compound are available for Present address, Management Information and Data Systems Division, Environmental Protection Agency, Washington, D.C. 20460. (1) H S Hertz R A Hites. and K Biemann Anal C h e m , 43, 681 (1971)
searching. Figure 1 shows the current options of the system. For details of those not described, the reader is referred elsewhere ( 2 - 4 ) . Experience with the Present System. The evolution and use of this system has helped to confirm the observation ( 5 ) that compounds can be identified from their mass spectra by using only a few peaks. Generally it has been found that a choice of 2-4 peaks from the abbreviated spectrum results in a list of a small number (usually less than 10) of possible (or probable) answers. The system is used an average of 25 sessions per day by over 200 scientists in the United States and Canada. Each session usually consists of 3-6 separate searches and well over 90% of these are just the peak and intensity searches. It has also been found that the intensities of peaks are, in general, of minimal value, in narrowing down possible an-
S. R. Heller, Anal. Chem., 44, 1951 (1972). S. R. Heller, H. M . Fales, and G . W. A. Milne, Org. Mass Specfrom., 7, 107 (1973). S. R. Heller. R. J. Feldrnann, H. M . Fales, and G. W. A. Milne, J. Chem. Doc., 13, 130 (1973). R. G. Ridiey, Chapter 6. "Compound Identification by Computer Matching Mass Spectrometry." in "Biochemical Applications of Mass Spectrometry," G. R. Waller, Ed., John Wiley, New York. N.Y., 1971, and references therein. A N A L Y T I C A L C H E M I S T R Y , VOL. 46, N O . 7, JUNE 1974
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