V O L U M E 26, NO. 4, A P R I L 1 9 5 4 fairly well, With Zephiran as a diluent all of the values agreed with the predicted values within the manipulative variation of this procedure. In preparing all of the solutions in Table IV it was necessary a t some point in the procedure to pipet and work with Evans Blue standards made up in distilled water. The larger reading variations observed with the saline solutions were associated with a difficulty in repeating spectrophotometric readings on rapidly changing solutions. When all manipulations were carried out in Zephiran-containing solution, duplicate dilutions prepared and read on different days agreed within u = 0.003 absorbance, or O . i % relative error, and all values showed excellent agreement with Beer’s law.
715 Vierheller (18) reported the dependence of absorption values upon the shape of the spectrophotometer cell. It seems possible that these results are analogous to those in Table I1 and are explainable in the same fashion. Errors of 30 to 40% were occasionally observed by Gregersen ( 7 ) in his work with a visual color comparator. If one compares dyed plasma with a protein-free dye solution in a Duboscq-type colorimeter, adsorption effects might conceivably account for errors of this size because of the relatively large glass surface involved. .illen and Gregersen ( 2 ) report that when dyed plasma is diluted the absorptivity of the dye changes. Wherever deviations from Beer’s law are observed in Evans Blue work, adsorption effects should be studied for the pwsihle cause.
DI scus SIOY
Alany of the problems and anomalies in the Evans Blue literature (1, 2, ?-10, 18) may be the direct result of a failure to appreciate the analytical significance of these adsorption effects. This may not he surprising, for in most other cases in analytical cheniistry (4, 12-24, 27) adsorption errors have proved to be negligiblv small. Gregersen et al. report that Evans Blue is unstable in phyyiological saline solution (9, 10) and that its absorbance decreases progressively with increasing salt concentrations (8). Allen (1) criticizes IIorris’ (16) method on the basis that Evans Blue is unstable in acid solutions. The present work also confirms that lower results are obtained in solutions containing salt (Table IFr) and acid (Table 111). Recovery experiments have demonstrated, however, that these effects are due to increaspd adsorption rather than to a destruction of the dye. C‘ontrary to the views expressed by A y e s (S), the fact that a standard curve does not obey Beer’s law indicates that there is an uncontrolled and usually an unknown factor operating in the anal>-tical system Tvhich may be a source of analytical error. This factor may be chemical, physical, or instrumental ( 5 ) . In the present case this deviation from Beer’s law was one of the inost marked and easily measured indicatois of this adsorption ePirct.
LITERATURE CITED Allen, T. H . , Proc. SOC.Exptl. B d . X e d . , 76, 146 (1951). -illen, T. H . , a n d Gregersen, XI. I., A n i . J . Physiol.. 172, 377 (1953). .%ires, G. H . , =isa~. CHEM.,21, 652 (1949). Bird, L. H., Sew Zealand J . Sci. Technol., B30, 334 (1949) C a s t e r , W. O., As.iL. CHEM., 23, 1229 (1951). Chinard, F. P., a n d E d e r , H. A , , J . Exptl. M e d . , 87, 4 i 3 (1948). Gregersen, 31. I., J . Lab. Clin. Med., 2 3 , 4 2 3 (1938). Gregersen, 31. I.. a n d Gibson, J. G . , Jr., A m . J . Phusiol.. 120, 494 (193i). Gregersen, 31. I., a n d Rawson, R . -I., Ibid., 138, 698 (1943). Gregersen, 31. I., a n d S t e w a r t , J. D., Ibid., 125, 142 (1939). Hartwell. J. L., and Fieser. L. F., Oig. Syntheses, Coll. T-ol., 2 , 145 (1943). Hensley. J. W., Long, 11. 0..a n d Willard, b. E . , I d . Eng. Cheni., 41,1416 (1949). Hershenson, H. 31., a n d Rogers, L. E.. A S ~ L C. H n r . , 24, 219 (1952). H o r n . D. IT., A m . J . Pharm., 108,324 (1936). lIorris, C . J. 0. R., Biochem. J . , 38, 203 (1944). van Porat. B., Acta M e d . Scand., Suppl.. 140, KO.256, 1 (1951). Schoonover, I. C . , J . Research S a t l . Bur. Standards, 15, 377 (1935). Vierheller, F., Seniana mdd. (Bztenos dires), S u p p l . diario, 51, S o . 2651. 885 (1944).
RECEIVEDfor review Xovenibes 10, 1952, -4ccepted Janriary 10, 1954. Work carried out under Contract AT(ll-1)-106 between the Pnivessity of Minnesota and the .4tomic Energy Commission.
Microscopic Identification of Wustite In Presence of Other Oxides of Iron RALPH G. WELLS Research and Development Laboratory, Pittsburgh, Pa., United States Steel Corp.
In the microscopic examination of partly reduced iron ores, wustite is often overlooked because of its resemblance to magnetite. RIicroscopic methods of identifying wustite, hematite, and magnetite are described. . i n etch solution of saturated alcoholic stannous chloride will attack wustite in 1 or 2 minutes, but it does not affect the otheroxides. Wustite is easily scratched with a steel needle, but magnetite is scratched with difficulty o r not at all. Wustite has a Knoop hardness of 155; magnetite, 361. Powdered wustite is black, isotropic, and nonmagnetic. Powdered magnetite is black, isotropic, and magnetic. Hematite has a greater reflectivity than the other oxides; it is anisotropic, and forms a red nonmagnetic powder when scratched. The described techniques used with reflected-light microscopy of partly reduced iron ores offer simple and accurate means of determining the types and amounts of the oxides present.
I
S STVDI-ING the reducibility of iron ores, it is desirable to
know which of the three forms of iron oxide-wustite (FeO), hematite (Fe2O8),and magnetite (FesOl)-are present in partly reduced ore or sinter. Chemical analysis will show whether the iron is ferrous or ferric, hut will not reveal which of the oxide phases are present. X-ray diffraction analysis will show the major constituents present, but will not detect oxide phases below certain concentration thresholds. Xcroscopic methods are the easiest, cheapest, and quickest; nith these methods, it is possible not only t o identify the oxide phases but also to determine the quantity and distribution of each phase in a given specimen. However, in a microscopic examination of partly reduced iron ores or sinters, wustite is often overlooked because of its resemblance t o magnetite. Methods by which wustite can be distinguished from magnetite microscopically are described in the present paper.
716
ANALYTICAL CHEMISTRY
Table I. Iron Oxides Identified in Reduced Iron Ores Fhaaes Identified By m i ~ m s c o ~ i c Stage of Examination BY x-ray methods methods Group 13 Ore A s received FeaOa, Fe10a.H30 Fe,Oi, FeiOi.H.0 o.5-minute treatment' FeaOl, FeaO4 Fer01 Fez04 5-minute treatment" FeaOa. FeO, Fe,O* 17erO4: FeO, FezOa 10-minute treatmentFc, FeO Fe, FeO. Fez04 l i t e r 15-minute treatment" Fe Fe, FeO Fontana Ore A s reoeived FenOa FezOa, Fer04 5-minute trehtmentb FeO FerO. FeO F e . 0 ~ FesOa 10-minute treatmenth FeO: FeaOa. FeaO,, Fe FeO: FelO; FeJ04. F e 15-minute treatment& Fe Fe, FeO ?-hour treatmenth Fe Fe, FeO Heated a t 800" C. in hydrogen for specified time then water wenched. b Heated a t 800' C. in hydrogen for Specified ti&. then furnace-cooled in nitrogen.
EXPERIMENTAL
Specimens of Group 13 iron ore (a hematite ore) were heated in hydrogen a t 800" C . (1475" F.) for 0.5, 5, 10, and 15 minutes. At the end of each period, a specimen w~diremoved from the furnace and ouenehed in water. Each of these mart,lv reduced specimens, a s k e l l as an unheated ore specimen, was cut in half. Half of each specimen was pulverieed so a8 to pass a 100-mesh I
Figure 2. S a m e Ore Specimen a f t e r Etching in S a t u r a t e d A l o h o l i c Stannous Chloride for 1 Minute FeO ie well etched, hut FssOa and FaiOa arc unaffected. Original magnification xi25, re. dured
'/i
in reproduotion
"
Several etchants were tried, but the one that gave the best re sults was ethyl alcohol saturated with stannous chloride. Cooke (8) suggests a mixture of stannous chloride in hydrochloric acid to distinguish wustite from magnetite. It is believed, however, that alcoholic stannous chloride is preferable hecause the reaction is slower and more easily controlled. This etchant cannot be stored; i t must be mixed fresh for each use. The etching operation was usually carried out by placing a drop of the etchant upon a selected area of the specimen by means of a smitll wire loop made of platinum. The length of time that the etchant should mmain upon the specimen was determined experimentally and depended upon the severity of etch desired. After being etched, the surface of the specimen was wiped with a dry lens tissue. With the alcoholic stannaufi chloride etchant, 1 to 2 minutes seemed to he optimum for etching wustite; magnetite was not affected hy as much a.8 5 minutes in the etchant (Figures 1 and 2). HARDNESS METHODS
Figure 1. Ore Specimen (Originally Fe208)a f t e r Heating for 10 M i n u t e s in Hydrogen at 800" C. Followed hy Quenching in W a t e r Ares illustrated show all three forms of iron oxide in the unetched condition. Original magnifiontion X125, reduced ' / r in reproduotion
specimens of Fontana ore (an ore containing both hematite and magnetite) were treated in the same manner as the Group 13 ore specimens, except that the heating times were 5, 10, and 15 minutes, and 7 hours. The specimens heated far 5 and for 10 minutes were quenched in water; those heated for 15 minutes and for 7 hours were cooled in a. nitrogen atmosphere in the furnace. ETCH METHODS
The properties ordinarily used in identifying iron-ore consti& uents by reflected-light microscopy are reflectivity, form rtnisotropy, reaction to etchitnts, hardness, and magnetism. However, the reflectivity, form, and anisotropy of wustite are so nearly identical with the corresponding properties of magnetite that they cannot be used in differentiating the two phases. Consequently, reaction to etchants, hardness, and magnetism were the properties employed in distinguishing between wustite and magne tite.
The hardness of magnetite is given by (3) as 5 ! / 2 to 6L/2 on the Mohs scale of hardness. N o information could he found concerning the hardness of wustite. However, experience in preparing metallographic specimens containing wustite inclusions has indicated that the hardness of wustite is less than that reported for magnetite. To test this observation, a steel needle wa8 drawn under moderate pressure x r m s a field containing both wustite and magnetite. The surface of the magnetite was not scratched, only slightly marked. The wustite, however, was scratched readily (Figure 3). To obtain quantitative evidence of the difference in hardness between the two phases, a series of Knoop diamond indentations, 100-gram load, was made aci-08s a field containing both wustite and magnetite (Figures 4 and 5 ) . The procedure was repeated on a number of different fields each eontaining both wustite and magnetite. Wustite had an average Knoop hardness of 155; magnetite an merage hardness of 361. MAGNETIC METHODS
Under certain conditions, the magnetic property of magnetite may he used in distinguishing it from the other two oxides. When the point of a magnetized needle is pushed along the surface of ore specimens, a powder is formed. Hematite powder is red and nonmagnetic. Magnetite powder is black and clings to the point of the needle. Wustite powder is also black hut nonmagnetic. Often, however, the magnetite is intermingled with
V O L U M E 26, NO. 4, A P R I L 1 9 5 4
717
the other oxides to such a n extent that this method is not applicable; identification made with other methods should then be confirmed by using the etching techniques. EFFECT OF VARIABLE OXYGEN CONTENT ON REFLECTIVITY
Hematite can usually be identified by its relatively great reflectivity: in partly reduced 01%~.metdlie iron is the only commonly encountered phase that has a reflectivity greater than that
Figure 3.
flectivity than does magnetite; however, the difftrence cannot always be detected by eye. In polarized light, hematite exhibits anisotropy, hut magnetite and wustite are isotropic. After the phases have been identified, their concentrations may be determined by the pointdaunt method (1). COMPARISON O F MICROSCOPIC AND X-RAY METHODS
The use of x-ray diffraction techniques for identifying iron oxide phases is limited because each phase has a concentration threshold helow which' i t cannot be detected by x-ray methods. Furthermore, silicate pha'es in the ore gangue may mask mall concentrations of iron oxide when x-ray methods are used.
Same Ore Specimen a f t e r Scratching
~-..
Scratch produced by drawing sharppointed -.>,. ...... -> ~ : - ~ ~~
~
~~
- . ~5. Same - ~ Ore Specimen ~ after Light Etching with S a t u r a t e d Solution of Alcoholic S t a n n o u s Chloride Light etching aeeenfu~fesFeO ens.
of hematite. The reflectivity of magnetitt. II U B U W ~ LWB ~ an that of hematite. However, it is possible for magnetite to contain oxygen in excess of its stoichiometrical composition, which results in B proportionate increase in reflectivity. Such an excess of oxygen ismost likely to occur in ores that have been wholly or partly fused and rapidly cooled; all or part of the excess oxygen ii the magnetite is retzinined in solution during fast cooling an,d makes the reflectivity of the magnetite similar to that of heme tite.
"
4 e.101e examinatinn nf ranirllu-~onlerl nm mill ..___ _._ Ir___i _.I ..... ~
..
t > a n s I l x r vhnr ".V I_"
hematite arranged in a Widmanstatten pattern within the magnetite grab. If the fused ore has been slowly cooled, hematit? will be prevalent, and the reflectivity of the magnetite will he less than that of the hematite. Wustite usually hrts slightly less re-
Oriainal magnification X 125, reduced %/a in reproduction
In this laboratory, reducibility studies on iron ores have shown that some iron oxide remains even after 7 hours of reduction in hydrogen a t 800" C. Microscopic examinations, Table I, canfirmed this finding, although x-ray analysis did not show the pres?me of iron oxide after only 15 minutes of hydrogen reduction a t 8000 c. Occasionally, when the gritin Bise of the phases is extremely s m d and the phases are intermingled, microscopic methods may not be adequate and x-ray analysis or chemical analysis must be used. Usually, microscopic m&ods Kill movide the desired information. LITERATURE CITED (1) Chayes, F., Am. Minealogist. 34 (1/2), 1-11 (1949).
(2) Cooke,
5. R. B., and Ban, T. E., Tram. Am.
R n ms.,
Znst. Mining Met
193, 1053-8 (1952).
RECE~VED for review Mkroh 28. 196.3. Accepted Janunry 27, 1RJ4. Presented before the Division of Indualrial and Engineering Chemistry. Symposium on Recent Developments in Ceramics and Glass. st the 123rd Meetinc of the I r n n i c ~ aC x ~ x r:AL c Socrsru. Lo8 Angeles, Calif. Other pspen ;hed ~nthe Sanusry 1954 iswe of Industrial a m1 in the S Y ~ P D S ~were~ublir U ~ En~ineeringChemiairy.
Determination of lake KIWIS-VUI
Figure 4.
-..
Same Ore Specimen w i t h D i a m o n d Hardness 1 n d e n t a t i o n s
Indentations made eores8 FeO and Fe104areas. Original magnification X125, mdiaced s/& in repmdnction
IGGIIUII
I n the article on "Determination of Calcium and Magnesium in Lake Waters" [ANAL. CnEx., 26, 347 (1954)l the formula a t the bottom of Table I, page 348, should read.
%,"s = -
100
1: 1
(C
- c)2
cdk=r
V. W. MELOCHE
