Pennington and Meloan (21). However, their method was applied only to liquid organic compounds. The presence of elements such as bromine, chlorine, and fluorine does not interfere with the analysis or change the calibration factors. These materials are effectively removed by reaction with the copper in the combustion train. No extraneous peaks were noted when samples containing halogens were analyzed. However, in the combustion of samples containing iodine, high values were obtained for carbon, hydrogen, and nitrogen on the second and subsequent runs. It was observed at this point that there was a decrease in flow of helium from 32 to 31 ccjmin. This was attributed to the formation and fusion of cuprous iodide (mp 605 "C) in a part of the 1-cm section of copper which was allowed to extend inside the combustion furnace at this end, where the temperature was about 700 "C. The helium flow was sufficiently restricted to change the response of the thermal conductivity cell. The simplest solution to this problem is not to allow any of the copper to extend inside the combustion furnace, so that it will not be heated appreciably above 600 "C, which is the temperature near the end of the reduction furnace. Because of the equilibrium and adsorption effects described above, the apparatus must be properly conditioned prior to calibration by burning a 2- to 3-mg sample of an organic material containing the elements that are to be determined. The oxygen content of a sample is determined by conversion to carbon monoxide using carbon in the combustion tube at 1100-1 120 'C in a helium atmosphere. Under these conditions the oxygen in the sample is converted to CO. This is trapped (21) S. Pennington, and C. E. Meloan, ANAL.CHEM., 39,119 (1967).
in the molecular sieve and measured by thermal conductivity as described by Meade (22). Because of its low freezing point (-207 "C) CO is not retained on the Carbowax but passes to the molecular sieve column where its retention is about 4 minutes from the time of removal of the liquid nitrogen trap. Because of their higher freezing points, other gases such as H2S (-82.9 "C), CS? (-111.5 "C), and COS (-138.2 "C), if present, will be retained and separated from the CO by the Carbowax column in liquid nitrogen. Figure 6 shows peaks obtained for carbon monoxide. A small positive blank of about 0.012 mg is obtained for oxygen. This is attributed partly to impurities in the helium and partly to air introduced with the sample. In the latter case 2 or 3 minutes reverse sweeping with helium before closing the sample-introduction end of the combustion tube would reduce this source of blank. In the determination of oxygen a standard deviation of 0.14 was obtained when dehydroabietic acid was used as the standard. The precision and accuracy of the results are equal to those obtained with the slower pyrolysis methods in current usage. Nine minutes are required for a determination. ACKNOWLEDGMENT
The authors gratefully acknowledge the contribution of James F. Carre who made many of the analyses reported here. RECEIVED for review October 17, 1968. Accepted December 23, 1968. (22) C. F. Meade, D. A. Keyworth, V. T. Brand, and J. R. Deering, ibid.,39, 512 (1967).
Study of Groves' Method for Determination of Ferrous Oxide in Refractory Silicates Elsie M. Donaldson Mineral Sciences Dicision, Mines Branch, Department of Energy, Mines and Resources, Ottawa, Canada
Groves' method for determining ferrous oxide in refractory silicates (by decomposing samples by fusion with sodium metafluoborate in a platinum boat in an inert atmosphere) is unreliable. Reduction occurs during fusion because of the tendency of iron compounds, particularly oxides, in the molten state to dissociate to lower oxidation states and metallic iron under the experimental conditions of high temperature and oxygen-free atmosphere used for sample decomposition. This effect increases in the presence of platinum and leads to low ferrous oxide values for samples containing predominantly iron(ll), and to high values for samples containing mostly iron(ll1) or moderate amounts of both iron(l1) and iron(ll1).
RECENT studies of pleochroism in some refractory iron-bearing silicates (tourmaline and cordierite) ( 1 ) have suggested that this phenomenon is related to iron(I1) -,iron(II1) electronic interaction. Because a relatively accurate knowledge of the iron(I1) content of the above minerals was essential to these studies, the present investigation was undertaken to assess existing methods for determining ferrous iron in refractory (1) G. H. Faye, P. G. Manning, and E. H. Nickel, Amer. Mineral., 53, 1174 (1968).
materials and to determine their applicability to the samples under consideration. Most of the methods for determining ferrous iron in silicates and refractory materials, described in a recent review by Schafer ( 2 ) , are not applicable to tourmaline and cordierite because they involve acid-dissolution procedures which are unsuitable for decomposing these minerals. However, several methods by Rowledge (3), Hey (4), and Groves ( 5 ) involving fusion of the sample were considered of interest in the present work because most acid-resistant silicates can usually be completely decomposed by fusion with a suitable flux. Rowledge's method involves heating a mixture of sample (staurolite, tourmaline, axinite, garnet) and flux (sodium metafluoborate) in a sealed borosilicate glass tube to 900 "C in a sand bath. Hey applied this method on a micro scale but used an evacuated tube to avoid difficulties arising from the oxidation of iron(1I) by air enclosed in the tube. Groves used the same flux but minimized air oxidation by fusing in a carbon dioxide (2) H. N. S. Schafer, Analyst, 91, 755 (1966). (3) H. P. Rowledge, J. Proc. Roy. SOC. West. Aust., 20, 165 (1934). (4) M. H. Hey, Mineralog. Mag., 26, 116 (1941). (5) A. W. Groves, "Silicate Analysis," 2nd ed., George Allen and Unwin Ltd., London, 1951, pp 181-6. VOL. 41, NO. 3, MARCH 1969
501
dominantly iron(II1) or moderate amounts of both iron(I1) and iron(III), and to low results for samples containing mostly iron(I1). EXPERIMENTAL
Figure 1. Apparatus for fusion method A. B. C. D. E.
To nitrogen tank
Glass-T connecting tube To titration apparatus Gas-control unit Gas-absorption bulb (silica gel) F. Quartz wool G. Gas-absorption bulb (Ascarite) H. Rubber stopper I. Vycor tube (25 X 31 X 800 mm) J. Closely-packed wire-form copper (bed length 10 in.) K. Split-type furnace (length 13 in.) held at 500 "C L. Flexible tubing (2 ft.) to facilitate removal of tube from furnace for cooling M . Vycor tube (25 X 31 X 1000 mm) N . Split-type high-temperature furnace (length 13 in.) 0. Platinum boat (10 X 10 X 60 mm) P . Gas-outlet bottle (10% sodium hydroxide)
atmosphere at 950 "C, using a platinum boat in a silica tube. Because preliminary tests with Hey's method revealed that fusion with a fluoride-containing flux in an evacuated glass tube has certain inherent disadvantages (difficulties of manipulation, attack on the tubes by the flux, cracking of the tubes during cooling, difficulty in dissolving the cooled melt away from the glass), Groves' method was considered to have the greatest potential in the present investigation. Groves' method for sample decomposition has been advocated recently by Gumbar (6) and Mikhailova et ai. (7) for the determination of iron(I1) in acid-resistant rocks. However, no data have been presented by these authors, or by Groves, to show that the method is reliable. Consequently, to assess Groves' method, it was applied to the well-known granite G-1 and diabase W-1, and to six silicate rock standards recently processed by the US.Geological Survey (8) (to provide new reference materials to supplement G-1 and W-1). In addition, because the ferrous oxide contents of the rock standards have not been fully established, these standards were first analyzed by a conventional acid-dissolution method to obtain better data for comparison. The results presented in this paper show that Groves' method for determining ferrous oxide in refractory silicates is not reliable. Reduction is inherent in the method because of the inclination of iron compounds, particularly oxides, to dissociate to lower oxidation states and metallic iron under the conditions of high temperature and oxygen-free atmosphere used for sample decomposition. This effect is increased in the presence of the platinum boat used as decomposition vessel and leads to high results for samples containing pre(6) K. K. Gumbar, Tr. Vses. Nauch-Issled. Geol. Inst., 125, 170 (1966). (7) Z . M. Mikhailova, R. V. Mirskii, and A. A. Yarushkina, Zh. Anal. Khim., 18, 856 (1963). (8) F. J. Flanagan, Geochim. Cosmochim. Acta, 31, 289 (1967). 502
ANALYTICAL CHEMISTRY
Apparatus. The polyethylene decomposition vessel employed for the acid-dissolution method was similar to that devised by Schafer (9), except that a separate acid-inlet system and a small, separate gas-outlet hole were included. The acid-inlet system was a small Nalgene funnel attached to Tygon tubing and reaching to within l / 8 inch of the bottom of the vessel. The nitrogen-inlet tube was constructed from a 1-ml Nalgene pipet and connected with rubber tubing to a nitrogen tank via a gas control unit. Figure 1 illustrates the apparatus employed for the sample fusion process. Tygon tubing was used to connect the various components of the fusion apparatus. The copper used in tube furnace K was prepared by reducing wire-form copper oxide with hydrogen gas at approximately 250 "C. A Leeds and Northrup pH meter was utilized for the potentiometric titration of ferrous iron. The titration vessel was a 400-ml graduated beaker equipped with a tightly-fitting rubber stopper, through which nitrogen-inlet and outlet tubes, a 10ml buret, a platinum electrode, and a calomel reference electrode were inserted. The nitrogen-inlet tube was a verticaltype, gas dispersion tube, which was bent so that the fritted glass end was parallel to the bottom of the titration vessel. The flow of nitrogen during titration was regulated with a needle valve, and stirring was accomplished with a Tefloncoated magnet in conjunction with a Magne-stir magnetic stirrer. Reagents. STANDARD CERICSOLUTION, 0.01N IN 1 N SULFURIC ACID. The solution was prepared by adding 55 ml of concentrated sulfuric acid to 10.9632grams of ceric ammonium nitrate in a dry 1-liter beaker, mixing for 2 minutes, and then successively adding 25-ml portions of water and stirring for approximately 1 to 2 minutes after each addition until the material was completely dissolved. This solution was diluted to 2 liters with water and standardized potentiometrically against ferrous ammonium sulfate. This was prepared according SODIUMMETAFLUOBORATE. to Rowledge (3) by heating a mixture of 84 grams of sodium fluoride and 70 grams of boric anhydride in a platinum dish at approximately 1000 "C until a clear, fluid melt was obtained. The resultant sodium metafluoborate [(NaF)2B2031 was cooled rapidly and then ground in a small ball mill. Alternatively, an unfused mixture, stored in a drying oven at 130 "C, was also used for "in situ" preparation of flux during fusion. Procedure for Acid-Dissolution Method. In the following procedure, the water and all solutions were saturated with nitrogen before use. A silicate sample of 50 to 100 mg, depending on the expected ferrous oxide content, was transferred to the plastic decomposition vessel and moistened with a few drops of water. The apparatus was assembled, then immersed to about one third of its depth in a water bath maintained at approximately 80 "C, and flushed with nitrogen for 1 minute at a flow rate of 0.5 lpm. Then, through the funnel, were added, in succession, 14 ml of 50% sulfuric acid solution and 5 ml of 48% hydrofluoric acid, and the apparatus was swirled (both the nitrogen-inlet and acid-inlet tubes being kept well below the surface of the solution) to disperse the sample. Swirling was repeated several times during dissolution. When decomposition was complete (generally 45 to 90 minutes), 40 ml of 5 boric acid solution were added and the flow of nitrogen continued for 2 minutes until the solution was well mixed. Then the gas flow was stopped, the apparatus was removed from the water bath and the contents were transferred, without delay, to the titration vessel containing 150 ml of water previously (9) H. N. S. Schafer, Analyst, 91, 763 (1966).
Table I. Determination of Ferrous Oxide in Standard Silicate Samples by the Acid-Dissolution Method FeO found, % FeO previously reported, % Flanagan (8) Goldich Carmichael Results of replicate Average Arithmetic mean Sample (Average values) et al. (10) et al. (11) Others determinations result value, yo G-1 0.89; 0.89; 0.90 0.98a (granite) 0.91 w-1 8.78; 8.74; 8.77 8.74a (diabase) 8.78 G-2 1.48 1.56 1.50 1.49; 1.45; 1.46 1.50 (granite) 1.45; 1.44 GSP-1 2.35 2.53d 2.37 2.45; 2.37; 2.39 2.31 (granodiorite) 2.37; 2.38 AGV-1 2.06 2.15 2.07 2.11; 2.16; 2.14 2.10 (andesite) 2.13; 2.17 PCC-I 4.91d 5.51 5.17d 5.490 5.54; 5.38; 5.46 5.49 (peridotite) 5.48; 5.45 DTS-1 6.89d 7.48 6.81d 7.36b 7.41 ; 7.33; 7.37 7.40 (dunite) 7.34; 7.38 BCR-1 9.08d 9.17d 8.87 8.860 8.94; 8.99; 8.96 8.91 (basalt) 8.93~ 8.93; 8.96 QFleischerand Stevens (14)-arithmetic mean of preferred values. *Peters (12)-average values. cKiss (13)-average value. dNot included in calculation of arithmetic mean values. saturated with nitrogen for about 5 minutes. The solution was diluted t o 250 ml with water and immediately covered with the rubber cap fitted with the electrodes. The flow of nitrogen was continued and the ferrous oxide content of the sample was determined potentiometrically with standard ceric solution. Procedure for Fusion Method. The silicate sample was mixed on platinum foil (or glazed paper) with 1 gram of either sodium metafluoborate or the unfused sodium fluorideboric anhydride mixture, and the resulting mixture transferred t o a dry platinum boat. The boat was placed in the center of the Vycor tube (Figure 1) via the gas-outlet end, which was then sealed tightly by insertion of the rubber bung connected t o the gas-outlet tube. Nitrogen was passed through the apparatus a t a flow rate of 1 lpm for 15 minutes to displace air from the system. Then the furnace containing the sample boat was allowed to heat for about 30 minutes to approximately 900 “C,or until a clear fluid melt was obtained. The Vycor tube was then removed from the furnace t o a cooling rack. The nitrogen was stopped after 10 minutes and the boat was removed and immediately dropped into the titration vessel containing a hot sulfuric-boric acid solution (14 ml of 50% sulfuric acid solution and 40 ml of 5 % boric acid solution diluted to 250 ml) previously saturated with nitrogen for 5 minutes. The titration vessel was immediately re-covered with the rubber cap fitted with the electrodes and the flow of nitrogen was continued. After the melt was completely dissolved, the solution was cooled t o room temperature and titrated potentiometrically with standard ceric solution. The ferrous oxide content of the sample was calculated after correcting for a reagent blank (approximately 0.10 ml) which was carried through the entire procedure. RESULTS
Determination of Ferrous Oxide by an Acid-Dissolution Method. Results for ferrous oxide in the U S . Geological Survey rock standards by several investigators (8, 10-13) d o not agree well; consequently, the samples were first analyzed by a n acid-dissolution method to obtain data to be compared later with results by Groves’ fusion method. The method (IO) S.S. Goldich, C. 0.Ingamells, N. H. Suhr, andD. H. Anderson, Can. J. Earth Sci., 4, 747 (1967). (11) I. S. E. Carmichael, J. Hampel, and R. N. Jack, Chem. Geol., 3, 59 (1968). (12) A. Peters, Neues Jahrb. Mineral. Monatsh., 314 119 (1968). (13) E. Kiss, Anal. Chim. Acta, 39, 223 (1967).
employed was a modification of Schafer’s method (9), in which the sample was decomposed with sulfuric and hydrofluoric acids in an all-plastic apparatus in a nitrogen atmosphere, and the ferrous iron was titrated potentiometrically, in the same vessel, with standard dichromate solution. Ceric solution was used as titrant because of its stability and freedom from induced side reactions (particularly with organic materials) that often occur with both dichromate and permanganate. Because the available electrode assembly did not permit the direct use of the decomposition vessel as the titration vessel, boric acid was used t o complex excess hydrofluoric acid, and thus inhibit air oxidation of iron(I1) during the transfer of the sample solution to the titration vessel. The results obtained with this modified method for the rock standards, including G-1 and W-1 analyzed concurrently as controls, are given in Table I. Although the results obtained for G-1 (Table I) are slightly low because the sample was not completely decomposed, those obtained for W-1 are in good agreement with the preferred value given by Fleischer and Stevens (14) and indicate that the modified version of Schafer’s method yields reliable results. The results of replicate determinations for the U.S. Geological Survey samples (G-2 to BCR-I) by the aciddissolution method are suitably consistent and the average results agree favorably with those reported by most of the previous investigators. The values listed in the last column of the table, except for G-1 and W-1, are the arithmetic means of the average results obtained in the present work and some results by other analysts (values not included are indicated in the table). These mean values are considered to be reasonably accurate estimates of the ferrous oxide contents of the samples and were used for comparison with data by Groves’ method. Preliminary Investigation of Groves’ Method. Because some details of Groves’ method were not clear, it was necessary t o study some factors that could influence the final results. It was also considered that certain features of the method could be improved. CHOICE OF GASAND METHOD OF PURIFICATION: Preliminary attempts to apply Groves’ fusion method in the recommended carbon dioxide atmosphere were unsuccessful because the (14) M. Fleischer and R. E. Stevens, Geochim. Cosmochim. Acta, 26, 525 (1962). VOL. 41. NO. 3, MARCH 1969
503
Table 11. Determination of Ferrous Oxide in Standard Silicate Samples by the Modified Fusion Method Arithmetic mean value, yo Sample (from Table I) Sample wt taken, mg FeO found, % G-1 0.98 50 1.31 100 1.30; 1.32Q 50 w-1 8.74 8.82Q;8.68; 8.63; 8.55 75 8.70 100 8.75 G-2 1.50 100 1.76; 1.85; 1.79; 1.72 GSP-1 2.37 100 3.03; 2.80; 2.90; 2.98 AGV-1 2.10 100 2.82; 2.19; 2.80 PCC-I 5.49 50 5.99 75 5.86 100 5.89; 5.80 DTS-I 7.40 50 6.93; 6.91 100 6.96 BCR-1 8.91 50 9.85; 9.93; 9.50; 9.52; 9.90 Fused sodium metafluoborate employed. Q
available gas contained too much oxygen. However, Groves also advocated the use of nitrogen, which is inert and more easily purified than carbon dioxide; therefore, high-purity nitrogen (oxygen content less than 20 ppm; moisture content less than 5 grains/1000 cubic feet) was employed in the present work. Preliminary tests with high-purity nitrogen showed that a scrubbing bottle containing alkaline pyrogallol solution was not satisfactory for oxygen removal at moderate gas-flow rates because the solution foamed up into the top of the bottle. However, it has been shown that copper maintained at temperatures of approximately 400 to 600 “C can remove relatively large amounts (up to 4 %) of oxygen from nitrogen feed gas at flow rates up to 1.8 lpm (15); therefore, hot wire-form copper was used in subsequent work. Silica gel and Ascarite were used to absorb water and carbon dioxide. The removal of carbon dioxide was considered necessary because, if present in the nitrogen, it would be reduced to carbon monoxide by passage through the hot copper, and this gas, in turn, would probably cause reduction of iron compounds in the sample during fusion. EFFECTOF GAS-FLOW RATE: Tests carried out at varying flow rates with 50-mg samples of W-1 showed that the optimum flow rate of nitrogen required for a n air-free atmosphere in the fusion apparatus is approximately 1 lpm. Although the rate can be reduced to 0.25 to 0.5 lpm during fusion with no significant detrimental effect, provided the apparatus is adequately flushed at the higher rate, a constant rate of 1 lpm was employed in the present work. METHODOF FLUX PREPARATION : Sodium metafluoborate, prepared by Rowledge’s method (3), is extremely hard and difficult t o grind and is subject to some risk of contamination during fusion and grinding. Tests with G-1 and W-1 (Table 11) showed that both the fused flux and the unfused mixture yielded identical results; therefore, the mixture was used in the present investigation. Application of the Modified Fusion Method to Standard Silicate Samples. To determine whether Groves’ fusion method is reliable, the modified method described under “Experimental” was applied to the silicate samples previously analyzed by the acid-dissolution method. Table I1 shows that, for most of the samples, the results of replicate determinations are not in good agreement. Although results for DTS-1 and some for W-1 are lower than the respective arithmetic mean (15) D. S . Gibbs, H. J. Svec, and R. E. Harrington, Ind. Eng. Chem., 48, 289 (1956). 504
ANALYTICAL CHEMISTRY
values calculated previously, those obtained for the remaining samples are all considerably higher, indicating that partial reduction of ferric iron occurred during fusion. Investigation to Determine the Cause of Reduction. Although some sample components (sulfides, organic material; manganese, vanadium and, probably, chromium and titanium in lower oxidation states) could probably cause reduction of some ferric iron during fusion, attempts to relate the positive error (or amount of reduction) obtained for most of the samples by the fusion method to the respective contents of these components (8, 10, 14) were unsuccessful. Organic material in the flux mixture was apparently not the cause because analysis indicated that very little, if any, carbon was present. Therefore, it was necessary t o investigate other factors in the fusion step to determine the cause of the observed reduction. POSSIBLE REDUCING IMPURITIES I N THE NITROGEN : To determine whether reduction during fusion was caused by reducing impurities (hydrogen, carbon monoxide, ethane, methane) in the nitrogen used to maintain a n inert atmosphere, several tests were carried out with sample AGV-1 (chosen because of its high ferric iron content) using a n Arneil catalyst [copper(II) oxide 4% iron(II1) oxide] which is known to oxidize and thus eliminate these impurities at about 520 “C (16). For these tests, the tube containing copper was replaced by one containing 6 inches of copper followed by 4 inches of catalyst. This tube was maintained at about 550 “C and the gasabsorption bulbs were connected to the gas-outlet end to absorb possible oxidation products. The results obtained in these tests (2.75 and 2.74%) showed that impurities in the gas were not responsible for the observed reduction. This was confirmed by the result obtained ( 2 . 8 2 z ) in a test carried out in vacuum. POSSIBLE REDUCTION BY THE PLATINUM BOAT:To determine whether the platinum boat was causing reduction, 50-mg samples of ferric oxide [prepared from ferric nitrate and found free of iron(I1) after dissolution with hydrochloric acid in a sealed tube filled with argon] were carried through the fusion procedure, using both platinum and silica (Vitreosil) boats. These tests showed that reduction occurred with both boats [4.7 and 1.8 mg iron(I1) obtained, respectively] but was more pronounced with the platinum boat. It was observed in the test with the platinum boat that some metallic iron had alloyed with the boat, suggesting that iron(I1) is also reduced during fusion. This was verified by the low result obtained [29.5% iron(II)] for a sample of vivianite
+
(16) A. Arneil, J. SOC.Chem. Ind., 53, 89T (1934).
z
[Fe3(P04)2*8H20] containing 32.8 ferrous and 0 . 4 z ferric iron, and by the fact that the total iron content of the solution after fusion (29.9%) was low by an equivalent amount. DISCUSSION
Because ferrous iron was obtained in the foregoing tests with ferric oxide when both platinum and silica boats were employed, it is evident that other factors besides the platinum boat, are contributing to the reducing processes which were found to occur when the fusion method was applied to the silicate samples. Attempts to elucidate these unknown factors experimentally were unsuccessful. However, a search of the literature pertaining to studies of iron(IItiron(II1) oxide equilibria in the molten state in related materials (iron oxides, iron silicates, glasses) revealed that, regardless of the type of crucible used to contain the melt, the above equilibria are dependent on temperature and the oxygen potential of both the molten phase and gas phase employed (17-22). According to Muan (21, 22) oxides of iron at high temperatures and low oxygen pressures are only moderately stable and dissociate partly to lower oxides, metal, and oxygen. With ferric oxide, this instability gives rise to iron(I1) in the molten oxide phase, thus explaining why iron(I1) was obtained in the test with the silica boat. Although the temperature used in the present work is lower than that required for significant decomposition of pure ferric oxide (approximately 1400 "C) (19), the fact that iron(I1) was obtained in this test indicates that the sodium metafluoborate flux lowers the effective dissociation temperature. Muan and other investigators (17, 20-23) have also established that platinum crucibles cannot be used to study ferric-ferrous oxide equilibria in the materials cited above, particularly at high temperatures and low oxygen pressures. Under these conditions, reduction, or more specifically, increased dissociation of iron oxides occurs because metallic iron formed in the process alloys with the platinum container. This effect increases as the oxygen potential of the system decreases (23) and causes significant changes in equilibria because of the removal of a component from the molten phase. Consequently, on the basis of the above observations, it can readily be seen that reduction is inherent in Groves' method because of the low oxygen potential of the nitrogen phase, the platinum boat, and the high temperature employed for sample decomposition. From the above observations on the effect of platinum on iron(I1)-iron(II1) oxide equilibria at high temperatures and low oxygen pressures, it is apparent that, when Groves' method is used to decompose iron-containing materials, the initial ferric-ferrous oxide ratio in the sample will determine, to a large extent, the resultant ratio in the melt after fusion. Consequently, it determines whether low or high results for ferrous oxide will be obtained. This is obvious if the following simplified equations
+ +
2 F e 2 0 3+ 4 FeO O2 (1) 2 FeO + 2 Fe O2 (2) are applied to iron oxides dissolved in the flux. It is evident that with samples containing predominantly iron(II1) (high (17) N. L. Bowen and J. F. Schairer, Amer. J. Sci., 24, 177 (1932). (18) L. S. Darken and R. W. Gurry, J. Amer. Chem. SOC.,67, 1398 (1945). (19) L. S. Darken and R. W. Gurry, ibid.,68, 798 (1946). (20) T. Baak and E. J. Hornyak, Jr., ibid.,44, 541 (1961). (21) A. Muan, Amer. J . Sci., 256, 171 (1958). (22) A. Muan, Bull. Am. Cer. SOC.,42, 344 (1963). (23) R. B. Sosman and J. C . Hostetter, J. Wash. Acad. Sci., 5, 293 (1915).
initial ferric-ferrous oxide ratio), if both equations are considered to represent the reactions occurring during fusion, a high result for ferrous oxide will be obtained (cf. test with ferric oxide) because the first reaction proceeds at a greater rate than the second. With samples containing predominantly iron(I1) (low initial ratio) the principal reaction occurring during fusion is represented by Equation 2 and consequently, a low ferrous oxide value will result (cf. test with vivianite). Finally, with samples containing moderate amounts of both ferric and ferrous iron, both reactions would occur and would, in general, lead to a high result.In this case and that of samples containing mostly iron(", the reaction Fe F e 2 0 3+ 3Fe0 would probably also occur, to some extent, and contribute to high results. Consequently, because of their initial ferricferrous oxide ratios, it is readily understandable from the above interpretation why the silicate samples yielded both high and low results for ferrous oxide by the fusion method. Low results would be expected for W-1 and particularly DTS-1 (ratios 0.17:l and 0.06:1, respectively), while high results would be expected for AGV-1 (ratio 2:l) and most probably for the remainder of the samples (ratios 0.9 to 0.4:l). The ferric-ferrous iron ratio after fusion is not dependent, to any great extent, on the absolute amounts of iron(I1) and iron(II1) present in the sample, but is primarily a function of the initial ratio, as contended above. This is supported by the fact that the fusion method yielded approximately similar results for ferrous oxide (Table 11) when different initial sample weights were employed. However, the resulting ratio also depends on the time of contact of the melt with the platinum boat. This was shown experimentally when fused melts of sample AGV-1 were maintained at approximately 900 "C for 1- and 2-hour intervals (2.89 and 2.63% ferrous oxide obtained, respectively). Because the time required to obtain a clear melt varied slightly from test to test, this factor is considered responsible for the variable results obtained for some samples. From the results obtained in the present investigation it is concluded that Groves' method for determining ferrous iron in refractory silicates is not reliable. Furthermore, because ferric-ferrous iron equilibria at liquid temperatures in an oxygen-free atmosphere are dependent on the oxygen potential of the molten phase, which, in turn, is a function of the composition of the melt (22), it is concluded that the method cannot be improved either by using other fluxes or other decomposition vessels. Although Mikhailova et al. ( 7 ) state that sodium fluoborate (NaBF4)can be employed in the fusion method with a platinum boat, this flux was found to be unacceptable in the present work. Tests in which W-1 and vivianite were decomposed with this flux yielded extremely low results [7.90% ferrous oxide and 27.8 % iron(II), respectively], presumably because of increased dissociation in the presence of platinum caused by the inherent decrease in the oxygen potential of the molten phase.
+
ACKNOWLEDGMENT
The author expresses sincere appreciation to the following members of the Mines Branch staff: to R. C. McAdam for his proposal of this investigation, to G. H. Faye for his helpful suggestions, to L. G. Ripley for his aid in the sealed-tube tests, to J. C. Hole for his analysis of the prepared ferric oxide, to R. G. Sabourin for his determination of the carbon content of the flux mixture, and to A. H. Webster and N. F. H. Bright for their critical review of this manuscript. RECEIVED for review September 30, 1968. Accepted November 26, 1968. Crown Copyright Reserved. VOL. 41, NO. 3, MARCH 1969
505
