barium precipitation, as originally described by Krishna (3), eliminates potentially interfering substances and leaves cyclic AMP in the supernatant. Adenyl cyclase activity in rat brain homogenates by this method compares closely with those reported by Krishna. The potential ability to collect and count radioactivity in the chromatographic effluent containing cyclic AMP should also make this method useful for the determination of specific activity of cyclic AMP produced from radioactive precursors. Cyclic AMP has been measured in guinea pig and rabbit atria using this method. A preliminary purification is necessary to remove interfering substances. The results of these studies will be reported elsewhere.
ACKNOWLEDGMENT The cooperation of Varian-Aerograph for providing the chromatograph during the inception of this project is appreciated. The expert technical assistance of Sharon Laws and Felicidad Avila, the encouragement and advice of Roger Jelliffe and David Blankenhorn, and the secretarial assistance of Georgene Denison is gratefully acknowledged.
RECEIVED for review April 9, 1970. Accepted June 11, 1970. This work was supported by a Grant-in-Aid from the Los Angeles County Heart Association, an Emergency Grant from the American Heart Association, and USPHS General Research Support Grant NO. SO1 FR05466-03.
New Semimicroprocedure for Determination of Ferrous Iron in Refractory Silicate Minerals Using a Sodium Metafluoborate Decomposition Robert Meyrowitz United States Geological Survey, Washington, D . C . 20242
THISPAPER is the fifth in a series (1-4) describing micro- and semimicroprocedures for determining the common rockforming elements as part of a scheme for the complete analysis of small samples of silicate minerals. This paper concerns the semimicrodetermination of FeO in refractory silicate minerals. The purpose of this work was to develop a new semimicroprocedure (25-35 mg samples) using a Groves fusion decomposition followed by solution of the melt in the presence of excess standard potassium dichromate. Many of the difficulties in determining FeO in refractory rocks ( 5 ) are not encountered in analyzing refractory silicate minerals because of the inherent homogeneity of minerals. The two important problems remaining are the decomposition (including solution) of the mineral and the prevention of oxidation of the ferrous iron before the determinative process has begun. The simplest manipulative procedure for the decomposition of refractory minerals is Groves’ (6) method of decomposition by fusion with sodium metafluoborate, (NaF)?B203, in a platinum boat in an inert atmosphere. Hey’s microprocedure (7) is the only one hitherto proposed for determining FeO in refractory minerals using samples of less than 50 mg. It is the only procedure wherein the metafluoborate melt is dissolved in excess oxidant (iodine monochloride). Hey’s procedure is manipulatively difficult because the sample is fused in an evacuated sealed tube. After the fusion the tube is broken with mortar and pestle and the melt is freed from glass as much as possible and is transferred to the titration flask. ~
~
(1) R. Meyrowitz, Amer. Mineral., 48,340(1963). ( 2 ) Ibid., 49,769 (1964). (3) R. Meyrowitz, U.S. Geol. Surv., Prof. Pap., No. 650-B,136 (1969). (4) Ibid., No. 700-D(1970),in press. (5) J. A. Maxwell, “Rock and Mineral Analysis,” Interscience Publishers, New York, 1968,pp 202-210. (6) A. W.Groves, “Silicate Analysis,” 2nd ed., George Allen and Unwin Ltd., London, 1951, pp 181-6. (7) M. H.Hey, Mineral. Mag., 26,116(1941). 1110
An almost complete history of the metafluoborate decomposition methods has been given by Schafer (8) and also Maxwell (5). Since then Gumbar (9) and Donaldson (10) described procedures essentially that of Groves (6), and Novikova (ZI) proposed decomposing the sample in an atmosphere of helium or argon. Donaldson (IO) and Novikova (ZZ) studied the effect of ferric oxide-ferrous oxide ratio of the sample on the reliability of their methods. EXPERIMENTAL Apparatus. Figure 1 illustrates the apparatus used for the fusion of the sample. The split type furnace used is that described by Meyrowitz and Massoni (12). A Mariotte bottle is used to calibrate the flowmeter. Reagents. Standard potassium dichromate, 0.02000N. Standard ferrous ammonium sulfate, 0.01N. One liter of solution contains 3.922 grams of Fe(NH4)?(SO&.6Hz0 and 28 ml of concentrated HzS04. Sodium metafluoborate. This is prepared according to Groves (6). A well-mixed batch of 21 grams of sodium fluoride and 31 grams of crystalline boric acid is placed in a 250-ml platinum dish covered with a platinum lid and gradually heated in an electric furnace to 350 “C and allowed to stand at this temperature overnight. The temperature is raised to 1000 “C. When the mass is completely molten, the contents are mixed well and again heated until completely molten, removed from the furnace, and allowed to cool. The (NaF)2B203is crushed and ground using a mortar and pestle and sieved using a plastic or other nonmetallic sieve with silk bolting cloth. The portion between 82 and 109 mesh is used for the decomposition of the sample. Procedure for Decomposition of Sample. Weigh 25-35 mg of powdered sample in the platinum boat. Weigh 350 (8) H. N. S . Schafer,Analyst, 91,755(1966). (9) K. K. Gumbar, Tr. Vses. Nauch.-lssled. Geol. Inst., 125, 170 (1966). (10) E. M.Donaldson, ANAL.CHEM., 41, 501 (1969). (11) Y. N. Novikova, Zh. Anal. Khim., 23, 1057 (1968). (12) R.MeyrowitzandC. J. Massoni, ANAL.CHEM., 27,475 (1955).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
mg of 82-109 mesh (NaF)2B203. With a small spatula, add a small portion of the flux to the boat and mix carefully using a platinum wire (No. 21 B and S Gauge) that has been flattened at one end. Brush off wire into boat. Again add a small amount of flux and mix well. Settle the mixed powders by tapping with the small spatula across the top of the boat. Again add a small amount of flux and mix. Tap to settle mixture. Continue this series of operations until a small amount of flux remains. Use the remainder to form a layer of flux above the mixed powders. Tap to settle the mixture. The level of the contents of the boat will be at least 1 mm below the top. Clean the bottom and sides of the boat with a brush. Position the split furnace over the Vycor tube. Place boat in the center of the furnace. Pass argon at approximately 50 ml per minute through the tube for 10 minutes. Heat the furnace so that at the end of 10 minutes the temperature will be approximately 500 O C . At this time raise the temperature of the furnace so that at the end of 5 minutes, the temperature will be approximately 850 “C. Maintain this temperature for 5 minutes. Turn off furnace and then open it. After the melt has solidified, move furnace from its position over the platinum boat to hasten cooling while maintaining the argon flow. Procedure for Solution of Melt and Determination of Excess Dichromate. Transfer the boat to a 125-ml Erlenmeyer flask containing 7/8 X 3//s inch plastic-covered magnetic stirring bar and a solution of 500 mg of crystalline H3B03,50 ml of 3.6N H2S04, 2.0 ml of 22 N &Pod, and approximately 9 ml of standard 0.02000N K2Cr207(use a buret, precision bore, Class A, 10 ml, graduated at 0.02-ml intervals). Cover the Erlenmeyer flask and stir with a magnetic stirrer. A piece of wood or similar insulating material should be between the flask and magnetic stirrer to prevent the conduction of heat from stirrer to flask. The solution of the melt should be hastened by breaking up the melt and removing it from the boat. This is done by spreading apart the sides of the boat using a pair of tweezers having long platinum tips and breaking the melt into small pieces using a heavy platinum wire (B and S gauge No. 14) formed into a chisel. Rinse off tweezers and platinum rod. The boat is not removed from the flask. The melt will dissolve completely within 30 to 75 minutes and the solution will remain clear. Titrate with standard 0.01N ferrous ammonium sulfate (using the same type of buret used for the K2Cr207)until the yellow color of the solution just disappears. Add 1.0 ml of 0.005% (w/v) sodium diphenylaminesulfonate. Add the ferrous solution dropwise until the violet or purple color just disappears. Rinse tip of buret with water. Add the dichromate dropwise with constant stirring until the purple or violet color reappears. Rinse tip of buret with water. Add the ferrous solution, “cracking” the drops added, until the purple or violet color just disappears. The tip of the buret is rinsed with water after the addition of each “cracked” drop. Procedure for Standardization of 0.01N Ferrous Ammonium Sulfate. The titration is performed as described above except that the solution containing H3B03, H2S04, H3P04, and K2Cr207is diluted to approximately 70 ml before the addition of the ferrous solution which is contained in a 25-ml buret, precision bore, Class A, graduated in 0.1-ml intervals. The ferrous solution is standardized in duplicate each day it is used. RESULTS
In Table I the results obtained by the fusion decomposition semimicroprocedure for various silicate minerals and rocks are compared with results obtained by acid decomposition. Data presented here are mostly from nonrefractory minerals rather than rocks. Rocks containing ferrous-bearing refractory minerals would be expected to give low values for FeO by acid decomposition procedures and comparison between the two methods would not be valid.
11
c
H
II
A
Figure 1. Apparatus for fusion A , to argon tank; B, glass connecting tube; C, rubber stopper; D, split-type furnace (length approximately 60 mm); E , platinum boat (5 X 5 X 30 mm); F, Vycor glass tube (length 330 mm; 0.d. 14 mm); G, rubber tubing; H,glass wool; I, drying tube, straight (150 mm); J , fine metering valve; K,flowmeter
The values by the fusion procedure uncorrected for the procedural blank compare favorably with those by acid decomposition except as noted below. Seventeen procedural blanks were determined and they ranged from -0.009 to +0.069 mg FeO. The mean and standard deviation were 0.015 and 10.022, respectively. For a 25-mg sample, the smallest recommended size, the absolute error would be from -0.04z to +0.28 % FeO. It is recommended that each analyst verify that the procedure as used does have a blank that is small. DISCUSSION
The weight of flux used is determined primarily by the size of the boat used. Small amounts of flux in the molten state tend to “ball” or gather at one or both ends of the boat. Sufficient flux must be added to minimize this tendency which would result in some particles of sample remaining undecomposed. The level of the mixture in the boat must not be so high as to cause spillage and fusion of the boat to the Vycor tube. The use of a Coombs-Alber type platinum microcylinder or sleeve to protect the Vycor tube would still result in the loss of the sample because of the fusion of the sleeve to the boat by the flux. The particle size of the flux must be small enough to ensure good mixing with the sample so that if balling occurs, complete decomposition of the sample will still obtain. However, the particle size must be large enough so that the required weight of flux will be properly contained in the boat. The mineral samples were fused with 82-109 mesh flux and the rock samples with 109-150 mesh flux (Table I). The results obtained do not in general support the conclusions of Novikova ( 1 1 ) and Donaldson (IO) that the Groves method “gives high values for samples containing mostly iron(II1) or moderate amounts of both iron(I1) and iron(III).” Because of the known affinity of platinum and iron, especially ferrous iron (13, 14), low results can be expected for samples containing high concentrations of ferrous iron, for example, garnets. The bottom of the platinurn boats used for the anal(13) H.R.Shell, ANAL.CHEM., 26,591 (1954). (14) B. G. Russell, J. D. Spangenberg, and T. W. Steele, Talanta, 16, 487 (1969).
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Table I. FeO Content of Various Silicate Minerals and Rocks-A Comparison of Results Obtained by Acid Decomposition of Fusion Decomposition
Acid decomposition semimicroprocedure, FeOa 0.56
Fusion decomposition semimicroprocedure Sample Average Name size, mg Z FeO A FeOb ZFeO 34.15 Epidote 0.62 0.60 +O. 04 46.98 0.58 Antigorite 2.2 2.14 28.98 2.11 2.10 -0.04 27.91 1 . 7gC 23.12 2.20 1.99 23.99 Augite-18 6.08 5.77 33.77 5.29 5.68 -0.09 33.28 5.75 34.13 5.98 34.40 5.70 35.32 Augite-14 9.6 9.51 9.39 9.48 -0.03 9.14 27.38 8.92 33.67 25.16 9.58 9.48 37.57 24.32 9.74 9.42 24.15 9.87 32.18 9.77 35.83 Titano-augite 12.6 12.36 26.03 13.32 13.48 $1.12 34.43 13.12 27.20 11.98c 34.59 13.62 29.81 13.70 28.17 13.63 Garnet-30 22.8 22.93 21.95 -0.98 24.77 21.95 21.95 29.44 Garnet-26 29.45 23.77 -1.94 25.7 25.23 23.29 22.02 24.16 24.77 21.95 Garnet-29 25.8 25.98 24.38 -1.60 18.04 24.15 27.74 24.39 27.60 24.60 Garnet-13 26.1 26.98 23.91 24.85 -2.13 23.44 18.88 24.90 21.96 25.74 PCC-1 4.943 5.10 +0.21 5.496 38.29 5.66f 32.59 5.73f -0.26 7.14 30.49 7.01f DTS-1 6 .79d 7. 40e 35.72 7.26, -0.31 8.43 33.29 8.38, 8.740 w-1 8.749 38.83 8.48f +O. 37 8.91d 9.28 33.82 9.301 BCR-1 8.91e 34.59 9.25, a Meyrowitz ( I ) , unless otherwise noted. Per cent semimicro fusion minus semimicro acid decomposition. Omitted in calculation of average. Flanagan (15). e Donaldson (IO). f Analyzed by Ellen Lillie, U. S. Geological Survey. 0 Fleischer (16). Acid decomposition macroprocedure, FeOa 0.64
x
ysis of these minerals had a deep purple or violet stain which was impossible to remove completely, even after repeated cleanings by fusion with pyrosulfate and boiling in (1 1) HCl. Other samples caused stains of a lighter color. For samples containing approximately 5 1 0 % FeO and Fe203/ FeO ratios of less than 0.50, the fusion values for FeO never differed by more than +0.4z FeO absolute. Three of the four samples analyzed by Donaldson (Table 11) had differences greater than +0.4% FeO absolute. At the present
+
(15) F. J. Flanagan, Geochim. Cosmochim. Acta, 33, 81 (1969). (16) M. Fleischer, ibid., 33, 65 (1969). 1112
time no explanation can be given for the relatively large increase in per cent FeO of the titanoaugite determined by fusion. Donaldson and Novikova state also that samples containing predominantly ferrous iron (low initial ratio Fe203/ FeO) will result in low per cent FeO by fusion. This is not true of the titanoaugite. High FeO results by fusion were not obtained in the analysis of samples containing small amounts of ferrous iron (epidote and antigorite) as were obtained by Donaldson (G-1, G-2, and AGV-I) and Novikova (martitehematite, foyaite, and G A granite). The duration of fusion of the various methods differs markedly, this paper less than 10 minutes, Donaldson approximately 30 minutes, and Novikova 1 minute. Both Donaldson
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
Table 11. Comparison of Results Obtained by Acid Decomposition and by Three Different Fusion Procedures Acid Fusion Ratio decomposition, decomposition, Total 72 F e W Name FeO ZFeO A FeOa iron as Fen03 % Fez03 FeOb Data from this paper except as noted 0.56 0.60 $0.04 Epidote 10.28 9.66 17.2 2.14 2.10 -0.04 Antigorite 5.77 3.39 1.6 PCC-1 5.49c 5.70 $0.21 8.53 2.43 0.44 5.77 5.68 -0.09 7.81 1.40 0.25 Augite- 18 -0.26 7.40" 7.14 DTS- 1 8.85 0.63 0.09 8.74d 8.43 -0.31 11.09 1.38 0.16 w-1 BCR-1 8.91C 9.28 +0.37 13.50 3.60 0.40 9.51 9.48 -0.03 Augite-14 12.57 2.0 0.21 13.48 $1.12 Titano-augite 12.36 15.93 2.2 0.18 22.93 21.95 -0.98 Garnet-30 26.8 1.3 0.05 -1.94 25.23 23.29 Garnet-26 29.4 1.4 0.05 24.38 -1.60 Garnet-29 25.98 30.5 1.6 0.06 24.85 -2.13 Garnet-13 26.98 31.9 1.9 0.06 Data from Donaldson (10) except as noted 0.9gd 1.31 +O. 33 G-1 1.94 0.85 0.87 1.50 1.78 +O. 28 G-2 2.76 1.09 0.73 2.10 2.80 $0.70 AGV- 1 6.80 4.47 2.13 GSP-1 2.37 2.93 +O. 56 4.33 1.70 0.72 PCC-1 5.49 +0.40 5.89 8.53 2.43 0.44 DTS-1 7.40 6.93 -0.47 8.85 0.63 0.09 w-1 8. 74d 8.69 -0.05 11.09 1.38 0.16 BCR-1 8.91 9.74 $0.83 13.50 3.60 0.40 Vivianite 32.8 29.5 -3.3 36.8 0.4 0.01 Data from Novikova (11) Martite-hematite 0.70 1.39 $0.69 79.0 78.20 112 1.13 2.03 +o. 90 Foyaite 6.40 5.14 46 GA Granite 1.26 1.70 +O. 44 2.81 1.41 1.12 Syenite diorite 5.80 5.50 -0.30 8.80 2.36 0.41 BR Basalt 6.40 7.06 $0.56 12.91 5.80 0.91 Variolite 12.69 12.94 $0.25 16.53 2.43 0.19 Olivine Gabbro Norite 16.52 16.18 -0.34 19.11 0.76 0.05 Garnet 17.33 16.71 -0.62 20.27 1.02 0.06 Ilmenite 42.28 41.19 -1.09 50.93 3.96 0.09 Metadiabase 17.90 16.72 -1.18 22.08 2.19 0.12 a Per cent FeO fusion decomposition minus per cent FeO acid decomposition. b Acid decomposition. c (10). d (16).
z
z
and Novikova found that the per cent FeO increased with increase in fusion time. For AGV-1, Donaldson found that FeO increased from 2.80 to 2.89% after a n additional 30 minutes and then decreased to 2.63 % after another 60 minutes. Novikova, however, found a n increase from 1.90% FeO to 2.67 FeO after a n increase of but 3 minutes for the fusion of foyaite. Because of this difference in fusion time and the difference in the solution of the fusion melt (with and without the presence of excess oxidant), it is very difficult to compare
z
z
and correlate directly my results with those of Donaldson and Novikova. The data presented show that for most purposes this method is sufficiently accurate for the semimicrodetermination of ferrous iron in most refractory minerals. RECEIVED for review January 26, 1970. Accepted May 4, 1970. Publication authorized by the Director, U.S. Geological Survey.
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