ANALYTICAL CHEMISTRY
326 (3) Assoc. 0 5 c . Agr. Chemists, “Official and Tentative Methods of Analysis,” 6th ed., p. 600, 1945. (4) Buxton, L. O., IXD. ENG.CHEM.,AXAL.E D . , 11, 128 (1939). (5) Fraps, G. S., and Kemmerer, A. R., Ibid., 13, 806 (1941). (6) Kemmerer, A. R., and Fraps, G. S., Ibid., 15, 714 (1943). (7) Kemmerer, A. R., and Frapa, G. S., J . Am. Chem. Sac., 6 6 , 305 (1944). (8) Polgar, A . , and Zechmeister, L., Ibid., 64, 1856 (1942).
(9) Silker, R. E., Schrenk, W. G., and King, H. H., IXD.EXG. CHEM., ANAL.ED.,16, 513 (1944).
Strain, H. H., “Chromatographic Adsorption Analysis,” p. 141, New York, Interscience Publishers, 1943. Tall, M . E . , and Kelley, E . C., IND.ENG.CHEM.,A N ~ LED., . 15, 18 (1943). Zscheile, F. P., and Brunson, A. M., J. Am. Chem. (12) White, J. W., Sac., 64, 2603 (1942). (13) Zechmeister, L., and Cholnoky, L., ”Principles and Practice of Chromatography,” p 112, Xew York, John Wiley & Sons, 1944. (14) Zbid., p. 115.
RECEIVED Octoher 20,
1948.
Determination of Total Iron in Silicates and Other Nonmetallic Materials MASKIEL R. SHELL, Electrotechnical Laboratory, Bureau of Mines, Norris, Tenn. A method is presented for determining total iron in various refractory oxides, silicates, limestone, and fluorspar. The sample is fused in a cast silver crucible, and the iron is determined colorimetrically with o-phenanthroline. No separations are made. Accurate results are obtained with a variety of materials. Crucibles of platinum or platinum-3.5qo rhodium are not suitable for the fusion because they retain an indefinite amount of the iron.
D
URISG the operation of an experimental electric arc fur-
nace for melting various refractories, the need arose for a rapid and accurate method of determining total iron. 9 colorimetric method was judged to be best suited for the quantity of iron involved. Of the reagents suitable for colorimetric determination of iron, o-phenanthroline appeared to be the best. It is stable, colorless, and water-soluble, and is a sensitive and selective reagent for ferrous iron, which M orks equally well betm een pH 2.5 and 8.5. The color conforms to Beer’s law and under some conditions is so stable that after 6 months’ standing (plus 100 hours under ultraviolet radiation) there is no appreciable change in either hue or intensity ( 5 ) . .4n iron-sensitive phenanthroline, 2-methyl-( 1,lO)-phenanthroline, was first discovered by Gerdeissen (6). However, he did not mention the ferrous iron complex, which was first reported by Blau (1,8 ) , who in 1898 synthesized o-phenanthroline ( 3 ) . Application of the ferrous iron-o-phenanthroline complex as an oxidation-reduction indicator did not come until 1931, when it was introduced by Walden, Hammett, and Chapman (IO). Use of o-phenanthroline as a reagent for ferrous iron was reported by Saymell and Cunningham in 1937 (9) and by Hummel and Willard in 1938 (8). I n 1938 an exhaustive study of the reaction was made by Fortune and Mellon ( 5 ) . Since 1938, numerous reports have appeared on further applications of o-phenanthroline as a redox indicator and as a reagent for ferrous iron. At present, o-phenanthroline is the most widely used reagent for determining small amounts of ferrous iron, or, after reduction, of total iron. Bowen and Schairer (4)found that platinum crucibles removed too much iron to be usable in a phase-equilibrium study of the ferrous oxide-silica system. They stated that platinum has a marked disadvantage for this purpose. Except under strongly oxidizing conditions it reduces iron from its compounds and takes iion into solid solution; in all cases some iron was removed from the charge by the platinum crucible (4). The mixtures used by Bowen and Schairer were very high in total iron content and were not in contact with a large excess of flux. These conditions were different from those of the present study.
EXPERIMENTAL
Inasmuch as solution of the samples in hydrofluoric and other acids is not necessarily complete, a fusion is necessary. However, when a fusion with sodium carbonate or mixtures of sodium carbonate and sodium borate pentahydrate is made in platinum crucibles, the results for iron are totally unreliable. The platinum crucibles themselves remove part of the iron from the sample. This phenomenon works a twofold disadvantage: Assuming a clean platinum crucible to start with, iron is lost to the crucible from the first sample, and lorn results are obtained; and this iron is retained in the crucible and released to later samples, particularly those low in iron. If a fusion method \?-eret o be successful, other types of crucibles had to be used. Sickel ions from nickel crucibles interfere with the colorimetric determination of ferric oxide ( 5 ) . Porcelain or quartz crucibles are not sufficiently resistant to the melt, and alumina, titanium dioxide, and zirconium dioxide crucibles are too porous. Thick-walled silver crucibles proved to be the best of those tested. SPECIAL EQUIPMENT
+
4 rrucible cast from 99.9 fine silver is needed, although one could be turned on a lathe from a silver rod. Dimensions of a 40-ml. crucible are: height, 4.5 cm.; inside diameter, average, 2.8 cm. with taper; thickness of wall and bottom, 3 mm. The silver is melted in a graphite crucible and then poured into a graphite mold machined to give the above dimensions. PROCEDURE
Grind the sample to a fineness that eliminates nonhomogeneity as a factor in the analysis. This fineness will always be a t least -200-mesh and may be lower. A hard, iron-free mortar and pestle of agate, mullite, or boron carbide is recommended. For samples with 0.2 to 10% iron, take a 100.0-mg. sample; for samples xvith 0.00 to 0.2’33 iron, take a 300.0-mg. sample. With either, weigh accurately, and transfer to a thick-walled, silver crucible, which should be of about 30- to 50-ml. capacity. Add 1.0 gram of sodium carbonate and 1.0 gram of sodium borate penta- or decahydrate and mix thoroughly. Heat over a lo^ flame until water is driven off and the flux has melted. Finish dissolution of sample by briskly heating the crucible, a t the same time rotating it in the flame with a pair of tongs. When fuqion is
327
V O L U M E 2 2 , N O . 2, F E B R U A R Y 1 9 5 0 Table I .
Loss of Ferric Oxide from Sample to Clean
Platinum-3.57~Rhodium Crucibles >.,I
I1Illr
T e 2 0 j Prruent,
Fen08 rollnd.
Difference,
1 2Q
0.5 3.3 4.7 5.2 1 . 6.5 4.20
-2.7 -1.3 -0.8 -0.39 -1.40
('or dieri te Cordierite Cordierite Cordierite Plastic clai- 9si. Ha,irite (i96 a
s;c
7c
6 0"
6 O' 6 oa ? 04 .a 6 ci
70 - n_ .7.
Determined bl- standard titration vrocedures ( 7 ) .
b BiireaiI of Standards saniple.
__~ ~
~-
-
Table 11. Gain of Ferric Oxide into Samples from Ferric Oxide-Contaminated Platinum-3.570 Rhodium Crucibles Fezor Present,
I-egOa F o u n d ,
Ilifferenre
?amplea Argillaceous limestone 1.4
1.63
1 . 73
+ 0 . 10
1 68
2 14 -..I
+n. .iii ..
Burnt refractory 77 Flint clay 97 Phosphate rock 56.4 Silica brick 102
0.90 0.98 2.18 1).66
1.25 1,67 3,56 1.00
+0.35 + O . 69 t1.38 -0.34
5
C7r
lar moment. After the melt has been removed from the crucible, several heatings in an oxidizing atmosphere, with intervening leaching with hot hydrochloric acid, are usually necessary to remove completely the iron held by the platinum in solid solution. The iron is diffused to the surface of the crucible, where it is oxidized and can be removed. In the analysis of iron-bearing rocks and minerals where a sodium carbonate fusion has been made in platinum, the use of the same crucible for both the ignition of the silica and the original fusion is a necessity. Furthermore, the crucible should be heated in an oxidizing atmosphere after solution of the nonvolatile matter from the silica to ensure complete recovery of iron. Any blackening of the crurible on rehpating usually indicates iron.
R
Table 111.
Direct Colorimetric Estimation of Ferric Oxide after Fusion in Silver Crucibles"
Electrotrchnical Laboratory bamples
FenOa Prebent, c'o
FezOa Foundb, %
+
complete, run the melt onto the side walls of the crucible. Thip practice aids solution; It is well to remember that the melting point of silver is 960.0 C. Cool to room temperature in air or in a water bath. Dissolve directly in the crucible, by using 25 ml. of 1 to 4 hydrochloric acd ( 1 part of concentrated hydrochloric acid to 4 parts of water by volume) and setting the crucible on a hot plate. Do not place the crucible in a beaker for solution of sample. Flakes of silicic acid may settle out from samples high in silica, but they apparently do no harm. When solution is complete make to 100 ml. in a volumetric flask. Silver chloride, which comes from reaction of the dissolved silver and chloride, will turn the solution cloudy and must be removed. There are several ways of doing this: by centrifuging, by filtration, and by gravity settling. Silver chloride must be complete1)- removed by one of these methods. Khatever the method, an aliquot of the clear liquid is used for analysis. If iron is 0.Z70 Fez03or lower, as much as 75 ml. should be used. If iron is higher, the aliquot should hr within the range of the instrument used for measuring color intensitj-. I n either case, a transfer pipet should be used. Transfer the sample t o a 100- or 150-ml. beaker, and add 2.5 nil. of 10% hydroxylamine hydrochloride solution and 20 ml. of 0.1% o-phenanthroline solution (both by weight in distilled water). Vsing a glass electrode pH meter, adjust the pH of the solution to 2.5 to 3.0 with a sodium carbonate-free sodium hydroxide solution. Transfer to a 100-ml. volumetric flask, make to the mark, and mix. Measure color intensity with any suitable colorimetric equipment. With a Beckman spectrophotometer, the absorption \vas at a maximum a t 512 mp. Compare the color intensity with previously prepared standards, and calculate per cent ferric oxide. DISCUSSIOR AND DATA
The most commonly used crucibles of the analytical laboratory-those of platinum and /or platinum-3.5% rhodium-could not be used for the fusion. \Then clean platinum-3.5% rhodium crucibles are used, the samples lose iron to the crucible, giving the low results shown i n Table I. However, if platinum-rhodium crucibles, which are apparently clean, but actually are contaniinated with iron. are used, the samples gain iron from the crucible, giving the high results shown in Table 11. The iron contamination is evident on heating for 20 to 30 minutes at 1OOO" C in an olidizing atmosphere. These data clearly indicate that platinum-rhodium crucibles cannot be used for the fusion unless, after solution and removal of the melt, the crucibles are reheated and leached with hot mid until all iron lost to the crucible is recovered. The analyses of nonmetallics in platinum crucibles reported i n Tables I and 11 show that the iron compounds present in the melt are in equilibrium n i t h metallic iron ( 4 ) The platinum rmcible actually acts as a reservoir to take up or release iron to the melt. according to the equilibrium conditions a t the particu-
(corundum niiillite) 232 Electrocast refractory (corundum mullite) 2321 Kenya kyanite, 2637 Cordierite 1 Cordierite l a Cordierite 3 Ball clay 2290
+
0.72 0.34 1.20 1,20 6.00 0.71
0 . ti8 0.36 1.20 1.25 6.05 0.72
1.63 2.18 5.66 0.03 0.90 0.13 0.084 0.081 2.05 0.067 0.60 7.06 0,039
1.60 2.11 5.71 0.032 0.92 0.12 0.081 0.073 2.07 0.073
Bureau of Standards samples .4rgillaceous limestone 1.4 Phosphate rock 56a Bauxite 69 FeldsDar 70
0.68 6.98 0.040 ,lea) .... 0.007 a A b o u t one half of determinations are single, remainder are averages of two o r more values. b "0 1 ; ~ ~ Ofound a corrected for blank.
;\Ian)- types of crucibles were tried but eliminated ior various reasons, such as interference with colorimetric reaction, insufficient chemical resistance, and too high porosity. Silver crucihies proved to be the best available. Gold crucibles might be satisfactory, but no data are av:rilai)le at this time. The results obtained with silver crucibles by using the procedure on a number of Electrotechnical Laboratory and Bureau of Standards samples are shov-n in Table 111. The agreement between ferric oxide present and found is shown to be excellent, but results such as these are obtained only after considerable effort to obtain suitable iron-free silver crucibles. Some of the commercial thinwalled, spun-silver crucibles do not have a sufficiently low blank, even after several fusions, to be usable for the lower iron samples. The thin-walled-type crucibles are easy to overheat in spots, with resultant holes, and to crack a t strains that might have been formed during the spinning process used in making them. K i t h the cast crucibles, a thickness of 3 mm. was enough to give such uniform heating that after 50 fusions the crucibles were in excellent condition, with no evidence of cracking or melting. Some of the crucibles had high initial blanks which may have been caused by a slight contamination with iron from the graphite crucible and mold. A crucible and mold other than graphite, such as refractory oxides, might be more satisfactory. However, after four fusions to clean the crucible, the blank remained constant at 0.007 (+0.001) yoferric oxide. Because some of the samples contained only 0.09 mg. of ferric oxide, precautions had to be taken to prevent contamination of the sample. Dust n-as carefully avoided. To prevent contami-
ANALYTICAL CHEMISTRY
328
nation of the crucible with other metals, the crucibles were held with silver-wrapped Nichrome tongs and set on a silver sheet. Solution of the fusion was always accomplished by pouring the acid into the crucible-in no case was the crucible set in a beaker. Once the crucible was clean, these precautions helped to produce a low and constant blank. The silver must be removed from the solution before addition of the o-phenanthroline, because a chemical reaction takes place which produces a precipitate. Therefore, no advantage would be obtained by the use of any acid other than hydrochloric acid to dissolve the melt without precipitating any silver salts. Of the methods for removing silver chloride, centrifuging is the easiest and fastest; a clear supernatant liquid is obtained in a few minutes. Centrifuging was the method largely used in this study. After coagulation, the precipitate was easily removed by filtration. If sufficient time were allowed, the precipitate settled ~ J Ygravity alone. With samples high in silica, such as feldspar or silica brick, a precipitate of silicic acid was obtained, but it apparently did not interfere in any way. It was removed along with the silver (ahloride. Buffers of ammonium acetate are used frequently in iron determinations to produce the correct pH, but these reagents cannot be used under the conditions described. The ammonium acetate would produce a heavy gel of silicic acid that would interfcre seriously. The pH of the solution must be adjusted with a glass-electrode pH meter, or by other means that introduce no e\traneous color or interfering ion. Ahcurateresults were obtained with a wide variety of samples, including various silicates, phosphates, and fluorspar. The number of ions interfering with the determination, and the extent of their interference, arc low ( 5 ) . Among ions that might interfere somewhat are nickel, bismuth, molybdate, tungstate, and cyanide. If present a t all, the percentage of these ions T\ ould be very low in the materials for which this procedure was tried and for Jvhich it is recommended. However, any change in hue or any precipitation should serve as a warning for further
study of the material. Silicon carbide, which is apparentiv not wetted very well by the molten flux, could not be completely decomposed a t the temperature used. Single determinations have been made in 15 minutes when all apparatus was ready and when the removal of silver chloride caused no difficulty. The method is easily applied concurrently to a large number of samples. CONC LU SlON
Platinum-rhodium crucibles cannot be used for the precise estimation of iron after a sodium carbonate-borate fusion unless the iron lost to the crucible is recovered by repeated heating and leaching with hot acid. €Ion-ever, excellent results without loss of iron are obtained with a wide variety of materials when the fusions are made in thick-walled cast silver crucibles. ACKNOWLEDGMENT
The author wishes to extend his thanks to Lloyd L. Hall for some of the determinations. LITER.4TURE CITED
(1) Blau, F., Ber., 21, 1077 (1888) (2) Blau, F., Monatsh., 10, 376 (1889). (3) Ibid., 19, 666 (1898). (4) Bowen, N. L., and Schairer, J. F., Am. ,J. Sci., 24, 184 (1932). ( 5 ) Fortune, M’. B., and Melloii. 41.G . , ISD. ENG.CHEJI.,-41.1~
ED.,10, 60 (1938). (6) Gerdeissen, Ber., 22, 245 (1889). (7) Hillebrand, IT.F., and Lundell, G. E. F., “Applied Inorganic bnalvsis.” New York. John Wilev 8: Sons. 1929. (8) Hummil, F. C., and Willard, H. H:, ISD. ENG.CHEM., ANAL. ED.,10, 13 (1938). (9) Saywell, L. G., and Cunningham, B. B., Ibid., 9, 67 (1937). (10) Walden, G. H., Hammett, L. P., and Chapman, R. P., J . Am. Chem. Soc., 53, 3908 (1931). June 1, 1919. W o r k done in cooperation with t h e Tennessee Valley Authority.
RECEIVED
Photometric Determination of Molybdenum by Acetone Reduction of the Thiocyanate ROSCOE ELLIS, JR., AND R . Y. OLSON ICunsus Agricultural Experiment Station, Manhattan, hkn.
A photometric method for the determination of small amounts of molybdenum in solutions is presented. 4 s in previous methods, molybdenum is determined by the yellow-amber color of its thiocyanate. The use of acetone as a reducing agent increases the sensitivity and eliminates the rapid fading of the color complex encountered when other reducing agents are used.
T
HE yellow-amber color developed by molybdenum, a thio-
cyanate salt, and a, reducing agent has been used for a number of years for the colorimetric determination of small amounts of molybdenum. Stannous chloride has been the most common reducing agent used. Hurd and Allen ( 2 )made a study of solvent extracting solutions, concentrations of reagents, and other variables involved, and determined the conditions which allowed the maximum color development and minimized the rate of fading of the color complex. Grimaldi and Wells ( 1 ) dispensed with solvent extracting solutions and developed the color in a water-acetone solution using stannous chloride as a reducing agent. This method stabilized the color somewhat. I n trying to determine the amounts of molybdenum in soils, the above methods were used by the authors and attempts were made
to evaluate the color with an Evelyn photometer. Rapid color fading was found to occur n?th each method. Although it was possible to standardize the procedure somewhat by establishing a fixed period of time before the readings were taken, it was thought highly desirable to find a reducing agent that would give color stability over a long period of time. It was felt that color stability would certainly improve the accuracy and convenience of the determination. Therefore, a method was developed in which acetone is used directly as a reducing agent and by which color stability is obtained for a period of 48 hours. The sensitivity of the determination is also increased over the two previous methods. REAGENTS
Potassium thiocyanate (water solution). Dissolve 10 grams of potassium thiocyanate in 100 ml. of distilled water.
