923
Anal. Chem. 1981, 53, 923-924
ACKNOWLEDGMEMT I thank M. E. Vernon and W. H. Con1e;y for their excellent technical assistance, H. M. Kunishi for helpful discussions, and Monsanto Chemical Co., St. Louis, MO, for supplying the glyphosate.
LITEIRATURE CITED (1) Arkansas Cooperative Extension Service, "Recommended Chemicals for Weed and Brush Control"; University of Arkansas' Division of Agrlculture, 1979; p 75. (2) McKlbben, G. E., Proceedings of the 32nd Annual Corn and Sorghum Research Conference, Chlcago, IL, Dec 1977; American Seed Trade Association: Washington, DC, 1977; pp 72-79. (3) Rueppel, M. L.; Brlghtwell, B. 8.; Schaefer, J ; Marvel, J. T. J . Agric. Food Chem. 1977, 25, 517-528. (4) Bronstad, J. 0.; Friestacl, H. 0. Analysf(London) 1976, 107, 820-824.
15) . , Elkstrom. G.: Johansson, S. Bull. Envlron. Confamln. Tox/co/. 1975, 14, 295-296. (6) Ragab, M. T. H. Chemosphere 1978, 7 , 143-153. (7) . . Rueppel, M. L.; Suba, L. A.; Marvel, J. T. Homed. Mass Specfrom. 1976; 3, 28-31. (8) Yoza, N.; Ishibashi, K.; Ohashi, S. J . Chromafogr. 1977, 134, 497-506. .- . - - - . (9) Hosokawa, I.; Ohshima, F. Water Res. 1973, 7 , 283-289. (IO) Sprankle, P.; Meggitt, W. F.; Penner, D. Weed Scl. 1975, 23, 229-234. (11) Glass, R. L., unpublished data, 1979.
RECEIVED for review November
19,1980. Accepted February 2, 1981. Trade names are used for the convenience of the reader and do not constitute any preferential endorsement by the U.S.Department of Agriculture over similar products available.
Spectrophotometric Determination of Reduced and Total Iron in Glass with 1,I0-Phenanthroline David R. Jones IV," William C. Jansheskl, and Don S. Goldman Owens-Corning Fiberglas Corporation, Technical Center, Granvllle, Ohio 43023
T h e reduced and total iron content in glasses is an important parameter in the glass industry. Ferrous iron in glass is a powerful absorber of long wavelength radiation. Window glass containing ferrous iron will retard heat loss in buildings through low-temperature radiation. Conversely, it will keep out some solar radiation in warm weather. This is an important consideration in building more energy efficient homes (1). Iron as Fe3+ imparts a yellow color i o glass, while Fez+ imparts a greenish blue color. Since in practice both species exist simultaneously in glass, knowledge of the iron species present is important in controlling the final properties of the glass. There are presently two separate methods for the determination of iron species in glass. Total iron (FezO3) is determined by the ASTM procedure (2),which is a spectrophotometric method employing 1,lO-phenanthroline. Ferrous iron (FeO) is usually determined by an oxidation-reduction method such as the method of Close et al. (3). This method requires a platinum dirih and inert atmosphere (COJ for digestion, followed by a titration using ceric sulfate. The end point is monitored colorimetrically by use of ferroin indicator or potentiometrically with platinum electrodes (4). Other methods suggePited for the determination of FeO in glasses include the use of 0.05 N potassium dichromate as a titrant, with a diphenylamine end point (5), back-titration of an excess amount of ceric sulfate with ferrous ammonium sulfate (6),and several types of electrochemical determinations of FeO and Fe203 (7-9). Additionally, several papers have been published on the measurement of FeO absorbance a t approximately 1100 n m in glass slabs (3, 10-14). The method described in this paper is an adaptation of a method by Begheijn (15)for the determination of reduced iron in rock, soil, and clay. The method elirninates the use of platinumware and requires no inert atmosphere during the digestion. A single sample is used to determine both reduced and total iron, resulting in a time savings of a t least 50% over t h e earlier methods. The original paper of Begheijn demonstrated excellent recoveries of FeO from USGS Geological Standards. T o further corroborate the agreement between this work and earlier methods, we undertook a study of the absorbance due 0003-270M/81/0353-0923$01.25/0
t o Fez+ in polished slabs of glass vs. the colorimetrically analyzed FeO content of those glasses. The early work of Bacon and Billian (10) demonstrated the correlation of absorbance at around 1100 n m vs. FeO content determined wet chemically. Review articles by Bamford (11)and Tomozawa and Doremus (12) confirm this and indicate the variation of the absorption coefficient for FeO as a function of the chemical composition of the glass. Generally, the absorptivity for FeO is greater in glasses containing more alkali (11).
EXPERIMENTAL SECTION Apparatus. The sample digestion was carried out in 100-mL Teflon evaporating dishes, which are available from the major chemical supply houses. Six samples and a blank can be run a t the same time. Reagents. Deionized water was used to prepare all solutions. The 1,lO-phenanthroline solution (0.25% (w/v)) should be stored in an amber bottle and discarded if the solution becomes colored. The potassium hydrogen phthalate (KPH) buffer solution (0.5 M) and the boric acid solution (4% (w/v)) are stable and can be stored in glass or plastic containers. The iron stock solution (LOO0 g/L as Fe) can be prepared from iron wire, or 1000 ppm iron atomic absorption standard may be used. The iron standard solution (0.01 g/L as Fe203)is prepared by using volumetric pipets to transfer 7.00 mL of iron stock solution to a 1-L volumetric flask. Five milliliters of concentrated HC1 is added and the mixture diluted to the mark with deionized water. Procedure. Samples to be analyzed were ground to pass a 100-mesh sieve and magnetically separated if they had been in contact with steel or iron apparatus. A 150-mLglass beaker and a 100-mL glass volumetric flask were prepared for each sample and the blank before beginning the analysis. Each beaker contained a stir bar, 15 mL of boric acid solution, 10 mL of KHP buffer, 4 mL of phenanthroline solution, and 2 mL of concentrated ammonium hydroxide. A 100-mg sample was weighed to the nearest 0.1 mg into a clean, dry Teflon dish. It has been found that occasionally a static charge will build up on the Teflon dishes, resulting in the sample scattering when put into the dish. Wetting the dish with a few milliliters of water and then shaking it dry alleviates this static buildup. In a fume hood, 0.5 mL of concentrated sulfuric acid was added to each dish. The dish was swirled to completely wet the sample, and tilted so that the sample and acid were in a single "bead" at the side of the dish. A 1.5-mL 0 1981 American Chemlcal Soclety
924
ANALYTICAL CHEMISTRY, VOL. 53, NO. 6, MAY 1981
Table I. Comparison with the Ceric Sulfate
Table 11. FeO Concentration vs. Absorbance at 1100 nm
Titrimetric Method glass sample no. A
B C D
glass 1 FeO, “2
titrimetric
this work”
0.07 0.114 0.164 0.142
0.071, 0.077 0.112, 0.113 0.161, 0.164 0.136, 0.137
a Same sample submitted to analyst “blind”, 2 weeks apart
portion of hydrofluoric acid was cautiously added to the other side of the dish, and the two acids were allowed to run together and mix. The H F reacted vigorously with the silica in the sample. After the reaction had subsided (-15 s), the dish was swirled to ensure complete mixing. There is some technique involved in the initial digestion to prevent spattering of the sample when the hydrofluoric acid is added. Allowing the acids to mix very gently overcomes the problem. After digestion was completed, 10 mL of boric acid solution was added, and again the dish was swirled to mix. The contents of the dish were quantitatively transferred to the previously prepared beaker, rinsing with deionized water. By use of a pH meter and ammonium hydroxide, the pH was adjusted to between 3.3 and 3.5 while stirring. The contents of the beaker were quantitatively transferred to a 100-mL volumetric flask and diluted to the mark. If the diluted sample was cloudy, a portion was filtered through medium filter paper, and the absorbance due to FeO measured in a 1-cm cell at 510 nm, against a water blank. In the case of both the FeO and Fe203determinations, the cloudiness is due to precipitation of the KHP buffer and not incomplete dissolution of the sample. Total Fe203was next determined by adding approximately 20 mg of hydroquinone to each of the 100-mL volumetric flasks prepared above, shaking to mix, and allowing the color to develop for 30 min. After this time, the absorbance due to total iron was measured at 510 nm against a water blank. If the solution is cloudy after addition of the hydroquinone and color development, a portion can be filtered as above, and the filtrate used for the absorbance measurement. Calibration Graph, A calibration graph was constructed by preparing five 150-mL beakers with the following (added in the order given): stir bar, 25 mL of boric acid solution, 10 mL of KHP buffer, 4 mL of phenanthroline solution, 2 mL of ammonium hydroxide, 0.5 mL of concentrated sulfuric acid, and 1.5 mL of hydrofluoric acid. By use of volumetric pipets, 0, 10.0, 20.0,30.0, and 40.0 mL of the iron standard was transferred to the beakers. The pH was adjusted to 3.3-3.5 as above, and the beaker contents were quantitatively transferred to 100 mL volumetric flasks. A 20-mg portion of hydroquinone was added, and the flasks were diluted to volume. The color was allowed to develop for 30 min, the samples were filtered if necessary, and the absorbance was measured at 510 nm against a water blank. After we corrected for the reagent blank, the absorbance was plotted vs. milligrams of FeO and Fe203. Each 10 mL of iron standard solution contains 0.100 mg as Fe203or 0.090 mg as FeO. Standard practice in Owens-Corning Fiberglas laboratories is to calculate an average slope (mg/A) of the FeO and Fe203data and use these slopes and the sample weights to directly calculate the percent FeO and total Fe20s in the glass sample. RESULTS AND DISCUSSION Recovery of the method is good, although there are no standard glasses available with certified values for FeO. Comparison of FeO data for the titrimetric method and this
glass 2
A/cm
% FeO
A /cm
% FeO
1.94 1.44
0.303 0.223 0.172 0.176 0.092
1.33 0.68 0.45 0.19
0.153 0.082 0.057 0.025
1.11 1.09 0.57
covariance S,. = 0.03 90, correlation coeff y x y = 0.9993
,S , yxy
= 0.0265
= 0.9997
Table 111. Comparison with the ASTM Procedure glass sample no. 1 2 3 4 5
total Fe,O, ASTM this worka 0.24 0.33 0.45 0.453 0.38
0.242, 0.239 0.3 29, 0.337 0.451, 0.461 0.451, 0.457 0.380, 0.385
a Same samples submitted to analysts “blind”, 2 weeks apart
method shows good agreement (see Table I). In this work, FeO content for two borosilicate glass compositions was determined by using the wet-chemical colorimetric method. Absorbances were measured at 1100 nm for solid pieces of the same glass. Glass 1 contains less than 1% NazO, and glass 2 contains around 15% NazO. T h e absorbance vs. FeO concentration data are shown in Table 11. These data show the linear correlation between the FeO values determined by this method and the absorbance at 1100 nm. T h e recoveries of total iron as compared t o those using t h e ASTM procedure demonstrate that sample digestion is complete (see Table 111). LITERATURE CITED Hayes, D. Chem. Eng. News 1980, 58,28. Lukens, R. P., Ed.; “Annual Book of ASTM Standards”, 1980 ed; Amerlcan Society for Testing and Materials: Philadelphia, PA, 1980;Part 17,Standard Cl69-175, Sectlon 18.
Close, Paul; Shepherd, H. M.; Drummond, C. H. J .
Am. Ceram. Soc.
Close, Paul T.; Hornyak, E. J.; Baak, T.; Tillman, J.
F. Mlcrocbem. J .
1958,41, 455-460.
1968, 10, 334-339. Shchegiova, M. D.; Maksimovich, S. I. Steklo Keram. 1972, 9 , 36; Chem. Abstr. 1977, 155715a. Abed, U. Anal. Chlm. Acta 1989, 4 7 , 495-502. Beyer, M. E.; Bond, A. M.; McLaughlin, R. J. W. Anal. Chem. 1975, 45, 479. Moore, W. M. Anal. Chim. Acta 1979, 105, 99. Dieker, J. W.; Van Der Linden, W. E. Anal. Chlm. Acta 1980, 114, 267-274. Bacon, Frank R.; Bllllan, Carroll J. J . Am. Ceram. Soc. 1954, 37, 60-66.
Bamford, C. R. In “Glass Science and Technology”; Elsevier: Amsterdam, 1977;Vol. 2, p 35-38. Tomozawa, M.; Doremus, R. H. In “Treatiseon Materials Science and Technology”; Academic Press: New York, 1977;Voi. 12, pp 45-48. Paul, A. Glass Techno/. 1985, 6 , 22-25. Zaman, M. S.;Paul, A. Glass Techno/. 1969, IO, 93-98. BeghelJn, L. Th. Analyst (London) 1979, 1055-1061.
RECEIVED for review September 12,1980. Accepted February 12, 1981.
