Anal. Chem. 1981, 5 3 , 193-190 (5) Blaedel, Walter; Jenkins, Rodger Anal. Chem. 1875, 47, 1337-1343. (6) Gullbault, George 0.; Cserfalvi, Tamas Anal. Left. 1876, 9 , 277-289. (7) Thomas, Lawrence C.; Christian, Gary D. Anal. Chlm. Acta 1975, 78, 271-276. (8) Christian, Gary D. “Ion and Enzyme Electrodes in Biology and Medicine”; Urban. and Schwarzenberg: Muchen-Berlin-Weln, Germany, 1976. (9) Smith, Marilyn D.; Olson, Carter L. Anal. Chem. 1875, 47,
-
-
i.n 7 . ~. - 1. 0.7,7.
( I O ) Smith, Marilyn D.; Olson, Carter L. Anal. Chem. 1974, 46,
1544-1 547. (1 1) Thomas, Lawrence C.; Christian, Gary D. Anal. Chim. Acta 1876, 82, 265-272. (12) Cheng, Frank S.; Christian, Gary D. Anal. Chem. 1977, 49, 1785-1788. (13) Cheng, Frank S.; Crlstian, Gary D. Clin. Chem. (Winsfon-Salem, N. C.) 1878, 24, 621-626. (14) Gullbault, George G. “Theory, Deslgn and Biomedical Applications of Solid State Chemiail Sensors”; CRC Press: Cleveland, OH, 1978, p 193. (15) Barker, A. S.; Somers, P. J. Top. Enzyme Ferment. Blofechnol. 1978, 2 , 120. (18) Peel, J. L. “Methods In Mlcroblology”; Academic Press: New York, 1972.
193
(17) Purdy, W. C. “Electroanalytical Methods in Blochemisby”; McGraw-Hill: New York, 1965. (18) Thomas, Lawrence C.; Chrlstlan, Gary D. Anal. Chim. Acta 1875, 77, 153-161. (19) Nicholson, Rlchard S.; Shain, Irving Anal. Chem. 1964, 38, 706-723. (20) Adams, Ralph N. “Electrochemlstry at Solid Electrodes”; Marcel Dekker: New York, 1969. (21) Manual of Model 174A, Polarographic Analyzer; Princeton Applied Research: Princeton, NJ, 1977. (22) Reo, P. S.; Evans, R. G.; Mueller, H. S. Biochem. Blophys. Res. Commun. 1977, 78, 648-654. (23) Peck, H. D., Jr.; Deacon, T. E.; DavMson, J. T. Blochim. Blophys. Acta 1865, 96, 429-446. (24) S P S Z , Gabor; Gerhardt, Wlllie; Gruber, Wolfgang; Bernt, Erlch Clin. Chem. (Wlnston-Salem, N.C.) 1976, 22, 1806-1811. (25) Szasa,Gabor; Gruber, Wolfgang; Bernt, Erlch Clin. Chem. (WinsfonSalem, N.C.) 1978, 22, 650-656. (26) Szasz, Galln; Gerhardt, Wlllle; Gruber, Wolfgang Clin. Chem. ( Winston-Salem, N.C.) 1877, 23, 1888-1892. (27) Yuan, C. L.; Kuan, S. S.; Guilbault, G. G. Anal. Chlm. Acta, In press.
Received for review August 14,1980. Accepted October 31, 1980.
Simultaneous Determination of Iron(II), Iron(III), and Total Iron in Sphagnum Moss Peat by Programmable Voltammetry on a Graphite Tubular Electrode F. J. Srydlowskl” and D. L. Dunmire Raltech Scientific Servhces, St. Louis, 900 Checkerboard Square Plaza, St. Louis, Missouri 63 188
E. E. Peck and R. L. Eggers Raiston Purina Company, Health Industries Products Division, Bridgeton, Missouri 63044
W. R. Matson Environmental Sciences Associates, Inc., 45 Wiggins Avenue, Bedford, Massachusetts 0 1730
An integrated step scan voltammetric program was used In conjunction with a pyrolytic graphite tubular electrode to determine the concentration and oxidation state of iron in sphagnum moss peat without Iron(111) reductive errors due to organic matter. Good correlation was obtained by this method when compared with colorimetric ( f = 0.973) and atomic absorptlon spectrometric ( r = 0.978) procedures; total iron results were compared to the sums of irorr(I1) and iron(111) obtained electrochemically. Total iron recoveries from sphagnum moss peat samples spiked with iron( 11) sulfate ranged from 88% to 109%. Copper interference was not slgnlficant at levels found in thee sphagnum moss peat.
The objective of this study was to develop a reliable iron speciation method for sphagnum moss peat used as a medication carrier for neonatal pigs. Neonatal pigs gain weight rapidly going from 3 to 12 Ibs in 3 weeks. If the pigs do not have access to soil (taken away by modern husbandry techniques) or other sources of iron, anemia can occur causing retarded growth, lower disease resistance, and death (1). Sphagnum moss peat is a complex matrix composed primarily of humus residues and numerous metal complexes (2). Iron’s oxidation state in sphagnum moss peat has been given some attention since animal nutritionista regard the iron(I1) species as the primary biologically usable form of iron (1,3). 0003-2700/81/0353-0193$01.00/0
If the iron(I1) concentration in a sphagnum moss peat product can be accurately determined, a supplemental amount of iron(I1) sulfate could be added to bring the product up to the potency level needed as an anemia-prevention agent. Additionally, iron(II1) salts are toxic, causing lesions in animal’s stomachs (4). If uncomplexed, an organism is able to reduce only a small portion of the iron to its biologically active form. There then is a need for an iron speciation method in the feed industry not only for potency considerations but also for toxicological considerations. A method developed by Sullivan (5) using a,a’-bipyridyl and colorimetric analysis of iron(II), iron(III), and total iron in drugs was used in the preliminary studies of sphagnum moss samples. Due to the complexity of the peat matrix, it was necessary to modify the procedure (5) to permit iron quantitation. Metal is fixed in peat’s humic acids by adjacent phenolics and benzene carboxylic acid groups which must be degraded to release iron for analysis. These additional degradation steps lengthened the procedure more than desirable for routine laboratory use and simpler methods were sought. Degradation also affected iron(II), iron(II1) ratios because of organic matter (6). Various polarographic (7)and electrochemical techniques ( 4 9 ) of determining iron(II)/iron(III) levels were investigated. Since on a graphite electrode, iron ions are reduced from solutions containing the complex elemental ions to the metal, they can be determined by anodic stripping techniques 0 1981 American Chemical Society
194
ANALYTICAL CHEMISTRY, VOL. 53, NO. 2, FEBRUARY 1981
analog equivalent to an X-Y recorder. Thus, each point on the scan represents approximately
Cell Head
Stirring (Graphite) Shalt
--&
Counter Electrode Wire
-Reference
Electrode Wire
-
Counter Electrode shalt (Teflon)
I I-
Test Electrode
Stirring Propeller (Spiral)
\ I -
Detail 01 Reference Electrode Compartment
Figure 1. Graphite cylindrlcal electrode.
(10-12). Although the technique is highly sensitive, it does not lend itself to simultaneous speciation, and when performed by classical polarographic techniques the iron determination is strongly disturbed by copper (IO, I I ) and lead (IO) found in equal amounts. T o avoid this problem, we chose to use an iron(III)/iron(II) charge-transfer technique (8, 9). On platinum or graphite electrodes, iron(II1) reduces to iron(I1) a t about $0.5 V vs. SCE. The rate of the electrode reaction depends on the medium and is reversible in certain cases. In a previous study (8),4.8-7.6 M HCl produced waves for iron in which iron(II1) and iron(I1) responded equivalently within 4-5%. Cyclic voltammetry scans in the region of +0.5 V vs. Ag/AgCl in 6 M HC1 were therefore used in this study.
EXPERIMENTAL SECTION Apparatus. AU samples were homogenized with a Krupps mill, Model 203. All atomic absorption analytical measurements were made with a Perkin-Elmer Model 5000 atomic absorption spectrophotometer, equipped with UV/VIS background corrector, hollow cathode lamps, and Model 56 chart recorder. A Beckman Model DB-GT spectrophotometer with 1-cm cell was employed for all colorimetric measurements. Environmental Sciences Associates, Inc. (ESA) Model 3040 charge-transfer analyzer (CTA) was used for all electrochemical measurements in this study. The CTA was equipped with a Heath-Schumberger X-Y recorder, a tubular pyrolytic graphite electrode (2.54 cm X 0.95 cm) with an active inner surface (12 cm2),a clockwise rotating right-hand spiral stirring propeller (Figure l),and a 200-mL cell attached by a threaded collet. Pyrex (No. 1040) 200-mL tall-form beakers were used as analysis cells. An in-line gas purifier (Diamond Tool and Die, Inc.) was used to reduce oxygen levels in purging nitrogen gas. Reagents. A certified atomic absorption standard (1000 pg/mL made from FeC13,Fisher) was used to prepare the iron standard curve for atomic absorption measurements and to add iron(II1) standards for the cyclic voltammetry measurements. Reagent grade ferrous sulfate (
