5,6-Dimethyl-l,lO-phenanthroline Spectrophotometric Constants as Ferrous Complex and Use a s Redox Indicator for Determination of Iron by Oxidation with Dichromate G . FREDERICK SJIITH AND WARREN W. BRANDT, University of Illinois, Urbana, 111. The spectrophotometric constants and conformance to Beer's law of 5,6-dimethyl-1,lO-phenanthroline ferrous sulfate have been demonstrated. Its formal oxidation potential in 1 F sulfuric acid was found to be 0.97 volt by a potentionietric evaluation. This dimethylferroin was shown to he a desirable indicator for the titration of ferrous ion by dichromate in sulfuric acid and hydrochloric acid solutions.
T
Potassium Dichromate and Ferrous Sulfate. Pure potassium dichromate and ferrous sulfate heptahydrate were dissolved in 1 F sulfuric acid in sufficient amounts to make approximately 0.1 N solutions. Formal Sulfuric Acid. One molecular weight of sulfuric acid (56 ml. of sulfuric acid, specific gravity 1.84) was diluted to 1000 ml. Dimethylferroin. A 0.01 Jf solution prepared by reaction of 0.03 mole of the dye base with 0.01 M of ferrous sulfate heptahydrate. Apparatus and Operational Technique. The potentiometric apparatus consisted of a student potentiometer and the usaal accessories, including a sensitive lamp and scale galvanometer. The electrode pair consisted of a saturated calomel electrode and internal salt bridge with contact to the test solution through an unsealed asbestos fiber microleak. A bright platinum wire completed the electrical circuit. Solutions were stirred with a magnetic stirring device. Ample time intervals were allowed for the attainment of equilibria before electrode potentials were recorded. The spectrophotometric studies were made using a G.E. recording spectrophotometer, using cells of 1-cm. thickness.
HE phenanthrolinium ferroub ion [fen oin) , as an oxidatioiireduction indicator, was first described by Walden, Hammett, and Chapman ( 7 ) ,and its use has been extensively adopted particularly in cerate oxidimetry (4). A series of substituted 1,lO-phenanthrolinium ferrous ions for use as redox indicators has been described by Smith and Richter (5). Ferroin as a redox indicator is oxidized from an intense red form to the faint blue of the phenanthrolinium ferric ion, the oxidation potential of the system being 1.06 volts in formal acid solutions (2). At 2 formal acidity, and successively 3, 4, 5 , and 6 formal acidity, the oxidation potentials are, respectively, 1.03, 1.00, 0.96, 0.925, and 0.80 volt (5). The 5-methy1-l,lO-phenanthrolinium ferrous ion (methylferroin) has the oxidation potential 1.02 volts in formal acid solutions. At 2 formal acidity, and successively 3, 4, 5, and 6 formal acidity, its oxidation potentials are, respectively, 1.00, 0.96,0.93,0.86, and 0.81 volt ( 6 ) . Both ferroin and methylferroin are stable redov indicators in 1 to 8 F mineral acid solutions, provided they are not heated (3). Seither indicator is completely satisfactory for rapid and convenient use in the ovidimetric determination of iron by dichromate a t less than 5 or 6 F sulfuric acid, which is inconveniently high. Such determinations are therefore carried out a t lower acidities using the diphenylamine series of indicators as less desirable substitutes. The present work has for its objective the determination of the spectrophotometric con-1stants of dimethylferroin (5,6-dirnethyl-l,10phenanthroline ferrous ion), the evaluation of its formal oxidation potential, and the demonstration of its practical use in the titration of ferrous iron by dichromate.
POTENTIOMETRIC TITRATION OF FERROUS IRON B Y DICHROMATE
The results of the potentiometric titration (in a 1 F sulfuric acid solution throughout) of potassium dichromate with ferrous sulfate and ferrous sulfate by potassium dichromate are shown in Figure 1. The two titrationr are seen to he markedly different. I
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I
I
I
I
I
PREPARATION OF REAGENTS
5,6-Dimethyl- 1,lO-phenanthroline. This compound was prepared by Case, and data concerning the synthesis of this substituted phenanthroline together tyith ot,her substituted polymethyl 1,lO-phenanthrolines have been published ( I ) . The reagent being described and the dimethylferroin indicator made from it are now commercially available (G. Frederick Smith Chemical Company, Columbus, Ohio). Sulfatoceric Acid. h sample of pure ceric hydroside, Ce(OH)4,made from pure ammonium nitratocerate, (X"&Ce(K03)6, by precipitation using a twofold excess of ammonium hydroxide served as starting material. A% sample was dissolved in sufficient hot dilute (1-4) sulfuric acid to dissolve the hvdroxide and provide for dilution to a 0.1 S sulfatocerate solution which was 1 F in free sulfuric acid. (Pure ceric hydroxidr is commercially available.)
Figure 1.
948
Oxidanf in mf. Potentiometric Titration of Ferrous Iron
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V O L U M E 21, NO. 8, A U G U S T 1 9 4 9
949 to carry out these titrations successfully in both directions indicates a high degree of stability for both the oxidized and reduced forms of the indicator.) SPECTROPHOTOMETRIC CONSTANTS OF DIMETHYLFERROIN COMPLEX ION
Solutions of the 5,6-dimethyl-l,lO-phenanthrolinium ion containing approximately 1 to 7 p.p.m. of iron mere prepared and finger printed Epectrophotometrically v i t h the results shown in Figure 3.
90
80
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5
10
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15 20 25 ffxidcrnf /i7 m,!
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30
35
70
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b’igtire 2. Oxidation Potential of DiniethylferroinDimethylferriin Indicator System
60
k
4 Table I. Fe P.p.m.
Calculation of Extinction Coefficient of Solutions of Dimetbylferroin Transrnittancy
70 58.4 36.4 21.9 12.5 8.3 4.8 3.1
Diinethylferroin .Concn. .UoZe/liter 1 . 7 6 7 X 10-5 3 . 5 9 6 X 10-5 6 , 3 3 4 x 10-5 6 . 9 7 8 X 10-5 8 . 8 1 2 x 10-5 1 0 . 5 7 x 10-5 1 2 . 1 2 x 10-0
Extinction Coefficient
30 4v.
12362
The abrupt fall in potential of the ferrous sulfate titration of dichromate gives an equivalence point break in potential from 1.22 to 0.85 volt. For this titration ferroin may be employed as indicator. The reverse titration of ferrous sulfate by dichromate produces an equivalence point break from 0.85 to 1.00 volt. Obviously, the ferroin-ferriin transition is not applicable as an indicator in this case. iln indicator with a transition potential of some value between 0.85 and 1.00 volt is required. Barium diphenylamine sulfonate has been employed. The transition point in this case is 0.84 volt. .\--Phenylanthranilic acid ( 6 ) is not suitable because its oxidation potential is 1.08 volts. By the uqe of dimethylferroin it favorltble indicator potential ie available. DETERMINATION O F OXIDATION POTENTIAL OF DIRIETHY LFERROIK-DIMETHY LFERRIIN REDOX INDICATOR SYSTEM
The oxidation potential of this system in 1 F sulfuric acid solution was determined by the titration of a mixture of ferrous sulfate and dimethylferroin. The sulfatocerate ion was ernployed as oxidant and the sulfuric acid solution was held at 1 I.‘ throughout. By attaining correct oxidation pot,entials for the Fe(II1)-Fe(I1) system and for the Ce(1V)-Ce(II1) system the determined oxidation potential for the indicator system was considered to be reliable. This determination was made in both the forward and reverse titrational scheme and the same result was obtained for the unknown oxidation potential. The data are shown graphically in Figure 2. From these titrations the oxidation potential of the dimethylferroin-dimethylferriin inten1 in 1 F acid is shown to br 0.97 volt. (The ability
20
l0
0 Wuve/engfh /R rn9 Figure 3. Spectrophotometric Constants of Diniethylferroin Complex Ion From an examination of the data shown graphically in Figure 3 the wave length of maximum absorption is found to be 5208 mu. This value is constant over the entire range 1 to 7 p.p.m. Conformity to Beer’s law t m s verified by plotting log transmittancy against parts per million of iron, n-hich gives a linear plot. LThPn the plot of data shown in Figure 3 \\-a? duplicated using the same solutions after a period of 180 days, duplicate results were obtained within 1.8%; the change was at,tributed to change in concentration of the solutions, even though they iyere stored in glass-stoppered containers. The molecular extinction coefficient calculated from data given in Table I was found to be 12,860. This value is the highest by a small amount of any phenanthroline derivative so far studied. Titration of Ferrous Iron by Dichromate with Dimethylferroin as Indicator. Thiq proccdurr may be applied in either hydrochloric or sulfuric acid solutions n-hich are 1 t o 2 I; in
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ANALYTICAL CHEMISTRY
Table 11.
Standardization of Approximately 0.05 N Ferrous Sulfate
(Forward titration employing dimethylferroin indicator. Reverse titration usine ferroin as indicator) F e + + Normality, F e + + Normality, Acid Forward Reverse No. of Formality Titrations Titration Titration I
1
F HtSO,
2
F HtSO,
6 3 6 6 6
0.04781 0.04733 0.04698 0.04749 0.04626
0.04785 0.04734 0.04702 0,04763 0.04630
Diphenylamine Sulfonate Forward Titration 1 F €IC1
6
0.04958
F HCI
6
C ,04972
LITERATURE CITED
0.04961
Potentiometric 2
point by alternate dropwise excess of ferrous and dichromate solution additions. Thirty such reversals over a period of 30 minutes did not affect the sharpness of the color change. A 1-ml. excess of 0.1 N dichromate did not affect the sharpness of the indicator color change in a titrated solution during 1 hour’s time. The use of 0.05 ml. of 0.025 M dimethylferroin in a volume of 150 ml. of solution gives a sharp indicator color change. Comparison titrations of ferrous iron by dichromate using the new indicator with the reverse titration of dichromate by ferrous iron using ferroin as indicator are recorded in Table 11. These latter tests were made in hydrochloric acid solution.
0.04969
acid concentration. Dimethylferroin gives, as color change, a transition from orange to green if hydrochloric acid is present and no phosphoric acid is employed to complex ferric ions. The color change in the presence of sulfuric acid is from red to yellowish green. The indicator may be reversed indefinitely at the equivalence
(1) Case, F. H., Abstracts of 112th Meeting, AM.CHEM.SOC., p. 35L, 1547. (2) Hume and Kolthoff, J . Am. Chem. SOC.,65, 1855 (1943). (3) Kolthoff, Lee, and Leussing, ANAL.CHEM.,20, 585 (1948). (4) Smith, G. F., “Cerate Oxidimetry,” Columbus, Ohio, G. Frederick Smith Chemical Co., 1942. (5) Smith, G. F., and Richter, F. P., “Phenanthroline and Substituted Phenanthroline Indicators,” Columbus, Ohio, G. Frederick Smith Chemical Co., 1944. (6) Syrokomsky and Stiepen, J . Am. Chem. Soc., 58, 928 (1936). (7) Walden, Hammett, and Chapman, Ibid., 53, 3908 (1931). RECEIVED July 22, 1948.
‘Determination of Starch and Cellulose with Anthrone FREDERICK J. VILES, JR., AND LESLIE SILVERMAN Haraard School of Public Health, Boston, Mass.
A procedure is presented for colorimetric analysis of starch and cellulose at a wave length of 625 mp, using a 0.1% solution of anthrone in concentrated sulfuric acid. The method is accurate for ranges of 10 to 200 micrograms and sensitive to 2 micrograms of these substances. Because of the instability of the reagent, a known standard must be used with each set of analyses to determine the correct Beer’s law constant. Color intensity studies of the effect of heat upon the reaction between anthrone reagent and starch and cellulose are presented. Spectral transmittance curves of carbohydrate-anthrone colors prepared under different conditions are also included.
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REYWOOD ( 1 ) initially demonstrated the use of anthrone
in a specific qualitative test for carbohydrates and suggested its possible quantitative use. Morse (3) used anthrone for determining lovi concentrations of sucrose, and Morris ( 2 ) , i n a report that appeared while this article !?as in preparation, investigated its applications t o carbohydrates and some conditions of the reaction. He also studied the relationship of color intensity with various carbohydrates. The authors have applied this reagent to analysis of starch and cellulose (cotton lint) in air samples collected in plants manufacturing cotton textiles. The discussion that follows includes only analyses of these two substances and pertinent facts concerning the reagent and reaction conditions. Anthrone can be made as described by other investigators (1-3) or obtained commercially (Paragon Testing Laboratories, Orange, S. J., Panrone Chemical Company, Farmington, Conn., and National Biochemical Co.. 3106 West Lake St.. Chicago 12, Ill.). For the present work, a commercial product obtained in 1946 from the Paragon Testing Laboratories was used without purification. The test is made by rapidly adding a solution of anthrone (0.05 to 0.20%) in concentrated sulfuric acid t o an aqueous solution or
suspension of the carbohydrate and mixing immediately. Under controlled conditions the amount of green color produced is proportional to the carbohydrate content. The heat produced by mixing acid and water appears to be a necessary part of the reaction. In analyses for cellulose, sulfuric acid (60% by volume) is used to digest the material prior to analysis and therefore is present (in aliquots of 0.5 ml.) mith water and the anthrone reagent. (For dissolving cellulose, 60% sulfuric acid was found to give optimum results regarding rapidity of solution. Cotton is the only form of cellulose refcwed to in this work.) Morse (3) used unfiltered and AZorris ( 2 ) filtered light, for photoelectric determination of carbohydrates. Seither investigator presented any spectral transmittance data for the anthronecarbohydrate color to permit proper selection of wave length for maximum sensitivity. Morris ( 2 ) arrived at an adequate choice (620 mp) from measurements with three light filters. SELECTION OF WAVE LENGTH FOR COLOR MEASUREMENT
The purpose of this study was the proper selection of wave length for colorimetric analysis of starch and cellulose with anthrone. Spectral transmittance curves (Figure 1) of the colors
