V O L U M E 23, N O . 6, J U N E 1 9 5 1
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ing p-hydroxybenzoic acid and omitting the diisopropyl ketone allowed the violet color due to nornicotine to appear, but color development continupd even over 20 hours. Further experiments showed that diisopropyl ketone (new) added to the reagents acted as a reaction stabilizer and the violet color 1 hour after starting the reaction was stable for over a 1-hour period.
shake to mix, and place the flasks in the dark for 60 minutes. Determine color density of the solution with the colorimeter, using a No. 54 filter, and plot the readings against the concentration.
ANALYTICAL PROCEDURE
ACKNOWLEDGiMENT
Reagents. Acetone, A.C.S. Diisopropyl ketone, Eastman's P3244. p-Hydroxybenzoic acid, Eastman's 1520, 2% by weight/ volume in diisopropyl ketone. l,%Diketohydrindene, Eastman's P3565, 0.3% by weight/volume in diisopropyl ketone. Nornicotine. Preparation of Standard Curve. Dissolve 0.0400 gram of nornicotine in 500 ml. of acetone. Place 2, 3, 4, and 5 ml. of the resulting solution in glass-stoppered flasks and add acetone to bring to 5 ml. where necessary. Add to each flask 15 ml. of diisopropyl ketone followed by 2 ml. each of the p-hydroxybenzoic acid and 1,3-diketohydrindenc reagent solutions. Stopper the flasks,
The authors wish to thank -4bner Eisner, Eastern Regional Research Laboratory, Bureau of Agricultural and Industrial Chemistry, Agricultural Research Administration, United States Department of Agriculture, Phihdelphia, Pa., for the synthetic nornicotine.
Figure 1 shows that natural and synthetic nornicotine react alike. The same quantity of nicotine and anabasine gives very little more color than the reagent blank.
LITERATURE CITED
( I ) Feinsteiii, L., and McCabe, E. T., Science, 112, 534 (1950). RECEIVED July 29, 1950.
Contribution from the United States Department of Agriculture, Agricultural Research Administration, Bureau of Entomology a n d Plant Quarantine, Beltsville, Md. Report of a s t u d y under the Research a n d Alarketing Act of 1946.
Determination of Formal Oxidation Potentials of Ferric-Ferrous and Dichromate-Chromic Systems Selection of Substituted 1,lO-Phenanthroline Indicators for Determination of Iron ut Low Acidity G. FREDERICK SMITH, University of Illinois, Urbana, I l l .
'HE determination of iron in hydrochloric acid solution by dichromate oxidation, according to existing procedures, is accompanied by irksome inhibitions. Diphenylamine-type indicators, which are ordinarily employed, leave much room for improvement. Because the oxidation of iron by dichromate gives a green solution due to the chromic ion, 1,lO-phenanthroline-type indicators would be preferable. The formal potentials involved have not been previously studied systematicallr, particularly in the lower range. The problem is complicated by the low oxidation potential of the dichromate-chi omic ion system, particularly at low acid concentrations. The present work has for Its objectlve determination of the requisite formal oxidation potentials and selection of suitable 1,lOphenanthroline-type ferrous complex ions to serve as indicators in the determination of iron by dichromate oxidation. The procedures described involve the use of 0.1 to 1.5 F hydrochloric acid. Previous Investigations. The study of polysubstituted I ,lophenanthrolines for use as ouidation-reduction indicators in the form of their ferrous complex ions together with the determination of various pertinent physical constants has been made by Brandt and Smith (1). The determination of iron by dichromate oxidation in 2 F sulfuric or hydrochloric acid employing the 5,6dimethyl-l,l0-phenanthroline ferrous ion as indicator has been described by Smith and Brandt ( 3 ) . Phenanthroline-type ferrous complex ions for use as oxidation-reduction indicators are now known which have color transition potentials covering the range 0.84 to 1.27 volts. The lower potentials, 0.84 to 1.10 volts, are represented by the various methyl substituted types ( I ) , and the higher values, 1.10 to 1.28volts, by the chloro-, bromo-, and nitrosubstituted types described by Smith and Richter ( 5 ) . -4pplications in volumetric microdeterminations of iron, arsenic, calcium, and the oxalate ion have been described by Smith and Fritz ( 4 ) and by Salomon, Gabrio, and Smith ( 2 ) . Ferroin-type indicators have not previously been known and available for use in the titration of ferrous iron by dichromate in 0.1 to 0.25 F hydrochloric acid. The dichromate titration of iron using a series of familiar indicators has been studied by Stockdale (6).
DETERMINATION OF OXIDATION POTENTIALS
Solutions of known or determined concentration of ferric chloride and potassium dichromate of 0.1 .V strength are required. In addition, hydrochloric acid solutions of graded formalities of 0.1 to 4.0 with small increments of increase in strength between 0.1 and 1.5 F were to be prepared. Solutions of the indicators, 4,7-dimethyl- and 3,4,7,&tetramethyl-l,IO-phenanthroline ferrous complex ions, which were 0.01 Ar are prepared by reaction of weighed portions of the organic base with hydrated ferrous sulfate, addition of water to promote solution arid complex formation, and dilution to calculated final volume. The ferric chloride solutions were prepared as follows: One-tenth molecular weight of ferric chloric hexahydrate was dissolved in 8.245 ml. of reagent hydrochloric acid (specific gravity 1.19, 37.5% hydrochloric acid) and diluted wit'h ivater with stirring. The solution thus obtained was transferred to a 1000ml. graduated flask, diluted to the mark, and thoroughly mixed. The procedure was repeated with appropriate increase in added hydrochloric acid t,o prepare approximately 0.1 N ferric iron in 0.25, 0.50, 0.75, 1.0, 1.5, 2.0, 3.0, and 4.0 F hydrochloric- acid in addition to the 0.1 F hydrochloric acid solution described. Solutions of the same hydrochloric acid formality w r v prcpared to serve for volume dilutions. Solutions of potassium dichromate in 0.1 to 4.0 F hydrochloric acid were prepared as follows: Samples of pure potassium dichromate weighing 4.9035 grams (0.1 equivalent weight) were accurately weighed and transferred to 1000-ml. beakers, dissolved in water, and diluted to 800 to 900 ml. with water. With constant stirring the proper volume of reagent hydrochloric acid was then added (8.245 ml. for 0.1 F acidity, 82.45 ml. for 1 F acidity, etc.). Finally, the beaker contents were transferred quantitatively to a 1000-ml. graduated flask, diluted to the mark, and thoroughly mixed. Solutions of potassium dichromate above 1 F in hydrochloric acid form some free chlorine upon storage and should be made up just before use. The presence of a very low chlorine content will become evident through the attainment of high results for the oxidation potential of the chromate-chromic ion couple. For this reason the solutions are prepared precisely as described to eliminate the presence of free chlorine, and solutions more than a few hours old are discarded. The potential and potentiometric titration studies were evaluated using an assembly of a Leeds & Northrup student potentiometer and decade resistance box, Weston standard cell, and lamp and scale galvanometer. The electrode system was a saturated calomel reference electrode and a platinum mire indicator elec-
ANALYTICAL CHEMISTRY
926
higher than 0.88 volt because the reduced color form (red) predominates in intensity over the oxidized form, which is faint blue. This necessitates a slightly higher potential to bring about (24.40 ml. of 0.1 N K $ h O , are equivalent t o 25.00 ml. of ferrous chloride sample) the maximum color change (Figure I). 100% Excess Vol. of Iron 50%+ *Equivalent Starting Potential The vertical break in potential for the correPoint e.m.f.. Cr& e.m.f., Ref. H n Electrode, Oxidized Fe CrsOr MI. Volt F e + + e.m.f., Volt Volt Volt sponding oxidation of iron by dichromate in 0.1 F Soln. Soh. Soh. Soh. Soln. Soln. Soh. Soln. hydrochloric acid, curve I1 is 0.85 to 0.91 volt. 2 2 1 1 2 1 2 1 For 0.1 F sulfuric acid the vertical break is 0.81to ... , . . ... ... ... 0.87 volt. These values call for the use of an indicator of lower oxidation potential than that found serviceable in 0.5 F acid concentrations, and the ferrous sulfate complex of 3 , 4 , 7 , 8 - t e t r a methyl-1,lO-phenanthroline was applied, having an oxidation potential of 0.85 volt. This indiTable 11. Formal Oxidation-Reduction Potentials of Dichromatecator gave excellent color transitions at points Chromic and Ferrous-Ferric Systems in Various Strengths of Hyindicated by the arrows in Figure 1. Here again drochloric Acid the color change was observed at a somewhat (Values in parentheses are for sulfuric acid solutions) higher e.m.f. These ferroins not only have the 3.00 4.00 Acid Formality 0.10 0.25 0.50 0.75 1.00 1.50 2.00 proper visual oxidation potentials but have the F e + + + - F e + +system, volt 0.73 0.73 0.72 0.71 0.70 0.70 0.69 0.68 0 . 6 6 (0.68) . . . (0.68) , . . extremely favorable molecular extinction coefCr+++++'-Cr*++ system, 0.93 0.96 0.97 0.99 l'.OO
