2,2 -Bipyridine Ferrous Complex Ion as Indicator in the Determination of Iron F. WM. CAGLE, JR., and G. FREDERICK SMITH Noyes Chemical Laboratories, University of Illinois, Urbana, III,
The 2,2'-bipyridine ferrous complex is suitable for use as an oxidation-reduction indicator in the determination of iron by cerate oxidimetry, following the use of sulfuric acid solutions with the Jones reductor for reduction of iron. The indicator reaction may be repeatedly reversed at the equivalence point by addi-
ion without deuse of the indicator in the form of a saturated aqueous solution of the sparingly soluble ferrous perchlorate complex is justified. Preparation of indicator is described and advantáges on the cost basis are pointed out.
the spectrophotometric
Indicator Solution. An approximately saturated aqueous solution of bipyridine ferrous perchlorate (molecular weight 822.72) results from the solution of 1.65 grams of the complex perchlorate in 1000 ml. of water. One milliliter of this solution is employed for each titration. The color change in 200 to 300 ml. of solution upon oxidation is vivid and reproducible. Twentyfive milliliters of 0.1 N ferrous sulfate in 1 to 2 formal sulfuric acid may be titrated without the use of phosphoric acid to decolorize the ferric ion formed upon oxidation. The sulfato-cerate ion is used as oxidant. A 0.025 formal solution of bipyridine ferrous sulfate may be employed as indicator solution. In this case the amounts of reagents above specified are to be used without conversion to the perchlorate, but with direct dilution to 1000 ml. In this case one drop of indicator solution is employed for each titration. The former procedure using the perchlorate indicator is preferred. The addition of the ferrous sulfate indicator, 1 drop, often inadvertently results in the addition of 2 drops in place of 1. Using a 1-ml. pipet to measure the perchlorate indicator solution into the titration beaker obviates this possibility and assures the analyst a correct indicator blank (0.02 ml. of 0.1 N oxidant). Sulfato-Cerate Solution. Ceric hydroxide, Ce(OH)4, is now commercially available. This material is of theoretical composition and is readily soluble in hot dilute sulfuric acid. To prepare an approximately 0.1 N solution of sulfato-ceric acid, H4Ce(S04)4, a sample of 20.8 grams of ceric hydroxide is placed in a 400-ml. beaker and dissolved, after the addition of 120 ml. of water, by the slow addition of 38 to 39 ml. of concentrated sulfuric acid (specific gravity 1.84) with continued stirring. The resulting solution is diluted to 1000 ml. and is then found to be approximately 0.1 N and 0.5 formal in sulfuric acid. Ceric hydroxide is soluble in warm concentrated nitric acid which serves as a procedure in the preparation of nitrato-ceric acid, H2Ce(N03)6. Ceric hydroxide is not readily soluble in cold 1 formal hydrochloric acid. The use of heat or stronger hydrochloric acid is not feasible because of the reduction of the quadrivalent cerium with the liberation of chlorine. Warm concentrated 72% perchloric acid dissolves ceric hydroxide definitely, but very slowly.
tion of
determination
of iron 2,2'reagent (1-7, 9). This procedure results from its property of forming with the ferrous ion a complex cation [(CioH8X»)3Fe]+ + which has an intense red color (wave length of maximum absorption equals 522
INbipyridine has found extensive application
II
The specific structural configuration, =N—C—C—N=, accounts for similar complex cations in the case of 1,10-phenanthroline and 2,2', 2' '-terpyridine (7, 9), and in substituted methyl, nitro, and halogenated 1,10-phenanthrolines (8). The 2,2'-bipyridine ferrous cation (bipyridine ferroin) may be oxidized to the corresponding ferric cation (bipyridine ferriin) at a formal oxidation potential of 0.97 volt (9), the color change being from an intense red to a faint blue. The oxidized form is somewhat less stable than the reduced form in mineral acid solutions. It has hitherto been assumed that this would prevent the use of the ferrous and ferric complex bipyridine ions as oxidation-reduction indicators. Since 2,2'-bipyridine may be obtained at a much reduced price, as compared to 1,10-phenanthroli*e which is commonly employed, the former if proved applicable should find extensive use. It is the object of the present work to show this goal to be attainable in the determination of iron in sulfuric acid solution employing the sulfato-cerate ion, [Ce, as oxidant. (S04) s] 2,2'-Bipyridine (molecular weight 156.18, melting point 69.50 C., molecular extinction coefficient at 522 µ of 8650) may be purchased in regular trade channels in pound lots at 50 cents per gram.
µ).
PREPARATION
ferrous ion
or
cerate
=
as a
II
excess
composing the indicator compound. The
OF MATERIALS
For use as a redox indicator 2,2'-bipyridine is very conveniently employed in the form of its ferrous perchlorate complex, (CioHsN2)3Fe(C104)2. To 11.7135 grams of the dye base is added a freshly prepared solution of 6.9505 grams of pure ferrous sulfate heptahydrate. The addition with stirring of the white crystalline bipyridine to the ferrous sulfate solution results in the rapid formation of the ferrous sulfate complex, (CioHsN2)3FeS04, which is intensely red in color and easily soluble in water. The slightly soluble dye base goes into solution as a result of forming the soluble complex ion with ferrous sulfate. For this reason the ferrous sulfate solution does not require the addition of sulfuric acid to prevent hydrolysis if freshly prepared and used at once. It is preferable to carry out this reaction without sulfuric acid, since below a pH 2.5 to 3 the time required for formation of the complex ion is increased in relation to the amount of acid present, and at high acid concentrations the reaction is almost completely retarded. To the aqueous solution of the bipyridine ferroin diluted to 300 ml. in a 400-ml. beaker there is added a sufficient amount of 72% perchloric acid (1 volume diluted to 10 volumes with water) to precipitate the dye base completely. The bipyridine ferrous ion, [(CioHsN2)3Fe] ++, immediately forms the sparingly soluble bipyridine ferrous perchlorate complex. The red insoluble precipitate is filtered in a sintered-glass filtering crucible, washed with generous portions of distilled water, and dried at 90° to 100° C. This reagent may now be purchased in the regular reagent trade channels.
BIPYRIDINE
FERROUS PERCHLORATE
AS REDOX
INDICATOR
For test procedure 25-ml. portions of an approximately 0.1 N solution of ferrous sulfate in 1 formal sulfuric acid from a calibrated pipet were transferred to a 400-ml. beaker and diluted to 150 ml. using 1 formal sulfuric acid. One milliliter of saturated bipyridine ferroin was then added as indicator, producing a deep pink coloration. The solutions were titrated, using an approximately 0.116 N solution of sulfato-ceric acid in 1 formal sulfuric acid as oxidant. The results of the analyses are given in Table I. The color change at the equivalence point is from orange to practically colorless. The indicator blank was a little less than 0.02 ml. of the oxidant per titration. An examination of the data of Table I indicates that the titration of precisely measured volumes of ferrous sulfate solution gives strictly concordant results in titration with the cerate ion. The test of the indicator for reversibility and stability in the presence of ordinarily encountered excess of oxidant for titrimetric application gives very satisfactory results. 384
JUNE 1947 Table
385
I. Titration of Ferrous Iron in Sulfuric Solution Using Bipyridine Ferroin as Indicator
Acid
hydrogen atoms bonded to carbon. the nitrogen is likewise not likely.
Titration (25.00 ml. of approximately 0.1 ferrous sulfate per titration. volume 150 ml. Sulfuric acid concentration, 1 formal, with sulfato-ceric acid as oxidant) Sample Ce+-++ Required Ml.
Test of Indicator Stability
1
21.50
Indicator reversibly oxidized and reduced 12 consecutive times at equivalence point without diminution of color
2
21.50
3
21.50
excess oxidant added at equivalence point. After 10 minutes stirring indicator color was restored using one-drop excess of ferrous iron solution 0.5-ml. excess of cerate oxidant added. 7 minutes’ stirring did not destroy indicator, as shown by addition of minute excess of ferrous iron
4
21.51
1.0-ml. excess of cerate oxidant added. 5 minutes’ stirring did not destroy indicator, as shown by same test with excess ferrous ion
5
21.49
One-drop excess of oxidant followed by stirring for 2 hours was required to destroy indicator property of reversal in color upon reduction
intensity
One drop
INDICATOR
CHARACTERISTICS IN PRESENCE OF HYDROCHLORIC ACID
The data given for sulfuric acid solutions provide conditions suitable to the use of the Jones (amalgamated zinc) reductor for the preparation of ferrous sulfate solutions for oxidimetric determination. In order to use the Walden silver reductor for similar purposes it is necessary to work in the presence of hydrochloric acid. Titrations of the same nature as those given in Table I, except for an approximately 0.1 N solution of ferric chloride in 1 formal hydrochloric acid, were taken and reduced in a Walden silver reductor and titrated, using the same cerate solution as previously employed. The results were of equal accuracy. In this case, however, the color change is from orange to yellow because of the color of complex ferric chlorides formed. It is not satisfactory to add phosphoric acid to form a complex ferric iron if quadrivalent cerium is to be used as oxidant because of the formation of insoluble cerium phosphates. LITERATURE
The instability of the indicator in its oxidized form may be due to a combination of effects. If the bipyridine ferrous ion, upon oxidation to the ferric form, dissociates to give 2,2'-bipyridine and ferric ions, these may fail to recombine when the ferric ion is again reduced because of the low pH of the solution. Another possible explanation consists in the plausible oxidation of the 2,2'-bipyridine to form addition products which no longer form complexes with the ferrous ion. The former explanation is the more acceptable, since the pyridine ring is known to be extremely stable in resisting both oxidation and substitution for any of the
Direct oxygen bonding to
(1) (2) (3)
CITED
Blau, Monatsh., 19, 647 (1898). Bode, Wochschr. Brau., 50, 321 (1921). Gerber, Claassen, and Boruff, Ind. Eng. Chem., Anal. Ed., 14, 364 (1942). Gray and Stone, Ibid., 10, 415 (1938). Harrison, J. Assoc. Official Agr. Chem., 24, 215 (1941). Hill, Proc. Boy. Soc. London, B107, 208 (1930). Moss and Mellon, Ind. Eng. Chem., Anal. Ed., 14, 862 (1942). Moss, Mellon, and Smith, Ibid., 14, 931 (1942).
(4) (5) (6) (7) (8) ‘ (9) Smith and Richter, ‘Phenanthroline and Substituted Phenanthroline Indicators,” Columbus, Ohio, G. Frederick Smith Chemical Co., 1944.
Determination of Carbon and Hydrogen in Petroleum Distillates A Lamp Technique M. C. SIMMONS1 Petroleum
Experiment Station, Bureau of Mines, Bartlesville, Okla.
determining simultaneously the carbon of organic materials, such as content petroleum distillates, is based on their combustion in a special lamp suitable for aromatics as well as other hydrocarbon types. The lamp combustion procedure simplifies difficulties normally encoun-
A method
and hydrogen
petroleum distillates often of carbon and hydrogen. However, customary methods of analysis for carbon and hydrogen, which involve burning the sample from a boat or other container within a combustion tube, become tedious and time-consuming when volatile materials must be analyzed. Recognizing the inadequacy of these Liebig-type determinations for petroleum distillates, Hindin and Grosse (1) recently reported an analytical method for determining the hydrogen content of such materials which simplifies their combustion by using an A.S.T.M. method D90-41T sulfur lamp. The procedure was intended primarily for rapid, routine analyses, and relatively large samples were of
CHARACTERIZATION requires accurate determinations
Present address, Houston Refinery Research Laboratories, Shell Oil Company, Inc., Houston, Tex. 1
tered in the ultimate analysis of volatile samples, and refinements improve accuracy and permit use of relatively small samples. Routine determinations can be made, and maximum deviation for repeated is about 0.01% when 0.5- to analyses from mean 1.0-gram samples are used. employed to obtain good precision and agreement with theoretical values—for example, the average deviation of repeated analyses from the mean was about 0.02% when 3- to 6-gram samples The lamp employed by Hindin and Grosse was not were used. suitable for burning compounds such as aromatics, because a smoky flame was obtained, nor for the simultaneous determination of carbon, apparently because of incomplete combustion. The authors pointed out these difficulties with their procedure and indicated means whereby it might be improved. The present paper describes an improved lamp technique for the determination of both carbon and hydrogen, based largely on the suggestions made by Hindin and Grosse. The technique comprises: (1) burning the sample in a lamp suitable for all volatile petroleum oils, including aromatics; (2) completing the
