Assay of Alkyl and Hydroxyalkyl Ferrocenes by Potentiometric Titration D. M. KNIGHT and R. C. SCHLITT Methods Development Section, Analytical Chemistry Department, Aeroiet-General Corp., Sacramento, Calif.
b The per cent purity or the equivalent weight of alkyl and hydroxyalkyl (dicyclopentadienyliron) ferrocenes can b e determined potentiometrically b y titrating with standard ferric chloride solution. The procedure is rapid, convenient, and can b e easily adapted for routine analysis to give a precision of ~0.22. Ferrocene derivatives that have a carbonyl adjacent to the cyclopentadiene ring do not titrate, thus it is possible to determine the amount of alkyl ferrocene in a mixture with alkyloxo ferrocene.
S
the discovery of ferrocene, numerous derivatives have been prepared and new applications for these substances are being investigated. To aid in the investigations, various analytical procedures have been developed for the detection of ferrocenes in complex mixtures. Behum ( 2 ) detected ferrocenes in reaction mixtures by noting the colors produced with potassium ferricyanide. Ferrocene and its derivatives have also been detected in mixtures by paper (4, 5 ) and thin-layer (10) chromatography, where the chromatograms can be developed with an oxidizing agent, or in combination with other reagents. Mulay ( 7 ) et al. have made a qualitative study of ferrocenyl compounds using broad-line proton magnetic resonance absorption. Mason and Rosenblum (6) have demonstrated that aryl ferrocenes can be titrated potentiometrically with dilute potassium dichromate and the E” values for the oxidation can be calculated. Quantitative and qualitative analysis of crystalline ferrocene derivatives was accomplished by x-ray diffraction (1). Mass spectrometry (8) or lorn voltage mass spectrometry (8) of ferrocenes removes an electron from the ferrous iron, to produce the ferrocinium ion, which can be collected without fragmentation, making it possible to identify the compound and determine the molecular weight. One basic procedure for estimating the purity of ferrocenyl compounds is elemental analysis, or simply determining the iron content. ;1convenient method for the determination of iron in ferrocenes was developed by Rosenberg INCE
470
ANALYTICAL CHEMISTRY
and Riber (9). However, there is a disadvantage in measuring the total iron in a sample. Should a ferrocenyl compound contain an iron impurity a misleading result would be obtained. To overcome this difficulty a new procedure was developed, that is based on the oxidation of ferrocene to the ferricinium ion as reported by Wilkinson et al. (ff ) and later by Wolf ( 1 2 ) . In this laboratory, alkyl and hydroxyalkyl ferrocenes were first titrated potentiometrically with 0. lx’ potassium dichromate in acetic acid. Erratic values greater than 100% were obtained, while the iron content indicated the compounds to be less than 100%. These observations were interpreted to mean that dichromate, in strong acid, oxidizes some of the organic material. To correct this problem, ferric chloride was used as the oxidizing agent and methanol was substituted as the solvent. A larger potentiometric break was also observed with methanol solvent than with acetic acid. The procedure described in the experimental section is a general method that can be applied to assay a number of alkyl or hydroxyalkyl ferrocenes. In the assay of ferrocene, five determinations gave the following percentages: 99.73, 99.68, 100.22, 99.95, and 99.82 with an average of 99.88, standard deviation of 0.22. The per cent based on iron content was 99.7 Table I compares the equivalent weight of some alkyl and hydroxyalkyl ferrocenes with the theoretical value. -4number of alkyloxo ferrocenes were titrated with ferric chloride. All of these compounds, which are listed in the discussion, failed to give a potentiometric break. The amount of alkyl ferrocene contained in a mixture of alkyl and alkyloxo ferrocenes was determined (Table 11).
*
methanol to make 1 liter. The ferric chloride reagent is standardized daily by determining the iron content. This is accomplished by pipetting 15 ml. of ferric chloride into a solution of 50 ml. of water and 20 ml. of concentrated hydrochloric acid. The solution is boiled to expel the methanol, and cooled to approximately 80’ C. A 3-drop excess of 10% stannous chloride in dilute hydrochloric acid is added, followed by 250 ml. of water, and 15 ml. of saturated mercuric chloride solution. Then 10 ml. of concentrated phosphoric acid is added, and the mixture is titrated with standard potassium dichromate to the diphenylamine sulfonic acid end point. Apparatus. The potentiometric titrations of ferrocenyl compounds are performed using a Precision-Dow Recordomatic-Titrator equipped with a calomel reference electrode (Beck-
Table I.
Compound 1,l‘-Diethyl ferrocene
Equivalent Weight of Ferrocenes Eq. wt.
Average 261.0 260.7 260.8 260.3 Butyl 247.2 246.8 246.4 f en-ocene 247.0 Hydroxy218.6 218.3 218.2 methyl 218.1 ferrocene Hydroxyethyl 231.8 232.1 ferrocene 232.4 232.1 259.5 259.3 Pentyl ferrocene 259.4 259.1 Hexyl 273.5 274.0 ferrocene 274.6 274.1 1,l’-Dibutyl 305,8 305.5 ferrocene 305.2 305.7
Theoretical 242.1 242.1 216.1 230.1 256.2 270.2 298.3
EXPERIMENTAL
Reagents. Standard potassium dichromate solution, O.lN, prepared by accurately weighing 4.9 grams of potassium dichromate standard reagent, dried at 110’ C. for 2 hours., and by dissolving in enough distilled water to make 1 liter. Ferric chloride reagent,: 27 grams of reagent grade ferric chloride hexahydrate dissolved in enough ACS grade
Table II.
Standard Additions
Composition, Found, Mixture % 70 1,l’-Dibutyl ferrocene 60.22 60.15 1,l’-Dibutanoyl ferrocene 1,l’-Diethyl ferrocene 72.31 72.22 1,l’-Diacetyl ferrocene
man No. 39170) and a platinum inlay electrode (Beckman KO.39273). Procedure. A 1.0-1.5 meq. sample, weighed to the nearest 0.1 mg., is added to an electrolytic beaker and dissolved in 150 ml. of methanol. Then 10 ml. of 70y0perchloric acid is added and the solution is titrated immediately with standard ferric chloride reagent. DISCUSSION AND RESULTS
An acidified solution of alkyl ferrocene turns blue upon standing, indicating the presence of ferricinium ion. It was also observed that the longer the delay before titrating the acidified solution the lower the assay. To overcome the oxidation of the sample, the titrations were conducted immediately after addition of perchloric acid. The presence of ferrous ion in ferrocene does not affect the determination. This was demonstrated by adding several drops of 1% ferrous ammonium sulfate to the ferrocene solutions. Titration of the samples was the same as if ferrocene alone were present. However, ferric ion as a contaminate will
lower the assay. Addition of ferric ion to the sample is equivalent to adding the titrant. Carbonyl substituted ferrocenes do not titrate with ferric chloride. Investigated compounds include; ferrocene-1,l ’-dicarboxylic acid, ferrocenoyl propionic acid, acetyl ferrocene, 1,l’-diacetyl ferrocene, 1,l’dibutanoyl ferrocene, and hexanoyl ferrocene. By making use of the difference in reactivity, it is possible to determine the amount of alkyl ferrocene in a mixture with alkyloxo ferrocene. An experiment was conducted where a mixture was prepared from weighed portions of alkyl ferrocene and its keto counterpart. A purity correction, as determined in Table I, was applied, and the mixture was titrated. The composition of the mixture and the results are found in Table 11.
LITERATURE CITED
(1) Baun, W. L., ANAL.CHEX 31, 1308
(1959).
(2) Behum, J. D., Talanfa 9, 83 (1962). (3) Clancv, D. J., Spilners, I. J., ANAL. 2 ).
ACKNOWLEDGMENT
The authors thank R. C. Olberg for synthesizing and donating the compounds used in this study.
RECEIVED for review Augusb 20, 1964. Accepted October 27, 1964. Published by permission of the Aerojet-General Corp.
Computational Aids for Identifying Crystalline Phases by Powder Diffraction LUDO K. FREVEL Chemical Physics Research laboratory, The Dow Chemical Co., Midland, Mich.
b A comprehensive computer program has been devised to carry out the entire searching-and-matching process of phase identification by powder diffraction. The input information consists of the powder data and elemental data of a sample. The output of the program is in the form of a report listing the experimental diffraction data in juxtaposition to the 10 most intense lines for each matched standard.
D
two decades the Joint Committee on Chemical Analysis by Powder Diffraction (20) has made available a file of some 8000 powder patterns and several index books for these tentative standards. Many analytical laboratories have found this compendium useful in the qualitative identification of corrosion products, the phase study of fluxes, the detection of inorganic fillers in plastics, the analysis of minerals, the recognition of polymorphs, and generally in the phase identification of crystalline substances in unknown mixtures. For this type of analysis it is tacitly assumed “that the URIKG THE PAST
same substance always gives the same pattern; and that in a mixture of substances, each produces its pattern independently of the other, so that the photograph obtained with a mixture is the superimposed sum of photographs that would be obtained by exposing each of the components separately for the same length of time” (11). Unfortunately these statements are not unequivocally true for any of the powder diffraction methods (9, 18). Moreover, the most crucial assumption that the powder pattern yields a chemical analysis of a sample is theoretically unsound (21). VALID INFERENCES FROM POWDER DIFFRACTION
An x-ray powder pattern of a crystalline solid consists of diffraction lines usually listed in descending order of interplanar spacings measured in angstroms. The intensity of a diffraction line is generally recorded as peak intensity measured in arbitrary units. Thus the measured data can be represented conveniently by a paired set { d v , I v ] . For a mixture of crystalline
phases the set of interplanar spacings { d v ) corresponds to the sum of the observed spacings for each crystalline phase present, i.e. {dvl
=
c
(1)
( 4 P
where { d ) designates the set of interplanar spacings for phase p . The d’s for any particular phase, however, are not totally independent of each other but are related functionally according to expression 2 d-2
=
h2a*2
+ k2b*2 + 12c*2 +
2klb*c*cos a*
+ 21hc*a*cos p* + 2hka*b*cos y* (2)
where a*, b*, c*; a*,8*, and y * are the reciprocal cell constants. From Equation 2 it is seen that the d-data for any crystalline phase can be accounted for by no more than six independent cell constants. The only valid information deducible from the spacings data pertains to the geometry of the unit cells of the various crystalline phases present. Each powder reflection, however, yields not only a &value b u t also a V O L 37, NO. 4, APRIL 1965
471
,
