( 5 ) Erkelens, P. C. van, Anal. Chim. Acta 25, 129 (1961). (6) Heggen, G. E., Strock, L. W., ANAL. CHEM.,25, 859 (1953). (~, 7 ) Koch. 0. G.. Microchim. Acta 1, 92 ( 1958).’ (8) Lundell, G. E. F., Hoffman J..I., “Outlines of Methods of Chemical Analysis,” p. 115, Wiley, New York, 1 Q.54 ----.
(9) Mitchell, R. L., Scott, R. O., J . SOC. Chem. Ind. 66, 330 (1947).
(10) Mitchell, R. L., Scott, R. O., Spectrochim. Acta 3, 367 (1948). (11) Nachtrieb, N. H., “Principles and Practices of Spectrochemical Analysis,” p. 135, McGraw-Hill, New York, 1950. (12) National Bureau of Standards, Monograph 32, Part I, “Tables of Spectral Line Intensities,” p. V, 1961. 113) Pohl. F. A.. Z. Anal. Chem. 142. ‘ (l), 19 ‘(1954). ‘ (14) Silvey, W. O., Brennan, R., ANAL. CHEM.34, 784 (1962).
(15) Thiers, R. E., “Separation, Concentration and ,,Contamination,” in “Trace Analysis, J. H. Yoe and H. J. Koch, eds., p. 637, Wiley, New York, 1957.
RECEIVEDfor review July 26, 1965. Accepted September 28, 1965. Division of Analytical Chemistry, 151st Meeting, ACS, Phoenix, Aris., January 1966.
Diphenylpicrylhydrazyl as an Organic Analytical Reagent in the Spectrophotometric Analysis of Phenols
‘
G. J. PAPARIELLO and M. A. M. JANISH Research Department, ClBA Pharmaceutical Company, Summit, N. J. A study has been made of diphenylpicrylhydrazyl as a colorimetric reagent for the analysis of phenols. Diphenylpicrylhydrazyl is an intense, violet-colored, stable, free radical which becomes discolored on reaction with phenols. The decrease in violet color is used as a measure of the quantity of phenol present. Use of this reagent provides the analytical chemist with a sensitive and reproducible procedure for phenol analysis. It also presents the possibility of analyring one phenol in the presence of another. The solvent used for the reaction was found to influence the reaction rate, and, consequently, dielectric constant-rate relationships were studied.
T
is the second report devoted to studying the utility of diphenylpicrylhydrazyl, a stable free radical, as a colorimetric analytical reagent. The initial work on this reagent dealt with the use of the reagent in the colorimetric analysis of amines ( 7 ) . Knowledge about the reagent uncovered during the prior study can be applied with some modifications to this work. The advantages that this reagent was found to bring to amine analysis, it also brings to the analysis of phenols. The reagent’s intense color and the great differences in reaction rate between various phenols enables both sensitivity and selectivity to be obtained when using this reagent. This phenol work benefits from studies by a number of workers (4-6, 8) on the mechanism and kinetics of reaction of phenols with diphenylpicrylhydrasyl. McGowan et al. ( 6 ) suggested that the rate-determining step in the reaction involves the removal of hydride from the phenolic hydroxyl group with the formation of an ion with a positive charge. ‘They base this contention on
a comparison of the relative rates (the ratio of the rates for substituted to unsubstituted compound) of solvolysis of substituted a,a-dimethylbenzyl chlorides in 90% aqueous acetone to the relative rates of reaction for the corresponding substituted phenols with diphenylpicrylhydrazyl. The great similarity found in the relative rates suggest that the mechanism of reaction is similar. The rate of solvolysis of a,a-dimethylbenzyl chlorides almost certainly involves the formation Of the carbonium ion(1). Thus, they conclude the reaction of diphenylpicrylhydrazyl with phenol involves the formation of the cation(I1)
HIS
I
I1
Hogg, Lohmann, and Russell (41, on the other hand, contend that the reaction involves the abstraction of a hydrogen atom from the phenol to give diphenylpicrylhydrazine and a phenoxy radical, as follows:
NO*
ON-N+NOz
4-
Although there is some disagreement as to the exact mechanism of reaction, there is general agreement that the reaction is usually a second-order reaction, that is, rate of disappearance of hydrazyl = ICz [hydrazyl] [phenol]. EXPERIMENTAL
substances Reagents* were used as received commercially. With the exception of the solvent study, methanolic solutions of the phenols were always used in this jvork a t phenol concentrations of 2 x 10-2 mmoles/ml. t o 2 x 10-5 mmoles/ml. 2,2-Diphenyl-l-picrylhydrazyl(Eastman, KO.7703) was used as received. A 2 X mmoles/ml. methanolic solution of diphenylpicrylhydrazyl mas used as the reagent solution except in the solvent study where a solution of the reagent was prepared in the solvent in question. The reagent solution should be prepared fresh daily. An aqueous 1N acetate buffer system, adjusted to a pH of 5.0, was used to control the pH. Analytical Procedure. Pipet into a glass-stoppered test tube 4 ml. of phenol sample solution, 1 nil. of methanol, 1 ml. of acetate buffer Sohtion, and 4 ml. of diphenylpicrylhydrazyl reagent. I n another test
ooH
Purple
NO,
Yellow VOL. 38, NO. 2, FEBRUARY 1966
21 1
tube prepare a reagent blank using 5 ml. of methanol, 1 ml. of buffer, and 4 ml. of reagent. After mixing, allow test tubes t o stand a t room temperature or 60" C. for 15 minutes to 1 hour. (The time and temperature of reaction depend on the rate of reactivity of the particular phenol being considered.) The absorbance of the sample and the reagent blank are measured against a methanol blank a t 515 mp in 1-cm. cells in a suitable spectrophotometer. The analysis value is obtained by finding the difference between the absorbance values of the blank and sample. The concentration corresponding t o this difference value is then read on a calibration curve made with known concentrations of the same phenol. RESULTS AND DISCUSSION
Phenol Reactivity.
The reactivity
of a number of phenols with the reagent was determined by periodically sampling and reading the absorbance of a solution of the phenol and the reagent under various assay conditions. I n this manner it was found that the mole ratios of phenol to reagent, the temperature, and the time were all factors which influence the reactivity of the phenols with diphenylpic-
Table
I.
Compound Monohydric phenols PhPnnl - __ - ..-.
rylhydrazyl. As with the amine work, 30 minutes' reaction time was found to be adequate for most compounds encountered and it is chosen as the usual reaction time for analysis. The results of the reactivity study are tabulated in Table I. The ring substituents affect the reactivity of the phenols in a manner similar to that of the amines. Thus, ring-activating groups such as methoxy or alkyl groups increase the reactivity of phenols with the reagent, and, conversely, ring-deactivating groups such as nitro, carboxyl, or halogen groups decrease the reactivity. Explanations for this behavior can be obtained by referring to the amine paper (7) and to the work of Hogg et al. (4)which correlates Hammett u values and the rate of reaction. Influence of Solvent. A quantitative description of the solvent influence on the reaction of the reagent with phenolic compounds was attempted. The derived relationship for reaction rate and dielectric constant of the reaction medium can be expressed as follows: In k = In k ,
'
N ( D - 1) U' - RT ( 2 D + 1) 7
Reactivity of Various Phenols"
Phenol:reagent ratio
Reactivity *
100: 1 Good 1:2 Good p-Methoxyphenol* 1:2 Good o-Methoxyphenol (guaiacol)* Good 10: 1 m-Methoxwhenol Fair 100: 1 p-Bromop&nol Good 100: 1 2,6-Dichlorophenol 100: 1 Good 2,4 6-Tribromophenol Good 1:5 PAllyl-2-methoxyphenol(eugenol)* 1:l Good 5-Methyl-2-isopropyl-1-phenol (thymol) Good 10: 1 &Hydroxy-3-methoxybenzaldehyde (vanillin) 100: 1 No reaction Methyl-p-hydroxybenzoate (methylparaben) Good 10: 1 p-Phenylphenol loo: 1 Poor p-Nitrophenol 100: 1 No reaction Salicylic acid 1:2 Good a-Tocopherol (Vitamin E)* 10: 1 Good Ethinyl estradiol 10: 1 Good Estradiol 10: 1 Good 17a-Methyl estradiol 1:l Good p-Naphthol 10: 1 Good 3-Hydroxy-2-naphthoic acid Good 10: 1 &Hydroxyquinoline Fair 100: 1 3-Hydroxypyridine No reaction 100: 1 4-Hydrox yridine Good 1:5 m-Xapht hT* Dihydric henols 100: 1 Good Hexachyorophene Good 1:l Diethylstilbestrol 1:l Good PHexylresorcinol Good 10: 1 Resorcinol Good 1:5 Hydroquinone* Good 1:5 Chlorohydroquinone* Good 1:lO Pyrocatechol* a All compounds were run at 60" C., with the exception of those marked with an asterisk, which were run at 25" C. b Evaluation of the reactivity of a phenol with the reagent is based on per cent discoloration caused by the phenol during a 30-minute reaction period using indicated mole ratio. Descriptive terms used are defined as follows: no reaction, 0-573 discoloration; Poor, 5 1 0 % discoloration; fair, 10-30% discoloration; and good 30-100% discoloration.
212
ANALYTICAL CHEMISTRY
where k is the rate constant in the medium of dielectric constant D and k, is the rate constant in a medium of dielectric constant unity; p represents the dipole moment, and r , the molecular radius; N , R, and T are Avogadro's number, the gas constant, and absolute temperature, respectively (1). The equation predicts that if the activated complex is more polar than the reactants, the reaction increases with an increase in dielectric constant. A plot of log kvs. ( D - 1 ) / ( 2 0 1) for reactions involving charged species in mixtures of two solvents should be linear, assuming there are no interfering influences. Consequently, a study was made to determine whether or not this relationship, Equation 1, was followed for the phenol-hydrazyl reaction in an ethanolbenzene system. An ethanol-benzene system was chosen because it offers the possibility of covering a wide dielectric constant range. This approach parallels that used by Hanna and Siggia in their study of the rates of bromination of unsaturated compounds (2). To make this study, it was necessary to obtain kinetic data. The collection of reliable quantitative data for this reaction was difficult, for, as others have pointed out ( 4 ) ,the reaction product, diphenylpicrylhydrazine, retards the reaction. The use of initial rates of reaction where the concentration of the retarding product is low is imperative. The method chosen to obtain the required kinetic data is a graphical one suggested by Wilkinson (IO). This method supplies the order of the reaction as well as the rate of reaction, One plots t/p us. t where p is the degree of completion of the reaction a t time t . For reactions up to third-order reactions, the plot is linear with a slope equal to n / 2 where n is the order of the reaction. The intercept is 1/K, where K is the value of the reaction constant k a t the initial concentration, Le., K = kC,n-'. With this method, rate data in ethanol-benzene mixtures were collected for p-methoxyphenol and 8hydroxyquinoline. Plots of log k US. ( D - 1 ) / ( 2 0 1) for 8-hydroxyquinoline and p-methoxyphenol reactions in the ethanol-benzene system were found to be linear over a great part of the dielectric constant range. There is, however, some deviation from linearity as one approaches the lower ethanol concentration, that is, below 10% ethanol. A possible explanation for this behavior is that ethanol forms a strong association with the reagent, enabling a small percentage of the ethanol in a benzene solution to complex with the reagent and consequently retard the reaction with the phenol. Once all the hydrazyl present has been complexed, however, further addition of ethanol will have no greater
+
+
retarding effect, but it will increase the polarity of the medium. This hypothesis gains support from the fact that the reaction rate in benzene alone is greater than that of the lower ethanolbenzene mixtures. This work indicates that Equation 1 is followed for the phenol-hydrazyl reaction as long a s complications such as association reactions are not met. It should be emphasized that dielectric constant is
Table
II.
not the only factor which influences the rate of reaction of hydrazyl with a given phenol, but it is a n important factor and one which can most readily be quantitatively correlated. Further quantitative indications of solvent effects on the rate of hydrazyl reaction with phenols were considered. When a variety of solvents are being considered, however, Equation 1 is not applicable and, hence, a simple plot of
Room Temperature Rate Data for Reaction of 8-Hydroxyquinoline with Diphenylpicrylhydrazyl in Various Solvents
Rate constant liters mole -1 minute-1 193
Dielectric Solvent system constant" Methanol 32.6 Ethanol 24.3 1-Propanol 20.1 1-Butanol 17.1 2-Propanol 18.3 2-Methyl-1-propanol 17.7 Benzyl alcohol 13.1 1-Oct anol 11.3b 9 . 17,Ethanol-90. 970 benzenec 4.2 18,37, Ethanol-81. 770 benzenec 6.1 37. 37, Ethanol-62.77, benzenec 10.1 57.27, Ethanol-42.8% benzenec 14.7 21.1 78.1y0 Ethanol-21. 9yc benzenec a Values obtained from references (3)and (9). b This value was determined in this laboratory a t 25" C. Chemical Oscillometer. c Weight-weight percentages.
Table 111.
89
49 45 28 17 47 24 5 13 48 76 83
Order of reaction 2.0 2.0 2.0 2.0 2.0 1.9 1.8
2.0 4.4 2.1 2.0 2.2 2.3
using- a Sargent Model V
Room Temperature Rate Data for Reaction of p-Methoxyphenol with Diphenylpicrylhydrazyl in Various Solvents
Dielectric constanta Solvent system Methanol 32.6 24.3 Ethanol 1-Prooanol 20.1 l-Butanol 17.1 2-Propanol 18.3 9. lYc Ethanol-90.97, benzeneb 4 2 18 370 Ethanol-81. 77, benzeneb 6 1 37 3% Ethanol-62.77" benzeneb 10.1 57.2% Ethanol-42.8% benzeneb 14.7 78 1%Ethano1-21.9% benzeneb 21.1 8 9% Ethanol-11 1% benzeneb 22.9 94 4% Ethanol- 5 670 benzeneb 24.4 Talues obtained from references (3) and (9). b Weight-weight percentages.
Table IV.
Phenol 6-Kaphthol PHexylresorcinol Hexachlorophene Eugenol Hydroquinone a-N apht hol Diethylstilbestrol
Results of Analysis
Theoretical amount, mg. 0.058 0.078 0.362 0.066 0.011
0.014 0.107
Rate constant liters mole -1 minute -1 5760 5000 4630 3400 900 910 1550 4240 5700 6440 7350 10415
Order of reaction 2.1 2.4 2- . 6_
2.6 2.5 2 0
1.9 2~.4 ~
2 2
3.0 2.8 3.3
of Some Typical Phenols Amount found, mg. 0.058, 0.058, 0.058 0.078, 0.078, 0.077 0.326, 0.326. 0.326 0.066; 0.066; 0.066 0.011,0.011,0.011 0.014, 0.014, 0.014 0.107, 0.107, 0.107
Table V.
Results of Phenol Analyses in Phenol Mixtures
Theoretical, Mixture c-Methoxyphenol Vanillin o-Methoxyphenol Vanillin Pyrocatechol Phenol Pyrocatechol Phenol
%
Found, %
50 50 75 25 50 50 75 25
50, 50, 49
76, 75, 74 50, 50, 50 76, 75, 75
log k us. dielectric constant must be made. If one plots the rate data listed for the various alcoholic solvents in Tables I1 and 111,one finds that there is a n increase in the reactivity for both the 8-hydroxyquinoline and the p methoxyphenol as the dielectric constant increases. .Is a matter of fact, if only the normal chain alcohols are considered, a linear relationship is obtained. It is noted in Tables I1 and I11 that the computed order of reaction for most phenols considered is close to second order. This is in agreement with the findings of earlier workers in the field. Practical Applicability in Phenol Analysis. The results of the analyses of a number of phenols are summarized in Table IV. The reproducibility and accuracy of these results are all one could ask for. However, use of a calibration curve prepared from standards run along with the samples v, as necessary t o achieve such results. I t was found that a sensitivity down to 10-3 to 10-5 mmoles of phenol could be obtained. Attempts were made a t assaying one phenol in the presence of another. Successful analysis of such mixtures depends upon the difference in the rate of reaction between the phenols in question. The analysis of o-methoxyphenol in the presence of vanillin was the first determination attempted. oMethoxyphenol is a starting material in the synthesis of vanillin and could very well be a contaminant of the vanillin. Because the reaction of o-methoxyphenol with the reagent is rapid a t room temperature, whereas the vanillin reaction is only appreciable a t elevated temperatures, viz., 60" C., o-methoxyphenol in vanillin could be determined, see Table V. Successful analyses of pyrocatechol in phenol were also accomplished, see Table V. The analysis of many more phenolic mixtures is possible under the proper reaction conditions. However, this procedure should be applied with caution to the analysis of a phenol in a n unknown mixture since both aromatic amines and mercaptans are reactive with diphenylpicrylhydrazyl. VOL. 38, NO. 2, FEBRUARY 1966
213
LITERATURE CITED
(1) Frost, A. A., Pearson, R. G., “Kinetics and Mechanism,” 2nd ed., p. 140, Wiley, New York, 1961. (2) Hanna, J. G., Siggia, S., ANAL. CHEM.37, 690 (1965). (3) Hodgman, C. D., “Handbook of Chemistry and Physics,” 42nd ed., p. 2513-22, The Chemical Rubber Publishing Co., Cleveland, Ohio, 1961.
(4) Hogg, J. S., Lohmann, D. H., Russell, K. E., Can. J. Chem. 39, 1588 (1961). (5) McGowan, J. C., Powell, T., J. Chem. Soe., 2106 (1961). (6) McGowan, J. C., Powell, T., Raw, R., Ibid., 3103 (1959). ( 7 ) Papariello, G. J., Janish, M. A. M., ANAL.CHEM.37, 899 (1966). (8) Venker, P., Herzmann, H., Naturwissenschuften 47, 133 (1960).
(9) Washburn, E. W., “International Critical Tables of Numerical Data, Physics, Chemistry and Technology,” Vol. 6, p. 102, McGraw-Hill, New York, 1929. (10) Wilkinson, R. W., Chem. & Ind. (London) 1961, 1395. RECEIVEDfor review July 27, 1965. Accepted December 2, 1965.
Spectrofluorometric Trace Determination of Trivalent Samarium, Europium, Terbium, and Dysprosium in Sodium Tungstate Solutions GlULlO ALBERT1 and MARIA A. MASSUCCI laboraforio di Chimica delle Radiazioni e Chimica Nucleare del CNEN, lstifufo di Chimica Generale ed Inorganica, Universifi di Roma, Italy Sodium tungstate acts as a specific reagent for enhancing the fluorescence intensity of samarium, europium, terbium, and dysprosium in aqueous solutions. Maximum fluorescence intensity is obtained by irradiating at 265-270 mp lanthanides dissolved in 0.6M sodium tungstate solution with a final pH of 9 and a temperature of 2 0 ” c. All measurements are related to the fluorescence intensity of a quinine sulfate solution. Correction for quenching by inorganic ions and conditions for decreasing positive interferences are given, Sensitivity is 10-1-1 0-3 Clg./ml.
I
that the analytical problems encountered in the determination of the lanthanide ions, especially as traces in solutions, arise from their very similar chemical properties. Therefore, it is extremely difficult to find specific reactions for the individual ions. Spectrophotometric methods produce satisfactory results for only high lanthanide ion concentrations, since the molar absorptivities of the individual ions are rather low (8). Better results are obtained using optical emission spectra (9,13) or activation analysis (6),but it is still difficult to determine traces of individual lanthanides in complex mixtures. Fluorometry has been only partially successful because the fluorescent intensity of solutions of common soluble salts of the lanthanides-e.g., sulfates, chlorides, and nitrates-are fairly low (10, 12, 14). A previous paper (4) described certain characteristic fluorescence tests for lanthanide ions on filter paper. It was possible to identify with sodium tungstate, 10-2 pg. of Samarium, 2 X T IS WELL KXOWN
214
ANALYTICAL CHEMISTRY
pg, of dysprosium, and pg. of europium; with sodium oxalate, pg. of terbium; and with sodium tetraborate, 5 pg. of cerium (111). The specificity of these reactions and the ease of separation of lanthanide ions on ion exchange paper make these tests fairly useful for qualitative and semiquantitative analyses of traces of these ions, but not for quantitative estimations. We reported (3) that in sodium tungstate solution the molar absorptivities of all the lanthanides in the ultraviolet range (260-270 mp) were considerably increased (about lo4 times). These studies showed that samarium, europium, and dysprosium in sodium tungstate solutions are from 104 to 105 times more fluorescent than in mineral acid solutions. It seemed appropriate, therefore, to investigate the possibilities of determining these ions in tungstate solutions by spectrofluorometry. THEORY
The transfer mechanism of electronic excitation has been extensively studied in recent years. It is well established that excitation transfer may occur not only between different molecules, but also between separate electronic systems of the same molecule (11). The excitation transfer between different molecules may occur if the absorption spectrum of the acceptor overlaps the fluorescence spectrum of the sensitizer (resonance transfer). Since the individual trivalent lanthanide ions have different but characteristic absorption spectra, it is possible to transfer selectively the absorbed energy only to a given ion. This is achieved by the choice of a suitable sensitizer whose fluorescence spectrum overlaps solely,
or more completely than any other ion, the absorption spectrum of the lanthanide ion to be determined. Crosby ( 7 ) has established that the requirement for the intramolecular energy transfer in a given lanthanide complex is that the lowest triplet state energy level of the complex must be nearly equal to or must lie above the resonance level of the lanthanide. In this case, too, there is the possibility of transferring the energy selectively to certain lanthanide ions by the suitable choice of a complexing agent, so that the lowest triplet state will be in a position favorable only to the resonance level of such ions. Since the energy transferred to the lanthanide ions may be emitted again as fluorescence in the visible region, this transfer of energy can be employed to cause selective fluorescence of given lanthanide ions. Since the molar absorptivities of lanthanide complexes are much greater than those of the simple ions, it should be possible to obtain also in these cases high fluorescence intensities. Such a possibility is of particular importance for the estimation of traces of single lanthanide ions in mixtures. The selective increase in the fluorescence intensity of samarium, europium, and dysprosium in scdium tungstate solutions, and of terbium in oxalate solutions (1, 3) must be connected with ionic association or chelate formation of lanthanides which absorb in the ultraviolet region and with energy transfer to the lanthanide ions of that complex. Since this energy cannot be transferred to the gadolinium or cerium ions, these ions, although highly fluoresc ent in chloride solutions, do not show appreciable fluorescence in aqueous tungstate or oxalate solutions.
