t h a t their means were equal except for 2:31Pawhere t h e F ratio was barely significant. Additional data were obtained on precision by applying t h e procedure t o 500-ml samples of a uranium mill process waste known to contain about 3 mg of cerium and four times t h e 100-pg limit for t h e electrodeposition of 2,'3nTh. This sample was chosen to represent one of the worst cases for t,he determination of thorium. T h e results of the five replicate analyses are summarized in Table V, and an alpha spectrum of the thorium fraction is shown in Figure 4. Alpha spectra were obtained under the conditions described by Sill a n d Olson (31 ) to minimize recoil at~omcontamination of the solid state detector. T h e counting efficiency for the ""Ac fractions was reduced to correct for the additional precipitate formed from cerium present in t h e sample. Thorium recoveries were determined with ','j4Th tracer and ranged from only 8 t o 25% because of large losses in t h e electrodeposition. In spite of t h e large thorium losses. t h e relative deviation of an individual result from the mean was only 4')/0 for r:ioTh, b u t 9, 16, and 5% for 2:iaTh, ?.STh, and "25Th, respectively. A major factor in the variance of t h e ?;'"Th results was t h e large correct,ion applied for t h e contribution of degraded ':ioTh alpha particles in the ';"'Th energy region. T h e relative uncertainties of just the numbers of counts were approximately 8 to 12% a n d 2 to 4% in the '"Th a n d 22TThenergy regions, respectively. T h e wide variance of t h e results for '2,'i1Pawas due to its large a n d erratic losses on silica from the process waste sample and also in the electrodeposition. I n such cases, thorium recoveries are not reliable estimates of ',jlPa recoveries. and the results were included in Table V merely to emphasize this point. T h e relative deviations of individual results about
the means for 2 2 7 A ~228Ra, , and 2zsRa were 2 to 4%, about t h e same as t h e precision for 2 2 7 Aand ~ 22sRa in t h e series of pitchblende ore analyses. Sensitiuity. T h e following detection limits for 500-ml liquid and 100-m" air samples were calculated for paired observations (32) and are merely intended t o illustrate t h e sensitivities of t h e procedure under the conditions specified. T h e blank activity for the determination of 2zsRa by either gross-alpha counting' or the emanation method is about 25 cph, which results in detection limits for gross counting and emanation counting, respectively, of 1 X 10-9 a n d 3 X 10-lo pCi/ml for liquid samples and 4.5 X 10-15 and 1.4 X pCi/ml for air samples. T h e 228Rablank of about 195 counts per 50 minutes is equivalent to a detection limit of 2.5 X 10-9 pCi/ml for a liquid sample and 1.2 X pCi/ml for a n air sample. However, decay of t h e 6.13-hour 228Ac used for the indirect determination of 2L'xRaresults in a practical detection limit 1.5 to 2 times these values. T h e blank for ";Ac is about 7 cph, and the detection limit is 6 X pCi/ml for liquid and 3 X pCi/ml for air samples. Gross alpha blanks of thorium fractions are roughly 25 cph making t h e detection limit 9 X 10-l') pCi/ml and 5 X 10-15 pCi/ml for liquid and air samples, respectively. T h e detection limits for individual thorium isotopes determined by alpha spectrometry may be higher or lower than for gross counting depending on t h e presence or absence of interferences from higher energy alpha emitters. T h e detection limits given above represent 1 X lo-' to 3 X lop4of RCG for 500-ml liquid samples and 5 X lo-? to 7 X lo-* of RCG for 100-m:i air samples.
(31) C. W . Sill and D. G. Olson, Anal. Chem., 42, 1596 (1970)
(32) L. A . Currie. Anal. Chem., 40, 586 (1968).
RECEIVEDfor review December 3, 1973. Accepted J u n e 13, 1974.
Excited Singlet State Acidity Constants for Hydroxy and Amino Substituted Anthraquinones and Related Compounds: A Comparison of Excited-State pK Measurement Procedures H. H. Richtol and 6. R. Fitch Department of Chemistry, Rensselaer Polytechnic Institute, Troy, N. Y
Excited singlet state acidity constants of hydroxy and amino derivatives of anthraquinones, fluorenes, fluorenones, and a xanthone were investigated in 33 YO dioxane-water. The excited singlet state acidity constants were determined by four methods. Three of these are based on the experimental determination of the energy differences between the excited state acidic and basic forms measured by absorption, fluorescence, and by an average of absorption and fluorescence energies. The fourth method involved the measurement of fluorescence intensity as a function of solution acidity. Experimental values showed increases in acidity of from 5 to 16 orders of magnitude over the ground state. Values obtained from fluorescence titration of both conjugate acid and base forms and from the averaging of fluorescence and absorption data give the best internal agreement.
12787
Initial observation of "abnormal" fluorescence due to protolytic dissociation was reported by Weber ( I ) . T h e effect was interpreted by Forster ( 2 ) who first demonstrated t h e occurrence of excited state dissociation with solutions of hydroxy- and amino-pyrene sulfonates. Weller ( 3 ) outlined a method whereby the value of pK* can be estimated from absorption and fluorescence spectra, if equilibrium is established in the excited state. H e was able to calculate the value of pK* provided t h e value of the ground state pK was known independently. Bartok, Lucchesi, and Snider ( 4 ) measured t h e effects of (1) K.Weber, Z. Phys. Chem. (Leipzig), 815, 18 (1931). (2) Th Forster, 2. Elektrochem., 54, 42 (1950). (3) A . WeIiL,, 2.Elektrochem., 56, 662 (1952). (4) W . Bartok. P. J. Lucchesi, and N. S . Snider, J. Amer. Chem Soc., 84, 1842 (1962).
A N A L Y T I C A L C H E M I S T R Y . VOL. 46, N O . 12, OCTOBER 1974
1749
electronic dissociation of para-substituted phenols, estimating the values of p K * from spectroscopic data. They found t h a t apart from large increases in acidity as a result of excitation, t h e dissociation constants of the excited phenols could not be correlated. Haylock, Mason, and Smith ( 5 ) ,carried out a critical examination of the methods employed for determining the prototropic equilibria of electronically excited molecules. Mason e t a1 (6) state t h a t the theoretical values of the acid dissociation constant of an excited-state equilibrium calculated from absorption and emission data may differ one from the other by as much as six pK, units, and both may diverge substantially from the value obtained experimentally from the variation of the fluorescence spectrum with the acidity function of the medium. Wehry and Rogers ( 7 ) measured protolytic dissociation of phenol and numerous monosubstituted phenols in the lowest triplet and first excited singlet states by means of fluorescence, phosphorescence, and ultraviolet absorption spectra. I t was shown that excited state acidities can be correlated well with ground state substituent constants. Schulman and Fernando (8, 9) have measured t h e excited state pK's of halogenated 8-quinolinols by the fluorimetric titration method. Using the Hammett acidity scale they were able to determine p K * values as low as minus eleven. Goldman and Wehry ( I O ) , having disputed the excited state equilibrium hypothesis, studied the fluorescence of 8-quinolinol in several media and concluded that the quenching of 8-quinolinol fluorescence is due to hydrogen bonding by hydrolytic solvents. However, Schulman ( 1 1 ) defended the excited state prototropic equilibrium hypothesis stating that while hydrogen bonding is probably of considerable importance in determining t h e quantum yields of the various species derived from 8-quinolinol in different solvents, t h e fluorescence phenomena observed in solutions of differing acidities and basicities are due primarily to prototropic equilibria in the lowest excited singlet states of the various prototropic species derived from 8quinolinol. Rosenberg and Brinn ( 2 2 ) have determined singlet excited state dissociation rate constants for naphthols and rationalized the results on the basis of a change in chargetransfer character. Recently, Lasser and Feitelson (13)have shown that fluorescence of either the acidic or the basic form of a molecule alone does not necessarily yield a p K value of t h e excited state. One of the main problems encountered in obtaining acidity constants in t h e excited singlet state is t h e method employed in their calculation and the assumptions made in each of these methods. This paper is concerned with the study of excited state acidity constants in the singlet state of certain anthraquinones, fluorenes, fluorenones, and hydroxyxanthone. Each of the acidity constants is measured by several methods, and a comparison is made of the results obtained by each method. EXPERIMENTAL Chemicals. Matheson Coleman and Bell spectrograde dioxane was used without further purification. I-Hydroxvanthraquinone, (5) J. C. Haylock, S. F. Mason, and B. E. Smith, J. Chem. Soc., 4897 (1963). (6) S. F. Mason, J. Philip, and B. E. Smith, J. Chem. Soc.. 3051 (1968). (7) E. L. Wehry and L B. Rogers, J. Amer. Chem. Soc., 87, 4235 (1965). (8)S. G. Schulman and Q. Fernando, J. Phys. Chem.. 71, 2668 (1967). (9) S. G. Schulman and 0 . Fernando, Tetrahedron, 24, 1777 (1968). 10) M. Goldman and E. L. Wehry, Anal. Chem., 42, 1178 (1970). 11) S. G. Schulman, Anal. Chem., 43, 285 (1971). 12) J. L. Rosenberg and I. Brinn, J. Phys. Chem., 76, 3558 (1972). 13) N. Lasser and J . Feitelson, J. Phys. Chem.. 77, 1011 (1973).
1750
A N A L Y T I C A L C H E M I S T R Y . VOL. 46,
NO. 12.
1-aminoanthraquinone, 2-aminoanthraquinone, l-hydroxyfluorene, 2-hydroxyflucirene, 9-hydroxyfluorene, %aminofluorene, 4hydroxy-9-fluorenone, 1-amino-9-fluorenone were all available commercially from either Aldrich Chemical Company, Chemical Procurement Laboratory, o r James Hinton Associates. All were recrystallized three times from ethanol. 1-Hydroxyxanthone and 2-hydroxyanthraquinone (14 ) were synthesized in this laboratory and recrystallized three times from ethanol. Buffer solutions of 33% dioxane-water in the p H range 1-14 were prepared by using appropriate mixtures of the following in deionized water solutions: HCI, HOAc, NaOAc, KHyPOd, iYaOH, H.&O:+ Each buffer was brought up to constant ionic strength hy adding an appropriate amount of KC1. For very weak bases, H a m mett indicators were used in HzSOd-HrO mixtures. T h e p H of the solutions were measured on an Orion Model 801 digital p H meter using glass and calomel electrodes. Calibration was made with Harleco standard buffer solutions. Absorption measurements were made on a Beckman DK2-A recording spectrophotometer or a Beckman DU spectrophotometer. Fluorescence measurements were obtained on an instrument constructed in this laboratory ( 1 5 ) , with quinine sulfate in 0.1.V sulfuric acid used as a standard for relative intensities.
RESULTS Ground State Acidity Constants. For the measurement in this work of acidity constants in the ground state, a technique introduced by von Halban ( 1 6 ) and later simplified by Robinson and Biggs ( I 7 ) was used. Further work by Robinson and Kiang (18) and by Biggs (19) helped to simplify procedures. By use of the following equation:
K , the dissociation constant can be obtained, where
m H is the hydrogen ion molality and YH,7.4 are the activity coefficients of the hydrogen ion and the respective anions, that of the uncharged molecule being almost unity a t the concentrations used. C Y is the fraction of anion present a t t h e particular pH. As calculated from the data of Robinson and Biggs (171, the p H is equal to -log Y H m H . From the extended Debye-Huckel(20) equation, the value of -/A can be estimated. Therefore, from changes in the absorption spectra a t varying pH, the dissociation constant can be calculated. T h e majority of t h e compounds studied showed distinct changes in the spectra of the protonated and anion forms of the acids, with clear isosbestic points. The results are tabulated in Table I and compared to literature values which were obtained in different solvent systems. pK Values of the Excited Singlet State. The dissociation constants of organic acids and bases in the first excited singlet state often differ from the corresponding ground electronic state value by several orders of magnitude because of the redistribution of electronic charge density on excitation ( 2 6 ) .Various methods are available for the evaluation of these excited state acidity constants and the
(14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26)
OCTOBER 1974
A . G. Perkins and T. W. Whattam, J. Chem. Soc., 121, 289 (1921). H. H. Richtol and F. H. Klappmeier, J. Chem. Phys., 44, 1519 (1966). H. V. Halban and T. Ebert, 2.Phys. Chem., 112, 359 (1924). R. A. Robinson and A . I. Biggs. Trans. faraday Soc., 51, 901 (1955). R. A . Robinson and K. Kiang, Trans. Faraday SOC.,51, 1398 (1955). A . I. Biggs. Trans. Faraday Soc., 52, 35 (1956). E. A. Guggenneim and T. D. Schindler, J, Phys. Chem., 38, 533 (1934). H. Treadwell and G. Schwarzenbach, Helv. Chim. Acta., 11, 386 (1928). E. Gerasimenko, S. S. Tkachenko and Y . B. Shteinberg, Reakts Sposbnost Org. Soedin, 6, 1065 (1969). P. H. Grantham. E. K Weisberger and J. H Weisberger. J. Org. Chem.. 26, 1008 (1961). V P. Kreiter. W. A . Bonner. and R. H. Eastman, J. Amer. Chem. Soc., 76, 5770 (1954). L. Sillen, "Stability Constants of Metal-Ion Compounds," 2nd ed., The Chemical Society. London, 1964, p 676. A . Weller, "Progress in Reaction Kinetics," VoI. 1. G. Porter, Ed., Pergamon, Oxford, 1961, p 189.
Table I. Ground State pK Values This Work
Iieporled !Ref.)
1-Ilydroxyanthraquinone
9.3
8.8
(96% EtOH) (21)
2-Hy droxyanthra-
7.1
5.7
(96% E t O H ) (21)
0.52
0 . 5 7 (H2SO:)
quinone 1-Aminoanthraquinone 2-Aniinoanthraquinone
(22)
0,76 9.3 9.1
1-hydroxy fluorene
2-H> droxyfluorene
14.6 4.3 8.4
9-hydroxy fluorene
2-Aniinofluorene 4-Hydroxy-9flu orenone 1-Amino-9-fluorenone
-0.3
1-Hydroxyxant hone
1 1 . 4 (70% EtOH) (23) 1 1 . 6 (70% EtOH) (23) 9.5 (H2O) ( 2 4 ) 4.4
(70% EtOH) (23)
-0 3
(7070 EtOH) ( H S O . ) 123)
10 7 125)
the ground state p K values. For 1-hydroxyfluorene, 2-hydroxyfluorene, and 9-hydroxyfluorene, the absorption maximum of t h e acidic form was obtained directly from the absorption spectra. However, the long wavelength absorption band of the basic form was determined more exactly from excitation spectra as it appeared either as a broad band or as the shoulder of a more major peak. Results for all compounds are summarized in Table I1 where p K is the dissociation constant for the ground state species. pK," is t h e dissociation constant for t h e species in the lowest excited singlet state, ua is the wavenumber of the long wavelength absorption maximum of t h e acid form, and l'h is the wavenumber of t h e long wavelength absorption maximum of the basic form. From Fluorescence Spectra If t h e analogous assumption is made with respect to fluorescence spectra t h a t was made for absorption spectra--l.e , t h a t t h e maximum of the fluorescence bands of the acid and base occur an equal amount above the 0-0 transition-then similar computations can be performed using fluorescence data. This meth~
~~
Table 11. Excited Singlet State Acidity Constants Determined from Absorption Spectra in 3 3 6 Dioxane-Water Solutions' Compound
Ph'
1-Hydroxyanthraquinone
9.3 7.1 0.52 03 6 9.3 9.1 14.6 4.3 8.4 -0.3 10.7
2-Hydroxyanthracjuinoiie
1-Aminoa~ithrac~uinone 2-Aminoant hracluinone 1-Hydroxyflucirene 2-Hydroxyfluorene 9-Hy droxyfluorene 2-Aminofluorene 4-Hydroxy-9-fluorenone 1-Amino-9-fluorenone 1-Hydroxyxant hone Precision for all values
1'1 rt 2
u,icin-')
X 10-1
Y ,,icm-1)
2.49 2 67 3 02 3 07 3.40 3.20 3.68 3.33 2.34 3.04 2.27
x
2.04 2 12 2.08 2.23 3.19 2.94 3.17 3.13 1.99 2.31 2.03
10 - 1
-\PK
9.5 11.5 19.7 17.6 4.4 5 .0 10.7 4.2 7.4 15.3 5 .O
pK.
:
-0.2 -4.4 -19.2 -16.8
4.9 4.1 3 .9 0.1 1. o - 15 , 6
5.7
.
Table 111. Excited Singlet State Acidity Constants Determined from Fluorescence Spectra in 3 3 5 Dioxane--Water SolutionL C'oinpound
1-Hydroxy fluorene 2- Hydroxy fluorene
9-hydroxy fluorene
2-Aminofluorene '' Precision
PK
v i cm-1) X 10-1
9.3 9.1 14.6 4.3
2.99 2.97 3.10 2.94
ui,(ctn-1)
X 10--
2.44 2.43 2.44 2.70
QK
pK,"
11.5 11.3 13.8 5.0
-2.2 -2.2 0.8
-0.7
for t h e above ~ a l u e sis = 2 ' , .
values obtained by these methods can also often differ markedly. From Absorption Spectra. The Forster cycle (2) is applicable only to the 0-0 transition of either absorption or emission spectra. Unfortunately, the position of the 0-0 band is generally difficult to evaluate in solution. If the assumption is made t h a t the maximum of the absorption bands of the acid and base occur equal amounts above the 0-0 transition, then absorption spectra alone can be used in a Forster cycle to determine the energy differences between t h e ground and t h e excited states and hence to calrulate the first excited singlet state pK*. T h e equation used is
pK
-
1E - 1E' - 2.303RT
pK* -
where AE and AE' are the absorption energies of the acid and anion, respectively. For most of t h e compounds under investigation, data could be taken directly from t h e spectra used to determine
od was also attempted and the results are tabulated in Table 111. It should be noted that excited singlet state acidity constants could be calculated, using this method, only for those compounds which gave fluorescence of both the acid and its conjugate base. From t h e Average of Absorption and Fluorescrnce. Rartok, Lucchesi and Snider ( 4 ) sought a more accurate indication of the 0-0 transition. They felt this could be achieved by taking an average o f the energy values obtained in both absorption and fluorescence. However, this method too has the limitation t h a t it can be applied only to those compounds which fluoresce in both acidic and hasic forms. Table IV presents the results for those compounds whose excited singlet state acidity constants were measured by this method. In the table, p K is the dissociation constant for the ground state species; (uat1 + ~ ! 1 ) / 2is the average value of the wavenumbers of the long wavelength of absorption and the short wavelength of fluorescence for the acidicform: (v,kl' ut.1')/2 is the average value of the wavenumbers of the long wavelength of absorption and the
+
A N A L Y T I C A L C H E M I S T R Y . VOL. 46. NO. 12, OCTOBER 1974
1751
Table IV. Excited Singlet State Acidity C o n s t a n t s D e t e r m i n e d f r o m an Average of A b s o r p t i o n and Fluorescence Spectra in 33 % Dioxane-Water Solutionsi ",I)
C'onipound
PK
1-Hydroxyfluorene 2-Hydroxyfluorene 9-Hydroxyfluorene 2-Aminofluorene
9.3 9.1 14.6 4.3
'' Precision
for a11 Idilues
IS
+
"fl
Y*l>'
3.20 3.09 3.39 3.14
4- V i l '
2
2
2.82 X l o 4 2 . 6 9 X 10;
x 10' X lo4 X 104 X 101
pK. e
APK
2.81 X 10' 2 . 9 2 X 10'
8 .0
1.3
8.4 12.2 4.6
0.7 2.4 -0 . 3
12''.
T a b l e V. Excited Singlet State Acidity C o n s t a n t s D e t e r m i n e d by "Fluorescence Titration" PK
Compound
1-Hydroxyanthraquinone 2-Hydroxyanthraquinone
4-Hydrox~-9-fluoreiioiie
9.3 7.1 9.3 9.1 14.6 4.3 8.4
1-Ilydroxyxanthone
10.7
1-hydroxy fluorene
2-€Iydr
