determined b y the three parameters ui, S.E.,M, S.E., and knowing these, a system-to-system comparison may be proposed. Replacing (M2-MI) b y S.E.,M and (Q-u,) by S.E.,, and expressing the error as a per cent of the mean or dispersion ( E X P , ESP) , Equation 7 becomes
1.481 log (ESP) - 2.384
where is the mobility of the minimal impurity which can be detected by the system. This purity detection index would appear to be the logical basis on which to compare two systems for their ability to establish chromatographic purity. It also provides a single number for expressing the chromatographic purity of a compound in a given system and can therefore supply the basis for a rigorous purity criterion.
(9)
This purity function may be used in the following fashion: Purity detection index =
LITERATURE CITED
(1) Ettre, L. S., ANAL.CHEM.36, 31A
(1964). (2) Finney, D. J., “Probit Analysis,]’ Cambridge Univ. Press, London, 1952. (3) Hastings, C. B., Jr., “Approximations for Digital Computers,” Princeton
Univ. Press, Princeton, N. J., 1955. (4) Klein, P. D., Simborg, D. W., Szczepanik, P. A., Pure Appl. Chem. 8 , 357 (1964). (5) Kunze, B., Tyler, S., Dipert, 31. H., “Semiannual Report of the Division of Biological and Medical Research,” p. 152, Arbonne Xational Laboratory, ANL-6823, January-July 1963. (6) ‘,‘%diochemistry Methods of Analysis, Proceedings of the IAEA Symposium in Radiochemical blethods of Analysis, Sulzburg, 1964, Vol. 2, p. 353, International Btomic Energy Agency, Vienna, 1965. (7) Tyler, S. A., Gurian, J., “Deterrnination of the LDSo, by Use of Probit, Angular and Logit Transformations,” Argonne National Laboratory Rept. ANL-4486, 1950. RECEIVEDfor review May 10, 1965. Accepted J ~ l 19, y 1965. Work supported by U. S. Atomic Energy Commission.
Structure and Behavior of Organic Analytical Reagents Some Aryl Azo 8-Quinolinols SUSUMU TAKAMOT0,l QUINTUS FERNANDO, and HENRY FREISER Department of Chemistry, University o f Arizona, Tucson, Ariz.
b The metal chelate formation constants of 5-benzeneazo-, 5-(o-hydroxyphenylazo)-, 5-(m-hydroxyphenylazo)-, and 5-(p-hydroxyphenylazo)-8-quinolinols have been determined potentiometrically in a 5070 v./v. dioxanewater medium a t 25” C. The acid dissociation constants of these ligands have been determined both spectrophotometrically and potentiometrically in a 5070 v./v. dioxane-water medium a t 2 5 ” C. The aryl azo substituents exert an acid strengthening effect on these reagents. The metal chelate formation constants are lower than the corresponding values for 8-quinolinol. As with other 8-quinolinols having electron-withdrawing substituents, the proton displacement constants are significantly higher. These reagents therefore have the added analytical advantage over the parent reagents, of forming metal chelates a t lower p H values.
of the chelates such as color and solubility. From the behavior of the alkylated and halogenated 8-quinolinols1 it would appear that electron-withdrawing substituents give rise to higher values of proton displacement constants. Since the chromophoric arylazo groups, wh n incorporated in a chelating agent such as 8-quinolinol, confer useful analytical properties to the reagent, i t is of interest to determine whether its electron-withdrawing behavior would have the predicted effect on the proton displacement constant of 8-quinolinol. An attempt has been made, by using the hydroxy substituted arylazo groups, to further modify the electron-withdrawing property of the arylazo group with a view to obtaining analytical reagents with large proton displacement constants.
0
Preparation of Azo Compounds. 5-Benzeneazo-, 5-( (o-hydroxyphenylazo)-, 5-(m-hydroxyphenylazo)-, and 5 - ( p - hydroxyphenylazo) - 8 - quinolinol were prepared b y a method similar to that used by Fox ( 2 ) for the preparation of 5-benzeneazo-8-quinolinol. 5Benzeneazo-8-quinolinol was purified by reprecipitation with acetic acid from a KOH solution and recrystallization from toluene, m.p. 189’ C. (dec.); lit. m.p., 170” C. ( 2 ) . 5-(o-Hydroxyuhenvlazo~-8-auinolinol was recrvstaliized’from’dioiane, m.p. 222’ C. (dec.); yo C, 67.82 found, 67.91 calcd. % H,
principal advantages of using organic reagents in analysis lies in the possibility of modifying the behavior of the reagent molecule by appropriate substitution. A wide range of electronic and steric effects of various substituents has been used t o bring about changes not only in those factors which affect the parameters that control the conditions of formation of metal chelates, such as the proton di placement constant ( 4 ) ,but also in those factors t h a t influence the properties NE OF THE
EXPERIMENTAL
4.37 found, 4.18 calcd. &(m-Hydroxyphenylazo)-8-quinolinol was recrystnllized from absolute ethanol, m.p. 210” C.; % C, 67.57 found, 67.91 calcd. % H 4.29 found, 4.18 calcd. 5-(pHydroxyphenylazo)-8-quinolinol was also recrystallized from absolute ethanol, m.p. 239’ C. (dec.); yo C, 67.72 found, 67.91 calcd. yo H, 4.27 found, 4.18 calcd. Reagents. 1,4-Dioxane was purified by refluxing over sodium metal for 48 hours and then fractionated through a 4-ft. column packed with glass beads. T h e distillate was collected between 99’ and 101” C., stored in t h e dark, and used within a week. All other compounds used were of reagent grade purity. Stock solutions of metal ions (0.01 to 0.002J1) were prepared from metal perchlorates and standardized gravimetrically. Apparatus. All potentiometric measurements were made with a glass and saturated calomel electrode pair and a Beckman Research p H Meter, standardized with a Beckman buffer solution a t p H 7.00 at 25” C. The titration vessel and the titration with carbonate free S a O H in a nitrogen atmosphere have been previously described in detail (3). Determination of pH M e t e r Correction and Ion Product of Water in 50% v./v. Aqueous Dioxane. A 0.01 J1 solution of HCIOl in 50% v./v. aqueous dioxane and at a constant ionic strength of 0.1, was titrated with ‘On leave from the Department of Physics and Chemistry, Gakushuin University, Tokyo, Japan. VOL 37, NO. 10, SEPTEMBER 1965
1249
Table 1.
Metal Chelate Formation Constants of Azo Substituted 8-Quinolinols in Aqueous Dioxane at 25” C. and tonic Strength 0.1
50% v./v.
5-Azo substituent
Metal ion Ni(I1) Ni(I1) Ni(1X) Ni(I1) Co( I1) Co(I1) Co(11) Co(11) Pb(I1) Pb(I1) Pb(I1) Pb(I1) Zn(I1) Zn(I1) Zn(I1) Zn(I1) Cd(I1) Cd( XI) Cd(I1) Cd(I1) Mn(I1) Mn(I1) hln(1I) Mn(I1)
in 8-quinolinol Benzeneazo o- Hydroxyphenylazo m-Hydroxyphenylazo p-H ydrox yphenylazo Benzeneazo o-Hy droxyphenylazo m-H y droxyphenylazo p-Hy droxypheny lazo Benzeneazo o-Hydroxyphenylazo m-Hydroxyphenylazo p-H ydrox yphenylazo Benzeneazo o-Hydroxyphenylazo m- Hydroxy phenylazo p-Hydroxyphen ylazo Benzeneazo o-H ydrox yphenylazo m-Hydroxyphen ylazo p-H ydroxyphenylazo Benzeneazo o-hydroxy phenylazo
m-Hy droxyphenylazo p-Hydroxyphenylazo
a standard N a O H solution a t 25’ -f 0.1’ C. T h e difference between t h e calculated hydrogen ion concentration and t h e corresponding p H meter reading a t every point in the titration was tabulated. The meter readings were found to be uniformly higher (by 0.10) than the corresponding values of -log [H+]. This correction was applied to all p H meter readings obtained in 50y0 v./v. aqueous dioxane medium. A standard solution of NaOH was added to a 0.lM solution of KC1 in 50yGv./v. aqueous dioxane at a constant ionic strength of 0.1 at 25’ C. The value of p , K , in this solution was calculated from the stoichiometric values of p H and pOH throughout the titration and found to be 15.38. Potentiometric Determination of Acid Dissociation Constants. A weighed quantity of the azo dye and standard HC104 solution in a 50yG v./v. aqueous dioxane solution was titrated with standard NaOH. In t h e titration for the determination of t h e second and third stepwise dissociation constants t h e HCIOa was replaced by a standard KCl solution. The following equations were used t o calculate the successive acid dissociation constants in the appropriate buffer regions of the titration curve.
Kz
=
~
[ H + ] S’ TE - S’
where S = [Clod-]
+ [OH] [H+l -
S’= [Na+] 1250
+ [H+]- [OH-]
ANALYTICAL CHEMISTRY
(4) (5)
Log
Log 81
82/i31
Log 8 2
9.69 9.73 9.65 9.87 8.81 8.25 8.81 9.09 8.57 8.46 8.77 9.22 8.57 8.17 8.89 9.23 7.50 7.14 7.72 7.85 6.20 7.11 6.56 6.64
8.64 8.65 8.61 8.91 7.93 7.87 8.17 8.10 6.52 6.58 6.28 6.78 7.87 7.76 7.71 7.89 6.85 6.67 6.58 6.61 6.37 5.90 5.96 6.02
18.33 18.5s 18.26 18.78 16.74 16.12 16.98 17.19 15.09 15.04 15.05 16.00 16.44 15.93 16.60 17.12 14.35 13.81 14.30 14.46 12.57 13.01 12.52 12.66
and T R represents the analytical concentration of the azo dye. I n the above equations, the electrolyte components that are used for maintaining a constant ionic strength are omitted. Equations 2 and 3 give approximate values for K Z and K 3 since the second and third buffer regions overlap. The method of successive approximations was used t o calculate K Z and K3 from Equations 6 and 7.
Kz
The values of log O1 and log p z are listed in Table I. Each value is the mean of a t least three values obtained from independent potentiometric titrations. The logarithmic values for the overall formation constants, log pz, are reproducible t o 10.05. The corresponding stepwise formation constant values, log p1 and log p2/p1 are somewhat less reproducible, varying from 20.1 t o 1 0 . 2 depending on the proximity of the values of the successive constants. Spectrophotometric Determination of Acid Dissociation Constants. T h e absorption spectra of the azo derivatives were measured between 300 and 500 mp in 50% v./v. dioxanewater at various p H values by means of a Beckman DB recording spectrophotometer. The solutions used for these measurements consisted of the azo dye (2 X 10-5LU)and appropriate concentrations of acetate or borate buffers in 50% v./v. aqueous dioxane. Perchloric acid and sodium hydroxide were used to control the p H a t the extreme ends of the p H scale. The ionic strength of each solution was maintained at 0.1 and absorbance measurements were made on these solutions in 1-em. silica cells at a wavelength corresponding to an appropriate absorption maximum. Values of pK1 were readily obtained from the equation,
=
[H+IPS’ ( T R- 8’) ([H+]+2 K3’) K3 =
[“+I ((S’- Td(Kz’
+ K3’S’
+
2[H+l [H+] * ( ~ T R - 8’)) Kz’ . (2TB - S’)
(6)
+ (7)
where Kz’ and K3’ are approximate values of K Pand K 3 . Potentiometric Determination of Metal Chelate Formation Constants. Metal chelate formation constants were determined potentiometrically b y the titration of a solution containing t h e azo dye, metal perchlorate sodium perchlorate ( O . l M ) , and perchloric acid with standard N a O H in a 50% v./v. aqueous dioxane medium. Details of the titration procedure have been published ( 3 ) . All computations were carried out with the aid of a n I R M computer. Values of the Bjerrum formation function, ii, and corresponding values of (HR-1, the free ligand anion concentration, were computed and used to plot the formation curve R us. -log [HR-1. Selected values of ii and [HR-] were used to calculate the successive chelate formation constants, and p2/p1,by the use of the method of least squares on a linear form of the formation function Equation 8.
where AH,R+,A H ~ Rand , A represent the absorbance of the protonated form of the azo dye, the neutral form and that of a mixture of the two forms, respectively. Values of pKz and pK3 were also obtained from equations analogous to Equation 9. The values of pKz and pKI for the 5(0-hydroxyphenylazo) derivative were too close together to be calculated by the above method and were obtained by a method of successive approximation from the equation,
where A is the absorbance of a solution containing a mixture of the three forms of the azo dye, H2R, H R - and R-2, and AH,Rthe absorbance of a solution containing only the neutral species, and A . the absorbance of solution, containing the species H R - and R-2, at a wavelength a t which the molar absorptivities of H R - and RP2 are equal. The value of pKz was calculated by using an approximate value of K 3 . Values of pK1, pK2 and pKa determined a t 25’ are summarized in Table 11.
RESULTS AND DISCUSSION
The acid strengthening effect of the electron-withdrawing substituent is evident in the decrease in the pK1 and pKz values of 5-benzeneazo-8-quinolinol from those of 8-quinolinol itself. A comparison of the effect of the substituent on each of the two basic groups of the ligand should involve a consideration of the nature of the substituent, the susceptibility of t.he basic group to eIect'ronic influences, and t'he degree to which such influences can be communicated through the moIecule. With respect to the latter factor, which depends on the position and the nature of t,he substituent, it might be predicted from the calculated charge distribution in 8-quinolinol that substituents in the 2, 4, and t o a much lesser extent, the 6 positions in 8-quinolinol, can transmit electronic effects more readily to the nitrogen atom than those in the 3, 5 , and 7 positions. Conversely, electronic effects could be transmitted more readily to the phenolic group from substituents in the 5 and 7 as well as from the 3-position, t'han from the other positions. The relative estent of the changes observed in the p K values of 5-benzeneazo-8-quinolinol (Table 11) is generally in accord with these considerat'ions. It must be kept in mind, however, t h a t the possibilities of hydrazone-azo tautomerism in these compounds make the measured pK values correspond to apparent constants. I n the hydroxy subst>ituted ligands the values of pK2 probably correspond to the loss of the proton from the 8hydroxy group and those of pK3 correspond to the proton loss from the hydroxy group on the phenyl ring. This seems more likely than the alternative of reversing the assignments of pK2 and pKo. Obviously, when the hydrosy group is ortho to the azo group, the strong intermolecular hydrogen bond that will be formed will have a significant acid-weakening effect. In a comparable case such as that of 1-(2pyridy1azo)-2-naphthol, (PAN) , the p K of the hydroxy group is 12.3 ( 1 ) . From the slight but significant decrease in the p K of the 8-hydroxy group as well as in the pKl it would appear t.hat the ohydroxyphenylazo substituent has a somewhat greater electron-withdrawing influence than the benzeneazo group. When the hydrosy group is in meta position its influence on the dissociation of either the %hydroxy or the protonated quinoline nitrogen would appear to be negligible. The hydroxy group in the para position causes an increase in the values of pKl and pKz
Table II.
Acid Dissociation Constants of Azo Substituted 8-Quinolinols in 50% v./v. Aqueous Dioxane at 2 5 " C. and Ionic Strength 0.1
Substituent in 8-quinolinol None 5-iBinzeneazo) &(o-Hydroxyphenylazo) 5-(m-Hydroxyphenylazo ) 5-( p-Hydroxyphenylazo) a
11 5 4 --.l-
3: Oj(3.13)" 2.9q2.99) 3 . 1l(3.09) 3 31(3.29)
8.84(8.98) 8.51(8.37)
8.84(8.68) 9.15(9.26)
12.0( 12.8) 11 ,2111 . 2 )
10.6(10.8)
Values in parenthesis determined spectrophotometrically.
either as a direct result of its electron releasing influence, or indirectly as a result of the possible formation of a hydrazone tautomer ( 5 ) . Finally the sequence of the pK3 values (0 > m > p ) is understandable from the following considerations. The pK3 values of both the meta and the para compounds are lower than that of the ortho compound since the hydroxy-azo hydrogen bond is absent. The pK3 of the meta compound is essentially the same as t h a t of phenol because the hydroxy group is isolated from electronic effects from the rest of the molecule. The metal stability sequence in each of the arylazo substituted 8-quinolinols is the same as that of the parent coinpound -i.e. , Ni(I1) > Co(1I) > Zn(I1) > Pb(II)> Cd(II)> Mn(I1). This would be expected from compounds in which the substituents are remote from the chelating groups. A quantitative comparison of the effect of a substituent on the f l z value of a particular metal chelate reveals t h a t the estent of the change in this value depends somewhat on the metal involved. As has been pointed out previously, the proton displacement constant, K p d , is of a greater importance to the analytical chemist in describing the formation of a metal chelate than is the chelate formation constant, 8, itself ( 4 ) . This will be true whenever the predominant reagent species does not correspond to the species described in the chelate formation reaction-Le., the predominant reagent species is protonated. If the chelating form of the reagent is capable of being multiply protonated, as is the case with the 8quinolinolate anion, then the values of K p dwill involve one or more acid dissociation constants depending on the pH range in which chelation with a particular metal ion occurs. With 8-quinolinol and the series of reagents studied here, a metal such as nickel is chelated a t pH values below the pK1 of the reagent. Hence, in
calculating the Kpd values for nickel, the following equation must be employed. Xii2 f 2HzOx+
2R O x 2 + 4H+ (11)
in which
Kpd
=
Pz Ki2Kz2
(12)
From the Kpd values thus obtained, i t may be calculated that the p H of formation of the nickel chelates of the arylazo reagents is from 0.9 to 1.3 (ApH = A log Kpd) lower than that of 8-quinolinol itself. For a metal such as manganese(I1) where the p H range of chelate formation is one in which the neutral species predominates, the proton displacement equation can be written as:
+ 2HOx 2 hfnOx2+ 2H+
r\In+2
(13)
in which
Kpd = PzKz2
(14)
For manganese(I1) then, the formation of the arylazo chelates will take place a t pH values from 0.5 to 0.8 lower than that for the 8-quinolinol. With all the metal ions studied here the ortho compound would form chelates at the lowest pH values. LITERATURE CITED
(1) Corsini, A., Yih, I. M., Fernando, Q., Freiser, H., ANAL. CHEW. 34, 1090
(1962). (2) Fox, J. J., J . Chem. SOC.97, 1339 (1910). (3) Johnston, W. D., Freiser, H., J . ilm. Chem. Soc. 74, 1383 (1952). (4) Sun, P. J., Fernando, Q., Freiser, H., ANAL.CHEM.36,2485 (1964). ( 5 ) Zqjlinger, H., "Diazo and Azo Chemistry, p. 322, Interscience, New York, 1961. RECEIVEDfor review April 23, 1965. Accepted July 3, 1965. Work supported by the U . S. Atomic Energy Commission.
VOL. 37, NO. 10, SEPTEMBER 1965
1251
