Equilibrium Constants for 2,3-Dihydroxypyridine and its Complex with Iron(II1) in 1 M Hydrochloric Acid

Transcription

1 Equilibrium Constants for 2,3-Dihydroxypyridine and its Complex with Iron(II1) in 1 M Hydrochloric Acid K. E. CURTIS AND G. F. ATKINSON Deporr~iienr o/'clreriris/ry, Uniurrsi/.r y/ Wo~rrloo, Wo~erloo, Onlurio Received November 4, Improved values for two acidity constants of 2,3-dihydroxypyridine have been computed using program PITMAP. The composition of the complex with iron(111) and its conditional and overall stability constants in I M hydrochloric acid have been evaluated. On a calcule par ordinateur les valeurs ameliorees des deux constantes d'acidite du 2.3-dihydroxypyridine en utilisant le programme PITMAP. On a evalue aussi la composition du complexe avec le fer(i11) et ses constantes de stabilite globales dans I'acide chlorhydrique I IM. Canadian Journal of Chemistry (1972) In a previous report from this laboratory (1 ), 2,3-dihydroxypyridine (DHP) was shown to react with iron(ii1) in mineral acid media 1.0 M in hydrogen ion to form blue complex species of metal-to-ligand ratios of I : I. A spectrophotometric procedure for the analysis of iron(i11) in 1.0 M hydrochloric acid has been developed (2). This work reinvestigates the acidity constants of DHP and reports on the coinplex composition and the conditional and overall stability constants of the complex with iron(ii1) in 1.0 M hydrochloric acid solution. Experimental Relrget~ls Iron(111) chloride solutions were prepared by dilution with 1.0 M hydrochloricacid solution froma x M stock solution prepared and standardized as before (I). Solutions of DHP were prepared by dissolving accurately weighed amounts of the sublimed reagent in a known volume of the appropriate solvent. Other chemicals were reagent grade and were tested as necessary for the absence of interfering impurities. Water used was deionized, distilled, and deaerated with nitrogen. Appuro /us Absorbance measurements were made at 25 C with a double beam Cary 14 recording spectrophotometer using fused quartz cells (I0 mm) throughout. An Orion Model 801 ph meter with a glass and calomel electrode pair calibrated as a concentration probe was used to measure hydrogen ion concentration (3). All volumetric ware was Class A and all pipettes were calibrated. Computations were performed on an IBM 360,'75 computer with graphical output plotted by a Calcomp 763 digital incremental plotter. De/ertiiino/ion oj'rlle Acidiry Consronrs oj'dhp The ph of twenty-two solutions of DHP concentration x lo-" M was adjusted with standardized potassium hydroxide and/or hydrochloric acid solutions to give a range of [HC] = 9.50 M to a ph of The ionic strength of solutions between ph 0 and 14 was maintained at 1.0 by means of potassium chloride solution but constancy was sacrificed at higher acidities in order to determine the "approximate protonation constant" (4) of DHP. The spectrum of each of these solutions was recorded between 220 and 315 nm against a reference solution of a solvent blank, corrected for baseline, and absorbance values at every 5 nm tabulated. The ph of each solution between 0 and 14 was measured while the spectrum was being recorded, while those of the solutions of higher acidities were calculated from known hydrogen ion concentrations. These data were then submitted to the computer program PITMAP (5). /tiues/igo/iotis o/ /lie DHP-Irotr(III) Cotiiplex Two procedures were used : ((I) Following the method of Heller and Schwarzenbach (6) as modified by Budesinsky and Haas (7, 8) and the method of proportional absorbances of Budesinsky (9, lo), three sets of solutions were prepared so that while iron and DHP were kept equimolar in all cases, the iron and DHP, the chloride ion, and the hydrogen ion concentrations respectively were varied within each set. The hydrogen ion concentration was altered or kept constant at 1.0 M as required, with standardized perchloric acid solution, while the chloride ion concentration was similarly controlled by means of a standard sodium chloride solution. Details of all solutions are given in Table I. The spectrum of each solution was recorded from 700 to 500 nm against an appropriate solvent blank, corrected for baseline and absorbance values digitized every 50 nm. (h) Eight spectra were recorded from 700 to 500 nm against the appropriate solvent blanks, for solutions of constant iron(1ii) concentration x lo-' M but with varying DHP concentrations. In this way, metal-to-ligand molar ratios of 0.5, 1.0, 2.5, 5.0, 10.0, 15.0, 20.0, and 25.0 in 1.0 im hydrochloric acid solut~on were obtained. After correction for baseline, the absorbance was digitized at 5 nm intervals. The ph of each solution was measured and these data, along with other known information, were submitted to PITMAP for computation of the overall stability constant of the complex.

2 n > z > TABLE I. Composition of solutions used for the method of Budesinsky (7-10). g > z Set I Set 2 Set 3-5 C c,, = cdhp (IM x 10') c,,. ( M) c,,. (M) c~,=c~~,(mxio~) c,.(m) C,,-(M) CFe=CDHP(M~103) CH+(M) CCl-(M) P - z ,064 F $ , X , I B P , < ,508 1, r u 0

3 CURTIS AND ATKINSON: ON EQUILIBRIUM CONSTANTS Results and Discussion Tlic Acidity Constrrnts of' D HP The accepted mechanism for the stepwise deprotonation of DHP: Evidence has been produced to support this order of proton removal, especially the tautomeric shift to the lactam structure for H,P (1 1). Comparison with the known pk, values for similar reagents also lends weight to the mechanism. In an extensive study of the spectral properties and ionization constants of substituted 2-hydroxypyridines, Spinner and White (I 1) have given values for pk,, and pk,, for DHP, with the third constant being reported as inaccessible. However, the value for pk,,, of the order of 0.2, was reported with a rather large degree of uncertainty. The study of the reagent in media I.O M in hydrogen ion made it necessary to obtain a better value. Consequently, the computer program PITMAP, written by Metzler (12) and extensively modified in our laboratory (5) was used to compute a new set of pk, values. The results are shown in Table 2, along with the wavelengths of maximum absorbance- for each species. No evidence for the dissociation of the third proton was found up to a ph of 14. PITMAP also supplies molar absorptivities of the individual species over the wavelength range, which may be plotted as shown in Fig. 1. When these values, along with the constants obtained, are submitted to program PLOT (5), spectra based on the optimized mathematical model are derived and compared graphically with the experimental input absorbances. Five such comparisons are shown in Fig. 2 and excellent agreements were also obtained with the other seventeen spectra. It was already known from previous work (1) that the ratio of ui:n was 1 : 1. The absolute values of ni, i,j, and 11 were found and the overall stability constant determined by the method of Budesinsky and Haas (7-9), taking into account the side reactions of the ligand with hydrogen ion and of the metal with chloride ion. It was assumed that HP- rather than P2- was the most deprotonated species of the ligand, a valid assumption when working in acidities of I.O M. The overall stability constant was therefore represented by /?zijll to indicate this modification. The conditional stability constant was determined by the method of proportional absorbance~ (9, 10) supplemented by a table of computed values for logy and x,,,, (13). The mathematical treatment of the various relationships in these procedures gave the expression ['I log /?:ijn = log Kl:l,r + l)l log ~ F~(cI) where + 1 log a,,(,,+ ip[ci] + (j- 1 )p[h] rx represents the coefficient of a side reaction (1 4) p*.. = l,f,]ll [Feti,CliH jp,j [Fe]"'[C1]i[H]'-ll[HP]" During the determination of K,,,,,, the absolute Thecomplex with ir.on(iii) in 1.0 Mhydrochloric values of and were found to be unity, as acid solution shown in Table 3. Equation 2 then became The general equation for the formation of the complex may be written as [31 log BTijl = log K1 I + log UFC(CI, [I] Fe,,CI, + HjP,$Fe,,CI,HjP, + 1% UHP(H) + ip[cll I)P[HI

4 1652 CANADIAN JOURNAL OF CHEMISTRY. VOL. 50, 1972 TABLE 2. pk, values for the deprotonation of DHP Species imnx (nm) PK, u 'I o m P2 - not observed 'Value of pk, = 8.7 found by polentiometric titration. 2.3-n:nlmnxI- PIRIOILC o PKRIII 0.11 PKRIZI 8.69 FIG. I. Computed spectra of DHP species. Then, at constant CF,, CDHp, and [H'], eq. 3 reduced to [4] log K,,ccFCccl, = constant - ip[cl] The calculated dependence of log K,,aF,,cl, on p[c1] was plotted in Fig. 3, in which the slope of the curve indicated a value of - i = - I, making the complex composition FeCIHjP. Similarly, at constant C,,, CDHp, and [CI-1, eq. 3 became ~1. ; A O.Oo o WR\'ELENGTH (Nti 1 FIG. 2. Experimental (symbols) us. calculated (solid lines) spectra of DHP solutions. Symbols for various ph values are *, 4.60; 0, 6.07; Y, 8.81; 0, 10.15; X, TABLI; 3. Determination of IJI and n and the conditional stability constant -log C,, s,,,,,* -logyll log Kl t - I O ~ S. ~ log ~ K2?t *Lim.Y,, = 2.00 and Lim.yZ2 = 8.00 under the experimental condi- y- cc lions. The observed variation of r,,, therefore indicates r1r = 11 = 1. tby inspection, log K,, values are more constant than those for log KZZ. confirming 111 = 11 = 1. [5] log K,,z,,, =constant - Cj- I)p[H] This plot is also shown in Fig. 3, a slope of - Q- I) = 0 being obtained. The final complex composition was thus FeCIHP. Introducing this

5 CURTIS AND ATKINSON: ON EQUILIBRIUM CONSTANTS P [HI 1 2,91,, 2.8,, P [CII FIG. 3. Determination of the coefficients i and j in the complex FeCIiHjP. 0, left and lower scales show I = I; A, right and upper scales show (j- 1 ) = 0. information into eq. 3 gave Calculations then gave log P;",,, = with a standard deviation of Using this value as an initial estimate of the true answer, program PITMAP was used to compute a value for the overall constant.' Subroutiile EQLIIL, which calculatesconcentratioi~s of the individual species present, was rewritten for the proposed model FeClHP. Other input data for the program included molar absorptivities for all the metal and ligand species present (conveniently zero in ail cases as neither absorbed in the wavelength region studied), the pk, values computed above, logp values for the iron(ii1) chloride system (15). and for each solution, the ph, concentration of iron and DHP, and the digitized spectrum. An answer of log Pr,,, = with a standard deviation of was obtained, in satisfactory agreement with the previous value found. Figure 4 shows the experimental us. calculated spectra. 'More recent and convenient versions of the computer programs used may be obtained by requesting "Computer Programs for the Analysis of Electronic Absorption Spectra" lioln Dr. J. A. Thomson, Department of Biochemistiy and Biophysics, lowa State University, Ames, lowa 50010, U.S.A. Wavelength, nm FIG. 4. Experimental (symbols) us. calculated (solid lines) spectra of the complex FeClHP at various concentrations of DHP. Symbol for various [L]/[M] ratios are 0, 0.5; x. 1.0; 0,2.5; +,5.0; A, 10.0; *, 15.0; ; Y,25.0. Summary Improved values for two of the acidity constants of 2,3-dihydroxypyridine have been computed using program PITMAP. Using these values, the overall stability constant of the complex with iron(ii1) in 1.0 M hydrochloric acid solution has been evaluated by two different procedures. One of these procedures also shows the composition of the complex to be FeClHP and gives a value of the conditional stability constant. The values for the constants found are pk,, = 0.10, a = for H,P' -+ H2P + H' pka, = 8.69, o = for H2P -+ HP- + Hf This work was supported in part by a National Research Council of Canada grant. One of us (K.E.C.) also acknowledges with thanks the award of an American Chemical Society Analytical Fellowship sponsored by the Society for Analytical Chemists of Pittsburgh, and of an N.R.C.C. Scholarship. We wish to thank Dr. J. A. Thomson for making the computer programs available and for his assistance in their use, and Dr. B. Budesinsky for valuable discussions. I. K. E. CURTIS, J. A. THOMSON, and G. F. ATKINSON. Anal. Chim. Acta, 49, 351 (1970). 2. K. E. CURTIS and G. F. ATKINSON. Presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, 2nd March 1971.

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