Stability constants for lactato and B-hydroxypropionato complexes
of Co(II), Ni(II), and Cu(II) were obtained from spectrophotometric
data, using Bjerrum\u27s method of corresponding solutions.
Measurements were made in the visible range and the
results plotted for graphical evaluation by Fronaeus\u27 method.
Graphically determined values for each stability constant were
utilized for averaging and least-squares calculations with a digital
computer programmed according to R. S. Tobias (»Gauss Z« program)

I. Filipović

J. Savić

M. Savić

English

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CR 0 AT IC A CHEM I CA ACT A 44 (1972) 305 CCA-710 545.82:541.122 Original Scientific Paper Spectrophotometric Determination of Stability Constants of Lactato and B-Hydroxypropionato Complexes of Cobalt(ll), Nickel(II), and Copper(II)* J. Savic, M. Savic, and I. Filipovic Institute of Chemistry, University of Sarajevo, 71000 Sarajevo, Bosnia and Hercegovina, and Institute of Inorganic and Analytical Chemistry, University of Zagreb, 41000 Zagreb, Croatia, Yugoslavia Received January 15, 1972 Stability constants for lactato and B-hydroxypropionato com-plexes of Co(II), Ni(II), and Cu(II) were obtained from spectro-photometric data, using Bjerrum's method of corresponding so-lutions. Measurements were made in the visible range and the results plotted for graphical evaluation by Fronaeus' method. Graphically determined values for each stability constant were utilized for averaging and least-squares calculations with a digital computer programmed according to R. S. Tobias (»Gauss Z« pro-gram). So far only a few papers have dealt with stabilities of complexes formed by cobalt, nickel, and copper with ions derived from hydroxycarboxylic acids1-s. Lactato complexes .of these metals have been examined by con-ductometric1, potentiometric2- 4 , polarographics, ion-exchange3, and spectro-photometric methods6•7 ; less work has b een devoted to B-hydroxypropionato complexes, which have only been studied by solubility8, and spectrophotometric methods7 . The p resent work is concerned with an attempt to obtain comparable values for the stability constants of complexes specified in the title. In obtaining such values Bjerrum's spectrophotometric method of corresponding solutions was utilized, a method particularly well suited to the investigation of processes involving successive complex formation. It should be vbserved that this aspect has not always received due consideration in earlier spectro-photometric studies6•7• The results presented in this paper are intended to constitute part of a comprehensive study of complex stability, including complexes of the mentioned metals with a series of unsubstituted monocarboxylic acids, for which several essentially different methods are being employed5•9 •10 . EXPERIMENTAL Absorbance were measured at constant temperature, 25 + 0.1° C, with a Unicam SP 500 Quartz Spectrophotometer, using silica cells. All systems investigated had * Correspondence should _be addressed to: Jelena Savic, Institute of Chemistry, Vojvode Putnika 43, 71000 Sarajevo, Yugoslavia. 306 J. SA VIC, M. SA VIC, AND I. FILIPOVIC the same ionic strength of 2, and nearly the same pH-values (lactate solutions with cobalt and copper pH 3.2 ± 0.2, lactate solutions with nickel had 2.9 ± 0.2, and all ~-hydroxypropionate solutions had pH 3.8 ± 0.2). Absorbances of the same solution, repeatedly measured, were reproducible within ± 0.001 unit, while the reproducibility for two identical solutions prepared at different times lay within ± 0.003 units. All chemicals were analytically pure, only sodium ~-hydroxypropionate (a product from Fluka, Buchs, Switzerland) had the grade purum. This salt was recry-stallized from water. Its concentration in solutions was ascertained by potentio-metric titration with standard sodium hydroxide solution, and checked with a cation exchanger. ~-Hydroxypropionate buffer solutions with constant acid-to-salt ratios [1 : 1 for Co(II) and Ni(II); 2 : 1 for Cu(II)] were made up the day they had to be used by adding a sufficient amount of perchloric acid to the salt solution. Lactate buffer solutions (1 : 1) were prepared as described before11• Perchlorates of metals were prepared by adding excess perchloric acid to the respective salts; thus copper perchlorate was prepared from basic copper carbonate, and the perchlorates of cobalt and nickel from their nitrates. The solutions thus obtained were evaporated until all nitric acid and most of the perchloric acid were removed, and the residual perchlorates were recrystallized twice. Metal concentrations in stock solutions were determined by electrolysis12, while the concentrations of free perchloric acid were estimated potentiometrically; free perchloric acid concentrations had to be allowed for when calculating total ligand concentrations (CL) in experimental solutions. Ionic strength,was adjusted to the specified value by addition of sodium perchlorate. RESULTS Experimental conditions and the way in which the metod of cor-responding solutions is to be applied were described in a previous paper10• By inspection of absorbance curves (recorded in the visible range, Figs. 1-3) the wavelengths were selected for subsequent measurements so as to ensure the best possible resolution of metal-ion and metal complex bands. Absorbance measurements were carried out a t the following wavelengths: for nickel solutions, 660 nm; for lactate cobalt solutions, 520 nm; for B-hydroxy-propionate cobalt solutions, 510 nm; for lactate copper solutions, 790 nm; for B-hydroxypropionate copper solutions, 745 nm. Data for experimental absor-bance vs. total ligand concentration curves were recorded for three or four different total metal ion concentrations (CM), selecting proper optical path---lengths (d) in order to satisfy the condition CM · d = const13• All ligand-free solutions of metal ions obeyed Beer's law. The results of these experiments are shown on Tables I - III. When absorbances were measured at the specified wavelengths, absorption due to free ligand was usually negligible, ex cept with B-hydroxypropionate systems (Fig. 1). But even in this instance ligand absorption had only to be considered in the range of lowest metal concentrations because of large optical path lengths encountered in this range (e. g. Cru: = 10 mM, d = 40 mm); although the abso~ute ligand absorption is very slight, a correction of total absorbance values proved necessary. These corrections were made by subtracting the amount of absorbance by an equal ligand concentration from the measured value. Such procedure neglects the influence of metal-bound ligand, but this was believed permissible because of a large ligand excess. Measurements at higher ligand concentrations in systems containing B-hydroxypropionato copper complexes and lactato complexes of cobalt and nickel were precluded by precipitation occurring at such ligand concentrations. It is likely that the precipitated species were neutral complexes. On the other hand, measurements with lactato complexes had to be limited towards higher ligand concentrations because of deformations arising in experimental curves. STABILITY CONSTANTS Cobalt CM 20 mM o Lactate CL 150 mM CM 20mM .. CL 0 mM d= 20mm CM 10 mM e 13-hydroxypropionate CL 600 mM 0.400 CM 0 mM ct 13-hydroxypropionate CL 500mM 0.300 c 0 .... u c .... x lJJ 0.200 0.100 400 500 600 J.., nm Fig. 1. Light absorption curves of Co(II) solutions. 307 Absorbance vs. total ligand concentration curves served to establish cor-responding solutions, i.e. solutions exhibiting equal absorbances at different total metal and total ligand concentrations. Within the limits of experimental error each group of corresponding solutions exhibited a linear CM vs. CL relationship in the entire range of ligand concentrations employed. These results suggest that no polynuclear complexes were formed in the systems under investigation. Best lines were · fitted to the experimental points by the least-square method, and from these lines both the average number of ligands bound (n), and free ligand concentrq.t- •"' ions ([L]) were obtained as slopes and intercepts, respectively. The val~es ~for:)F. n and [L] were then utilized to determine stability constants by the ;Frf:fn~~· · graphical method14• These stability constants plus the values of log [t'[,'·11;"-an(f::~· CM were used as input data for executing the Gauss Z program for nQ?r-li»ear least-square refinement calculation according to R. S. Tobias15• Calculations were carried out with a Gamma 30 digital computer. The program provided for ten refinement cycles of the stability constants. ' ,·. _::.. :1·; ,. ')" . .. ···"" x~ ,; TABLE I CobaLt and Nickel Solutions Cobait lactate, /.. ·= 520 nm Nickel lactate, /.. = 660 nm CivrfmM: CM/mM 10 I 20. I 40 I 80 20 I 4 0 CL . d = 40 CL d = 20 . CL d = 10 . CL d = 5 CL d = 40 CL d = 20 EI E 1· EI E EI EI /mM mm /IT).M mm I /mM mm /mM mm /mM mm I /mM mm 0.9 1.9 2.9. 4.9 7.9 9.9 14.9 19.9 29.9 39.9 49.9 59.9 79.9 99.9 119.9 149.9 179.9 0.187. 0.189 0.192 0.198 0.208 0.213 0.224 0.233 0.256-0.277 0.294 o.308 0.333 0,359 0.376 0.399 0.7 -1.7 2.7 4.7 7.7 9.7 14.7 19.7 29.7 39.7 49.7. 59.7 79.7 99.7 119.7 149.7 179.7 0.183 0.188 0.188 0.193 0.202 0.205 0.217 0.228 0.245 0.264-. 0.282 0.296 0.320 0.342 0.359 0.384 0.401 0.4 1.4 2.4 4.4 7.4 9.4 14.4 19.4 29.4 39.4 49.4 59.4 79.4 99.4 119.4 149.4 179.4 0.181 0.185 0.186 0.187 0.196 0.197 0.206 0.213 0.230 0.248 0.261 0.275 0.300 0.321 0.342 0.365 0.389 0.8 1.8 3.8 6.8 8.8 13.8 18.8 28.8 38.8 48.8 58.8 78.8 98.8 118.8 148.8 178.8 0.180 0.182 0.184 0.187 0.190 0.197 0.202 0.214 0.224 0.236 0.248 0.269 0.286 0.302 0.325 0.347 1.2 3.2 6.2 8.2 13.2 18.2 28.2 38.2 48.2 58.2 78.2 98.2 118.2 148.2 178.2 198.2 0.154 0.157 0.166 0.171 0.183 0.196 0.215 0.233 0.249 0.266 0.293 0.312 0.327 0.343 1.5 4.5 6.5 11.5 16.5 26.5 36.5 46.5 56.5 76.5 96.5 116.5 146.5 176.5 196.5 0.151 0.159 0.162 0.169 0.179 0.194 0.211 0.223 0.238 0.262 0.286 0.302 0.326 0.344 80 E CL d = 10 /mM mm 1 3 8 13 23 33 43 53 73 93 113 143 173 193 0.152 0.151 0.158 O.f64 0.175 0.185 0.197 0.206 0.224 0.244 0.258 0.282 0.307 0.319 CA:> ~ cc :-< Ul ;i.. < ...... _n ~ Ul ~ p ~ :z t:l t-< "'.l ...... t-< ...... 'd 0 < ...... (} c 0 .... 0.300 g 0.200 .... x w 0.100 STABILITY CONSTANTS Nickel o Lactate CL lUO mM CM 20 mM e /J - hydroxypropionate CL 700mM d = 40 mm " CL O m.M 600 700 BOO .A, nm Fig. 2. Light absorption curves of Ni(II) solutions. 309 Computer output data included the refined stability constants and their standard errors, and calculated formation functions. The values for stability constants obtained by both the graphical and the digital methods are present-ed on Table IV. DISCUSSION The stability constants for lactato complexes reported in this paper, which were determined by the corresponding-solutions spectrophotometric method, are in good accordance with values, obtained by potentiometric, polarographic, and ion-exchange methods, recently reported in the literature2-5• Admittedly our constants for lactato complexes of cobalt and nickel are somewhat below, and those for copper somewhat above the literature values. Thus with the lactato cobalt complexes our value for Bi is 19, while H. Thun et al. 4 .obtained 23.4 potentiometrically, and G. Calabro et al.2•3 give 25.7 as the value determined both potentiometrically and by ion exchange. For lactato complexes of nickel our value of Bi is 37, compared to 38.9 and 42.5, respect-ively, as reported by the authors cited. With lactato copper complexes we found Bi = 431 by spectrophotometry, .and 350 by polarography; the above authors give the values 309 and 316, respectively. Still, if certain differences in the composition of media are acounted for, and especially if the different capacities of methods are considered, we regard these accordances as satis-factory. TABLE II Cobalt and Nickel Solutions Cobalt B-hydroxypropionate, /, = 510 nm Nickel B-hydroxypropionate, 'A= 660 nm CM/mM CM/mM 10 I 80 I 200 20 I 80 I 160 CL d: 40 I CL d ~ 51 CL E CL d: 401 CL d!10i CL d=2 /mM mm /mM mm /mM mm /mM mm /mM mm /mM 49.3 0.208 99.3 0.225 94 0.217 85 0.210 99 0.172 96 0.167 92 149.3 0.235 144 0.229 135 0.221 149 0.180 146 0.173 142 199.3 0.244 194 0.238 185 0.231 199 0.187 196 0.181 192 299.3 0.260 294 0.255 285 0 .247 299 0.196 296 0.190 292 399.3 0.274 394 0.268 385 0.260 399 0.202 396 0.198 392 499.3 0.286 494 0.280 485 0.272 499 0.209 496 0.204 492 599.3 0.296 594 0.290 585 0.281 599 0.216 596 0.210 592 699.3 -694 0.300 685 0.292 699 0.218 696 0.213 692 742 --E d=5 mm 0.161 0.168 0.173 0.184 0.191 0.198 0.203 0 .207 0 .208 C'-' ...... 0 ;-.. [/J ::i> < H p ~ [/J ;J> < H p ::i> z tJ ~ "'J H t" H '"d 0 < H 0 c 0 .... 0.500 0.400 g 0.300 .... x UJ 0.200 0.100 600 STABILITY CONSTANTS ·Copper d = 40 mm 700 o Lactate CL 150 mM e .6-hydroxypropionate CL 120 mM 800 900 J..,nm Fig. 3. Light absorption curves of Cu(II) solutions. 311 However, by using a conductometric method W. P. Evans and C. B. Monk1 arrived at markedly higher stability constants for the lactato complexes of all three metals, viz. to B1-values of 79, 164, and 1047, respectively . For the B-hydroxyprnpionato complexes of copper other authors, M. Lloyd et al.8, determined B1 = 832 (the value extrapolated to zero ionic strength) by the solubility method, but they were working with a different medium. Strikingly, wide discrepancies between stability constant values are found throughout in spectrophotometric work. Thus M. Bobtelsky and I. Bar-Gadda6 obtained B2-values 48 and 501, respectively, for lactato cobalt and lactato copper complexes using Job's method. Employing the Asmus and the Mccon-nel-Davidson methods, S. Ramamoorthy and M. Santrappa7 were led to Bi values 126 and 102, respectively, for lactato copper complexes, while for the B-hydroxypropionato copper complex these methods gave Bi' s 501 and 363, respectively, and the Dey-Mukerji method gave Bi = 3987. It is quite obvious that these discrepancies must be primarily attributed to inadequacies of some TABLE III Copper Solutions Lactate, A. = 790 nm ~-hydroxypropionate, A.= 745 nm CM/mM 4 I 8 16 CL CL CL E I E I /mM d = 40 /mM d = 20 /mM 1.7 2.7 4.7 7.7 9.7 14.7 19.7 29.7 39.7 49.7 59.7 79.7 99.7 119.7 149.7 179.7 199.7 0.222 0.252 0.272 0.311 0.322 0.352 0.375 0.402 0.418 0.432 0.439 0.458 0.470 0.478 0.492 1.4 2.4 4.4 7.4 9.4 14.4 19.4 29.4 39.4 49.4 59.4 79.4 99.4 119.4 149.4 179.4 199.4 0.209 0.225 0.251 0.286 0.296 0.332 0.357 0.390 0.410 0.421 0.433 0.449 0.462 0.470 0.481 0.8 1.8 3.8 6.8 8.8 13.8 18.8 28.8 38.8 48.8 58.8 78.8 98.8 118.8 148.8 178.8 198.8 I 32 E I c E d = 10 /mk d = 5 0.203 0.208 0.228 0.247 0.263 0.293 0.322 0.362 0.390 0.407 0.418 0.438 0.452 0.467 0.480 0.487 0.5 2.5 5.5 7.5 12.5 17.5 27.5 37.5 47.5 57.5 77.5 97.5 117.5 147.5 177.5 197.5 0.190 0.202 0.218 0.223 0.248 0.263 0.303 0.333 0.358 0.380 0.409 0.428 0.442 0.458 0.474 0.479 4 CL E /mM d = 40 1.7 2.7 4.7 7.7 9.7 14.7 19.7 29.7 39.7 49.7 59.7 79.7 99.7 119.7 149.7 179.7 199.7 0.187 0.197 0.218 0.243 0.258 0.286 0.314 0.352 0.384 0.404 0.423 0.453 0.474 0.494 0.516 0.532 CM/mM I 8 I I c E I I 1mk d = 20 1.4 2.4 4.4 7.4 9.4 14.4 19.4 29.4 39.4 49.4 59.4 79.4 99.4 119.4 149.4 179.4 199.4 0.178 0.187 0.202 0.227 0.239 0.270 0.297 0.341 0.371 0.394 0.412 0.440 0.465 0.484 0.502 0.518 16 CL E I /mM d = 10 1.8 3.8 6.8 8.8 13.8 18.8 28.8 38.8 48.8 58.8 78.8 98.8 118.8 148.8 178.8 198.8 0.177 0.191 0.206 0.223 0.249 0.273 0.311 0.345 0.369 0.393 0.423 0.449 0.468 0.493 0.510 0.520 32 E CL /mM d = 5 0.5 2.5 5.5 7.5 12.5 17.5 27.5 37.5 47.5 57.5 77.5 97.5 117.5 147.5 177.5 197.5 0.168 0.176 0.188 0.197 0.217 0.236 0.274 0.304 0.330 0.353 0.392 0:420 0.443 0.476 0.499 0.510 ~ ...... Nl ~ en :i> < ..... .0 ~ en :i> < ..... _o :i> § !""' '>J ..... t"' ..... "C 0 < ..... () TABLE IV Stability Constants ( ± standard errors) Ligand Metal I Bi B2 graph. I comp. graph. comp. Co"'" 23 19 ± 1 309 337 ± 7 Lactate Ni2+ 39 37 ± 2 853 868 ± 19 Cu2+ 360 431 ± 21 14 x 103 (12.68 ± 1.02) x 103 co2 • 2.9 3.1 ± 0.1 B-hydroxy-Ni2+ 7.5 6.0 ± 0.4 19 21 ± 1 propionate cu2 • 67 58 ± 3 1582 1609 ± 42 Bs graph. comp. 16.1 x 104 (18.83 ± 1.06) x 104 en ~ t:!j ..... ["' ..... >--3 ><:: (') 0 z Ul >--3 ~ z >--3 en w ~ w 314 J. SAVIC, M. SAVIC, AND I. FILIPOVIC of the spectrophotometric method when used for systems capable of succes-sive complex formation, as well as to differences in the handling of data. Anyway, the stability constants presented in this paper indicate that the investigated complexes obey the natural order of stability given by Irving and Williams16 viz. Co < Ni < Cu. Similarly as with other monocarboxylate complexes of cobalt, nickel, and coppers,9,io the stabilities of copper complexes with either ligand greatly exceed those of the cobalt and nickel complexes, a relationship that can be explained by the stabilizing Jahn-Teller effect. The relative basicities of ligand would suggest a greater stability for B-hydroxypropionato complexes, and a smaller stability for lactato complexes. Since this expectation is not fulfilled by the relationship of the stability constants it seems obvious that lactato complexes are stabilized by chelate ring formation: their fivemembered chelate rings are well known to be the most stable kind of rings formed by saturated ligands.17 Conversely, the low stability of B-hydroxypropionato complexes, comparable in magnitude to the stabilities of acetato, propionato, and butyrato complexes9, indicates that the effect of chelate ring formation in B-hydroxypropionato complexes is either very small, or altogether lacking. Acknowledgement. The authors are indebted to the Federal Fund for Scientific Research for financial support. Also, they wish to express their gratitude to Dr. VI. Simeon for supplying the computer program. Our special thanks are due to the Electronic Data Processing Center of the firm Energoinvest, Sarajevo, for the permission to use their Gamma 30 computing unit for all our calculations. REFERENCES 1. W. P. Ev ans and C. B. Monk, J . Chem. Soc. (1954) 550. 2. G. D' Amore, P. Curro, and G. Ca I ab r o, Atti Soc. Peloritana Sci. Fis. Mat. Nat. 10 (1964) 229; Chem. Abstr. 64 (1966) 15361 b. 3. G. Ca I ab r o and P. Curro, Atti Soc. Peloritana Sci. Fis. Mat. Nat. 10 (1964) 357; Chem. Abstr. 64 (1966) 15361 c. 4. H. Thu n, W. Guns, and F. Verb eek, A nal. Chim. Acta 37 (1967) 332. 5. I. Filipovic, I. Pi 1 j a c, A. Med v e d, J . S av i c, A. Bu j a k, B. B a ch - Drag u tin o vi c, and B. Mayer, Croat. Chem. Acta 40 (1968) 131. 6. M. Bob t e Is k y and I. Bar - Gadd a, Bull. Soc. Chim. France (1953) 687. 7. S. Ram am o or thy and M. Sant rap pa, J. Inorg. NucZ. Chem. 30 (1968) 2393. 8. M. Lloyd, V. Wycherley, and C. B. Monk, J. Chem. Soc. (1951) 1786. 9. I. Fi 1 i po vi c, T. Matus in o vi c, B. Mayer, I. Pi I j a c, B. B a ch--D rag u tin o vi c, and A. Bu j a k, Croat. Chem. Acta 42 (1970) 541. 10. B. Grab a r i c and I. Fi 1 i po vi c, Croat. Chem. Acta 42 (1970) 479. 11. J. S. Savic and I. Fi 1 i p o vi c, Croat. Chem. Acta 37 (1965) 91. 12. A. I. Voge I, Quantitative Inorganic Analysis, Third Edition, Longmans, Lon-don, 1966, p . 608, 613. 13. S. Fron a e us, Acta Chem. Scand. 5 (1951) 139. 14. S. F r o n a e us, Acta Chem. Scand. 4 (1950) 72. 15. R. S. Tobi as and M. Yasuda, lnorg. Chem. 2 (1963) 1307. 16. H. Irving and R. J. P. W i 11 i ams, J. Chem. Soc. (1953) 3192. 17. A. E. Martell and M. Calvin, Chemistry of the Metal Chelate Compounds, Prentice-Hall, INC., Englewood Cliffs, N. J., 1952, p. 137, 143. STABILITY CONSTANTS IZVOD Spektrofotometrijsko odredivanje konstanata stabilnosti laktato-i B-hidroksipropionato-kompleksa kobalta(II), nikla(II) i bakra(II) J. Savic, M. Savic i I. Filipovic 315 Odredene su konstante stabilnosti laktatnih i B-hidroksipropionatnih kompleksa Co(II), Ni(II) i Cu(II) primjenom spektrofotometrijske metode korespondentnih oto-pina po Bjerrumu. Istrazivanja su vrsena u vidljivom podrucju spektra. Konstante stabilnosti prisutnih kompleksa procijenjene su grafickom metodom S. Fronaeusa. Ovako dobivene vrijednosti posluzile su kao osnova za izracunavanje konstanti sta-bilnosti i odgovarajucih standardnih pogreilaka pomocu digitalnog elektronickog ra-cunala, koristeci Gauss z program prema R. S. Tobiasu. KEMIJSKI INSTITUT UNIVERZITETA U SARAJEVU I INSTITUT ZA ANORGANSKU I ANALITICKU KEMIJU SVEUCILISTA U ZAGREBU Primljeno 15. sijecnja 1972. 

Spectrophotometric Determination of Stability Constants of Lactato and B-Hydroxypropionato Complexes of Cobalt(ll), Nickel(II), and Copper(II)

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