STABILITY AND IONIZATION CONSTANTS OF COMPLEXES OF CATECHOL AND BORIC ACID A THESIS PRESENTED IN PART REQUIREMENT FOR ADMISSION TO THE DEGREE OF MASTER OF SCIENCE IN THE UNIVERSITY OF NEW SOUTH WALES BY KWAT IE THE SUBMITTED JANUARY, 1962 TABLE OF CONTENTS PAGE ACKNOWLEDGEMENTS ABSTRACT ii CHAPTER INTRODUCTION 1.1 COMPLEX FORMATION OF BORIC ACID . 1.2 STRUCTURE AND PROPERTIES OF BORIC ACID 2 1.3 COMPLEX COMPOUNDS OF BORIC ACID WITH Dl- OR POLY-HYDROXY COMPOUNDS 3 1.4 PREVIOUS QUANTITATIVE STUDIES OF BORIC AC I D-D IOL SYSTEM 6 1.5 SUMMARY 10 CHAPTER THEORETICAL CONSIDERATIONS 11.1 METHOD 12 1 I .2 TITRATION CURVE 12 I I .3 MATHEMATICAL FORMULATION OF EQUILIBRIA 16 I I .4 DERIVATION OF THE THEORETICAL EQUATIONS 18 I I .5 APPLICATION TO THE CASE WHERE MOLE RATIO OF CATECHOL: BOR I C ACID = I*. I 22 11.6 EVALUATION OF K, 28 CHAPTER 111 EXPERIMENT I 1 1 .I MATER IALS 30 1 I I .2 APPARATUS 32 111.3 PROCEDURE 33 111.4 DISCUSSION OF THE METHOD 35 CHAPTER IV DISCUSSION OF THE RESULTS IV.I CASE I; [ C] IS NEGLIGIBLE 38 IV.2 CASE 2; [ C] IS NOT NEGLIGIBLE 40 IV.3 THE APPROXIMATE VALUES OF K&t AND K_ TABLE OF CONTENTS. (CONTD) PAGE CHAPTER IV DISCUSSION OF THE RESULTS (contd) IV. 4 CALCULATION OF K., K AND K. 42 a» 3 IV. 5 PROBABLE ACCURACY OF THE CONSTANTS .. 44 IV. 6 DISCUSSION OF THE VALUES OF [C] 45 IV. 7 CALCULATION OF THE CONCENTRATIONS OF THE VARIOUS SPECIES IN THE SOLUTION ., .. 45 IV. 8 DISCUSSION OF THE VALUES OF THE CONCENTRATIONS OF THE VARIOUS SPECIES 46 IV. 9 THE APPROXIMATE VALUE OF .. 59 IV.|0 JUSTIFICATION 0F THE ASSUMPTIONS 61 IV. I | EVALUATION OF K? FROM RIGOROUS TREATMENT 63 CHAPTER V CRITICAL EVALUATION OF PREVIOUS YORK ON CATECHOL-BORIC ACID COMPLEXES V.I ANTIKAINEN'S EQUATION FOR THE DETERMINATION OF BORIC ACID-CATECHOL COMPLEX CONSTANTS 65 V. 2 CRITICISM OF ANTIKAINEN'S WORK 68 V. 3 THE WORK OF ROY, LAFERRIERE AND EDWARDS 70 V.4 CRITICISM OF THE WORK OF ROY AND CO-WORKERS 73 V.5 WORK OF SCHAFER 76 • • V.6 criticism of schafer’s work .. 77 CHAPTER VI. THERMODYNAMIC STUDIES V INTRODUCTION 79 VI.2 EQUILIBRIUM CONSTANTS AND TEMPERATURE 80 V|.3 METHOD 82 V|. 4 RESULTS 82 VI.5 0 ISCUSSI ON OF RESULTS 87 SUMMARY 97 APPENDIX 1 101 I I IC7 I I I I 18 BIBLIOGRAPHY 13! * Sj« !jf # ACKNOWLEDGEMENTS THE AUTHOR IS GREATLY INDEBTED TO DR. A. BRYSON FOR HIS ENCOURAGEMENT, HIS FRIENDLY SUPERVISION AND GUIDANCE THROUGHOUT THIS WORK. HE ALSO WISHES TO THANK PROFESSOR D.P. MELLOR FOR PERMITTING THE USE OF FACILITIES IN THE SCHOOL OF CHEMISTRY, THE UNIVERSITY OF NEW SOUTH WALES, MR. E. P. SERJEANT, COLLEAGUES AND STAFF OF THE DEPART­ MENT OF ANALYTICAL CHEMISTRY FOR HELPFUL DISCUSSION ON f NUMEROUS OCCASIONS, AND FINALLY, TO THE COMMONWEALTH OF AUSTRALIA FOR THE COLOMBO PLAN SCHOLARSHIP i ABSTRACT Boric acid and catechol form a series of complexes similar to those formed with other diols,these being briefly specified by the formula HBC, HBC2 and HBC^,where the ratios of boric acid to catechol are respectively 1:1, 1:2 and 1:3* From titration curves of solutions with mole ratio of 1:1 with 0H“, it was found that the calculated pKa values showed a steady drift during the course of the titration.This drift has been correlated with the progressive formation of the ion BC2“ as neutralisation proceeds and mathematical equa­ tions have been derived from which the constants K^, Kg, K^, K and Ka2 for the following reactions: HB + C v HBC K HBC H+ + BC“ HBC + C HBC, hbc2 v H+ + BC “ BC~ + C ii have been evaluated. Of these constants,* K,1* , K at and K30 are able to be precisely defined, while and Kg are determined to a lower degree of accuracy.Further experiments with higher ratio of catechol to boric acid have enabled the values of Kg and Kg to be more accurately evaluated. From these constants the concentrations of various species have been determined at different points of the titration curve. Determination of the constants at five temperatures (10°C - 30°C) have enabled the thermodynamic quantities to be evaluated for the above reactions and these results have been used in assessing the factors responsible for establishing the various equilibria. iii 1 CHAPTER ONE INTRODUCTION I.1 Complex Formation of Boric Acid Boron having a group valency of three, has one un­ occupied orbital and hence has a considerable tendency to form complex compounds by completion of its electron shell. This tendency is so great that on occasion is done by way of nback co-ordination” as in Cl B in Cl / Cl this being indicated by the smaller interatomic distances. Normally however, the octet is obtained by combination be­ tween the boric compound which behaves as a Lewis acid and a base which provides the required electrons, thus forming quadricovalent compounds, A typical example of these com­ pounds is probably tetrafluo-boric acid. The tendency of boron to form complexes with diols was known as early as 1&42 when Biot reported that boric acid became acid to litmus upon addition of sugar. This obser­ vation has since been verified for many other organic 2 compounds having hydroxyl groups. In order to explain the unusual increase of acidity in solution of boric acid with increasing concentration of ( q } itself, Kohlenberg and Schreiner postulated the formation of polyboric acids. The presence of polyboric acids is in­ dicated by cryoscopic and ebullioscopic measurements and by measurements of partition and diffusion. Kolthoff^ postu­ lated that in concentrated solution of boric acid small quantities of tetraboric acid were formed. By means of conductometric measurements Thygesen (5) has arrived at the same conclusion as Kolthoff concerning the existence of tetraboric acid but his investigations indicate in addition that other polyboric acids also exist. Both of them found that in solution of boric acid less than 0.1 M no appreciable autocomplex formation occurred. This is con- firmed by pH determination made by De Witt Stettenv with glass electrode and also by the works of Antikainen (7) and Dales!8* I.2 Structure and Properties of Boric Acid Boric acid is a planar molecule, the OH groups being 120° apart. The acid is weak (pKa = 9*22)^ and a characteristic feature is the formation of a number of polyborates, the most important of which is the tetraborate 3 B^0?~. It is generally accepted that boric acid does not ionize by dissociation of its OH groups, but acts as a Lewis acidforming with OH” ions, initially the sym­ metrical B(OH)^” ion. The structure of the various poly­ borate ions subsequently produced is still a matter of dispute. A convincing picture of possible structures of these has been given in a recent paper by Dales. (8y) In order to achieve the tetrahedral configuration of the B(OH)^“ ion the boric acid molecule must be changed from trigonal to tetrahedral configuration in order to allow union with the OH”. This confers an added stability on boric acid and is held to be responsible in part for its weak acidic nature. 1.3 Complex Compounds of Boric Acid with Pi- or Poly-Hydroxy Compounds The formation of a complex between boric acid and certain hydroxy compounds has been demonstrated by the increase in acidity, the increase in mutual solubility and the change in optical rotation. The first real measure­ ments of conductance in this field were made by Magnanni in 1890-93* Thomsonfound in 1893 that boric acid,which cannot be titrated in the usual manner, could be determined by titration in the presence of various polyhydroxy com­ pounds, using phenolphthalein as indicator. The present 4 method of determination of boric acid with the aid of manni< tol is based on Thomson's observation. Hermans has shown that Is 2 and Is 3 diols of cis- configuration generally form complexes which are acidic and which may be isolated. He assigns these complexes two structures. One is a diester formed by splitting out two molecules of water between one hydrated borate ion and a glycols o o / i OH H0V -yOH ___^ /OH - + B ^---- [ ® \ + 2H20 1.1 OH H0/ X0H c - oy OH The other is formed by splitting out four molecules of water between one hydrated borate ion and two molecules of a glycol: \ / C - OH H0V - OH HO - C 'C - CL ,0 - C 1 + B I 1 1 C - OH ho' noh HO - C 'C - 0 0 - cx + tfl20 1.2 This nborospirantf structure has a centre of asymmetry and this would be expected to give rise to optical isomers. The (15) fact that some isomers have been resolved, support the proposed structure. (i Kolthofi reported the existence of diborotartaric 5 acid of the type ^2^2° means of solubility determinations (17) and Darmois and Peryraux claimed to have obtained com­ pounds of the type H2B2D from solutions which contained glucose, galactose and fructose in addition to borates. Schafer' 'prepared salts of the complexes of boric acid and catechol isolating the lithium, potassium, sodium, magnesium, strontium of the monocatechol complex and the sodium, potassium of the dicatechol complex. He also succeeded in isolating a tricatechol complex in the form of its pyridinium salt. To these he gave the structures: /°\ /°\ /\ M [CgH^ B - 0] xH,0 ; M [C^ B C6H ] V \ o / \ o/ OH (19) and C6H4' CcH,NH+ BN)- C6H4 5 7 °6H4 " 0H The salts are very largely hydrolysed in aqueous solution and Schafer calculated the equilibrium constant for the reaction: BC“ + C 6 I.4 Previous Quantitative Studies of Boric Acid-Diol System Despite the large amount of qualitative work on diol complexes of boric acid comparatively little work of a satisfactory nature has been done on the thermodynamics of complex formation and this section will describe the position up to the present time.