Talanta 86 (2011) 447–451 Contents lists available at SciVerse ScienceDirect Talanta j ourna l ho me page: www.elsevier.com/locate/talanta Short communication Determination of dissociation parameters of weak acids in different media according to the isohydric method Tadeusz Michalowski a,∗ , Boguslaw Pilarski b , Agustin G. Asuero c , Agnieszka Dobkowska b , Slawomir Wybraniec a a Faculty of Engineering and Chemical Technology, Cracow University of Technology, 31-155 Cracow, Poland b P.P.H.U. Cerko s.c., 80 299 Gda´ nsk, Afrodyty 9, Poland c Department of Analytical Chemistry, The University of Seville, 41012 Seville, Spain a r t i c l e i n f o Article history: Received 15 June 2011 Received in revised form 29 August 2011 Accepted 1 September 2011 Available online 7 September 2011 Keywords: Acid–base equilibria Aqueous solutions Titration Dissociation constants a b s t r a c t The isohydricity (pH constancy) principle is referred to the pair of solutions: weak acid (HL, C 0 mol/L) and strong acid (HB, C mol/L) when mixed e.g., according to titrimetric mode. Such a case takes place if the relation C 0 = C + C 2 × 10 pK 1 is valid, where pK 1 = -log K 1 , K 1 – dissociation constant for a weak monoprotic acid HL. This principle, outlined and formulated in earlier paper (Michalowski et al., Talanta 82 (2010) 1965), is the basis for a sensitive method of pK 1 determination, confirmed for a series of weak acids in presence of basal electrolytes or in water + organic solvent (dimethyl sulphoxide, methanol, isopropanol) media. The results of titrations were elaborated according to principles of regression analysis, with use of least squares method. A new criterion for precision of the results obtained according to this method is formulated. The pK 1 values obtained are comparable with ones found in literature. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The term “isohydric” refers to a pair of solutions having the same pH value. The isohydricity concept gained its quantitative utterance in the formulae derived in the paper issued lately [1], where were also stated, that the isohydricity property is limited to the systems where only acid–base equilibria are involved. In other instances, protons are generated/consumed in side (redox, precip- itation and/or complexation) reactions. The isohydricity principle can be categorized not only as an interesting curiosity of electrolytic systems, however. The isohydricity has also some relevance to the buffering action and buffer capacity [2,3], and shows some analogies with pH- stat action and pH-static titration [4,5]. Moreover, it can also be considered as a valuable and sensitive tool for determina- tion/validation/verification of acidity constants (pK 1 = -log K 1 ) of weak monoprotic acids (HL), where K 1 = [H + ][L - ] [HL] (1) In [1], the isohydricity concept was exemplified by the systems composed of different pairs of mono- or polyprotic acids or their salts. In particular, a weak acid HL (C 0 mol/L) and strong acid HB ∗ Corresponding author. Tel.: +48 12 628 21 77. E-mail addresses: michalot@o2.pl, michalot@chemia.pk.edu.pl (T. Michalowski). (C mol/L) form a pair of isohydric solutions, provided that the rela- tion C 0 = C + C 2 × 10 pK 1 (2) is valid. In this case, pH of the solution obtained after addition of V mL of C mol/L HB (as titrant, T) into V 0 mL of C 0 mol/L HL (as titrand, D) remains constant, irrespectively of V value. Similarly, pH is constant after addition of V mL of C 0 mol/L HL into V 0 mL of C mol/L HB, if C 0 and C are interrelated as in Eq. (2). Moreover, for the pair (HL, HB) of the isohydric solutions, the ionic strength (I 0 , mol/L) of the mixture remains unchanged (I 0 = C) after mixing the compos- ing solutions at different proportions, particularly when the mixing is carried out according to titrimetric mode, in quasistatic manner, under isothermal conditions; the last requirement is involved with possible changes in K 1 value, resulting from heat effects occurred during the titration. The true (not approximate) constancy inherent in the isohydric systems is a unique property, testifying on account of this method. The isohydric method is based on preparation of a series of solutions of both acids: weak acid HL (C 0i * mol/L) and strong acid HB (C mol/L), whose concentrations (C 0i * , C) are interrelated in the equality (see Eq. (2)) C ∗ 0i = C + C 2 × 10 pK ∗ 1i (i = 1, . . . , n) (3) where pK ∗ 1i are the pre-assumed numbers, not far from real pK 1 value (Eq. (2)). The n pairs {(HL, HB) i } of the solutions (i = 1,. . .,n) are mixed according to a common pH-metric titration procedure, 0039-9140/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2011.09.002