water mixtures

ANALYTICA CHIMICA ACTA

ELSEVIER

Analytica

Chimica

Acta 340 (1997) 133-141

Ionic equilibria in aqueous organic solvent mixtures The dissociation constants of acids and salts in tetrahydrofuran/water mixtures Urmas Muinasmaa,

Clara Rifols, Elisabeth Bosch, Marti Ros&*

Departament de Quimica Analitica, Universitat de Barcelona, Diagonal 647, E-08028 Barcelona. Spain Received

10 July 1996; revised 15 October

1996; accepted

20 October

1996

___-__ Abstract The dissociation constants of several acids (perchloric, hydrochloric, phosphoric, acetic and benzoic acids) and of some sodium salts (chloride, acetate and benzoate) have been conductometrically determined in tetrahydrofuran/water mixtures up to a 90% of tetrahydrofuran in volume. The results demonstrate that conductometry can be successfully applied to determine the dissociation constants of salts and moderately weak and strong acids in the studied mixtures. The dissociation constants of the acids and some bases taken from the literature have been fitted to solvent composition through a previously derived equation, which is based on a preferential solvation model. The fitting parameters obtained allow calculation of the dissociation constant for any solvent composition inside the applicability solvent composition range. From the pK value, the pH of any buffered solution, such as those used in liquid chromatography, can be calculated for the particular tetrahydrofuran/ water composition of interest. Appreciable ion-pairing for sodium salts and strong acids has been observed for tetrahydrofuran contents higher than 60% in volume. Therefore, the accurate calculation of the pH values of buffers in tetrahydrofuran-rich solutions must take into account the pK values of the acid and salt. Keywords: Acids; Bases; Ion-pairs;

Tetrahydrofuran;

Aqueous organic solvent mixtures;

1. Introduction Analytical applications of water/organic solvent mixtures have spread during the last few decades. Liquid chromatography (LC) is perhaps the analytical technique that makes a major use of mixed aqueous solvents. Methanol/water, acetonitrile/water, and tetrahydrofuramwater are the most used mobile phases in reversed-phase liquid chromatography (RPLC). The retention of a solute in a particular *Corresponding

author. Fax: +34 34021233.

0003.2670/97/$17.00 Copyright PII SOOO3-2670(96)00516-8

0

Preferential

solvation

chromatographic column depends strongly on the solvent composition of the mobile phase and in many instances on its pH [ 11. The accurate determination of this pH requires the establishment of reference buffer solutions at the different water/organic solvent compositions. A IUPAC document noticed the need of new data for standards in many non-aqueous and mixed solvents [2]. Regarding the three most used RPLC mobile phase systems, there is a huge number of acidity and basicity pK data in methanol/water that can be used for pH standardization [2-91. Methanol/water is by

1997 Elsevier Science B.V. All rights reserved.

134

U. Muinasmaa

et al./Analytica

far the most studied binary system, since 1925 [9]. The pK data in acetonitrile/water are more recent and less extensive, but nowadays the pK data of several acids and the pH of some standards have been determined [S,lO-151. However, there have been only a few pK studies about dissociation of acids and bases in tetrahydrofuran/water. Reynaud [ 16,171 determined the dissociation constants of acetic and benzoic acids and of several anilines at 10, 20, 30, 40 and 50% of tetrahydrofuran in volume by potentiometry. More recently, Niazi et al. [4,5,18] measured conductome&ally the dissociation constants of chloroacetic, salicylic, benzoic, 2-nitrobenzoic, 3-nitrobenzoic, and 4-nitrobenzoic acids in 10, 20, 30, 40 and 50% of tetrahydrofuran in weight. Some salts have also been studied in tetrahydrofuran/water mixtures, mostly alkaline chlorides and perchlorates 119-231. The low dielectric constant of tetrahydrofuran determines that salts and strong acids and bases are only partially dissociated in tetrahydrofuran-rich mixtures. An accurate computation of pH requires the knowledge of the dissociation constant of the salts contained in the buffered solution, since the formation of salt ion pairs affects the dissociation of acids and bases [24-281. In this paper, we determine the dissociation constants and limiting molar conductances of some strong and weak acids (perchloric, hydrochloric, phosphoric, acetic and benzoic) and of some sodium salts (chloride, acetate and benzoate) in mixtures of water and tetrahydrofuran up to an 80-90% of organic solvent in volume. Previously developed equations are used to relate the dissociation pK values with solvent composition. The equations and parameters obtained allow the calculation of the pK value of the electrolytes studied at any solvent composition between the most used mobile phase LC range. The pK values should allow the calculation of pH reference values of standards from the buffer composition. 2. Experimental 2.1. Apparatus A Radiometer CDM 83 conductometer and a Radiometer CDC 304 cell were used. The cell

Chimica Acta 340 (1997) 133-141

constant (1.000 cm-‘) was determined from conductance of 0.0100 m KC1 at 25f0.2”C.

the

2.2. Chemicals The chemicals used were: perchloric acid (Merck, Poole, UK. p.a. ACS 70-72%), hydrochloric acid (Merck, p.a. 25%), phosphoric acid (Merck, p.a. 85%), acetic acid (Carlo Erba, RS anhydrous), benzoic acid (Carlo Erba, RS-STD), sodium chloride (Merck, suprapur 99.5%), sodium acetate (Carlo Erba, RPE-ACS 99%), sodium benzoate (Carlo Erba, RPH 99%), tetrahydrofuran (Fluka, for HPLC), and triply distilled water.

2.3. Procedure Different measured amounts of a ca. lo-* M solution of the electrolyte were added to 50 ml of tetrahydrofuran and the conductance was measured after each addition. All the measurements were done in vessels externally thermostated at 25f0.2”C with a water jacket. 2.4. Calculation

methods

The dissociation constants of the electrolytes studied were calculated from the conductometric measurements by means of the Debye-HiickelOnsager (Eq. (I)) and Shedlovsky (Eq. (2)) equations ~291 A = cu(Ae - S&)

(1)

A = c&J - S(A/Aa)Jccu

(2)

with S=

8.18. 105h0 (ET)3’2

82 + 7+T)“2

In these equations, A and Ac are the measured molar conductance and limiting molar conductance (cm2 mol-’ n-i), E and 77the dielectric constant and dynamic viscosity @ = g cm-’ s-l) of the medium, and T the absolute temperature (298.2 K). c and Q!are the electrolyte concentration (mol I-‘) and degree of dissociation, which are related to the mean activity

(1. Muinasmaa

coefficient (v) and dissociation the well-known equation K

_

constant

et al. /Analytica

(K) through

y2Y2 I -a

A=

calculated

from

A&G 1 + BqJC(y’

50.29 (ET)“~

Bates-Guggenheim convention, which assigns a constant value of 0.456 nm to the a0 parameter, was used [2,30]. The fundamental parameters [31,32] of these equations for the tetrahydrofuran/water mixtures studied are given in Table 1. The limiting molar conductance, ho, and the electrolyte dissociation constants, K, of each solute at each solvent mixture were calculated by non linear regression using the Gauss-Newton-Marquat algorithm [33] to minimize squared A residuals. The Fuoss-Kraus and Shedlovsky [29] linearization methods were used for preliminary analysis of the conductance data in order to obtain good initial estimates of A0 and pK.

Table I Properties

The pK values computed by Onsager or Shedlovsky equations 0.02 unities.

The limiting molar conductances and dissociation constants of the studied sodium salts and acids are given in Tables 2 and 3, respectively. The values for pure water and tetrahydrofuran were taken from the literature [25,26,34-361. The limiting molar conductances have been also plotted in Figs. 1 and 2 against the solvent composition. All values are the average of at least three different series of solutions at different concentrations. The number of concentration points used for each series goes from 15 for strong electrolytes or water-rich mixtures to 5 for the weak acids (acetic and benzoic) at 90% of tetrahydrofuran. As the tetrahydrofuran contents increases, the dissociation of the electrolytes decreases and so does the conductance of the solutions. Therefore, for weak electrolytes in solutions with high tetrahydrofuran contents, only the most concentrated solutions give conductances sufficiently higher than the conductance of the pure solvent. Absolute standard deviations of pK values are between 0.02 for low tetrahydrofuran contents and 0.15 for high tetrahydrofuran contents. Table 2 Limiting molar conductances (cm2 mol ’ 12 ‘) and dissociation constants of sodium salts in tetrahydrofuran/water mixtures % THF (vol.)

of tetrahydrofuran/water %THF (weight)

XTHF

0

0.00 9.00 18.20 27.61 31.24 47.09 57.17 67.50 78.07 88.90

0.0000 0.0241 0.0527 0.0871 0.1292 0.1820 0.2502 0.3416 0.4707 0.6668 1.oooo

20 30 40 50 60 70 80 90 100

I00.00

THF = tetrahydrofuran.

NaCIOJ“

Density -3 (g cm

t

0.9907 0.9928 0.9887 0.9829 0.9744 0.9642 0.9503 0.9367 0.9211 0.9025 0.8819

78.54 72.39 65.95 58.56 50.53 42.34 34.18 26.39 19.41 13.18 7.39

IO 0.0089 0.0116 0.0144 0.0165 0.0175 0.0174 0.0160 0.0132 0.0099 0.0070 0.0048

20 30 40 50 60 70 80 90 100’

NaAc

NaCl

NaBx

:I,,

PK

A0

PK

Al,

pK

A,,

PK

117.4 89.4 73.5 63.0 56.2 52.5 51.9 55.0 63.2 80.2 -

<2 <2 <2 <2 ~2 <2 <2 <2 1.86 3.08 .-

126.5 100.4 88.0 73.8 64.0 55.0 50.0 46.2 44.6 50.1 -

<2 <2 <2 <2 <2 <2 1.82 2.22 2.72 3.76 -

91 72.0 62.4 53.8 49.5 42.4 39.4 39.1 39.5 -

<2 <2 <2 <2 <2 <2 1.95 2.24 2.86 -

82.5 67.9 56.3 48.5 44.9 40.0 39.5 38.6 39.6 50.2 68

<2 i2 <2 <2 <2 <2 2.04 2.30 2.74 3.91 6.87

mixtures Oh

‘%THF (vol.)

10

Debye-Hiickeldiffer less than

the

1.8246 . 10” @)3/Z -.

B=-.

135

3. Results and discussion

Activity coefficients were Debye-Htickel equation log?: = -

Chimica Acta 340 (1997) 133-141

_ ” Literature blank values [22,23]. h Literature values [34]. ’ Literature values [26]. pK <2 indicates electrolyte at the working concentrations. NaBz = Sodium benzoate.

a completely dissociated NaAc = Sodium acetate.

136

U. Muinasmaa

Table 3 Limiting molar conductances % THF (vol.)

HCI

HC104

0* 10 20 30 40 50 60 70 80 90 100’

0-l)

(cm* mol-’

et al./Adytica

and dissociation

Chimica Acta 340 (1997) 133-141

constants

of acids in tetrahydrofuran/water HBz

HAc

H3PO4

mixtures

Ilo

PK

A0

pK

Ao

PK

Ao”

PK

PP

Aoa

417.1 365.3 289.9 233.2 187.0 160.3 138.6 111.8 99.63 88.9 -208

<2 <2 <2 <2 <2 <2 <2 <2 2.23 2.99 1.6,l.l

426.2 315.3 297.3 239.1 204.9 167.6 126.6 106.6 82.23 56.95 -

<2 <2 <2 <2 <2 <2 <2 1.84 2.55 3.16 -

384.5 319.2 216.2 220.9 186.9 144.0 117.8 94.0 42.5 25.3

2.11 2.38 2.65 2.92 3.24 3.52 3.94 4.52 5.00 6.62 -

390.7 347.4 275.1 221.5 185.3 152.6 121.1 91.7 76.5 -

4.16 4.98 5.17 5.43 5.74 6.11 6.58 7.22 8.07

4.92 5.14 5.39 5.70 6.08 -

382.2 343.3 269.1 216.2 180.7 150.2 121.2 97.3 76.6 51.9 -

-

PK

PP

4.19 4.56 4.92 5.37 5.83 6.24 6.75 7.33 8.11

4.49 4.87 5.33 5.78 6.23 -

25.1

299.1 275.4 212.8 167.7 135.8 110.2 81.7 58.6 37.0 7.8

a Calculated from Xi+ - Xt=+ and the limiting conductance of the salt. b Potentiometric literature values [[ 16,17]]. ’ Average of (Aowx~) - AO(NaC104)) and (AO(HCI) - bO(NaCd see Fig. 2. d Literature values [[34,36]]. ’ Literature values [[25,26,37]]. HAc = Acetic acid. HBz = Benzoic acid.

A0

0

10

20

30

40

50

60

70

60

90

100

0

10

20

30

% THF

40

50

60

70

60

90

I”00

% THF

Fig. 1. Limiting molar conductances (cm2 mol-’ K’) of sodium salts in tetrahydrofuran/water mixtures. This work: l = sodium chloride, + = sodium acetate, n = sodium benzoate. Literature values: * = sodiumperchlorate [[20]], v = sodiumperchlorate [[22]], n = sodium perchlorate [[23]], 0 = sodium chloride [[22]].

Fig. 2. Limiting molar conductances (cm* mol-’ K’) of acids tetrahydrofuranlwater mixtures: A = perchloric acid, 0 hydrochloric acid, v = phosphoric acid, 0 = acetic acid, 0 benzoic acid, n = benzoic acid from literature [18], A

There were three independent literature series [20,22,23] for sodium perchlorate. Two of the series [22,23] agree very well (Fig. 1). One of the series covers up to 75% of tetrahydrofuran [23] and the other [22] up to 95% of tetrahydrofuran. Therefore, this electrolyte was not studied and the conductances given in Table 1 have been interpolated between the literature ones of these two series. The third series [20] gives very strange values (see Fig. 1) that do not agree with the ones of the other two series and this series was discarded. The pK values given by these

authors 2201 for sodium perchlorate are also markedly higher than the ones given by the other authors [22,23]. The A0 value of sodium perchlorate in pure tetrahydrofuran is not given in Table 1 because the three available values (A0 =18.2 [20], A0 =84 [25], and A0 =125.8 [35]) do not agree at all. The A0 values determined in this work for sodium chloride (Table 2) agree very well with the literature values of Taniewska-Osiliska et al. [22] (see Fig. 1). The conductances of the acids in pure tetrahydrofuran were too low to determine the dissociation

AO(HCIO~) -

Ao(N~cIo~),

l

=

AO(HCI) -

in = = =

b(NaCI).

(1. Muinasmaa

et al./Analytica

constant, and neither the acids, nor the salts were measured in this pure solvent. Acetic acid and sodium acetate could not be studied at 90% of tetrahydrofuran because of the low solubility of the salt in this medium. The A, and pK values for sodium dihydrogenphosphate are not given because this electrolyte gave too high conductance values which led to wrong results (negative constant values). One plausible explanation for these results is that the partial dissociation of the dihydrogenphosphate gives Hf ions with a conductance much higher than that of Na+ ions. For phosphoric acid this was not a problem because it is less dissociated than the salt and also because the first and second dissociation of the acid give ions of the same conductance (in fact the same H+ ion). Direct estimation of the A0 and pK values of the acetic and benzoic weak acids in tetrahydrofuran-rich solutions (ca. % THF ~50%) led to a poor reproducibility of the results between different series. The reason is that A0 is an extrapolated value to c=O, and the lower the measured conductances, the lower will be the precision on the extrapolation. This can be better seen from the linearizations of the DebyeHiickel-Onsager and Shedlovsky equations (known as Fuoss-Kraus and Shedlovsky methods) [29]. For simplicity we will here show only the linearized Shedlovsky equation, which is 1 --= AS(z)

-‘+ Ao

cAS(z)y2 KA;

(8)

Chimica Acta 340 (1997) 133-141

137

The AD of a weak acid (HA) can be accurately computed from the experimental conductances of a strong acid (HB) and the corresponding salts (NaA and NaB) by means of the Kohlrausch’s law of independent ion conductances. AO(HA) = A;+ + x0,

= AO(HB) -

Ao(N~B) -t Ao(N~A)

(11) The term AO(Ha)-AO(Naa) is equal to Xi+ - Xi,, and can be computed from perchloric or hydrochloric acids. Fig. 2 shows that both acid/salt pairs give concordant values and the average of the two values is given in Table 3. The A0 values of acetic and benzoic acids presented in Table 3 have been calculated from the A0 values of their sodium salts and the A;+ - X&, terms. This allows an accurate computation of the pK values of these two acids. The pK values are in very good agreement with the potentiometric results of Reynaud [ 16,171 for some solvent compositions (Table 3 and Fig. 3), but the results for benzoic acid do not agree completely with the conductometric results of Niazi et a1. [ 181 at different solvent compositions (Fig. 3). The reason of these discrepancies can be that they used a different conductance equation or perhaps that they estimated A0 and K simultaneously from only the benzoic acid conductances. Figures 1 and 2 show that the conductance behaviour of salts and acids in tetrahydrofuran/water mixtures is very different. The limiting conductances

where S(z) = [z/2 + JWZ12

(9)

and z=C?m30

(10)

The plot of the l/AS(z) term against cAS(z)y2 should give a straight line. The limiting molar conductance is the reverse of the intercept of the line and the dissociation constant the ratio between the square of the intercept and the slope. If the electrolyte is poorly dissociated, K is low, the slope of the plot is very high and the relative error in the intercept is large (sometimes even negative intercepts without meaning are found). In this instance, the error in the K value is also very high because it depends on the error in AO.

PK

0

10

20

30

40

50

60

70

80

90

100

% THF Fig. 3. Dissociation constants of neutral acids in tetrahydrofuran/ water mixtures: a = perchloric acid, v = phosphoric acid, 0 = acetic acid, + = acetic acid from literature [16,17], q = benzoic acid, n = benzoic acid from literature [ 16.171. * = benzoic acid from literature [ 181.

138

U. Muinasmaa

et al. /Analytica

of acids are high for water-rich mixtures and decrease monotonically with the increase in tetrahydrofuran contents at least up to a 90% of tetrahydrofuran (Fig. 2). It is not clear what happens in the 90-100% of tetrahydrofuran range, because the only literature [25] available at ho at 100% (for perchloric acid) seems to be too high when compared with that of 90%. But the limiting conductances of sodium salts (Fig. 1) are much lower and show a minimum at 7080% of tetrahydrofuran (50-60% for sodium perchlorate). This can be explained by the different mechanism of transference of H+ and Naf in waterrich mixtures. The hydrogen ion can be transferred from one water molecule to another by a simple change of the hydrogen bonds, whereas the sodium ion must move physically through the solution. In consequence, the conductance of hydrogen ion depends on the hydrogen bonding ability of the solvent, which decreases with the decrease in water contents, but the conductance of sodium ions depends on the viscosity of the solution, which is maximum at intermediate solvent compositions (Table 1). Table 4 shows that the Walden product (Acv) is relatively constant (about one) for sodium salts in the range O60% of tetrahydrofuran, but that it is much higher for acids in the same solvent range and with a maximum at 10% of tetrahydrofuran (20% for phosphoric acid). Such maxima have also been observed for mixtures of water with alcohols and to a minor degree with dioxane and acetone [5-8,38-40], but not with acetonitrile and dimethylformamide [5,12,21]. We attribute this behaviour to the viscosity of aqueous mixtures of alcohols, tetrahydrofuran, acetone and dioxane which show a maximum that the aqueous

Chimica Acta 340 (1997) 133-141

mixtures of acetonitrile and dimethylformamide do not show [5]. The pK values of Tables 2 and 3 for the sodium salts and the strong acids perchloric and hydrochloric demonstrate that formation of ion pairs is appreciable for tetrahydrofuran-rich mixtures, above 60% of tetrahydrofuran approximately. Ion association is mainly ruled by electrostatic interactions which become appreciable in low dielectric constant medium (usually at dielectric constants below 30, which corresponds to ca. 65% of tetrahydrofuran). For the weak acids (phosphoric, acetic and benzoic), the pK values are higher (Table 3) because in addition to the ion pair form in tetrahydrofuran-rich mixtures, they present the molecular form in the whole range of solvent compositions. We have set up several equations, based on preferential solvation models, that allow to relate thermodynamic (pK) and spectroscopic (transition energies) solute properties with the solvent composition [28,38,41-431. Application of the simplest equation to pK values in tetrahydrofuranlwater mixtures takes the form PK =

PKH~O +

avTHF

(12)

1 + bVTHF ’

where pKu*o is the acid pK value in pure water, VTHF is the volume fraction of tetrahydrofuran in the mixture, and a and b are constants. This equation can be employed for some binary solvent systems in the full range of solvent compositions and for almost any water/organic solvent system up to a mole fraction of organic solvent of about 0.5 [41,43]. Eq. (12) was successfully used to correlate the dissociation pK

Table 4 The Walden product bun (g cm mol-’ % THF

R-’

s-l)

for sodium salts and acids in tetrahydrofuran/water

NaC104

NaCl

NaAc

0

1.04

1.13

0.81

0.73

3.71

3.79

10

1.04

1.17

0.84

0.79

4.25

4.36

20

1.06

1.26

0.90

0.81

4.17

4.27

30

1.04

1.22

0.89

0.80

3.86

40

0.98

1.12

0.87

0.79

HC104

HCI

mixtures HAc

HBz

3.42

3.48

3.40

3.11

4.04

3.99

3.97

3.95

3.87

3.96

3.66

3.67

3.58

3.27

3.59

3.27

3.24

3.16 2.61

H3P04

50

0.91

0.95

0.74

0.69

2.78

2.91

2.50

2.65

60

0.83

0.80

0.63

0.63

2.21

2.02

1.88

1.93

1.93

70

0.73

0.61

0.52

0.51

1.47

1.41

1.24

1.29

1.28

80

0.62

0.44

0.39

0.39

0.98

0.81

0.42

0.56

0.35

0.35

0.62

0.40

0.18

0.76 -

0.76

90

0.40

U. Muinamaa

et al./Analytica

values of electrolytes with the solvent composition in mixtures of 2-methylpropan-2-01 with other alcohols [42]. In addition to simplicity, one additional advantage of Eq. (12) over other equations and models developed is that it allows the solvent composition to be taken in volume fraction instead of mole fraction, as it is required in the more complex models. Since in the tetrahydrofuran/water system, a tetrahydrofuran mole fraction of 0.5 corresponds to a 82% of tetrahydrofuran in volume, Eq. (12) should give good fits for the measured pK values of Table 3 in the range O-80% of tetrahydrofuran. These fits can be observed in Fig. 3. The pK data point for phosphoric acid in 90% of tetrahydrofuran does not fit well because it is outside the applicability range of Eq. (12). Another Eq. (12) with different fitting parameters (pKn,o, a and b) would be required for the range 82-100% of tetrahydrofuran. For the sodium salts and the strong acids there is not enough pK data in the range O-82% of tetrahydrofuran to test the applicability of Eq. (12). The feasibility of Eq. (12) to fit the pK data to solvent composition in the tetrahydrofuramwater system has been also tested for the conjugated acids of a set of aniline derivatives studied by Reynaud in the range lo-50% of tetrahydrofuran in volume [ 16,171. The pK values of the anilinium acids in pure water were available through reference [3], except for p-bromo-N,N-diisopropylanilinium. The fits obtained are presented in Fig. 4. The fitting parameters of Eq. (12) obtained for the studied phosphoric, acetic, benzoic and anilinium Table 5 Parameters

for dissociation

of weak acids in tetrahydrofuran/water

7

PK 5

,t,, 0

,,, ,, ,,,,,, 10

,,,,

20

., ,, 30

_______I 40

50

Fig. 4. Dissociation constants of cationic acids in tetrahydrofuranj water mixtures: 0 = anilinium, + = N,N-dimethylanilinium. l = NJ-diisopropylanilinium, n = p-bromoanilinium, A = pbromo-N,N-dimethylanilinium, 7 = p-bromo-N-ethylanilinium, 0 = p-Bromo-N,N-diisopropylanilinium.

acids are presented in Table 5. These parameters can be used to estimate the pK value of the acid for any solvent composition inside the range studied. The b values of Table 5 are higher than unity, which indicate that the acids are preferentially solvated by water rather than by the organic solvent (or in the more rigorous models, the “water-organic solvent” complex [28,43]). The a parameters of the neutral acids are very different from the a parameters of the cationic anilinium acids (Table 5). For neutral acids, positive a values are obtained which indicate that the pK of the acid increases if the contents in tetrahydrofuran increases (as Fig. 3 shows). The negative a values of

mixtures

PKH,o

a

Phosphoric acid” Acetic acid”,h Benzoic acid“.h Benzoic acidC Aniliniumh N,N-Dimethylaniliniumh NJ-Diisopropylaniliniumb p-Bromoanilinium’ p-Bromo-N,N-dimethylaniliniumb p-Bromo-N-ethylaniliniumb p-Bromo-N,N-diisopropylaniliniumh

2.14 4.16 4.19 4.20 4.62 5.20 8.19 3.89 4.30 5.87 8.22

2.14 1.68 3.30 2.15 -1.56 -2.41 -3.11 -2.95 -4.09 -5.16 - 10.30

’ Data from [TX].

,,,,

% THF

Acid

a This work. b Data from [16],[17]].

139

Chimica Acta 340 (1997) 133-141

h

sd

N

Range (% THF)

I .50

0.04 0.02 0.04 0.07 0.04 0.09 0.07 0.03 0.14 0.13 0.08

9 14 14 6 5 6 6 5 5 5 4

O-80% O-80% O-80% O-53% O&50% O-50% O-50% O-50% O-50% O-50% 2(3-50%

I .26 I .60 1.33 1.39 1.34 1.41 1.84 1.68 I .78 2.91

U. Muinasmaa et al./Analytica

140

Chimica Acta 340 (1997) 133-141

the anilinium acids show that the pK value of the acid decreases with the increase in tetrahydrofuran contents (Fig. 4). This can be explained from the different dissociation equilibria of a neutral (HA) and a cationic (HB+) acids.

pK value can be easily calculated for any solvent range of the composition inside the applicability equation.

HA*A-+H+

Acknowledgements

HB+&B+H+,

>

(13) (14)

In Eq. (13) the dissociation of the acid creates cations and anions which interact electrostatically. Therefore, the dielectric constant of the medium must have a strong effect on the dissociation of the acid. Since the dielectric constant decreases with the increase in tetrahydrofuran concentration, the ions are less stabilized and the dissociation decreases. However, in Eq. (14) there is no change in the number and charge of ions and therefore the dielectric constant of the medium must have a small influence on the dissociation. In this instance, specific solvation interactions determines the dissociation of the electrolyte. Typical plots of pK, vs. solvent composition for cationic acids in aqueous/organic solvent mixtures show a minimum at intermediate solvent compositions [3].

4. Conclusions Conductometty is a good method to determine the dissociation constants of acids and salts in tetrahydrofuramwater mixtures. Simultaneous calculation of A0 and pK is possible if the electrolyte is sufficiently dissociated (ca. pK ~5-6). If A0 is known or if it can be estimated from the conductances of salts and strong acids, dissociation constant of weak acids can be determined up to pK&3. Salts are only partially dissociated in tetrahydrofuran-rich solutions (ca. % THF ~60%) because of the low dielectric constant of these media. Accurate calculation of the pH of acid/salt buffers in these solvents must consider the dissociation constants of acids and salts. Equations have been already developed to take into account the partial dissociation of the salts in pH calculations [24,27]. Preferential solvation models can be successfully applied to describe the variation of the pK values with solvent composition. From the fitting parameters, the

The financial support of the DGICYT (Project PB94-0833) of the Spanish Government is gratefully acknowledged. This work has been partially supported by the CIRIT (GRQ93-1028) of the Catalan Government. U.M. work was supported by an ECTempus grant (JEP-06125) which is gratefully acknowledged.

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