Conductance studies in mixtures of water with DMF at 298.15 K. Part VI. Lithium and sodium nitrates, sodium perchlorate and propionate, potassium picrate and thiocyanate, and limiting ionic conductance

,.

~

journal of MOLECULAR

LIQUIDS ELSEVIER

Joumal of MolecularLiquids 79 (1999) 123-136

Conductance Studies in Mixtures of Water with DMF at 298.15 K. Part VI. Lithium and Sodium Nitrates, Sodium Perchlorate and Propionate, Potassium Picrate and Thiocyanate, and Limiting Ionic Conductance. Adam Szejgis ~ , Adam Bald*", Jerzy Gregorowicz" and Malgorzata Zurada" ~-Llniversityof L6d~, Faculty of Physics and Chemistry, PL-90-236 L6d2, Pomorska 163, Poland e-mail: [email protected], pl Received 11 February 1998; accepted 19 May 1998 ABSTRACT Conductance data of LiNO3, NaNO3, NaCIO4, C2H~COONa, KPic, and KSCN in mixtures of water with N,N-dimethylformamide (DMF), in the whole range of the mixed solvent composition at 298.15 K, are reported. The obtained data have been analysed by the FuossJustice equation in terms of the limiting molar conductance (A~), ionic association constants (KA), Walden products (Aorl), and parameter R. From the data obtained here and those reported earlier, individual limiting ionic conductance of the anions of, NO3, C104-, SCN', Pie" and C2H~COO', has been determined using Fuoss-Hirsch split. From the values of the individual limiting ionic conductance (~o') obtained in this manner, the values of the Walden products for single ions (ko-rl) were also calculated. The obtained results as well as the values of the limiting molar conductance and Walden products for individual ions have been discussed. Some literature data have also been presented for comparison. © 1999 ElsevierScience B.V.All rights reserved. INTRODUCTION This work is continuation of our studies on conductometric properties of univalent electrolytes in mixtures of water with N,N-dimethylformamide (DMF), which have been a subject of our interest for many years [1-5]. In our previous works in this series [1-5] we have reported on the conductivity properties of some alkali metal chlorides [1], bromides [2], iodides [3], some tetraalkylammonium iodides [4], and i-AmsBuNI and NaBPh4 [5] in binary mixtures of DMF with water at 298.15 K. In those papers [1-5] the obtained results were extensively discussed and compared with existent literature data, wherever it was possible. In this paper .we present conductivity data for solutions of lithium nitrate (LiNOs), sodium nitrate (NaNO3), sodium perchlorate (NaCIO4), sodium propionate (C2HsCOONa) potassium picrate (KPic) and potassium thiocyanate (KSCN). The solutions were investigated in the whole range of the mixed solvent composition, except for sodium propionate, because of its low solubility. The pertinent literature shows, that the solutions of nitrates of sodium and potassium were already investigated by Bahadur et al. [6 ] in mixtures of water with DMF up to 70 mol % of amide 0167-7322/99/$ - see front matter © 1999ElsevierScience B.V.All rights reserved. S0167-7322(98) 00107-X

Pll

124 in the mixture (only five composition). Chittleborough et al. [7] have investigated solutions of potassium thiocyanate in mixtures containing 50 and 75 mol% of amide; and Niazi et al. [8] have investigated solutions of NaC104 in mixtures of water with DMF in the range 0 - 35 mol% DMF. These investigations were performed at 298.15 K, so it was interesting to compare the fragmentary literature data [6-8] with those obtained in this paper.

EXPERIMENTAL

Water and DMF (Apolda, Germany) were purified according to the standard procedures as described earlier [1-5]. The salts used were mainly of spectral grade of Merck or Apolda, and were only dried at elevated temperatures for 12 hours. The experimental procedure used in this work is similar to those described earlier [1-5]. The conductivity measurements were performed with the use of Precise Component Analyser type 6425 (Wayne-Kerr, UK). All conductance values were the results of an extrapolation to infinite frequency [9]. Solvent properties like relative permittivity and viscosity, necessary for calculation, were derived from the literature [10-12], and were numerically interpolated, when necessary. Solutions and mixed solvent compositions were prepared by weight, and were accurate to within _+0.01 wt%. The temperature was kept constant to within _+0.005 K , and all the data were corrected at 298.15 K with the specific conductance of the solvent. Taking into account the purity of reagents, experimental procedure, and conductivity equation, we estimate the accuracy of the measured values of conductivity as better than 0.05 per cent.

RESULTS AND DISCUSSION

The conductivity data were analysed with the Fuoss-Justice conductance equation [13-15] using the following set of equations: A = oc

[Ao -S (otc) 1/2 + E (ore) in (ac) + J (otc) + J3t2(Or.C) 3/2 ]

(1)

I
(2)

In y+_= - (AotltZcl/2) / (1 + BRec 1~ c LI2)

(3)

All symbols in these equations have their usual meaning and were explained elsewhere [1-5] (see also source papers i.e. refs. [13-16]). The error of the R value i.e. AR was estimated as corresponding to 0.1 (o),),,i~ (for more see our earlier papers [1-5] as well as paper of Fuoss [17]). The values of A0, KA and R together with their standard deviation and estimated errors of the R values (i.e. ZLR)are given in Tables 1-6; for LiNO3, NaNO3, NaC104, C2H~COONa, KPic and KSCN solutions, respectively. The dependencies of Ao for all solutions investigated in this paper versus the mixed solvent composition (A0 = fix)) are given in Figure 1.

125 Table 1. Limiting molar conductance (/%), ionic association constants (KA), their standard deviations (a/%) and (aKA), parameters R and their estimated errors AR, and the Walden products (/%rl) for LiNO3 in water+DMF mixtures at 298.15 K

DMF/ mol% 0.00 2.50 5.00 10.00 20.00 25.00 35.00 50.00 70.00 80.00 90.00 100.00

Ao

o/%

KA

OKA

R

AR

Aorl

110.06 89.66 75.12 56.62 39.59 36.16 34.29 39.04 53.08 62.03 71.95 82.51

0.04 0.02 0.02 0.03 0.01 0.02 0.01 0.01 0.01 0.01 0.04 0.02

-----0.7 1.1 2.8 6.1 8.1 11.9 15.9

-----0.3 0.5 0.8 1.1 1.9 2.1 2.0

4.0 4.0 4.0 4.0 4.5 4.5 4.5 5.0 5.5 5.5 6.0 6.0

0.5 0.5 0.5 1.0 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

0.980 0.986 0.986 0.983 0.934 0.899 0.825 0.728 0.661 0.645 0.648 0.656

Table 2. Limiting molar conductance (A0), ionic association constants (KA), their standard deviations (aAo) and (aKA), parameters R and their estimated errors A_R, and the Walden products (Aorl) for NaNO3 in water+DMF mixtures at 298.15 K

DMF/ Ao mol% 0.00 121.63 2.50 100.42 5.00 85.28 10.00 65.94 15.00 54.18 20.00 47.54 i 25.00 43.54 35.00 41.27 50.00 45.40 70.00 59.22 80.00 67.79 90.00 77.18 100.00 87.10 i _ interpolated value

aAo

KA

t~KA

0.02 0.02 0.03 0.03 0.02 . 0.02 0.03 0.02 0.02 0.03 0.02 0.02

1.5 1.5 1.4 1.7 1.5

0.6 0.5 0.7 0.7 0.6 . 0.6 0.9 1.3 2.3 3.1 3.5 4.1

.

. 2.3 3.4 6.4 14.5 20.7 30.1 49.3

R

AR

Aorl

4.0 4.0 4.0 4.0 4.0

0.5 0.5 0.5 1.0 0.5

4.5 5.0 5.0 5.5 5.5 6.0 6.0

0.5 0.5 0.5 0.5 0.5 0.5 0.5

1.083 1.105 1.130 1.145 1.143 1.121 1.082 0.993 0.847 0.737 0.705 0.695 0.692

.

126 T a b l e 3. Limiting molar c o n d u c t a n c e (A0), ionic association constants (KA), their standard deviations (aAo) and (aKA), parameters R and their estimated errors A_R, and the W a l d e n products (A0rl) for NaCI04 in w a t e r + D M F mixtures at 298.15 K DMF/ mol% 0.00 2.50 5.00 10.00 20.00 25.00 35.00 50.00 70.00 80.00 90.00 100.00

Ao 117.35 94.95 78.91 59.28 42.71 39.53 38.47 44.21 58.82 67.05 75.03 82.43

oAo

KA

OKA

R

5~R.

0.01 0.02 0.02 0.02 0.01 0.01 0.01 0.04 0.05 0.04 0.02 0.01

--------0.9 1.4 2.8 3.9

--------0.3 0.6 0.9 1.1

4.0 4.0 4.5 4.5 4.5 4.5 4.5 5.0 5.5 5.5 6.0 6.0

0.5 0.5 0.5 1.0 0.5 0.5 0.5 0.5 0.5 0.5 0.5 1.0

Aorl 1.044 1.044 1.046 1.030 1.007 0.982 0.925 0.825 0.732 0.697 0.675 0.655

Table 4. Limiting molar c o n d u c t a n c e (A0), ionic association constants (KA), their standard deviations (oA0) and (OKA), parameters R and their estimated errors AR, and the W a l d e n products (Aorl) for C2HsCOONa in w a t e r + D M F mixtures at 298.15 K

DMF/ mol% 0.00 2.50 5.00 10.00 20.00 25.00 35.00 50.00 70.00

Ao 85.83 72.28 62.56 49.64 36.84 34.22 32.41 35.51 46.09

aAo 0.07 ' 0.07 0.06 0.04 0.02 0.01 0.02 0.04 0.04

KA

OKA

---0.7 2.2 3.7 6.6 15.9 61.7

--0.4 0.9 1.1 1.9 2.3 4.5

R

AR

Aovl

3.5 3.5 3.5 3.5 4.0 4.5 4.5 4.5 6.0

1.0 1.0 1.0 0.5 0.5 0.5 0.5 1.0 1.0

0.764 0.795 0.829 0.862 0.869 0.869 0.780 0.662 0.574

127

Table 5. Limiting m o l a r c o n d u c t a n c e (/%), ionic association constants (KA), their standard deviations (~/%) and (t~KA), parameters R and their estimated errors AR, and the W a l d e n products (/%rl) for KPic in w a t e r + D M F mixtures at 298.15 K DMF/ mol% 0.00 2.63 5.71 9.98 14.41 20.13 26.50 36.12 49.24 68.30 81.39 87.17 100.00

A0

0/%

KA

OKA

R

AR

/%rl

104.00 84.60 69.76 56.32 47.39 40.93 37.07 36.13 39.90 50.19 58.32 61.75 68.64

0.04 0.04 0.04 0.01 0.01 0.01 0.04 0.02 0.01 0.05 0.04 0.03 0.05

-------0.7 1.2 2.0 3.3 4.5 4.9

-------0.3 0.6 1.2 1.4 1.9 2.1

4.0 4.0 4.5 4.5 4.0 4.5 4.5 5.0 5.0 5.0 5.5 6.0 6.0

0.5 0.5 0.5 0.5 0.5 0.5 1.0 0.5 0.5 0.5 0.5 0.5 1.0

0.926 0.951 0.959 0.978 0.988 0.968 0.927 0.858 0.754 0.645 0.595 0.579 0.546

Table 6. Limiting molar c o n d u c t a n c e (/%), ionic association constants (KA), their standard deviations (0/%) and (OKA), parameters R and their estimated errors AR, and the W a l d e n products (/%rl) for K S C N in w a t e r + D M F mixtures at 298.15 K

DMF/ mol% 0.00 2.22 5.48 9.16 19.25 24.27 34.08 48.10 68.96 81.16 88.38 100.00

A0 140.04 115.80 91.16 73.68 51.33 46.76 44.09 48.02 62.22 72.86 79.54 90.30

o/%

KA

~KA

R

AR

AoB

0.03 0.02 0.03 0.02 0.03 0.02 0.02 0.02 0.02 0.02 0.03 0.04

-------0.6 3.0 4.5 5.7 7.4

-------0.3 1.0 1.5 1.9 2.0

4.0 4.0 4.0 4.0 4.5 4.5 5.0 5.0 6.0 6.5 6.5 7.5

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 1.0

1.247 1.256 1.235 1.227 1.199 1.157 1.069 0.929 ,0.790 0.747 0.733 0.718

Units in table 1-6; Ao and 0/% / S cm 2 mol ~ ; KA and OKA / d m 3 mol ~ ' R and AR / 10 "s cm ; A0r I / S c m 2 mol 1 P

128

160 .--o.-- LiNO= -.-o--- N a N O , =. NaClO, C=HsCOONa ,~ KPic • KSCN

140

2,5

120

;_100 o

E

Ix.

1,5 ~-

80 cO .<

6O

40 0,5 20

0

0

~ 20

40

40 60

80

1~

mol % DMF

Figure 1. Viscosity (rl) of water+DMF mixtures and limiting molar conductance (A0) for LiNO3, NaNO3, NaCIO4, C2HsCOONa, KPic and KSCN at 298.15 K. As shown in Figure 1, the values of the limiting molar conductance (A0) decreases abruptly, when small amounts of DMF are added to water, reaching minimum at ca. 30-35 mol% content of the amide in the mixture, and further increases with increasing of DMF content up to pure anfide. A similar character of the dependence of A0 = fix) was oboerved in the case of all the electrolytes investigated earlier by us [1-5]. This confirms the substantial influence of macroscopic viscosity on ionic mobility. The position of the minimum on the dependence A0 = f(x) corresponds to the maximum of viscosity at appropriate DMF contents in the mixtures. These dependencies evidently result from the Stokes law. Similar dependencies of A0 = f(x) were also observed in papers [6-8]. However, one should point here, that the values of A0 for scdium nitrate in mixtures of DMF with water up to 70 mol% of amide, obtained by Bahadur et al. [6] differ from those obtained in this work. Thus, for example for the mixtures containing 10, 30, 40, and 50 mol% of DMF, the values of A0 for NaNO3 obtained in paper [6] are 72.18, 45.58, 44.21 and 48.27 S cm 2 mol "~ respectively, while our values for. the same mixed solvent composition are 65.94, (41.5), (42.0), and 45.40 S cm 2 molk The values of Ao given in brackets were obtained by numerical interpolation of the data obtained in this paper, for better comparison. Although in paper [6] a different conductivity equatton was used i.e. Fuoss 1978 equation (ref. 8 in paper [6]), it could not explain the discrepancies mentioned above. On the other hand, for mixtures containing 70 mol% of DMF the agreement of our values of A0 for sodium nitrate with the literature value is quite good (i.e. 59.22 S cm 2 mo1-1

129

- this work and 59.02 S cm 2 mol "l - Bahadur et al. [ 6]). It is well known, that conductometric investigation of numerous electrolytes, for example univalent, with common cation or anion in the same pure or mixed solvent can confirm the correctness of the obtained values of A0, as it results from Kohlrausch law. Thus, for example, the differences of values of Ao for LiBr and NaBr, presented in our earlier work [2] are the same as those for LiNO3 and NaNO3 , presented in this paper. In our opinion, this confirms the coincidence of the present data with earlier findings [2]. On the other hand, relatively greater discrepancies between the values of Ao for NaCIO4 given in paper [8], and those obtained in this work can be observed. These discrepancies cannot be explained by the fact, that a different conductivity equation was used. Moreover, the dependence A0 = fix) presented in paper [8] seem to be somewhat composed in their character, and even suggest a presence of an inflexion point, in the range 0 - 36.5 tool% of DMF in the mixture. Furthermore, the character of the dependencies of A0 = f(x) for NaCIO4, suggests even the presence of a minimum on the mentioned dependence at greater DMF contents in the mixtures, so it differs from those observed for electrolytes investigated earlier [1-5], as well as from the results obtained in this paper. This fact should be pointed here, because these discrepancies can prove however, to misleading conclusions, if one to analyse the conductometric properties of electrolytes. In turn, the values of A0 for KSCN obtained in this paper are in excellent agreement with those given in paper of Chittleborough et al. [7]. 1,4

1,4

1,2

--o-- LiNO~ -,..o.-- NaNO 3 8 NaClO,, ,i. C=H~COONa • KPic

~ ~

1,2

1

1

~E 0,8

0,8

0,6

,0,6

0,4

0,4

0,2

.0,2

o

0 0

I 20

I 40

I 60

....

I 8O

0 100

mol % D M F

Figure 2. Walden products (A0rl) for LiNO3 KSCN in water+DMF mixtures at 298.15 K.

, NaNO3

,

NaCIO,, C2HsCOONa, KPic and

130 It is well known, that it is better to analyse the conductometric properties of electrolytes in terms of Walden products (A0rl), because of the substantial influence of the macroscopic viscosity on ionic mobility. The dependencies A0rl = f(x) for all salts investigated in this paper are presented in Figure 2. As shown in Figure 2, in the case of NaNO3, KPic, C2H~COONa, small additions of the DMF to the water lead to an increase in the values of Aovl, and distinct maximum appears on the dependencies A0rl = f(x) at ca. 20 mol% of DMF in the mixture. In turn, in the case of LiNO3, NaCIO4 and KSCN, the values of Aorl increase to a very small extent (see also Tables 2 and 6) or these values remain constant (Table 3). So, in the case of these three electrolytes values of Walden products in the range of a small amount of DMF contents in mixture become stabilised. Greater amounts of DMF in the mixture lead to the distinct diminution in the values of A0rl. In mixtures containing more than 70 mol% of DMF one can observe a slight diminution in the values of A0rl, particularly in the case of KSCN and NaCIO4, and distinct stabilisation of the values of A0rl in the case of LiNO3. It should be pointed here however, that the character of the dependencies of A0rl = f(x) for NaNO3, KNO3, N-I-hNO3 presented in paper of Bahadur et al. [6] is similar in form to the dependencies obtained in this paper for NaNO3 (in spite of the above-mentioned discrepancies between the values of the limiting molar conductance obtained in [6] and our values obtained in this paper). In contrast, Niazi et al. [8] suggested, that the values of Aorl for NaCIO4 pass by a distinct maximum, which in turn does not result from our investigations. We suggest, that this can result from the greater values of A0 obtained in paper [8], which was already pointed earlier. As is known, the conductivity properties of electrolytes result from the conductivity properties of ions.

Table 7.

The individual limiting ionic conductance (~o) in water+DMF mixtures at 298.15 K.

DMF/ NO3 CIO4" mol% 0.00 71.30 67.02 2.50 58.19 52.72 5.00 48.70 42.53 10.00 37.05 30.39 15.00 30.10 -20.00 26.29 21.46 25.00 24.12 20.11 35.00 23.49 20.69 50.00 26.55 25.36 70.00 36.43 36.03 80.00 42.69 41.95 90.00 49.63 47.48 100.00 57.07 52.40 Units in Table 7; Xo"/ S cmz mol1

SCN

Pic

CzHsCOO"

66.33 52.49 42.84 31.47 25.87 22.97 21.68 21.65 26.55 37.45 44.04 51.27 59.27

30.29 24.53 21.58 16.88 14.47 13.61 13.09 13.45 17.76 25.71 29.64 33.59 37.61

35.50 30.05 25.98 20.75 -15.59 14.80 14.63 16.66 23.30 ----

131 This results, for example, from the fact, that the character of the dependencies of A0rl = f(x) for LiNO3 and NaNO3 is different, and it surely results from the differences in the conductivity properties of lithium and sodium ions, as was already explained in paper [5]. Thus, it is better to divide the values of Walden products for salts into the ionic contribution, and then to analyse the dependencies of ionic Walden products (3,o-+11)on the mixed solvent compositions. For this reason, the values of the limiting ionic conductance (Zo) for N O r , CIO4-, SCN', Pic" and CzHsCOO" (Table 7) were calculated, on the basis of the values of the limiting ionic conductance (~*) of Li +, Na + and K + given in paper [5], which were obtained on the FuossHirsch split. The reasons for which Fuoss-Hirsch split [ 18] was used for these calculation were explained in our previous paper [5]. The dependencies ~o" = f(x) for the above-mentioned anions are presented in Figure 3.

8O

80

ClO, SCN 70

• Pic j. C2H5COO--o---

t 70

NO~

60

60

50

50

4o

40

3O

30

20

20

10

10

cO

0

Figure 3.

I 20

I 40

I 60

I 80

0 I00

mol%DMF

Limiting ionic conductance (Zo') in water+DMF mixtures at 298.15 K.

As shown in Figure 3, the character of the dependencies ~ = f(x) for all these anions is similar in form, and a distinct minimum appears at ca. 30-35 tool% DMF content in the mixture. This is evident in view of the influence of the macroscopic viscosity on the ionic mobility, as was already mentioned, analysing the dependencies A0 = fix), presented in Figure 1. Nevertheless, it is clearly visible, that in the case of Pic and C2H5COO ions, the values of limiting ionic

132 conductance in water are smaller in comparison with those for other ions, which can be connected with their considerably greater dimensions. Moreover, one can observe, that the values o f ~ for P i c and C2H5COO" decrease to a smaller extent in comparison with NO3-, C104 and SCN, when the small amounts of the DMF are added to water. The values o f ~ for Pic', and probably for C2H5COO, are smaller in pure DMF, in comparison with other anions, which seems to be connected with their relatively greater dimensions. Moreover the values of ~ - for Pic" are smaller in pure water than in pure DMF, so they are in reverse order than in the case o f NO3", CIO4- and SCN. However, taking into account the considerable influence of macroscopic viscosity on the ionic mobility, the analysis of the dependencies o f the ionic Walden products on the mixed solvent composition seems to be more advisable, because the changes in the ionic Walden products are evidently connected with effective ionic radii. The values o f ionic Walden products (Tv0"rl)are collected in Table 8, and the dependencies ~ r l = f(x) for all anions investigated in this paper are presented in Figure 4.

Table 8. Walden products for single ions (Lorl) in water+DMF mixtures at 298.15 K.

DMF/ mol% 0.00 2.50 5.00 10.00 15.00 20.00 25.00 35.00 50.00 70.00 80.00 90.00 100.00

NOs

CIO4

SCN

Pic"

C2H~COO"

0.635 0.640 0.645 0.644 0.635 0.620 0.599 0.565 0.495 0.454 0.444 0.447 0.454

0.597 0.580 0.564 0.528 -0.506 0.500 0.498 0.473 0.449 0.436 0.427 0.417

0.591 0.577 0.568 0.547 0.546 0.542 0.539 0.521 0.495 0.466 0.458 0.461 0.471

0.270 0.270 0.286 0.293 0.305 0.321 0.325 0.324 0.331 0.320 0.308 0.302 0.299

0.316 0.331 0.344 0.360 -0.368 0.368 0.352 0.311 0.290 ----

Units in Table 8; X~'lq / S cm 2 mol 1 P

As can be seen in Figure 4, the changes in the values o f ~-rl versus mixed solvent composition are small and relatively smaller than those which we obtained earlier for alkali metal cations [5]. On the other hand the dependencies ~ ' r l = f(x) for NO3", C104" , Pic', S C N and C:HsCOOare quite different.

133 0,7

0,7 CIO',

0,6 ,

•

SON"

•

Pic"

"~

~t 0,5 ~

"~o '

~

0,6

~ C=H,COO -o-- NO~

~

~

:

~

0,5

n

~

0,4

0,4

~t~o0,3 '

0,3

,<

0,2

0,2

0,1

0,1

o 0 Figure

4.

I 20

i 40

t 60

I o 80 1(30 tool % DMF

Walden products for single anions (~o rl) in water+DMF mixtures at 298.15 K.

The values of Xo'rl for Picand C2HsCOOions, which are relatively great ions, are considerably smaller in comparison with other ions, which could be expected considering to the values of ion radii. The changes of the values of Walden products of picrate anion versus mixed solvent composition are relatively small and 3imilar to the changes in the values of ko*rl observed for tetraalkylammonium ions [5], and particularly in the case of tetraethylammonium cation. This can confirm the close similarity of effective ion radii of Pic" and Et4N-* in the investigated mixtures of water with DMF. The increase in the values of ionic Walden products with increasing of DMF content in the mixture (as in the case of other tetraalkyammonium ions and tetraphenylborate ion) could be connected with the different type of hydration, that is hydrophobic hydration, as was mentioned in paper [5]. The changes in the water structure caused by the addition of DMF probably prove, that the water dipoles are less ordered in the surroundings of these ions, and so their mobility increases. However, ranging from 40 mol% DMF content in the mixture up to the pure amide the values of X0"rl for picrate ion increase insignificantly. The increment in the values of %0±rl for picrate ion in comparison with tetraalkylammonium cations and BPh4, accompanying the addition of DMF to water, seems to be less intensive. This can be connected with smaller charge delocalisation in the case of picrate ion. The second of the relatively large ions investigated in this paper i.e. propionate anion has hydrophobic part and carboxylate group of hydrophilic character. The relatively small ionic mobility, and therefore small values of the ionic Walden products of the propionic

134 ion can be explained by its relatively great dimension. However the appearance o f a distinct maximum on the dependencies k0"rl = f(x) at ca 20 mol% o f DMF content seems to be characteristic for the propionate anion. The observed increase in the values of ~o'rl for propionate ions seems to have two causes; i.e. hydrophobic dehydration (as in the case of other hydrophobic ions), and so-called ,,sorting effect" [19], resulting from the preferential interaction o f carboxylate group with water in comparison with DMF. The fact, that the closest surroundings of carboxylate group of propionate ion are richer in a water in comparison with mixed solvent can lead to a smaller drop in ionic mobility, than it could follow from the change o f the macroscopic viscosity, which in turn increases with increasing o f DMF content in the mixture. 65

65

60

60

55

55

50

50

45

45

40

40

s,...

35 30 25

25

20

20

15

15

10,

10 5

5 0

0

20

40

60

80 mol

o 10o % DMF

Figure 5. Values Of KA as a function of DMF content in water+DMF mixtures at 298.15 K for LiNO3, NaNO3, NaCIO4, C2H~COONa, KPic and KSCN in water+DMF mixtures at 298.15 K.

Greater addition of DMF to the water can lead to the gradual dehydration of the carboxylate group of propionate ion and it can be in consequence the reason for which one observe very intensive increase o f the values of ionic association constant (KA) of sodium propionate (see Figure 5). It was shown in paper o f Bahadur et al. [20], that the maximum o f the values o f Walden product on the dependence AorI = fix) for sodium acetate appears in a water-rich region i:e. at ca 16 mol% o f DMF contents in the mixture. However these investigations were performed at 308.15 K, and the values of ~,0rl were not given, so the direct comparison of the

135

conductometric properties of acetate and propionate ions in the mixtures of water with DMF is somewhat difficult. The values of ionic Walden products for N O 3 , S C N and C104 are relatively greater than those for picrate and propionate ions (see Figure 4 and Table 8). One can also observe, that in the water-rich region the dependence ~0-rl = f(x) for NO3" differs from that for ions SCN" and CIO4". In the case of nitrate ions the addition of DMF to water leads to an increment of the values of ionic Walden product (~'rl), while in the case of the thiocyanate ion and particularly in the case of the perchlorate ion these additions lead to diminution of the values of ~ r l . Above ca. 10 tool % of DMF content in the mixture with water the values of E0rl decrease gradually in the case of perchlorate and thiocyanate ions, while in the case of the nitrate ion, these values, after reaching the maximum, further decrease more intensively up to ca. 50 mol% DMF content. Ranging from ca. 50 mol% of DMF content in the mixture up to pure amide, the values of ~0-1] for all three anions are almost similar. However one can observe that the values of ~0"rl decrease gradually with increasing of DMF contents in the case of the perchlorate ion, while in the case of the thiocyanate and nitrate ions these values remain constant, and in DMF-rich region they increase insignificantly up to pure amide. In our opinion, the differences in the conductivity properties of the investigated in this paper oxoanions i.e. NO3" and ClOd in water-rich mixtures seem to be most visible. It is well known from literature [21-24] that the anion are poorly solvated by disubstituted amides. However in our previous paper [5] we suggested, analysing the differences in dependencies of ~'rl = f(x) for different halides anion, that chloride ions show a greater ability to preferential solvation by a water dipole in comparison with iodide ions. From this point of view, nitrate ions probably have greater ability to preferential solvation by water dipole in comparison with perchlorate ions, because either in the case of perchlorate and iodide ions, small addition of DMF to water leads to diminution in the values of ~'rl. Our suggestion can be confirmed by the fact, that increasingly with increasing of DMF content values of K^ increase in the greater extent in the case of nitrates of lithium and sodium in comparison with sodium perchlorate. These facts can be connected with preferential solvation by water dipoles, being stronger in the case of nitrate ion in comparison with perchlorate ions. The increment of the values of Z,0"rl for NO3, ranging from DMF-rich region up to pure amide, can result from the weak solvation of these anions by DMF dipole. In this range of the mixed solverU: composition the values of ~-rl for perchlorate anion decrease monotonically, which can additionally confirm the above-mentioned suggestion. Owing to the results given in paper [5] one can generally conclude that in the mixtures of water with DMF the conductometric properties of nitrate ions seem to be similar to chloride ions, while the conductometric behaviour of perchlorate ions is similar to iodide ions. Moreover, from the literature data [ 19,25-32] on the conductometric properties of perchlorate and nitrate ions in other mixtures of water with organic co-solvent it follows that addition of protic co-solvent to the water , for example methanol [25] and ethanol [19], leads to an increment in the values of 7%rl and to the appearance of a maximum on the dependence ~o-rl = f(x) in the case of perchlorate ions. On the contrary the addition of aprotic co-solvent to the water, for example TMS [26,27], DMSO [28,29] and HMPT [30] leads to a decrease in the values of ~0"rl for CIO4" ions. On the other hand, in the case of nitrate ions the addition to the water of both protic, (for example ethanol [31]) as well as aprotic co-solvent, (for example HMPT [30]) lead to an increase in the values of ~ r l , and to the appearance of a maximum on the dependence ~o'rI = f(x). However, the literature data on conductometric properties of nitrate ions in the mixtures of water with acetonitrile [31,32] suggest either increase [32] or decrease [31 ] in the values of ionic Walden products. It is worth nothing here, that dependence

136 ~o'rl = f(x) for NO2" ion in the water+DMF mixtures at 308.15 K presented in paper [20] is similar in form to those obtained in this paper for NOr. In our opinion additional conductometric investigations in the water+DMF mixtures at 298.15 K including nitrite, chlorate, acetate and others carboxylate ions should be made, in order to obtain more precise informations on these ions in the system under consideration. We intend to undertake these investigations.

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