Abstract
Silver nitrate can form homogeneous liquid phases with some organic nitriles and water, even when there is no miscibility between the pure liquid components. We determined the shapes of the single phase regions in the ternary phase diagram for the following systems: silver nitrate/RCN/H,0 with R = CH3, C3H7, C6H5, and C6H5CH2 at room temperature and for R = C6H5 also at 60 C and 0°C. Furthermore we studied kinematic viscosities, electrical conductivities, and densities of mixtures containing silver nitrate, RCN, and water with the mole ratios X/4/1 (0.2 ^ X S 3.4). In these cases also R = C2H5 and C4H9 were studied. The organic nitriles show different dependences of viscosity and conductivity on the silver nitrate content from the aliphatic ones. P h a s e E q u ilib ria a n d T r a n s p o r t P r o p e r tie s
Introduction
Organic nitriles, with the exception of acetonitrile, are not miscible with water. Upon the addition of silver nitrate homogeneous solutions can be formed. This phenomenon is based on heteroselective solva tion: silver ions and nitrate ions are preferentially sol vated by different constituents of the mixed solvents. Strehlow and coworkers [1, 2] were the first to describe this effect quantitatively. They measured the Hittorf transference numbers of acetonitrile and water in sil ver nitrate/acetonitrile/water (X:4:l) mixtures at small silver nitrate contents X and came to the conclu sion that the silver ion solvate complex contains about nine acetonitrile molecules and half a water molecule. The nitrate ion is surrounded by three acetonitrile molecules and three water molecules. Singh, Mac Leod, and Parker [3] measured the Waiden product of diluted silver nitrate solutions in acetonitrile/water mixtures (0 ^ xacetonitrile ^ 100 mol%). The Waiden product drops very sharply up to 20 mol% of aceto nitrile. This observation leads to the assumption that the solvodynamic ( = Stokes-) radius of the silver ion in acetonitrile/water mixtures increases with increas ing acetonitrile concentration.
In order to increase our knowledge about highly concentrated ionic solutions, we first investigated the phase diagrams of some ternary systems silver nitrate/ organic nitrile/water. Furthermore, we measured the kinematic viscosity, electrical conductivity and densi ty of some mixtures as functions of the silver nitrate content. In these measurements, solutions with X moles of silver nitrate per 4 moles of organic nitrile and 1 mole of water were used. X was varied between 0.2 and 3.4. In order to study the effect of the size and the shape of the nitriles on the transport properties, a variety of nitriles was used. Acetonitrile CH3CN (acn), propionitrile CH3CH2CN (ppn), butyro nitrile CH3(CH2)2CN (bun), and valeronitrile CH3(CH2)3CN (van) represented linear aliphatic spe cies. As aromatic nitriles we used benzonitrile C6H5CN (bzn) and phenylacetonitrile C6H 5CH2CN (pan).
Experimental

Preparation of the Solution and Determination of the Ternary Phase Diagrams
The solutions were prepared in weighing bottles of Duran glass with ground caps. About 20 g of silver nitrate is weighed in the bottle and the desired quanti ty of water is added. Then the bottle is closed and the mixture is stirred until silver nitrate is dissolved. Now the bottle is weighed again in order to detect possible losses of water. Finally, the desired amount of nitrile is added and the mixture is stirred for 10 min and than 0932-0784 / 88 /' 1100-0956 S 01.30/0. -Please order a reprint rather than making your own copy.
weighed again. The photosensitive mixtures are stored in a black desiccator under nitrogen atmo sphere.
The equilibrium curves in the ternary phase dia grams (tpd) are determined as follows: We used the same weighing bottles as described above. Portions of nitrile and water were added and their respective masses determined, until a desired mole ratio was established. The two phase mixture then was kept at 26 °C under continuous stirring. Now silver nitrate is added in small portions. After stirring the system for 2 minutes it is inspected visually. When the phase boundary has disappeared, the bottle is dried and weighed. Then more silver nitrate is added until a precipitate appears. The total amount of added silver nitrate is now determined by weighing. From the ini tial masses of nitrile and water and from the masses of silver nitrate at the limits of the one phase region, the mole fractions of the equilibrium phases can be deter mined. The same procedure was followed to deter mine the tpd for AgN 03/C6H 5CN/H20 at 60°C and at 0 C.
Electrical Conductivity Measurements
For the measurement of the electrical conductivity, we used a conventional cell, made of Duran glass with platinum electrodes. The cell constant was determined with 1 m KCl solution. An audio frequency oscillator (Philips function generator PM 5131) is used as a volt age source. The potential drop at the conductivity cell and the potential drop at a precision resistance of 508.2 Q in series with the cell were measured using a Philips voltmeter, model PM 2524. The potential dif ference at the circuit was held sue16@example.com V by a 1:1 booster. The resistance of the conductivity cell then was calculated by Ohm's law.
The cell resistance was frequency independent in the range 1 kHz -4-15 kHz. All conductivity measure ments were made at 5 kHz. The temperature in the conductivity cell was maintained at a constant value with a Braun Thermomix 1460 thermostat.
Viscosity measurements
The measurements of the viscosity were performed in Mikro-KPG-Ubbelohde-viscosimeters (SchottGeräte GmbH. App.-Nr. 68 225 K = 0.03057 and App.-Nr. 90 663 K = 0.009693). The efflux times were measured automatically with the electronic clock AYS 300 (Schott-Geräte GmbH). They appear as 6-digit print outs. The temperature of the viscosimeter was maintained at a constant value with a Braun Thermo mix 1460. Figure 1 shows Gibbs phase triangles for three sys tems. Only acetonitrile is completely miscible with water. The saturation curve in this system is in ac cordance with values reported earlier by Moolel and Schneider [4] . For the other systems, RCN and H20 are not miscible. By addition of silver nitrate, homo geneous three component regions appear. As seen in Fig. 2 , in the case of benzonitrile the one phase region at 26 °C is very narrow at higher water concentrations. At 0 °C two separated one phase fields occur. At 60 °C a larger one phase region is observed.
Results

Ternary Phase Diagrams
It is interesting to see that the maximum solubility of silver nitrate in R C N /H 20 mixtures seems to be quite independent of R and always occurs at a nitrile to water ratio close to 4:1.
Transport Properties
In order to be able to cover a wide range of silver nitrate concentrations, the experiments were done in RCN /H 20 mixtures with a mole ratio 4:1. The kine matic viscosity, electrical conductivity and density were measured in the temperature range from 20 to 60 °C. From these data the molar conductivities and the viscosities can be calculated. By interpolation, val ues for 30 °C are determined. These data are collected in Table 1 . Furthermore, they are displayed in Figs. 3,  4 , and 5. Arrhenius plots of the electrical conductivity and of the viscosity always lead to straight lines. The activation energies, obtained from these plots, are shown in Figs. 6 and 7.
Discussion
The system silver nitrate/acetonitrile/water has al ready been studied by Schneider and Strehlow [1] , Our findings, as displayed in Fig. 1 nitrile and water are completely immiscible. They be come miscible, when only ten mol% of silver nitrate are added to the mixture. In the case of aromatic nitriles, the miscibility gaps extend much more into the interior of the phase triangle, indicating that higher concentrations of silver nitrate are necessary to form a one phase region. This different behavior of aliphatic and aromatic nitriles cannot be traced back to a different affinity of the nitriles towards silver ni trate: the solubility of silver nitrate in the pure nitriles is nearly the same for all systems investigated in this study. We wanted to measure the transport properties over a wide range of silver nitrate concentrations without changing the composition of the solvent. The dotted lines in Figs. 1 and 2 , corresponding to mole ratios X A:1, characterize the investigated systems.
Only in the case of acetonitrile, a concentration range, in which the specific conductivity increases with increasing salt concentration can be realized, see Figure 3 . At X = 1 the specific conductivity shows a maximum, followed by a steep decrease. For the larger nitriles, this decrease at X ^ 1 also can be observed. Only in the case of valeronitrile, the conductivity is nearly independent of X.
The formal molar conductivities decrease with in creasing X in all investigated cases (Figure 4 ). This decrease is accompanied by an increase of the dy namic viscosity, Figure 5 . The decrease of the formal molar conductivity with increasing X can either be caused by strong ion pair formation or by the pronounced increase of the vis cosity. In the latter case, the Waiden rule should be followed by the systems. This rule can be stated as a relationship between the activation energies for the -1,4
Ag NO Nitrile Water 1
acetonitrile chäcn ■ acn a pr0pi0mtr1 le ch3ch2cn ■ Ppn □ butyronitrile ch3ch2ch2cn -B un o V A le r0 Nitrile chjCHjchj chjc n •V an Xbenzonitrile c6h5cn ■ B zn • phenyl -A C ETO N ITR ILE c«H 5ch2cn -Pan conductivity and for the viscosity:
In Figs. 6 and 7 we show the activation energies as functions of X. They were determined from Arrhenius plots. While the activation energies in the systems R = aliphatic group vary only little with X and show a maximum around X = 2.8, the activation energies for R = aromatic group increase monotonously with X. Nevertheless, there is a correlation between the activation energies of the transport properties: In Fig. 8 the ratios \E \IE k A\ are plotted versus X. This ratio is always > 1, but despite the fact that the activa tion energies themselves are found in the region be tween about 10 kJ/mol and 40 kJ/mol, the ratios al ways lie between 1.15 and 1.3 and do not vary systematically with either X or R. Bearing in mind that at higher X, the amount of salt is only little smaller than the amount of solvent, for mation of ion pairs appears to be very likely. These large ion pairs contribute to the increase of the viscos ity as well as to the decrease of the conductivity. We will discuss this matter in more detail in a forthcoming publication, in which measurements of self diffusion coefficients in such systems are reported [5] . 

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