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Electrical Conductivity of Aqueous Sodium. Hydroxide Solutions at High Temperatures. H. Bianchi, 1 H. R. Corti, 1,2 and. R. Fern&ndez-PrinF 2. Received May 2 ...

Journal of Solution Chemistry, Vol. 23, No. 11, 1994

Electrical Conductivity of Aqueous Sodium Hydroxide Solutions at High Temperatures H. Bianchi, 1 H. R. Corti, 1,2 and R. Fern&ndez-PrinF 2 Received May 2, 1994; Revised August 19, 1994 Electrical conductivities o f dilute sodium hydroxide aqueous solutions have been determined at 75, 100 and 150~ at 1.6 MPa using a recently developed DC-measuring technique especially suited for the study of aqueous solutions above room temperature. The data were analyzed with modern theories to obtain the infinite dilution conductivity and the association constant at the three temperatures. KEY WORDS: DC conductivity; sodium hydroxide; high temperatures; association constants,

1. INTRODUCTION Electric conductivity is a classical experimental technique which has proved to be a very precise and flexible tool for the determination of the transport and thermodynamic properties of dilute electrolyte solutions. Its success also depends on the fact that the available theoretical treatments for symmetric electrolytes are able to describe precisely the concentration dependence of the dilute solutions. (1) More recently the theory of electrolytic conductivity has been extended (2~) to unsymmetrical electrolytes and mixtures of electrolytes; its performance for mixtures of electrolytes containing unsymmetrical electrolytes has been tested at room temperature. (4) Thus, electrolytic conductivity can be an important tool for the study of the behavior of aqueous electrolytes at high temperature. This field was explored many years ago by Noyes, (s)

1Departamento QuLmica de Reactores, Comisi6n Nacional de Energfa At6micr Av. Libertador 8250, 1429-Capital Federal, Argentina. 2Member, Carrera del Investigador CONICET. 1203

0095-9782/94/I1G0-1203507.00/09 t 994 PIenmnPublishingCorporation

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Bianchl, Corti, and Ferndndez-Prini

after that there were important contributions from the schools of E.U. Franck, C6)of S. W. Benson, ~ W. L. Marshall (g) and W. Lindsay. (9~These pioneering studies were devoted essentially to the study of associated electrolytes and since they were previous to many of the more m o d e m theories, they were not able to extract information on the association constants of slightly associated electrolytes. To get the full advantage of the conductivity measurements it is necessary to have an overall precision better than 0.1 percent. With this purpose we have designed and built a DC conductivity apparatus and air oven which enabled us to accomplish such a goal) 1~ Here we report the results obtained for NaOH dilute aqueous solutions up to 150~ The limitation in the upper temperature that could be studied in this work was the result of the materials used for the gaskets which prevent leaks and electrically insulate the electrodes.

2. EXPERIMENTAL In order to study aqueous systems at high temperature, it is convenient to use metal vessels to contain the solutions, but this is likely to introduce spurious components in the equivalent circuit of an AC measuring system thus complicating the determination of the electrical resistance o f the solutions. For this reason we favored the use of the DC conductivity technique which had been introduced by Gunning and Gordon ~ to study solutions at room temperature and shown to be a good alternative to the standard AC measuring method in terms of the attainable precision. A drawback of this technique is that it requires the use of reversible electrodes, so that the measured electric resistance is only related to the ohmic component and no faradaic resistances contribute to it. In our case we decided to employ as the measuring reversible electrodes, platinum disks covered with platinum black, the solutions were saturated with gaseous hydrogen to guarantee reversibility of the electrochemical reaction leading to charge transfer at the electrodes. Only a brief account of the conductivity setup is given here since it has been described in detail elsewhere. (12) The cell consists o f five platinum disk electrodes insulated with teflon gaskets and pressed between anodized zircalloy rings. The resistance of the passivation layer on the surface of the zircalloy was more than a thousand times bigger than the highest resistance measured in the solutions. Each of the five platinized platinum electrodes were sandwiched between two teflon gaskets and introduced between two zircalloy cylinders, thus forming the conductivity cell illustrated in Fig. 1. The two extreme electrodes were used to introduce a DC-electfic pulse in the cell; the three intermediate

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Teflon Insulation

Zircalloy Rings

Fig. 1. Sketchof the injectionsystemand conductivitycell describedin the text. electrodes formed two independent conductivity cells and the differences of electric potentials between pairs of electrodes were monitored. The conductivity cell was thermostatted within +0.01~ (~z) in a high performance air oven and the pressure of the system was fixed with a back pressure regulator (BP). The operation of the conductivity setup and the oven used as thermostat was carried out through a digital controlling and data-acquisition unit. DC pulses not exceeding 300 mV and lasting 1 s were produced by the controlling unit; the pulses were used to determine the electric resistance of the cell comparing the electric potential differences produced in the conductivity cells with that observed in a standard resistor (RR). The DC pulse was controlled by a personal computer (PC) which monitored the ceils using a digital voltmeter (DVM) and an analog signal scanner (SC). To guarantee a low level of contamination of the working solution, the cell was designed to allow solutions to flow through them. Before entering the actual cell, the working solution was only in contact with stainless steel, teflon and sapphire. The hydrogenated solution was injected into the system through a high pressure liquid chromatography pump (HPLC). To avoid boiling of the solutions, all measurements

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were carried out at 1.6 MPa; the pressure was monitored downstream with a calibrated pressure transducer (T). Sodium hydroxide solutions were prepared by decomposition of sodium mercury amalgam in water under an inert atmosphere. The mercury employed to prepare the amalgam was purified using standard procedures involving a nitric acid treatment, an alkaline oxidation and finally vacuum distillation. The amalgam was prepared by electrolysis of an aqueous solution having 40 wt. percent of NaOH (Mallinckrodt pro analysi) using the purified mercury as cathode and a platinum foil anode. An approximately 0.1 molar sodium hydroxide stock solution was obtained after the amalgam was completely decomposed in water. This stock solution was weight titrated with 0.1 molar potassium biphthalate using a digital analytical balance, weighing burettes and a pHmeter. The titrations were carded out in a closed cell flushed with CO2-free nitrogen. The solutions which were studied were carefully prepared by weighing the appropriate amount of stock solution under a controlled atmosphere. The solutions were hydrogenated before they were injected into the conductivity cell to reduce the bias potential between the measuring (reversible) electrodes. Table I. Solvent Properties at 1.6 MPa ~C

p~ (g-cm "3)

lqa (P)

eb

50 75 100 150

0.98868 0.97553 0.95910 0.91770

0.0054730 0.0037825 0.0028223 0.0018279

69.94 62.27 55.44 43.96

aRef. 16. bRef.18.

The conductivity data for NaOH at 50 and 75~ obtained by Marsh and Stokes (13) were used to determine the cell constants at these temperatures. At higher temperatures the cell constants were corrected for the thermal expansion of the zircalloy rings a*) which did not contribute more than 0.1 percent to the cell constant between 50 and 200~ The density of the solutions were calculated from the partial molal volumes of the electrolyte(~s) and water densities (~) at the experimental temperature. The values of viscosity, density and dielectric constant of water used are summarized in Table I.

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Table H. NaOH Conductivity Data at 1.6 MPa 75~

a

100~

150~

103C=

Ab

103C~

Ab

103C =

Ab

1.6757 1.9627 1.9627 1.9627 4.1421 4.1421 4.1421 5.2894 5.7577 7.3296 7.3296 8.4841 9.2452 10.875 15.512 15.512 15.512

472.70 471.67 471.77 471.67 467.58 467.41 467.11 464.66 464.29 462.10 462~ 460.15 459.02 457.81 452.32 452.41 452.40

1.6475 1.9296 1.9296 1.9296 3.7589 3.9835 3.9835 4.0023 4.8217 5.4554 5.4554 5.4554 5.4554 7.9076 7.9076 8.3413 10.692 15.251 15.251

588.63 588.11 588.35 587.98 579.74 580.20 580.04 579.42 576.53 575.98 575.83 575.85 575.68 570.29 570.27 569.14 567.80 561.27 561.40

1.8463 1.8463 1.8463 2.4198 2.4198 3.8117 3.8117 6.8955 6.8955 7.9816 8.6977 10.231 14.594

799.47 799.56 799.47 795.19 795.85 785.90 786.36 772.91 772.47 770.41 767.52 762.64 752.39

mol-dm-3 .

b

S-cm2-mo1-1.

3. RESULTS The cell constants had values of 4.0045 and 4.0020 cm -x with an uncertainty of 0.05 percent. The bias potentials between pairs of electrodes, which were monitored before and after each determination of the difference in their electric potentials, were about 0.5 mV and were determined within 10 percent. Table II reports the measured conductivities of NaOH at 75, 100 and 150~ The performance of the conductivity cell improved at temperatures higher than 75~ mainly due to smaller bias potentials. In order to fit the experimental data we have used the well proven expansion of the Fuoss-Hsia equation (FHFP) (17) and also the Lee and Wheaton complete equation (LW) c2~which is the most successful of the available equations for the general case of dilute electrolyte mixtures/4~ Ion association cannot be neglected for NaOH at temperatures above

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ambient, an incipient ionic association is observed when analyzing the results of Marsh and Stokes for 75~ (1~ Ionic association corresponds to the process, Na+(aq) + 0H-(aq) Na0H(aq)

(1)

and the association constant KA is KA -

(1 -

a)C

(2)

where a is the degree of dissociation of the electrolyte and y• its mean activity coefficient. In the calculation of the association constant from the conductivity data, we have used the Debye-Htickel expression for the activity coefficients of the free ions and the distance of closest approach of free ions d was fixed at the value of the Bjerrum distance q. According to the FHFP equation, the conductivity of NaOH is given by, a-lA = A ~ - Sq"aC + E a C l n (aC) - J , ( d ) a C + J2(d)(aC) 3n

(3)

For the LW equation we have, $

)~j= ~j ( 1

S

+ zjEZE tv%pj [A~(~d)(13~:)+BvPOcd)(~) 2 + CvP(r.d)(~x:)']) p=2

v=l

IzjbceF

-6~q(1 + r,d)( 1 + ~')(r,d)(~c) + ~2)(r,d)(~l~:)2 + 1-l~5)rd/6) (4) where 1~ is e2/e&T and the other symbols are described in the original paper. (2) The conductivity of the NaOH solutions were calculated from the ionic conductivities using A = ct[~,(Na+) + ~.(OtV)]

(5)

The values of the dielectric constant and viscosity of pure water at different temperatures were taken from reliable sources, t16'ls) Table III gives the least-square best-fit parameters obtained with the two theoretical expressions for the data reported in the present work lower temperatures. and those reported by Marsh and Stokes (13) for 4 Figure 2 illustrates the deviation of the points as function of the concentration. In the case of NaOH at 75~ we have also included the deviations of the data obtained by Marsh and Stokes (13) which are about five times more precise than the ones obtained with our high tempera-

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0,5

h

--

I

I

I

'

75 ~ O

0.0

-0.5 1

100 ~

,b

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