Floyd Wilcox, Sr. Centrol Wesleyon Colleae Central, South Carolina 29630
I I
Ion Specific Electrodes in Fused Salts An experiment
Since the turn of the century, the glass membrane potential has been of considerable interest, both theoretically and practically. Most of the teaching has stressed the analytical application of the potential and few attempts are made a t teaching the theory upon which the operations of ion selective glass electrodes is based ( I ) . Even when the theory is presented in class, there are usually no ex~erimental~rocedures which demonstrate them 12). his is understandable for two reasons. First, the selectivitv constants for conimercial electrodes have been desighed so that they are extremely sensitive to only one ion (i.e. hydrogen, sodium, or silver) and almost insensitive to all of the other ions. Thus, it would be extremely difficult to apply the theory. For example, the Corning Number 5-30090-80 silver-sodium ion selective electrode has selectivity constants for silver, sodium, lithium, and ammonium equal to 106, 103, 12. and 1, respectively (3). The potential between a solution that is 1 M (assuming an ideal solution) in both sodium and silver ions and one that is 1 M in silver ions alone is 0.77 mv. Thus a system is needed in which the selectivity constants are as close together as ~ossihle.Otherwise. verv laree concentration differences are necessary. Secondly, the activity coefficients are not usually known for these mixed systems. It could be assumed that the activity coefficients are unity. However, a t the large concentration difference necessary to apply the equation
to commercial electrodes, the assumption would most certainly be completely invalid. Recently, Notz and Keenan (4) published an article in which they determined the selectivity constants of Pyrex membrane electrodes for sodium, silver, lithium, and potassium as 147, 47, 10, and l, respectively using ammnnium nitrate as an inert solvent a t 190°C. Doing the same type of calculation as before (i.e. 1 M in both sodium and silver versus 1 M in sodium alone) gives a change in the potential of 10 mV. This makes the system easier to study. The authors also demonstrated that the above systems were fairlv ideal over a concentration range of 0-4 -M- and activity c&fficients would be unnecessary. A further advantage of using the above svstem is that i t would provide students with an opportunity to do experiments in a fused salt system. This usually is not presented to undergraduates but it can provide several unique experiences. Therefore, an experiment for undergraduates presented below was developed to demonstrate the theory of the glass memhrane potential in fused salts. Theory
The ion-exchange model for the glass membrane potential was first expounded by Nicolskii (5) in 1937 and later refined by Cnnti and Eisenman (6). Their refined model considered the glass memhrane potential to he composed of a diffusion potential (due to the ions diffusing through the glass) and two phase boundary potentials which are due to the ion-exchange properties of the glass. The final equation from their deviation as applied to the system is
This paper was presented at the Southeastern Regional American Chemical Society meeting in Birmingham, Ala., November, 1972.
Volume 52, Number 2, February 7975 / 123
where Cis the contribution d u e to t h e solvent (i.e., t h e solvent itself and t h e impurities) and X i s the respective mole fraction of the major electroactive species.
and Pyrex hulh electrode were stared in fused ammonium nitrate when not being used. The Pyrex hulh electrodes were not preconditioned in any manner except far soaking them in molten ammanium nitrate when they were not being used. The extensive preconditioning noted by Notz (4) results in slightly better results for the selectivity constants and eliminates much of the scatter in the C' values. It is much more time consuming and was not considered necessary for the accuracy needed in this experiment. The general procedure for a run is as follows. A known amount of ammonium nitrate was introduced into s 250-ml Berzelius beaker, placed in the fused salt bath, and allowed to come to thermal equilibrium. The electrodes were cleaned by soaking them in a test tube of molten ammonium nitrate. They were then wiped gently with a Kimwipe and placed in the reactor. The solution was continuously stirred (at a slow rate to avoid splashing) with a glass propeller type lab stirrer. The leads from the electrodes were connected t o the pH meter and emf readings were taken periodically until constant to within 0.5 mV. A weighed increment of the desired solute was then added and the emf readings were taken periodically until constant within the above limit. Incremental addition was continued until the desired final concentration was reached. A t the- end of each run a bias ootential of the electrode svstem .~~ was measured hy placing the electrode palr in 8 brnker that run. tained approximarely 50 p, of the ,ame solution as that whwh was used to fill the eleutroder. The bias pormtiala w r e u,ualig lrss than 10 mV.
Experimental
Results a n d Calculations
The constant temperature bath was constructed according to the directions given by Notz (4). A drawing of the bath is given in Figure 1. The inside care, made of sheet tin, was covered with as-
A typical r u n as outlined above t a k e s approximately 3 hr. Nine typical s e t s of data c a n be found i n T a b l e 1.T h e data were obtained with several different electrodes. A
where R, T,and F h a v e t h e i r usual significance, at i s the activity of species i, A a n d B refer to t h e t w o sides of t h e m e m b r a n e a n d k i i s the selectivity r a t i o or c o n s t a n t which isdefined b y t h e e q u a t i o n
ki
= KI.,(P,/P~
Kl,, is the equilibrium constant for t h e exchange of ions 1 and i at the surface of t h e glass and (p;/pl) i s t h e ratio of t h e mohilities of t h e t w o ions in the glass membrane. B y definition K1,l i s unity. The s u m m a t i o n is over all species which are electroactive. If we fix the composition of the m e l t on one side of the glass membrane, eqn. (1) reduces t o
E
=
E'
+ (RT/F)ln
If t h e system contains n o more t h a n t w o electroactive species, b o t h of which a r e considered to be ideal, the equation reduces to
*
~~
~~~~
~
~~~~
~~
Table 1. The EMF Data for t h e Pyrex Membrane Potential f o r v a r i o u s Nitratesalts in Fused Ammonium Nitrate at18S°C. RU"
lSadivm
w f (mV)
Mole Percent Sodium
Run
Mole Percent Sodium
Mole Percent Silver
2Silver emf (mV)
Mole Percent Silver
-211.2
-132.2 -113.3 -88.8 -36.6 -1.1
and Sodium (mv) -215.5 -176.1 -158.7 -122.6
Run 3-Silver
-94.6
-66.8 -89.0
Mole Percent Potassium 0.0 0.494 2.113 0.253 12.199
Run 4Sodium Figure 1. The fused salt furnace.
hestos and then wound with Nichrome resistance wire (total resistance of approximately 25 ohm). The bottom heater was constructed by installing a commercial hot plate element (approximately 25 ohm) on fire brick. The inside core was then fitted over the bottom heater and rested upon fire bricks. The sides of the furnace were wrapped with 4-in. Fiherglas insulation and the furnace was held together with two $-in. Transite boards fastened with eight %-in. threaded rods. The heat transfer medium was contained in a 2-1 stainless steel heaker that fitted into the center of the furnace. The bath was maintained close to the desired temperature with the side and bottom heaters. The final bath temperature (185 1'C) was achieved using a n on-off regulator (YSI thermistemp) to control an immersion heater. The sensing element was a thermistor probe. The heat transfer medium was a ternary nitrate eutectic of lithium, potassium, and sodium (27, 55, and 18 wt %, respectively) which has a melting paint of 120°C. The bath was stirred continuously with a propeller type stirrer. The reactor was a 250-ml Berzelius beaker. The potentials were measured to the nearest 0.1 mV using a Coming Model 12 Research pH meter. The Pyrex membrane electrodes were prepared by blowing a bulb 12 mm in diameter on the end of a 24-cm piece of 7-mm Pyrex tubing. The bulb and 5 cm of the stem were filled with a solution that contained 0.1 mole AgNOs per kg of the ternary mixture mentioned previously. The electrical contact was made with a silver wire (99.9%). The reference electrode was of the asbestos junction type and was filled with the same silver nitrate containing eutectic as was the bulb. Both the reference electrode
+
124
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Journal of Chemical Education
emf (mv) -213.9 -131.7 -111.4
-84.9 -35.0 0.5 Run 6-Sodium and potassium emf (mv) -217.2 -208.5 -180.1
19.996
Mole Percent Sodium
Mole Percent
-143.0 -139.9 -39.4 Run ?Sodium emf (mv)
Mole Percent Sodium
Run 9-Lithium
-244.2
0.0
Potassium
-171.1
-134.8 -122.6
-87.3 -35.8 0 . 8 Run 10Sodium and Lithium emf (mv) -230.9 -214.8 -178.8 -1467 -104.2
-96.4
0.693 1.265 2.504 9.527 22.986 Mole Percent Sodium 0.0 0.0 0134 0.124
emf (mV) -239.1 -216.9 -201.9
-182:3 -1M)Z
-136.6
Mole Percent Lithium 0.0 0.507 0.506 7.159
Percent Lithium 0.0 0.653 1.476 3.348 7.098 13.813
Mole
..
Figure 2. Cell emf 0.Na: 0. Ag;
0
versus
a2 0.4 0.6 0.8 1.0 LOGARITHM X, the logarithm of the mole fraction MNOI; M
=
Li; 0.K.
Figure 3. The emf calculated using the average E'. C", and k , values the experimental emf for the mixed systems; NaN03-MN03;M =
Versus
O.Ag;O.K;..Li.
Table 2. Calculated Selectivity Constants (k), E', and Co Values for the Data Given in Table 1. slop
" , Run
E' imV)
1 2 4 5
-119.0
7
8
-121.5 -121.5
average
-119.4
lF1
C0
kh-.
kr.
k*
k,.
