constant before adding the next increment of europic chloride solution. Prior to the equivalence point, the potential became constant (drift not greater than 0.2 mV/S min) almost immediately. During this stage the precipitate was so translucent that the solution was only slightly turbid. Very close to the equivalence point, the precipitate transformed, rather suddenly, to a much more opaque, “white” form, and this change was accompanied by an increase in potential,-decrease of fluoride ion activity. When the 0.30 cc increment of EuC13 solution was added to reach the 26.80 cc point in Table 11, which is the theoretical equivalence point, the potential increased immediately from its previous value of 3.2 mV to 16 mV and remained there for about 4 min, the precipitate being still translucent, But, with the solution continuously stirred, the potential suddenly began to increase quite rapidly (while the solution rapidly became opaque) and finally it became constant at 44.3 mV after 20 minutes. Apparently the freshly precipitated EuF3is highly hydrated, and, prior to the equivalence point, it is kept in a nearly colloidal condition by the adsorption of fluoride ion. The translucent to opaque transformation close to the equivalence point probably results from the formation of a less hydrated form, and, as indicated by the concomitant increase in potential, this final stable form is considerably less soluble than the original precipitate. If one titrates so rapidly that insufficient time is allowed for this transformation to complete itself at the equivalence point, it occurs somewhat later. Consequently the maximal rate of potential change will occur somewhat after the true equivalence point, whereas under equilibrium conditions it actually occurs slightly in advance of the equivalence point because of the asymmetric character of the titration reaction. Under the near equilibrium conditions of Table 11, the maximal rate of potential change is at 26.65 cc, and thus 0.15 cc or 0 . 6 x before the true equivalence point, as expected. Referring to Table 11, the potential (-107.8 mV) observed with the original solution served to calibrate the electrode in terms of fluoride ion concentration, and the concentrations of fluoride ion beyond the equivalence point were calculated from the observed potentials. The concentrations of europic ion were calculated from the volume of excess europic chloride titrant solution added, with respect to the theoretical equivalence point volume of 26.80 cc, and with an additive correction for the europic ion contributed by the solubility of the precipitate (equal to one third the observed fluoride ion concentration). As shown by the last column in Table 11, combination of these concentrations leads to a value for the solubility product (Eu3+) (F-)3 of EuF3 (2.2 X 10-17) which is constant to * l o x over a very large range of excess concentration of europic ion. In view of the fourth power concentration dependence of this solubility product, this degree of constancy certainly is good, and it demonstrates the absence of the cationic species EuF2+ and EuFZ+. Note that this concentration solubility product of EuF3 is about ten times larger than that of LaF3 observed under the same conditions of ionic strength. Neither the quantity of +2 europium used to dope the lanthanum fluoride membrane to create fluoride holes, nor whether or not this really is necessary, is public knowledge.
Table 11. Titration Data and Solubility Product of Europic Fluoride in Aqueous Medium 100 cc of 0.03000M NaF titrated at 25.00” C with continuous stirring. Theoretical equivalence point is at 26.80 cc 0.03724M EuClB cc 0 25.50’ 26.20 26.50 26.80 27.10 27.40 28.60 32.00 40.00 50.00
(F-)
EmV -107.8 - 22.2 - 6.6 3.2 44.3 52.6 57.1 66.0 74.8 81.8 85.7
(Eu*+)(F)* x io+”
(Eu *+)
M X 10-6 0.0300
M
+
5.75 4.78 3.55 2.40 1.91 1.62
1.07 X 1.92 X 5.32 X 1.47 X 3.51 X 5.75 X
2.0 2.1 2.4 2.0 2.5 2.4 Av 2 . 2 5 0 . 2
10-4
lO-‘ lO-‘ 10-8
Because the solubility of EuF3 is somewhat greater than that of LaF3 the addition of large quantities of europium would seem to be undesirable because it should decrease the upper limit of pF to which the membrane would respond according to Equation l . Presumably, therefore, only a very small quantity of europium is used. In view of the fact that membranes of other pure, undoped substances (silver halides and barium sulfate) respond correctly to the corresponding activity of their anions, one is inclined to wonder if doping of the lanthanum fluoride membrane with +2 europium really is necessary. RECEIVED for review January 19, 1968. Accepted February 28,1968.
Correction Direct Determination of Fluoride in Tungsten Using the Fluoride Ion Activity Electrode In this article by Bruce A. Raby and William E. Sunderland [ANAL.CHEM.,39, 1304 (1967)l information regarding the equilibration time found for the p F electrode was not included. Therefore, the following comment should be considered an addendum or correction to the original manuscript. “The electrode which we used for our studies was one of the early Orion Model 94-09 p F electrodes with the white plastic body. This electrode had an interstice between the sensing crystal and the body of the electrode. Slow diffusion of the test solution into the interstice was responsible for the slow rate at which the system came to equilibrium. It should be noted that Orion’s new model p F electrode has its crystal cemented into the body to eliminate the interstice. Consequently, equilibrium is attained in less than 1-3 minutes at a 10-6M fluoride concentration. The response is more rapid at higher concentrations.”
VOL 40, NO. 6, MAY 1968
939
