DISCUSSION
It is no particular feat to obtain accurate results for fluoride in the absence of interfering ions. While most interferences can be eliminated by a Willard-Winter distillation, this step (despite its merits) is time-consuming and better avoided if possible, particularly if fluoride-retarding elements such as boron and aluminum are present, The chief merit of thermometric titration is that it provides a possibility for avoiding interference from sulfate, phosphate, borate, and silicate, and thus may eliminate the need for distillation in specific cases. It is not well suited for titrating the dilute solutions obtained in distillation. In 1954, Elving, Horton, and Willard (5) listed some 1400 references dealing with the determination of fluoride; Horton’s (6) recent revision and expansion of this work adds some 650 more, Nevertheless, he noted that “The determination
of ionic or elemental fluoride is still very difficult despite the continual appearance of new methods and the variations of older methods described in the scientific literature.” The present work has little effect on this situation, but it does represent an area not previously explored and it opens up some possibilities for handling certain special situations more expeditiously. RECEIVED for review July 3, 1967. Accepted August 31, 1967. (5) “Fluorine Chemistry,” Vol. 2, Chap. 3, P. J. Elving, C. A. Horton, and H. W. Willard, Eds., Academic Press, New York,
1954.
(6) C. A. Horton, Sec. A, Vol. 7, in “Treatise on Analytical Chemistry,’’ I. M. Kolthoff and P. J. Elving, Eds.
entiometric Titration of Fluoride with etraphenylantimony Sulfate James B. Orenberg and Michael D. Morris Department of Chemistry, The Pennsylcania State University, University Park, Pa. 16802 Aqueous solutions of fluoride ion may be titrated with tetra phenyla nt i mo ny su Ifate , [(CsH&S bJZSO4.The relative error is +I.% over the range 1 x lO-3M to 5 X 10-2M fluoride. Fluoride i s extracted into chloroform as the ion pair (CfH5)4SbF. The aqueous phase fluoride activity is monitored potentiometrically with a fluoride sensitive electrode. Equal initial volumes of aqueous phase (pH 4-5) and extractant are used. The aqueous sample is made O.1f i n sodium sulfate to inhibit emulsification and facilitate phase separation. Thirty seconds of intimate phase contact (stirring) and 15 seconds for phase separation are required between additions of titrant. Phosphate, arsenate, arsenite, and sulfate do not interfere. Nitrate and perchlorate interfere, but are readily removed by the addition of tetraphenylarsonium sulfate; nitrate forms an extractable ion-pair with the reagent, while perchlorate forms a very insoluble salt. Sulfite and nitrite interferences are removed by oxidation with hydrogen peroxide to sulfate, and the nitrite to removable nitrate. The halides and thiocyanate interfere and are removed by silver nitrate precipitation followed by extraction of the nitrate with tetraphenylarsonium sulfate as above. Conditional partition coefficientsfor the fluoride extraction have been measured and used to demonstrate agreement between theoretical titration curves and experiment.
THESEVERAL METHODS commonly employed for fluoride titration are not completely satisfactory. The popular thorium nitrate titration ( 1 ) has several difficulties. The stoichiometry frequently deviates from the theoretical ThF4 and numerous ions interfere. The most serious interferences are phosphate, arsenate, arsenite, sulfate, and sulfite, all of which precipitate the titrant thorium(1V). Lanthanum acetate has been suggested as a conductimetric titrant (2), but fluoride must be (1) C. A. Horton, “Treatise on Analytical Chemistry,” I. M. Kolthoff, P. J. Elving, and E. B. Sandell, Eds., Part 11, Vol. 7, Interscience. New York, 1961, p. 259. (2) Ibid., p. 264.
1776
a
ANALYTICAL CHEMISTRY
separated from interferences by a prior Willard and Winter distillation. Direct (3) and back (3) titrations based on the precipitation of lead chlorofluoride, PbClF, have been recommended. Indirect titrations based on precipitations of calcium fluoride and titration. of excess calcium have also been recommended ( 4 ) . Phosphate and sulfate interfere with methods based on lead chlorofluoride or calcium fluoride precipitation. Many of the difficulties of standard fluoride titrations are overcome by the use of tetraphenylantimony sulfate, [(C6Hs)4Sb]SOa, as an extractive titrant for fluoride. Tetraphenylantimony salts were first employed as analytical reagents by Willard and Perkins ( 5 ) , who used the bromide as a precipitant for perchlorate and several other inorganic anions. Affsprung and May (6) used tetraphenylantirnony sulfate to precipitate carboxylates. Morris ( 7 ) has used this reagent io titrate perchlorate, using amperometric end point detection. Moffert, Simmler, and Potratz (8) first proposed tetraphenylantimony cation as an extractant For fluoride. These authors noted that while about 9 7 x of the fluoride could be removed from the aqueous phase in three extractions, 10-20% of the other halides present were co-extracted. Bowen and Rood ( 9 ) have used extraction of tetraphenylantimony fluoride into carbon tetrachloride to obtain carrier-free F l 8 . These authors have made a detailed study of the extraction system. They note that extraction is most efficient at pH 3. In more acid solution, formation of undissociated HF decreases the (3) Ibid., p. 265. (4) Ibid., p. 266. (5) H. H. Willard and L. R. Perkins, ANAL.CHEM., 25, 1634 (1953). (61 H. E. Affmrune: and H. E. May, Ibid., 32,1164 (1960). (7j M. D. M&ris,>bid., 37,977 (565). (81 K. D. Moffert. J. R. Simmler. and H. A. Potratz, Ibid., 28, \
,
I356 (1956). (9) L. H. Bowen and R. T. Rood, J. Inorg. N L I CChem., ~. 28, 1985
(1966).
distribution coefficient, while in more alkaline solution, formation of tetraphenylantimony hydroxide ion pairs also decreases the distribution coefficient. Ross and Frant (10) have described a fluoride-sensitive electrode, in which the potential developed across a lanthanum fluoride crystal depends upon the ratio of the fluoride activities on either side of the crystal. They presented specimen curves for the potentiometric titration of fluoride with lanthanum acetate. The electrode has been employed for direct potentiometric determination of fluoride in tungsten samples by Raby and Sunderland (11). They were able to obtain a standard deviation of + 2 pg of fluoride over the range 2-100 pg per 50 ml of solution-Le., over the concentration range 2 x 10-6-1 X Mfluoride. Lingane has discussed the application of the fluoride sensitive electrode to fluoride titrimetry (12). Lingane used the electrode to follow precipitation titrations with thorium nitrate, lanthanum nitrate, and calcium chloride reagents. He observed that lanthanum ion gave the greatest rate of potential change in the region of the equivalence point, and recommended titration of neutral unbuffered fluoride solutions with lanthanum nitrate as the most satisfactory of the several precipitation titrations investigated. If care is taken to determine the equivalence point potential to within + 2 m y , then a precision of + O . l % is obtainable in titration of moderately large quantities of fluoride by this procedure. Lingane reported titrimetry of pure solutions of sodium fluoride only (12). He did not investigate the effects of other anions on precipitation titrations of fluoride. As Lingane has shown, potentiometric end point detection is far more convenient and reliable than the conventional color change indicators for these titrations. However, the introduction of even a vastly improved detection method does not remove the fundamental drawbacks of the titrations, their susceptibility to interference from anions such as sulfate, phosphate, and arsenate, which precipitate the titrant cations ( I , 2 , 4 ) . Galik (13) has discussed the principles of extractive titrations and has titrated copper, zinc, mercury, cobalt, and cadmium by extraction of their dithizonates into carbon tetrachloride (11). The absorbance of the organic phase is measured as a function of titrant volume added. The determination of two ions in a sample is possible when the partition coefficients are sufficiently different. Extractive titrations are widely used with isotopically labeled titrants (14). With a labeled titrant, the course of the titration can be followed by monitoring the activity of either phase. A wide variety of labeled titrants can be prepared. The method, while versatile, is cumbersome. In the present system, tetraphenylantimony sulfate is used as an extractive titrant for fluoride. Fluoride is extracted from an aqueous sample into chloroform as tetraphenylantimony fluoride. The fluoride ion activity in the aqueous phase is monitored during the titration with a fluoride sensitive electrode. Sigmoid potentiometric titration curves are obtained. Phosphate, sulfate, arsenate, and similar oxyanions do not interfere, while interfering anions such as halides and nitrate are readily removed.
TITRATION THEORY
The titration reaction exploited in this work is described by Equation 1. (CeHd4Sb+(aq)F-(aq)
(1)
The partition coefficient P,for this reaction, written in terms of concentrations, is given by Equation 2.
The unsubscripted concentrations are aqueous phase values. This constant will be a function of ionic strength, because of the dependence of aqueous phase activity coefficients on ionic strength. Furthermore, the apparent value of P will depend strongly upon the aqueous solution pH, as fluoride is the anion of a weak acid and as tetraphenylantimony hydroxide has a large formation constant (9). Thus, it is profitable to define (15) a conditional constant, P’,for the extraction according to Equation 3.
(3) The notation is that used in the description of complexation titrations (15). [F’]is the total concentration of fluoride present in the aqueous phase in all forms and [(CaH&Sb’] is the total concentration of tetraphenylantimony present in the aqueous phase in all forms. The alpha coefficients are defined as a~ = [F’I/[F-I and a S b = [(C,Hs)aSb’l/[(CaH6)4Sbfl. If fluoride exists in the aqueous phase as F- and undissociated H F only, Equation 4 holds. (4)
Similarly, if the only tetraphenylantimony species present in the aqueous phase are the cation and undissociated (C6H6)rSbOH,Equation 5 applies.
K,is the acid dissociation constant of HF, KOis the formation constant of (CGH&SbOH and K, is the ion product of water. These alphas are functions of solution pH only. If aqueous acid-base reactions were the only side reactions competing with reaction 1, P’would be a function of pH only. However, tetraphenylantimony fluoride forms aggregates, presumably dimers, in the organic phase (9) and P’ increases with the total concentration of tetraphenylantimony fluoride in the organic phase, and therefore with the total tetraphenylantimony concentration in both phases. For calculation of titration curves, it is not necessary to know explicitly the dependence of P’on reactant concentrations. If equal phase volumes are employed, and if dilution is neglected, then Equation 6 describes the titration curve at all points, even beyond the equivalence point. C o ( l -f)
(10) J. W. Ross, Jr., and M. S . Frant, Science, 154, 1553 (1966). (11) B. A. Raby and W. E. Sunderland, ANAL.CHEM., 39, 1304 (1967). (12) J. J. Lingane, Ibid..p. 881. (13) A. Galik, Tahnra, 13, 109 (1966). (14) T. Braun and J. Tolgyessy, “Radiometric Titrations,” Pergamon Press, New York, 1967, pp. 94-122.
+ (CoH&SbF(org)
]I
-P
+
(15) A. Ringbom, “Complexation in Analytical Chemistry,” J. Wiley, New York, 1963, pp. 35-60. VOL. 39, NO. 14, DECEMBER 1967
e
8777
C o is the initial concentration of fluoride (all forms) in the aqueous sample and f is the fraction titrated. Equation 6 follows from application of the conditions of material balance and electrical neutrality to the system and substitution into the conditional partition coefficient in the usual way, Removing the restriction of equal phase volumes and including a dilution factor leads to a similar, but more complicated expression. Equation 6 may be simplified if the partition coefficient is sufficiently large so that extraction is complete (stoichiometric before the equivalence point, 9 8 z complete at the equivalence point), In this case, approximations 6a-6c are valid.
[F'] = [F']
=
45, f
1
=
I
)f > 1
P Y f - 1)
2z
Equation 6a or 6c will generally hold except within 1 to of the equivalence point. Because P' does not affect the titration curve until quite near the equivalence point, when almost all of the tetraphenylantimony fluoride has been extracted, the value of P' corresponding to the equivalence point conditions may be used in titration calculations based on Equations 6-6c. EXPERIMENTAL
Reagents. Tetraphenylantimony sulfate was prepared from tetraphenylantimony bromide (City Chemical Corp.) according to the method of Morris (7). Tetraphenylantimony fluoride was prepared by precipitation from aqueous solution containing tetraphenylantimony sulfate and sodium fluoride. Tetraphenylantimony fluoride was recrystallized from an acetone-water mixture and dried at 110" C for 1 hour. Tetraphenylarsonium carbonate was prepared from tetraphenylarsonium chloride (City Chemical Corp.) by treating aqueous solutions with silver sulfate to obtain soluble tetraphenylarsonium carbonate. The carbonate was neutralized to pH 4 with sulfuric acid to give solutions of tetraphenylarsonium sulfate. All other chemicals were of ACS reagent grade. All solutions were prepared with distilled water. Apparatus. The titrations were carried out at room temperature, 21-27' C. Potentials were monitored with a Leeds & Northrup pH meter, Model 7401, whose feedback loop had been modified to make the meter read 280 mV full scale (16). A fluoride sensitive electrode (Orion Research Inc., Model 94-09) was used to monitor aqueous phase fluoride activity. Potentials were measured against a saturated calomel electrode or saturated mercurous sulfate electrode (Radiometer K 601, The London Co., Westlake, Ohio). Because commercial calomel electrodes sometimes leaked sufficient chloride into the solution to have a deleterious effect on the titration, the mercurous sulfate reference electrode was generally used. The zero of the potential scale was the potential difference between the fluoride electrode and the reference electrode immersed in a solution of 1.25 X lO-3M sodium fluoride at room temperature. This zero is arbitrary, but convenient. Measurements of pH were made with the k e d s & Northrup pH meter, without scale expansion. Atomic absorption spectrometric measurements were made with a Perkin-Elmer Model 303 atomic absorption spectrophotometer equipped with a multislot burner (17) and an antimony hollow cathode lamp. (16) Leeds & Northrup Co., Philadelphia, Bull. 177166 (1963). (17) E. A. Boling, Spectrochim. Acta, 22, 425 (1966).
1778
e
ANALYTICAL CHEMISTRY
Procedure. The titrate consisted of 30 ml of chloroform and 30 ml of an aqueous fluoride sample. The aqueous solution was made 0.1M in sodium sulfate to prevent emulsification and promote rapid phase separation. The initial pH of the sample was adjusted to approximately pH 7. Because of the excess sulfuric acid present in the titrant, the aqueous solution pH drops to about pH 4 during the course of the titration. After each addition of titrant, approximately 0.09M tetraphenylantimony (as the sulfate), the titrate was stirred vigorously for 30-40 seconds to allow extraction of tetraphenylantimony fluoride into the chloroform. The mixture was allowed to settle Tor 10-15 seconds, and the fluoride activity in the aqueous phase was measured with the fluoride ion electrode. The end point was taken to be the inflection point of the titration curve, a plot of potential us. volume of titrant added. Tetraphenylantimony sulfate solutions were standardized by titrations against known solutions of sodium fluoride of concentration approximately 5-6 X 10-3M. Nitrate is coextracted with fluoride and was removed by addition of excess tetraphenylarsonium sulfatt to the titration mixture. The titration mixture was then stirred for about 1 minute to equilibrate the phases. Tetrnphenylarsonium nitrate was quantitatively extracted into chloroform (18). Extraction of tetraphenylarsonium fluoride into chloroform is negligible unless a very large excess of tetraphenylarsonium cation is present (18). The chloroform containing the tetraphenylarsonium nitrate was used for the titration itself. Halides and thiocyanate are coextracted with fluoride and were removed by precipitation with silver nitrate. Digestion of the precipitate for about 4 minutes was necessary to coagulate it Unless the precipitate had been coagulated, it tended to form emulsions and prevent rapid, complete phase separation during the titration. The coagulated precipitate is innocuous and wds not removed from the titration mixture. After the digestion step, tetraphenylarsonium sulfate was added to the titration mixture to remove the nitrate added during the precipitation. Perchlorate was removed by precipitation with tetraphenylarsonium sulfate. It was not necessary to digest the precipitate or remove it from the titration mixture before beginning the titration. Both nitrite and sulfite interfere and were removed by oxidation with hydrogen peroxide in a solution about 0.03F in sulfuric acid. Nitrite was oxidized to nitrate, which was then removed by extraction with tetraphenylarsonium ion. About 6 ml of 0 . 3 x aqueous hydrogen peroxide was required to oxidize a 3 0 4 sample 5 X 10-3F in either sulfite or nitrite. Both oxidations were complete at room temperature within 5 minutes of the addition of hydrogen peroxide. The distribution coefficient, D = (Sb)org/(Sb)aq was measured by atomic absorption spectrometry (19) for several concentrations of tetraphenylantimony fluoride. Solutions of tetraphenylantimony fluoride in chloroform were equilibrated with equal volumes of an aqueous buffer, pH = 3.05, consisting of 0.1M sodium sulfate, 0.067M KH?P04, and 0.0067M H3P04. Phase equilibration was carried out in a constant temperature bath (25.0" =t 0.1" C ) for about 20 minutes. An 8-ml aliquot of the aqueous phase to be analyzed was withdrawn and to it was added 2 ml of isopropanol. The antimony concentration was measured by atomic absorption spectrometry. Dilution with isopropanol approximately doubled the absorbance of a given tetraphenylantimony solution. The calibration curve was prepared using solutions of tetraphenylantimony fluoride in a chloroform saturated aqueous buffer whose composition matched (18) R. Bock and G. M. Beilstein, 2. Anal. Chem.,192, 44 (1963). (19) M. D. Morris and L. R. Whitlock, ANAL.CHEM., 39, 1180 (1967).
160
Table I. Titration of Fluoride with [(C&)4Sb]&304
t-c
1
1
0
1
I
1
02
04
l I l i l I 06 0 8 IO 12
14
16
18
Taken 0.0153 0.0325 0,0779 0.1558 0.1559 0.2025 0.2043 0.2075 0.2338 0.312 0.624 0.779
20
Titrant added, ml
Figure 1. Titration of 0.1558 mmoles of fluoride with 0.0934F tetraphenylantimony pH = 3.5.
- Theoreticalcurve 0
Experimental potential differences
that of the test samples. The standard aqueous tetraphenylantimony solutions were diluted with isopropanol in the same manner as the unknowns. RESULTS AND DISCUSSION Analytical Data. Typical results for the titration of fluoride ion over a concentration range of 5.0 X 10-4M to 5.0 X 10-2M are shown in Table I. That some ions do not interfere and that the described procedures are effective in removing ions which do interfere are demonstrated by the data of Table 11. As all samples were 0.1M in sulfate, sulfate is not an interference. The data of Table I indicate that the titration is practical for the determination of fluoride at concentrations as low as 1 x 10-*M (0.032 mmole in 30-ml sample). Below this level the potential break at the end point is not large enough or of high enough slope to give results of high precision or accuracy. It is desirable to carry out the titration in a solution of p H 4-5. Tetraphenylantimony hydroxide is sparingly soluble, K,, = 1 X 10-8 (8), and this insolubility precludes the use of tetraphenylantimony salts in alkaline solutions. Hydrofluoric acid is a weak acid, pK, = 3.0, and the efficiency of the extraction decreases as the titrate is made strongly acidic and an increasing fraction of the fluoride is present as undissociated hydrofluoric acid. Bowen and Rood (9) find that the extraction of tetraphenylantimony fluoride into carbon tetrachloride is most efficient around pH 3. We have chosen to work at a somewhat higher pH to eliminate the possibility of loss of fluoride as volatile hydrofluoric acid. The fluoride electrode employed in this work responds only to free fluoride ion and will not sense hydrofluoric acid or other fluoride complexes (IO). Thus the solution pH must be kept high enough so that little undissociated hydrofluoric acid is present if there is to be an adequate potential break in the region of the titration end point. A specimen titration curve is shown as Figure 1. The end point is taken to be the mid-point of the potential break. The curve in Figure 1 was obtained in a p H 3.5 phosphate buffer in order to facilitate comparison of experiment with theory. In the unbuffered solutions employed for most titrations somewhat larger potential breaks, about 80 mV, were observed in the end point region. Because the indicator senses fluoride ion, the potential is well defined at all points in the titration curve. However,
Millimoles of FFound 0.0151 f 0.0002 0.0329 SC 0.0005 0.0778 f 0.0002 0.1560 rt 0.0005 0.1574 rt 0.0010 0.2000 zt 0.0025 0.2030 f O.OOO0 0.2060 rt 0.0002 0.2345 f 0.0020 0.3155 SC 0.0015 0.625 f 0.001 0.775 f 0.001
Error, mmoles -0,0002 +0.0004 -0.0001
%
Error -1.6 1-1.2 -0.1
+0.0002 +0.0015 -0.0025 -0.0013 -0.0015 +0.0007 $0.0035
+O.l $1.0
+0.001
+0.2
-0.004
-0.5
-0.1 -0.6
-0.7 +0.3 fl.1
Table 11. Effect of Interferences on Titration Approx. F- taken, mmoles of F- found, Error, % mmoles mmoles Error mmoles interference 0.1558 0.1558 0.1558 0.1558 0.1558 0.1558 0.1558 0.1558 0.1558 0.1558
0.80P04-3a
0.1559 zt 0.0001 +O. 0001 -0.0024 +O. 0006 -0.0007 -0.0008 0.2OC1-d 0.1551 f 0.0014 -0.0007 0.201-d 0.1550 zt 0,0000 -0.0008 0.20 SCN+ 0.1550 + 0.0007 -0.0008 0.20S03-2e 0.1543 f 0.0007 -0.0015 0.20 NOz-f 0.1550 rt 0.0015 -0.0008 O . ~ O A S O 0.1534 ~ - ~ ~ f 0.0000 0.40 AsOa-8" 0.1564 f 0.0006 0.30N03-* 0.1551 5 0.0014 0.20 Clod-c 0.1550 i 0.0000
(4.0
-1.5 +0.4 -0.5 -0.5
-0.5 -0.5 -0.5 -1.0
-0.5
No removal necessary. Removal by extraction into chloroform as the tetraphenylarsonium salt. Precipitation of tetraphenylarsonium perchlorate. Removal by precipitation with silver nitrate, extraction o tetraphenylarsonium nitrate. e Oxidation in acid solution by hydrogen peroxide to SO*-2. f Oxidation in acid solution by hydrogen peroxide to NO3-; concurrent removal of NOa- by extraction into chloroform as the tetraphenylarsonium salt. a
b 0
no attempt was made to closely control the pH of the titrant or sample, the temperature of the system, or its ionic strength. Thus, the fluoride activity coefficient and the fluoride-hydrofluoric acid ratio varied somewhat from titration to titration. Because of these variations the value of the electrode potential at the end point varied over a 10-mV range during a series of replicate titrations. This variation causes no difficulty in determining the end point of a potentiometric titration in which the titration curve is plotted point by point. Chloride, bromide, iodide, thiocyanate, nitrate, nitrite, and sulfite are all extracted by tetraphenylantimony cation, while perchlorate is precipitated by the reagent. If fluoride is titrated in the presence of an anion precipitated or coextracted by the reagent, the slope of the titration curve at the end point is decreased and a positive end point error is observed. The series of interference removal procedures described above have the advantage that they require no transfer of the sample from one container to another (distillation, filtration, etc.). In each case one or two reagents is added to the titration mixture before the titration is begun to eliminate interferences. No other operations except stirring, heating, and pH adjustment are required. These procedures depend in many cases upon the use of tetraphenylarsonium ion to extract nitrate without extractVOL. 39,
NO. 14, DECEMBER 1967
* 1779
Table 111. Extraction of Tetraphenylantimony Fluoride from pN 3.05 Phosphate Bu@erer, 0.1M Sodium Sulfate int0 Chloroform [(@~Ha)aSbl",
D
x x x
31.3
3.00
10-3 10-3
9.00 7.0 10-3 10.0 X 10-3
P' 1.01 x 106
x
15
11.13
99
1.32 X lo6
118
106
1.39 x 106
a Total concentration of tetraphenylantirnony species in both phases.
Figure 1 shows that the theoretical and experimental titration curves are coincident, within experimental error, until well after the equivalence point. The experimental points fall below the theoretical curve only when the fluoride activity is well below 10-5M-i.e., in the region where the electrode response begins to fall off ('10-12). In the region of the equivalence point, the slope of the titration curve is about 9 mV/0.01 ml. The readability of the microburets employed in this work is about &0.005 ml. With careful technique and carefully calibrated equipment, a standard deviation of * 0 . 3 x might be attainable. Realistically, the precision of the titration can be expected to be not much better than the standard deviation obtained in this work. At higher concentrations, however, where the slope of the titration curve is greater, greater precision and accuracy may be attainable. If the initial fluoride ion concentration is 5 X 10-4M, the potential change from 99% titrated to the equivalence point is slightly less than 4 mV. The partition coefficient for this system is too small to ailow titrations of samples less concentrated than about 1 X 10-3M. Because the electrode response is limited to solutions more concentrated than about 1 x 10-6M (10-12), the fluoride electrode would not be a satisfactory indicator for titrations of solutions much less concentrated than about 1 X 1Q-3Mbyany method.
ilz
ing fluoride. The partition coeficient for tetraphenylarsonium nitrate extraction is large and that for tetraphenylarsonium fluoride extraction is quite small (18), but finite, and a very large excess of tetraphenylarsonium cation is detrimental to the titration. If a two- to three-fold excess of tetraphenylarsonium ion over fluoride is present after removal of nitrate and other interferences, then about 2z of the fluoride will be extracted as tetraphenylarsonium fluoride and a 2% negative error will be observed. Titration Curve. In order lo ascertain whether the lower practical limit of the titration was governed by the magnitude of the partition coefficient or the limited response of the indicator electrode, conditional partition coefficients were measured at several different tetraphenylantimony fluoride concentraiions. The partition coefficients were measured at pH 3.0, but since the partition coefficient varies only slightly over the pH range 3-4 (9), the data may be used to predict titration curves obtained in a system buffered at pH 3.5. Distribution coefficient and partition Coefficient data are presented in Table 111. The data of Table 111 show that the conditional partition coefficient increases systematically over the range of tetraphenylantimony fluoride concentrations studied. This behavior is expected in a system in which the organic phase consists of a monomeric tetraphenylantimony fluoride in equilibrium with dimers or higher aggregates (9). The partition coefficients are significantly higher than those found by Bowen and Rood (9), who observed P' = 3.0 X IO4 at a fluoride concentration of 1 X l0-3M for extraction into carbon tetrachloride from a solution of pH 3. We have observed that the use of carbon tetrachloride instead of chloroform in the extractive titration gives smaller potential breaks at the end point. This observation Is consistent with the greater efficiency of extraction into chloroform. To compare the predictions of Equations 6-Gc with experiment, it is convenient to measure differences in potential. Because Alog(F') = Alog aF- Equations 6-6c can be used without knowledge of the fluoride ion activity coefficient or aFwhich remain constant through the titration. In the construction of the theoretical titration curve, P' = 1.13 X lo6, the value for total fluoride concentration of 5.0 X 10-3M, was used. The 0.1558 mole fluoride sample had an initial concentration of about 5.2 X I0-3M in the 30-ml titrant solution employed.
'1988
e
ANALYTICAL CHEMISTRY
CONCLUSIONS
The tetraphenylantimony extractive titration has strengths which are complementary to those of conventional precipitation titrations. In particular, the extractive titration may be carried out in the presence of phosphate, sulfate, arsenate, and arsenite. Other heavy, multicharged anions do not extract tetraphenylantimony cation and these would not be expected to interfere. These ions, however, precipitate conventional fluoride titrants, such as lanthanum and thorium. On the other hand, small, singly charged anions are extracted by tetraphenylantimony cation, although no other ion which we have tested is extracted as efficiently as fluoride. While simple procedures have been developed to remove these interferences, these ions in general do not interfere in precipitation titrations with heavy metal titrants. Consequently, the titrant of choice depends upon the nature of the sample. The partition coefficient for tetraphenylantimony fluoride extraction is not particularly large. More complete extraction, and sharper end points, could be achieved by increasing the ratio of chloroform to aqueous sample. However, this procedure would increase the interference problem, because the extraction of all tetraphenylantimony salts would be enhanced. A change of solvent would be useful only if the partition coefficient for fluoride extraction were increased by a larger factor than those for most of the expected interferences.
RECEIVED for review July 5, 1967. Accepted September 19, 1967. Presented in part, Division of Analytical Chemistry, 154th Meeting, ACS, Chicago, Ill., September 1967. Work supported in part by Public Health Service Grant 1 R 0 1 DH 00101 -01,
