Microdetermination of fluorine in organic compounds by potentiometric

Microdetermination of Fluorine in Organic Compounds by Potentiometric Titration Using a Fluoride Electrode Truman S. Light and Richard F. Mannion Research Center, The Foxboro Company, Foxboro, Mass. 02035

A method suitable for rapid and routine determination of organically bound fluorine is described using, as the titrant, a 0.005M solution of thorium nitrate in 80 volume per cent ethanol. A fluoride electrode is the end point detector. The titration medium is adjusted to 0.01M in nitric acid and 80% by volume in alcohol. The sample is decomposed by combustion in a polycarbonate oxygen flask using dodecyl alcohol as a combustion accelerator. Results of determination on typical microchemical fluorine standards in the sample range of 1to 10 mg are within the =k0.3% (absolute) values traditional for microelemental analysis. Phosphorus and sulfur, if present, may interfere, and prior separation is suggested.

LITERATURE on the determination of fluorine in organic compounds is quite extensive and has been reviewed in recent years by MacDonald ( I , 2) and about ten years ago by Ma (3, 4). Unlike other halogens and sulfur for which several straightforward decomposition and determination steps may be recommended (1, 3,the determination for fluorine has been beset by many difficulties in both the decomposition and determination steps (1-4). The decomposition of organic fluorides may be accomplished by oxidation methods such as combustion in oxygen, either in a flask or in a tube, or by sodium peroxide in a metal bomb. Another school of thought advocates not oxidation but reduction for cleavage of the carbon-fluorine bond. Reduction methods include alkali metals in a bomb or by sodium in liquid ammonia. The methods for determination of fluoride include gravimetric precipitation as lead chlorofluoride, volumetric determinations by acid-base or precipitation titrimetry and by spectrophotometry. MacDonald, in the middle of a study containing detailed and workable microanalytical methods for many elements, is not only unable to recommend a preferable method for fluorine but is moved to remark, “In analysis for fluorine, faith in one’s method seems to be as important as scientific rectitude” ( I ) . The possibility of substantial improvement in the determination of fluorine (as well as putting this determination on a less subjective basis) has been offered very recently by the invention of the fluoride electrode by Frant and Ross (6), by the study of the feasibility of potentiometric titrations by Light (7) and Lingane (8, 9), and by the recent availability of (1) A. M. G. MacDonald, “The Oxygen-flask Method” in “Ad-

vances in Analytical Chemistry and Instrumentation,” C. N. Reilley, Ed., Vol. 4, Wiley (Interscience), New York, N. Y . , 1965, pp 75-116. (2) M. E. Fernandopulle and A. M. G. MacDonald, Microchem. J., 11,41 (1966). (3) T. S. Ma and J. Gwirtsman, ANAL.CHEM., 29, 140 (1957). (4) T. S. Ma, ibid., 30, 1557 (1958). (5) A. Steyermark, “Quantitative Organic Microanalysis,” 2nd ed., Academic Press, New York, N. Y., 1961. (6) M. S. Frant and J . W. Ross, Jr., Science, 154, 1553 (1966). (7) T. S. Light, “Analytical Evaluation of the Fluoride Ion Elec-

trode,” paper presented at Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1967. (8) J. J. Lingane, ANAL.CHEM., 39, 881 (1967). (9) Zbid., 40, 935 (1968).

clear polycarbonate flasks as a suitable substitute for the borosilicate ones conventionally used in the oxygen flask combustion technique. This paper presents and discusses a method for the microdetermination of fluorine in organic compounds well within the traditional ~ k 0 . 3 z absolute error sought in microelemental analysis. EXPERIMENTAL

Apparatus. The oxygen combustion flask assembly consists of a 500-ml Erlenmeyer flask made of clear polycarbonate (Nalgene No. 4103). A No. 10 natural white rubber stopper has a platinum wire support embedded in it to which is secured a platinum basket (A. H. Thomas No. 6471-410) for holding the sample. The fluoride electrode is made by Orion Research (Model 94-09) as is the plastic sleeve type reference electrode (Model 90-01). A laboratory automatic titrator was assembled from a motor driven buret of 10-ml capacity (Sargent No. S11120), a Foxboro pH to current converter (Model 6991, and a FoxboroYEW 1-second response time recorder (Model ER). A control box permitted automatic stopping of the titration at the point of maximum inflection of the titration curve. Any one of several commercially available laboratory titrators could be used; alternatively, the titration curves could be plotted manually by point to point reading of the electrode output with any suitable expanded scale pH meter. A Cahn microbalance (Model G) was used for all weighings. Solutions and Reagents. All chemicals used were reagent grade insofar as practical. The fluorocarbon test compounds were British Drug House microanalytical standards when available; otherwise they were research grade experimental compounds. The thorium nitrate solution was 0.005M in 80% (by volume) ethanol, prepared by direct dissolving of thorium nitrate in water, and standardized as discussed below. The approximate calibration factor was 0.38 mg fluorine per ml. The ethanol was J. T. Baker No. 9400; the nitric acid was 1.OM; the dodecyl alcohol was Eastman No. 873; and the sodium fluoride, reagent grade, was dried at 110 “C for 2 hours prior to use. The paper sample wrappers were A. H. Thomas No. 6471-F and the indicator was brom cresol green in 0.1% ethanol. Procedure. A 2- to IO-mg sample of the fluorocarbon, containing 1 to 3 mg of fluorine, is weighed on a microbalance and transferred to the paper sample carrier. If the sample is liquid, it is weighed into a methyl cellulose capsule (A. H. Thomas 6471-G). The carrier is folded, placed in the platinum basket and one or two drops (not more than 50 mg) of dodecyl alcohol are placed on the folded part of the paper. The polycarbonate flask, to which 10 ml of distilled water has been added, is flushed with oxygen, the tail of the paper ignited and quickly inserted into the flask which is then inverted and held behind a clear plastic safety shield. The stopper is firmly held into the flask with heavily gloved hands and the flask is rotated slowly to keep the flame from scorching the wall. Caution: safety precautions consisting of hand and arm protective clothing, ignition behind a safety shield, and eye protection should be routinely observed when carrying out any oxygen flask combustion. VOL 41, NO. 1, JANUARY 1969

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Figure 2. Ethanolic solution titration of fluoride with thorium at various acidities (1 mg of F using fluoride electrode, SO% ethanol)

Figure 1. Aqueous solution potentiometric titration curves of fluoride with thorium, lanthanum, zirconium, and calcium (50 ml of 0.1M NaF)

When combustion is complete, the flask is rinsed under cold water and left standing with occasional shaking for 10 minutes or until the haze has disappeared in the flask. The contents of the flask are then transferred to a 150-ml plastic beaker by washing with three or four portions of ethanol. A measured volume of ethanol, totaling 40 ml, is used in this transfer. Two drops of 0.1 brom cresol green indicator are added and 1 M nitric acid is added dropwise to turn the indicator from green to yellow. Ten drops of 1 M nitric acid, totaling 0.50 ml, are then added to the 50 ml of solution, making this solution 0.01M in nitric acid. The solution is then titrated potentiometrically with 0.005M thorium nitrate to a potential, which is the point of maximum inflection, that has been predetermined by titration of a standard under the identical experimental conditions. The percentage of fluorine in the sample may then be calculated from :

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where V is the volume in ml, of the thorium nitrate titrant, K is its factor expressed as 100 (mg F/ml titrant), and W is the sample weight in milligrams. Standardization of Thorium Nitrate and Determination of End Point Potential. The thorium nitrate may be prepared: as a standard solution from material of known purity, as reported by Lingane (8); by standardizing with a simple titration, omitting the oxygen flask procedure, using dried reagent grade sodium fluoride; or by standardizing against recognized microchemical fluorocarbon standards, such as p-fluorobenzoic acid or trifluoroacetanilide that are available as British Drug House microchemical standards. Although all three procedures have been shown to be equivalent, thus proving the stoichiometry of the titraton reaction: Th(1V)

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the soundest analytical procedure is to standardize against an accepted microchemical standard. The initial titration will require the construction of a potentiometric titration curve as shown in Figures 1-4, either using an automatic titrator or manual plotting of data. Visual inspection of this titration curve is sufficient to determine the point of maximum in108

ANALYTICAL CHEMISTRY

flection and future titrations may be made directly to this potential. This end point potential has been shown to be stable for several weeks and the thorium nitrate solution is stable for at least several months, providing the alcoholic solution is protected from evaporation. However, good analytical practice would dictate frequent confirmation of the standardization factor and end point potential against a known aliquot of a sodium fluoride solution. Sodium fluoride solutions, stored in a polyethylene or polypropylene bottle, in concentration greater than 0.1 mg fluorine per ml have also been demonstrated to be stable for several months. RESULTS AND DISCUSSION

Performance of Fluoride Electrode. The fluoride electrode is a relatively new analytical tool, and has been discussed by several authors recently (6-9, IO). It is handled in many respects in a manner similar to that of the glass electrode. A rapid check may be made for the satisfactory performance of a fluoride electrode by measuring its response in each of two standard sodium fluoride solutions that are two decades apart in concentration--e.g., 10-2Mand 10-4M or 100 and 1 mg fluorine per liter. The change of emf between the two solutions should be between 56 and 59 mV per concentration decade, A third point check, in distilled water, is also desirable. In distilled water, the emf should read a potential equivalent to a concentration below 10-6M fluoride. The reference electrode used in conjunction with the fluoride electrode should also be examined critically. In general a sleeve type plastic reference electrode is recommended in order to eliminate spurious junction potentials and to remove glass (silica) from the system. A check on the reference electrode performance may be made rapidly by observing if there is any stirring effect on the magnitude of the measured emf. If the difference between no stirring and rapid stirring is more than a few tenths of a millivolt, there is probably a spurious potential existing at the reference electrode. Flushing of the sleeve type junction should eliminate this potential. Fluoride Titrations. The fluoride electrode by offering a sensitive potentiometric method for the detection of the end (10) K. Srinivasan and G. A. Rechnitz, ANAL.CHEM., 40,509 (1968).

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point, has permitted comparative studies of the various titrimetric methods for fluoride (7-9). In Figure 1, a comparison of the more popular titrimetric methods for fluoride is shown. Thorium, lanthanum, zirconium, and calcium are used as titrants for sodium fluoride in aqueous solutions at the 0.1 molar level. Figure 1 shows that thorium and lanthanum are more suitable than zirconium and calcium as titrants for fluoride because of their greater end point breaks and consequent smaller amounts of fluoride remaining soluble and untitrated after the end point. In order to determine microamounts of fluoride, it becomes advantageous to carry out the titration in alcoholic medium as suggested by Lingane (8, 9), where the solubility of the lanthanum and thorium fluorides is even further reduced. Figure 2 shows the titration of 1 mg of fluoride in 80% by volume ethanol with 0.005 M thorium nitrate. Because of the hydrolysis of thorium, a problem considered in some detail by Lingane, the effect of carrying out this titration at different levels of acidity is shown in Figure 2. Even in 0.001M nitric acid, formation of basic thorium hydroxide is evident as shown by the double inflection point and the occurrence of the end point slightly beyond the equivalence point. When the acidity is increased tenfold to 0.01M nitric acid, the shape of the titration curve appears ideal although the formation of some undissociated hydrofluoric acid tends to reduce the size of the potential break at the end point. In Figure 3, the titration of 1 mg of fluoride in 80% by volume ethanol with 0.01M lanthanum nitrate is shown, again as a function of the acidity. Although the hydrolysis is less pronounced than with thorium, it is visible in the curve at the 0.001M nitric acid level and has disappeared at the 0.01M nitric acid level. Again, as the acidity of the titration medium is increased, the sensitivity of the titration decreases. Figure 4 compares the titration of 2 mg of fluoride in 80% ethanol by volume and in 0.01Mnitric acid using both thorium and lanthanum as titrants. It can be seen that both titrants are feasible in this medium; however thorium is slightly more sensitive giving a greater end point break and leaving somewhat less fluoride remaining in the solution beyond the equiv-

Table I. Potentiometric Titration of Sodium Fluoride with Thorium Nitrate, Using the Fluoride Electrode Sample and Titrant Adjusted to 80% (by Volume) in Ethanol and 0.01M in Nitric Acid; Sample Volume 50 ml Weight of F, mg Wt. of NaF, Theor. of mg F Found Difference 0.224 0.101 0.102 0.001 0.342 0.155 0.154 0.001 1.500 0.678 0.680 0.002 2.350 1.063 1.061 0.002 2.800 1.266 1.263 0.003 5.184 2.345 2.347 0.002 ~

alence point. The 0.01 molar acid medium, in addition to eliminating hydrolysis of the thorium ion and the tendency to form a basic thorium fluoride, should also serve to minimize interference from other constituents which tend to form weak acids, notably sulfate and phosphate. For these reasons, thorium nitrate was chosen as the titrant in the medium of 80% ethanol and 0.OlMnitric acid. The feasibility of this titration over the range needed in microanalysis is shown in Table I. Samples of pure sodium fluoride ranging from 0.2 to 5 mg, corresponding to the fluorine content of 1 to 10 mg samples of fluorocarbons, may be titrated under the conditions described with an accuracy of 1 or 2 pg. Oxygen Flask Decomposition of Fluorocarbons. The decomposition of fluorocarbons in a glass oxygen flask has been a moot subject in the literature. Schoniger (11) described this decomposition as early as 1956. Some later authors found low results; one school of thought attributed these to incomplete decomposition and suggested combustion aids (12, 13); others attributed the low results to loss of fluorine by combination with the boron or aluminum present in borosilicate glass and suggested use of pure silica or quartz flasks (11) W. Schoniger, Microchimica Acta, 869 (1956). (12) A. Steyermark, R. R. Kaup, D. A. Petras, and E. A. Bass, Microchem. J., 3, 523 (1959). (13) B. 2. Senkowski, E. G. Wollish, and E. G. E. Shafer, ANAL, CHEM., 31, 1574 (1959). VOL. 41, NO. 1, JANUARY 1969

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( I , 2, 14, 15). More recently, the use of polypropylene and polyethylene flasks has been suggested ( I , 2, 16); these plastic flasks not only have the virtue of being less costly than glass, silica, or quartz, but also remove silica as well as boron and aluminum from the fluorocarbon decomposition system.

Table 11. Determination of Fluorine in Organic Compounds Following Combustion in a Borosilicate Flask Dodecyl Alcohol Used as a Combustion Aid Weight, Found, Error, Sample mg %F %F pFluorobenzoic acid 7.016 12.67 0.89 (Theor: 13.56% F) 8.166 12.84 0.72 12.66 0.90 8.888 Trifluoroacetanilide 6.876 28.43 1.71 (Theor: 30.14% F) 5.748 28.57 1.56 6.643 28.83 1.31 Polytetra3.028 65.28 10.70 fluoroethylene 3.802 66.12 9.78 (Theor: 76.0% F)

Table 111. Determination of Fluorine Using Polycarbonate Flask without a Combustion Aid Weight, Found, Error, Sample mg %F %F pFluorobenzoic acid 11.730 13.22 0.34 (Theor: 13.56% F) 10.010 13.06 0.50 13.36 0.20 10.438 Trifluoroacetanilide 3.162 29.20 0.94 (Theor: 30.14% F) 5.482 29.26 0.88 29.33 0.81 7.616

Table IV. Determination of Fluorine in Organic Compounds Following Combustion in a Polycarbonate Flask Dodecyl Alcohol Used as a Combustion Aid Error, Found, Weight, %F Sample %F mg +O. 18 13.74 6.616 pFluorobenzoic acid" +0.26 13.82 6.008 (Theor: 13.56% F) +0.25 13.81 8.626 +0.13 13.69 8.700 $0.01 30.15 7.926 Trifluoroacetanilide' 30.20 +O. 06 6.822 (Theor: 30.14% F) 30.21 +O. 07 7.508 -0.04 30.10 8.414 +o. 10 15.79 7.108 p-Fluorobenzonitdd 15.32 -0.37 9.498 (Theor: 15.69% F) 15.66 -0.03 6.806 -0.08 15.61 8,752 f O . 28 76.28 2.420 Polytetra-0.41 75.59 3.204 Fluoroethylend -0.25 75.75 3.344 (Theor: 76.00% F) -0.02 75.98 3.688 -0.31 16.94 5.704 o-Fluorotolueneb -0.26 16.99 5.772 (Theor: 17.25% F) +o. 01 17.26 9.730 -0.10 9.50 9.118 CeHsCF=CHGH5' -0.23 9.37 8,932 (Theor: 9.60% F) -0.05 9.55 7.804 -0.24 17.19 CeHsCFzCHzCeHs' 7.860 -0.24 17.19 (Theor: 17.43% F) 6.660 -0.10 17.33 7.304 a

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1 10

ANALYTICAL CHEMISTRY

Our own initial experiments with decomposition of fluorocarbons in borosilicate oxygen flask led to recovery of only 75 to 9 5 x of the fluorine. We eventually attributed this loss both to reaction with the glass and incomplete combustion. Table I1 reports the careful combustion in borosilicate flasks using the combustion accelerator and titration method in the final procedure. The results are significantly low in every case, the error increasing as the fluorine content increases. After experimentation with several classes of plastic flasks, 500-1111 polycarbonate Erlenmeyer flasks were adopted because of their transparency, rugged construction, and low cost. Table 111, reporting decomposition in the oxygen flask, shows the results obtained with two fluorocarbon standards, again using the procedure eventually adopted but omitting the combustion aid. The results of Table I11 all show low results for fluorine, although not as low as the combustion in the glass flask reported in Table 11. Indeed, some of the results in Table I11 are within the 0.3% absolute error traditionally allowed in elemental microanalysis; however, all the results are low, indicating incomplete recovery of fluorine. Many combustion aids have been suggested including sodium peroxide or nitrate, potassium chlorate, and organic materials ( I , 4, 12, 13) and water (17). Investigation of this list led to the elimination of inorganic compounds because of frequent cases of incomplete combustion and carbonaceous residue as well as there being no hydrogen in these materials; water appeared to suppress the flame. Dodecyl alcohol was selected as the combustion aid because it substantially aided Combustion, made excess hydrogen available, and may be conveniently added dropwise (m.p. 24 "C)in controlled amounts so as not to exceed the capacity of the oxygen flask. It should be noted that because of their great solubility, the fluoride combustion products of this decomposition only need to be collected in water; no need exists for alkaline or oxidizing solutions. It should also be noted that no greater manipulative skill is required for polycarbonate flask combustions than with the glass flasks. By keeping the flame from the wall of the flask, scorching never occurs and the flasks appear to last indefinitely. As a standard laboratory precaution, no oxygen flask combustion, whether in glass or plastic, should be undertaken without adequate protection between the operator and the combustion. The final procedure adopted for determination of fluorocarbons is combustion in a polycarbonate flask using dodecyl alcohol as a combustion accelerator. The products are collected in water, diluted fivefold with ethanol and titrated with 0.005M thorium nitrate using a fluoride electrode as a potentiometric indicator for the end point detector. The titrant is prepared with the same alcoholic content as the medium which is being titrated and is standardized against pure sodium fluoride; this gives the same titer as standardization against a microchemical fluorocarbon standard that has been carried through the entire procedure.

(14) C. A. Johnson and M. A. Leonard, Analysr, 86,101 (1961). (15) F. H. Oliver, ibid.,91,771 (1966). (16) J. E. Burroughs, W. G . Kator, and A. I. Attia, ANAL.CHEM., 40,657 (1968). (17) 0. Schwarzkopf and F. Schwarzkopf, "The Determination of Fluorine in Organic, Organometallic, and Inorganic Compounds," paper presented at the 155th National Meeting, ACS, San Francisco, Calif., April 1968.

Table IV shows the results obtained on seven microchemical standards or research grade fluorocarbons with fluorine content ranging from 13 to 76%. To study the replication and the distribution of errors, several analyses were made on each sample. The standard deviation of the absolute error, for the 25 analyses listed in Table IV is 0.20%; for 22 of these analyses, the error in the fluorine content is less than 0.3%. No significant bias is evident in these results; because the titrant is standardized against reagent grade sodium fluoride, it is believed that the fluorocarbons are totally decomposed and all of the fluorine is recovered as titratable fluoride ion. Interferences. The most common constituents of fluorocarbons, such as nitrogen and halogens, do not interfere with this analysis of fluorine. Two elements, occasionally present in fluorocarbons are known to form insoluble salts with thorium. These are sulfur and phosphorus which are converted to sulfate and phosphate in the combustion. Although these would tend to be complexed by the hydrogen

ion in the acidic medium of the thorium titration, they will also tend to be coprecipitated. The error would depend to some extent on their concentration. Procedures for separation of sulfur and phosphorus prior to the determination of fluoride are known. These include separation on ion exchange columns (15, 18) and the Willard and Winter separation of the fluorine by distillation as fluosilicic acid (3). ACKNOWLEDGMENT The authors thank Professor Joseph Bornstein of Boston College for making available some research grade samples of fluorocarbons. RECEIVED for review June 13, 1968. Accepted October 3, 1968. Paper presented at the 155th National Meeting, ACS, San Francisco, Calif., April 1968. (18) C. Eger and A. Yarden, ANAL.CHEM., 28, 512 (1956).

Analytical and Biochemical Measurements with a New, Solid-Membrane Calcium-Selective Electrode G . A. Rechnitz’ and T . M. Hseu Department of Chemistry, State University of N e w York,Buffalo, N. Y . 14214

Evaluation of an immobilized matrix type, calcium-ionselective membrane electrode shows several advantages over the earlier liquid membrane electrode. In particular, the electrode is shown to have a reduced selectivity for the hydrogen ion, resulting in a useful extension of the electrode’s pH range into the acidic region. Selectivity for calcium over common interfering univalent and divalent cations is satisfactory for most practical measurements. The new electrode is successfully employed for direct potentiometry, .POtentiometric titrations, and the study of three calcium complexes of biological significance.

brane and active ion exchanger and reference liquids, which necessitates a relatively high degree of skill and experience for the assembly and successful employment of the liquid membrane calcium electrode. In this paper, we report on the evaluation studies of a new calcium-selective membrane electrode, now commercially available, which is to a large extent successful in overcoming these limitations. It will be shown that this electrode, which is of the immobilized matrix type with a “solid” active membrane, displays an improved calciumto-hydrogen ion selectivity and is well suited for both analytical and biochemical measurements (8).

THErecently-developed calcium-ion-selective electrodes of the liquid membrane type have already attained wide acceptance for analytical and general chemical measurement purposes (1-7). While these electrodes represent a major improvement over previous means of calcium activity measurement, they nevertheless suffer from several limitations which restrict the practical use of the electrodes for such measurements. Chief among these limitations are, perhaps, the serious potentiometric interference of the hydrogen ion, which limits the useful range of the electrode in acidic media, and the complicated internal arrangement of inert support mem1 Alfred P. Sloan Fellow, to whom requests for reprints should be addressed.

(1) M. E. Thompson and J. W. Ross, Jr., Science, 154, 1643 (1966). (2) J. A. King and K. MuKherji, Nururwissenschufren, 53, 702

(1966). (3) S. C. Glauser, E. Ifkovits, E. M. Glauser, and R. W. Sevy, Proc. Soc. Exp. Bioi. Med., 124, 131 (1967). (4) J. W. Ross, Science, 156, 1378 (1967). (5) F. S. Nakayama and B. A. Rasnick, ANAL.CHEM., 39, 1022 (1967). (6) A. Shatkay, ibid., 39, 1056 (1967). (7) G. A. Rechnitz and Z . F. Lin, ibid., 40,696 (1968).

EXPERIMENTAL Chemicals. Ca(C10&.4 H20 was prepared (9) from calcium carbonate by dissolution in a slightly more than theoretical quantity of 70 HCIOd followed by evaporation of excess solvent. The crystals that separate out were centrifuged and dried in a desiccator. A stock solution of 0.2M Ca(ClO& was prepared by dissolving the Ca(CIOa)z.4 H z 0 in water and was standardized by the Ag+ ion indicator potentiometric titration method (10). Trans-l,2-diaminocyclohexane-N,N,N’,N’-tetraaceticacid (HIDCTA), obtained as Chel C D from Geigy Industrial Chemicals, was recrystallized from water to a product analyzed at 99.8% purity by potentiometric titration with NaOH in the presence of a slight excess of copper(I1) chloride. All other chemicals used were of analytical reagent grade. All solutions were prepared from water which had been both deionized and distilled. Apparatus. A Beckman Research model pH meter was used for all potentiometric measurements. EMF us. time (8) A. L. Lehringer, Physiol. Reu., 30,393 (1950). (9) G. Brauer “Handbook of Preparative Inorganic Chemistry,” Vol. I, Academic Press, New York, 1963. (10) J. S. Fritz and B. B. Garrola, ANAL.CHEM., 36,737 (1964). VOL. 41, NO. 1, JANUARY 1969

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