F. DRY I~OPROPYL ALCOHOL. Reflux 1 liter of isoPropyl alcohol with 15 grams of sodium borohydride for 30 minutes and distill. Protect from atmospheric moisture.
the development of this apparatus, and Frederick R. Jensen of the University of California, Berkeley, in whose laboratory part of the experimental work was performed.
ACKNQ WLEDGMENT
RECEIVED for review April 12, 1967. Accepted September 6, 1967. Study assisted in part by a grant from Parke, Davis and Co. This is paper IV in a series on Catalytic Hydrogenation.
The author thanks Herbert C . Brown of Purdue University who provided suggestions and laboratory space during
u r ~ ~ ~ iTitration ~ ~ t rof ~Fluoride c with Neodyrniurn(lll) John E. Roberts Department of Chemistry, University of Massachusetts, Amherst, Mass. 01002 1958, Brandt and Duswalt ( I ) investigated the Photometric end-point detection when fluoride was titrated with calcium or thorium ions. While the results were satisfactory for simple systems, the presence of phosphate or sulfate interfered seriously. Mention was made that titrations with cerium(II1) were also satisfactory, but no further details or supporting evidence were offered. The titration of fluoride with thorium (IV) has long been known to be nonstoichiometric requiring standardization against known fluoride under carefully controlled conditions. The lanthanide(lI1) cations precipitate fluoride quantitatively even in solutions of low pH. The solubility products of the lanthanide fluorides ( ~ 1 O - ~ 5lead ) to an equilibrium concentration of fluoride which is at least an order of magnitude smaller than that from calcium fluoride. Furthermore, in contrast to thorium fluoride, the lanthanide trifluorides are stoichiometric. The method reported here utilizes neodymium(II1) as the precipitant for fluoride in acidic solution with photometric determination of the equivalence point. Simple equipment is used and either macro- or micro-scale operation is satisfactory. IN
‘ EXPERIMENTAL
The solutions used were 0.1000M sodium fluoride, approximately 0.2M neodymium nitrate prepared from freshly ignited 99.99 % Nd& and a 50 % v/v water solution of polyethylene glycol 400. A seeding reagent was prepared by suspending enough precipitated and washed NdF3 in water to give a strong turbidity. Equipment used was as follows: Photometric end points were detected with a Fisher Electrophotometer 11. This was modified for magnetic stirring by mounting an airdriven magnetic stirrer (G. F. Smith Chemical Co.) below the cell compartment. This arrangement was inexpensive and had a sensitivity commensurate with turbidimetric end points. Two sizes of cells were used. A short length of 33 mm borosilicate glass tubing sealed and flattened at one end was satisfactory for 50-ml total volumes. For titrations on a smaller scale, the 25-ml cells supplied with the instrument were used. Regular 5-ml burets were used with the large cell; a Benedetti-Pichler style horizontal buret (2) was used for small scale operations with the 25-ml cell.
(1) W. W. Brandt and A. A. Duswalt, ANAL.CHEM.,30, 1120 (1958). ( 2 ) A. A. Benedetti-Pichler, “Introduction to the Microtechnique of inorganic Analysis,” Wiley, New York, 1942, p. 2 5 6 8
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Development of Procedure. INDUCTION PERIOD. When an acidic solution of fluoride was titrated with neodymium(II1) solution, the plotted results showed the expected two straight lines which could be extrapolated to an intersection at the equivalence point. The graph showed some curvature in the vicinity of the end point and also at the beginning of the titration, an induction period. This latter effect, apparently due either to solubility or precipitation-nucleation rate effects, was eliminated by addition initially of enough suspended NdF3 to produce a noticeable turbidity. This was zeroed out instrumentally before titration. While this effect did not affect accuracy for large samples, with 1-mg samples correct results were obtzined only after addition of the seeding reagent. PROTECTIVE COLLOID.During titrations with continuous stirring, variations in turbidity were occasionally noticed and were apparently related to particle size changes and coagulation of the precipitate. Addition of a protective colloid eliminated this effect. While Gum Arabic was effective, ethylene glycol 400 gave better stability of the suspension and was not susceptible to organic growths. INCIDENT LIGHT. The incident light color had only minor effects on the titration. A green filter (525 mp) yielded the steadiest most reproducible readings. Although aqueous neodymium(II1) absorbs strongly in the visible spectrum, this was never observed in this work; a much narrower band pass would be required to detect this effect. EFFECTOF pH. Titrations were carried out at acidities ranging from 0.1M H N 0 3 to pH 4.5. At pH 4.5 (acetate buffer) slightly high results were found suggesting either that acetate exerts a solubility effect on the precipitate or that some mixed complex in which the F:Nd ratio is less than 3 :1 is being formed. At pH 2.9 (monochloroacetate buffer) the results were satisfactory although in this case (as in some others in which a sizeable salt concentration existed) the initial induction period was not completely eliminated by introducing solid NdF3 before titration. In this connection, microscopic examination of the precipitate showed that the NdFa formed from chloroacetate buffered solutions had a particle size ranging from 3 to 15 microns (Martin’s diameter) whereas that from nitric acid solutions consisted of particles uniformly less than 1 micron. SOLVENT.Addition of up to 50% by volume of ethanol or acetone had no measurable effect on the titrations. INTERFERENCES. Ions which form insoluble compounds with Nd(lI1) under acidic conditions will interfere ; notable is oxalate. Ions which form insoluble compounds or stable complexes with fluoride will interfere; in this group are iron(III), aluminum(IPI), borate, caicium(II), thorium(IV), and perhaps others which were not investigated. It is especially noteworthy, however, that neither phosphate nor
sulfate, which commonly occur with fluoride, caused any interference whatsoever in concentrations up to 0.1M, even in the presence of alkali metal ions. Procedure. The sample should contain 1 to 6 mg of fluorine in 15 ml for micro scale operation or 10 to 20 mg of fluorine in 35 ml if the larger cell is to be used. With 1.OM H N 0 8 or l.0M NaOH, adjust the pH to between 1 and 2. Add 2 or 3 drops of 50% polyethylene glycol 400 and enough NdF3 suspension to produce a noticeable turbidity. Introduce a magnetic stirring bar and stir, with the cell in the instrument illuminated with green incident light until readings are constant. Titrate with standard neodymium solution added in small increments, reading absorbance after each addition, until the absorbance readings reach a constant value. Plot absorbance as ordinate and volume of titrant as abscissa and extrapolate the lines to intersection.
lternating
RESULTS
The method was applied first to solutions of sodium fluoride containing from 1 to 20 mg of fluorine. The average absolute error was 0.09 mg. Finally, samples of p-fluorobenzoic acid and thenoyl trifluoroacetone were combusted by the Schoniger oxygen flask technique using 0.002M soaium hydroxide in the flask to collect the fluoride. The resulting solution was treated according to the above described procedure. For pfluorobenzoic acid: calcd F : 13.56; found: 13.63. For thenoyl trifluoroacetone: calcd F: 25.65 ; found: 25.58. These results compare favorably with the usual results of other titrimetric methods for fluorine.
RECEIVED for review May 24, 1967. Accepted August 25, 1967.
ith io
Ronald F . Evilia and A i James Diefenderfer Department of Chemistry, Lehigh University, Bethlehem, Pa. 18015
WITHINthe past few years several designs for phase-sensitive alternating current polarographs have been presented (1-5). A design using sharply tuned twin-T filters has been presented (1); however, operation at different frequencies is extremely difficult. An analog computation method has also been presented (2), but the large amount of circuit wiring necessary makes this method unattractive. Another approach to phase-sensitive ac polarography is the lock-in amplifier technique. This technique was recognized earlier (6)but no instrument making use of this alternative has appeared. We have obtained excellent results by incorporating a commercially available lock-in amplifier (Princeton Applied Research Model HR-8) into an operational amplifier instrument of standard design. This instrument, which was built with a minimum of external electronic wiring, equals or exceeds the specifications of earlier instruments (1-4). THEORY OF OPERATION
The amplifier components of the instrument are normal, and their operation was described previously (7-9). The lock-in amplifier serves as both the ac signal generator and the phase sensitive detector. Because the detector is locked-in to the signal generator, any drift of the signal is automatically
(I) E. R. Brown, T. G. McCord, D. E. Smith, and D. D. Deford, ANAL.CHEM., 38, 1119 (1966). (2) S. W. Hayes and C. N. Reilley, Zbid.,37, 1322 (1965). (3) T. Takakashi and E. Niki, Talanta, 1, 245 (1958). (4) D. E. Smith and W. H. Reinmuth, ANAL.CHEM.,32, 1892 ( 1960). (5) G. C.Bard, Anal. Chirn. Acta, lS, 118 (1958). (6) D. E. Smith, “Electroanalytical Chemistry,” A. J. Bard, ed., Vol. 1, Marcel Dekker, New York, 1966, p. 118. (7) D. D. Deford, Division of Analytical Chemistry, 133rd National Meeting, ACS, San Francisco, Calif., April 1958. (8) W. M. Schwarz and I. Shain, ANAL.CHEM., 35, 1770 (1963). (9) D. E. Smith, “Electroanalytical Chemistry,” A. J. Bard, ed., Vol. 1, Marcel Dekker, New York, 1966, pp. 102-108.
I\ t----\ Triangular Genera t o r
Potential
MODE
-A C
_ _ _ _ -DC. - -- cy-1 c
Figure 1. Flow sheet-block ~~~r~~ of circuit showing modes of operation
compensated. The theory of lock-in amplifier operation has been presented previously (6, IO, 11). By using a lock-in amplifier as the phase-sensitive detector, one can examine either the in-phase or quadrature signal by simply switching from one mode to the other. By increasing the time constant (front panel switch) to several times the drop time, the drop oscillations are damped out at slow scan rates. In short, the lock-in amplifier approach is capable of performance equal to previously presented methods (1-4)9 but requires far less external electronic wiring.
(10) Instruction manual, Precision Lock-in Amplifier Model HR-8, Princeton Applied Research Gorp., Princeton, N. J., 1965. (1 1) “How the lock-in amplifier works,” Brower Laboratories, Inc.,
Westboro, Mass. V6b. 39, NO. 14, DECEMBER 1967
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