Spectrophotometric Titration of Microgram Amounts of Fluoride SIR: Several investigators have described spectrophotometric titration methods for determining microgram amounts of water-soluble fluoride using thorium nitrate as titrant and sodium alizarin monosulfonate as indicator (1,2, 4, 5 ) . However, these methods require special photoelectric filter photometers or lack adequate sensitivity and precision for determining microgram amounts of fluoride. A modified spectrophotometric procedure has been developed which overcomes these limitations; it is based on adaptation of a readily available instrument to spectrophotometric titration (1, 3) and titration of fluoride with thorium reagent containing a portion of the required indicator. The procedure can be used to determine fluoride in the range of 0 to 200 kg. with a precision and accuracy of ea. f l pg. EXPERIMENTAL
Apparatus. A light - tight interchangeable cell compartment was constructed for use with a Beckman Model DU spectrophotometer in the spectrophotometric titration. Titration cells having a path of ca. 7.5 cm. were conveniently prepared by shortening 400-ml. borosilicate glass beakers and marking them a t the 250- and 260-ml. volumes. -4 5-ml. microburet, graduated in 0.01 ml., was used. Reagents. Distilled water, fluorine-free (purified by demineralizer). Thorium nitrate solution, 0.1N aqueous. Sodium alizarin monosulfonate indicator solution, 0.006%. Sodium fluoride standard solution, 10 pg. F/ml. Thorium nitrate-sodium alizarin monosulfonate mixed titrant. Dissohe 80.0 + 0.1 mg. of the indicator in ca. 250 ml. of water contained in a 500-ml. volumetric flask. Add 25 ml. (pipet) of 0.1N thorium solution and 200 ml. of absolute ethyl alcohol. Swirl the mixture, dilute to volume with water, cool to room temperature, dilute to volume again with water, and mix thoroughly. Nitric acid, 0.001N. Adjust the pH of water potentiometrically to nearly 3 by dropwise addition of 1N nitric acid with efficient stirring. Then adjust carefully to pH 3.00 =t0.01 with 0.1N nitric acid. If a magnetic stirrer is used, stop stirring each time the pH is read during the final adjustments. Procedure. Pipet no more than 250 ml. of aqueous sample solution, containing up to 200 pg. of fluoride, into a titration cell which contains a glasscovered magnetic stirring bar. Pipet 5 ml. of the indicator solution and, if necessary, dilute to the 250-ml. mark with fluorine-free water. Place the cell
P
a300t
P
L 5
TITRANT. rnl
Figure 1 . Extrapolated straight portions of typical titration curves
on a magnetic stirrer. Adjust the pH to nearly 3 by dropwise addition of 1N nitric acid or sodium hydroxide and finally to pH 3.00 + 0.01 with 0.1N acid or base. Stop the stirring each time the p H is read during the final adjustments. Raise the electrodes to about 1 cm. above the solution, rinse with 0.001N nitric acid, and dilute the solution to the 260-ml. mark with the 0.001N acid. Wipe the outside of the titration cell and place it in position in the t-itration compartment. Support the buret so that its tip extends into the solution. Set the spectrophotometer to a wavelength of 525 mp and to an appropriate slit width. Adjust the instrument to zero absorbance and the stirring to the maximum feasible without producing a vortex that interferes with the passage of light. Add the thorium-indicator mixed titrant in appropriate increments and plot the absorbance against titrant volume. Stop the titration when the ratio of absorbance to volume of titrant added begins to decrease. Extrapolate the straight portion of the titration curve after the end point to zero absorbance. The intercept a t zero absorbance is the volume a t the end point (see Figure 1). Determine the fluoride content by reference to a calibration curve prepared from titration of 5-, lo-, and 20-ml. portions of the standard sodium fluoride solution. Titration and Calibration Curves. Figure 1 gives typical titration curves.
Table 1.
Reagent Lot
F- Level,
13 6 6 5 8 5
a
50 200 50 200 50
C
DISCUSSION
The precision and accuracy of the method were determined on standard solutions of differing fluoride concentration using different lots of titrant and indicator. Table I summarizes results obtained for three lots of reagent solutions prepared over a period of several weeks and standardized a t least once daily a t the 50- and 200-fig. fluoride levels. Fluoride was determined with a precision and accuracy of about f1% relative. The data suggest that only one standardization is needed for each lot of reagents, provided preparations are made weekly and room temperature does not vary sufficiently to cause errors due to volume changes. Satisfactory recoveries were obtained for 1-, 3-, lo-,
Precision and Accuracy Data for Fluoride
No. of Detns.
b
For the most accurate determination of end points, continue the titrations until absorbance falls off with addition of titrant. At low fluoride levels, addition of 0.05-ml. increments of titrant is required to obtain a sufficient number of points. Increments of 0.1 ml. are adequate a t the 50-pg. fluoride level, and 0.2 ml. a t the 200-pg. fluoride level: Since the straight Dortions of the titration curves al$ay$ pass through the origin for blank titrations, the origin is used as one of the points in the preparation of linear calibration curves.
pg.
200
Av . 50.0
200.0 50.0
200.0 50.0
200.0
Found,
pg.
F Range
49.3- 50.9 199.4-200.9 49.0- 50.5 199.4-200.9 49.0- 50.5 199.5-201.0
VOL 34, NO. 7, JUNE 1962
0.
0.58 0.57 0.64 0.66 0.56 0.88
863
20-, and 150-pg. amounts of fluoride as illustrated by the following data:
Table 111.
Direct Titration Results for Fluoride
Fluoride Found Sample City water Demineralized water with 100 pg. F added Water extract A Water extract B
F,
Added
Found
1 3 10 20 150
1 2. 3
io
20,20, 19 151
Some common interferences studied are listed in Table 11. Approximate concentration in the final solution and per cent relative error in titrant volumes
Table 11.
Anion Added
c1-
NOa-
C1O4POa-3
so,-'
Interference Data
Concn. of hdded Anion in Final soln., $& Relative Error P.P.M. 50 pg. F 200 pg. F +1.5 Nil 40 360 385 285 0.1 0.06 9 2
+1.5 f3.0 f0.7 -3.0 -0.7 1-12.5 +0.8
f0.5 f2.5
...
...
+i:5
...
rg.
P.P.M.
25, 23
0.10, 0.09
100, 100, 96 18 (4detns.) 7 ( 8 detns.)
at the 50- and 200-pg. fluoride levels are given. Fluoride in aqueous samples can be separated from most interfering substances by the Willard and Winter distillation (6) or by some other suitable means. This method has been used successfully in the author's laboratory for over three years. During this time the slopes of the linear calibration curves-sensitivity of the method-varied from 0.8 to 0.7. This change is attributed to variation in ambient temperature a t the time of preparation of reagents and consequent variation in thorium content of the reagents. It causes no decrease in the accuracy of the method as long as the recommended standardization procedures are followed. Typical direct titration results are
0 . 3 8 , 0 . 3 8 , 0.37 0.12 14 detns.) 0 . 0 3 (8 detns.)
given in Table I11 for various routine aqueous samples. LITERATURE CITED
(1) Dean, J. A,, Buehler, & H., I.Hardin,
L. J., J . Assoc. 0.j'icial Agri. Chenaists
40. 949 119571. \
,
(2) Gwirtsman, J., Mavrodineanu, Coe, R. R., ANAL. CHERI. 29, (1957). (3) Klingman, D. IT., Hooker, D. Banks, C. V., Ibid., 27, 572 (1955). (4) Ma, T. S., Gwirtsman, J., Ibid., 140 11937). (5) Mavrodkeanu, R., Gwirtsman, J., Contribs. Boyce Thompson Inst. 18, 181 (1956). ( 6 ) Willard, H. H., Winter, 0. B., ANAL. CHEM.5. 7 11933). I~
I
W.P. PICKHARDT Polychemicals Department E. I. du Pont de Nemours &- Co. Wilmington, Del.
Nature of Extraneous Peaks in Gas Chromatographic Analysis of Gira rd-T kola ted Car bonyls SIR: The use of Girard-T reagent [ (carboxymethy1)trimethyl-ammonium
chloride, hydrazide] by this laboratory for the isolation of carbonyls from multifunctional group mixtures, as described by Teitelbaum (4) and applied by Stanley et al. (S),results in the appearance of a great many extraneous peaks when the carbonyl fraction is analyzed by gas chromatography. These impurities have been traced to the formalin used for regeneration of the carbonyls. The terpenoid fraction of cold pressed orange oil, after being separated from the terpene fraction (1, 2) contained aldehydes, ketones, esters, and alcohols. This terpenoid fraction was treated with Girard-T reagent as described by Stanley et al. (3) with regeneration of the aldehydes using formalin. The residue, containing the aldehydes, was analyzed by gas chromatography (column 16-feet x '/,-inch Silicone 200 Fluid coated 30- to 60-mesh firebrick, temperature programmed 50' to 150' C. a t 2.9' C. per minute, helium flow rate 50 ml. per minute, hot wire thermal conductivity cell). The curve is reproduced in Figure 1. The components represented by the individual peaks were isolated and determined by infrared spectro864
ANALYTICAL CHEMISTRY
photometry. The chromatogram indicated the presence of octanal (peak g), nonanal (peak ll), decanal, and citronellal (peak 13), which were aldehydes isolated from the orange oil terpenoid fraction, The remaining peaks were attributable to the Girard-T process
except peak 7 , which may contain material from the terpenoid fraction in addition to an impurity introduced by the Girard-T process. Figure 2 shows a chromatogram of the extract obtained from a blank Girard-T run which was carried out
48
60
72
04
TIME, MIN
Figure 1. reagent
isolation of carbonyls from orange oil using Girard-T
