10th Edition of the Sational Formulary when that method had actually been wperbeded by an improved method publi.hed in the 11th National Formulary. I n my opinion this unfortunate and unintentional error is corrected in a fair manner by Mr. Levine's letter. DOLALD P. HOLLIS
Analytical Instrument Division S'arian Associates 611 Hansen LVay Palo Alto, Calif.
SIR: The following are my comments to RIr. Levine's letter. Our article was originally written very early in 1961 for
the September Meeting in Atlantic City, and therefore the reference (3) in Mr. Levine's letter was not available during the composition of the original draft. After the meeting, it was rewritten for publication essentially as read in Atlantic City. Upon review, it waz completely revised and some additional new material was incorporated. that time we noted the reference cited by Rlr. Levine, but since it was not official, we did not make references to it in our bibliography. R e are sorry for the omission. The reference (21) in our paper evidently was our error and may have
crept in during one of the revisions. We appreciate your bringing this to our attention. Our statement in the last line of paragraph one of our article referred particularly to effervescent compositions, but we still feel that the present method presented in our article is even faster than the new column chromatographic method of Kational Formulary, 11th Edition, for ACP compositions. Our technique can be speeded up to 15 minutes total time, if necessary. R. C. CRIPPEN Analytical Section Atlas Chemical Industries, Inc. FVilmington 99, Del.
Spectrophotometric Determination of Fluoride by Zirconium-Eriochrome Cyanine R SIR: Megregian (6) report'ed that zirconium-Eriochrome Cyanine R could be used as a colorimetric reagent for the determination of 0.00 to 1.40 mg. of fluoride per liter of a water sample. He recommended this reagent in preference to zirconium-alizarin ( 3 ) , aluminuniEriochrome Cyanine R ( l a ) , thoriumalizarin (j), titanium-peroxide (@, and some fluorimetric reagents (9) on considerations of sensitivity, stability, simplicity, and rapidity ( 7 ) . ZirconiumEriochrome Cyanine R is now used as one of the standard colorimetric reagents for determining fluoride in water and in distillate samples. This reagent was also reported as most suitable for use in an atmospheric fluoride analyzer (1). In the course of an investigation on the removal of toxic amount5 of fluoride from water, this author observed a high relative concentration error when fluoride was determined with zirconiumEriochrome Cyanine R a t certain concentrations recommended by Megregian. Therefore, an investigation wy8s made to find out the fluoride concentration range which is most suitable for determination using original and modified Alegregian reagents. Also, the feasibility of simplifying t,he determination by use of a factor instead of a calibration curve was investigated. EXPERIMENTAL
Apparatus. Beckman Model D B spectrophotometer and 0 998-cm. absorption cells Reagents. The composition of reagent h (Eriochrome Cyanine R) reagent 11 (zirconium chloride), and the reference solution a e r e the same a. th0.e uied by Xlegregian (6) Procedure. I standard solution containing 100 p p m of fluoride was 1684
ANALYTICAL CHEMISTRY
prepared by dissolving 0.2221 gram of 99.5% pure sodium fluoride (Matheson Coleman I& Bell, CH 721) in wat,er and diluting it to 1 liter. Other standard solutions were prepared by diluting aliquots of this solution to proper volumes. All t,hese standard solutions were stocked in polyethylene bottles and were used within less than a week. Having filled both the absorption cells with the reference solution, the R E F knob of the spectrophotometer was adjusted until the per cent' transmittance read 100.0. The sample cell was then filled with a solution obtained by mising 5.00 ml. of reagent A, 5.00 ml. of reagent B, and 50.00 i d . of a standard fluoride solution. Per cent transmittance of this solution was then read a t 528 mp maintaining the t'emperature within 25" i 1' C. Several other readings wer taken using 5.00 ml. of reagent A, 5.00 ml. of reagent 13, and 50.00 nil. of fluoride solutions of different concentrations. ht these transmittance readings, the absorbances and the per cent absorptances (100.0 - % T ) were calculated. -1graph was obtained relating absorbances with fluoride concentrations (Figure 1.1) and another relating per cent absorptances with the logarithms of the fluoride concentrations (Figure 2A1). A second set, of per cent transmittance readings was obtained with 50.00 ml. of the same standard fluoride solutions and 5.00 ml. of reagent A and 10.00 ml. of reagent I3 (Figures 1B and 2B); and a third set, with 50.00 ml. bf the fluoride solutions, 10.00 ml. of reagent A, and 5.00 of reagent I3 (Figures 1C and 2C). RESULTS A N D DISCUSSION
Figure I d was obtained with the reagent originally used by Megregian (6). The high slope of this curve between about 0.2 to 1.4 p.p.m. of
fluoride suggested that zirconium-Eriochrome Cyanine R solution used by Xegregian was a sensitive reagent for determining less than about' 1.4 p.p.m. of fluoride. A still further increase in the sensitivity of this reagent was observed by *%damsand coworkeh when they reduced its hydrochloric acid content by 50% ( I ) . However, in the analysis of samples containing certain metals, a reduction in the hydrochloric acid content may int,roduce additional interference in this method. De Boer (4)observed that, like zirconium, many other metals give colors with alizarin, but only the color due to zirconium was left unaffected on treatment with concentrated hydrochloric acid. This color of zirconium-alizarin was then reduced by fluoride, thus providing an indirect method for the determination of fluoride. I t is espected that zirconiumEriochrome Cyanine R will behave like zirconium-alizarin. An increase in the sensitivity of the reagent through a reduction in its hydrochloric acid content might also adversely affect the range of fluoride concentrations which could be determined with small relative concentration errors. The linear portion of the curve in Figure 1;1, between 0.2 to 1.4 p.p.m. of fluoride, gave a negative slope of 0.276. If Megregian's reagent contained 5.00 ml. of reagent A and 10.00 ml. of reagent B, this slope would reduce to 0.124 (Figure 1B). Although such a modified reagent decreases the sensitivity, it increases the range over which absorbance and fluoride concentration vary linearly (Figures 1.-i and 1B). If the reagmt were modified to contain 10.00 i d . of reagent h and 5.00 nil. of reagent B, the negative slope of the absorbance-concentration curve
“I
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Figure 2. Determination of optimum concentration range in the estimation of fluoride by zirconium-Eriochrome Cyanine R method A. 0
I
2 3 4 5 CONCENTRATION OF FLUORIDE, R R M .
6
7
Figure 1 . Calibration curve for the determination of fluoride by zirconium-Eriochrome Cyanine R method A. 8.
C.
B.
C.
50.00 ml. of fluoride soldion, 5.00 ml. of reagent A, and 5.00 ml. of reagent B 50.00 ml. of fluoride solution, 5.00 ml. of reagent A, and 10.00 ml. of reasent B 50.00 ml. of fluoride solution, 10.00 ml. of reagent A, and 5.00 ml. a1 reagent B
50.00 ml. of fluoride solution, 5.00 ml. of reagent A, and 5.00 ml. of reagent B 50.00 ml. of fluoride solution, 5.00 ml. of reagent A, and 10.00 ml. of reagent B 50.00 mi. of fluoride solution, 10.00 ml. of reagent A, ond 5.00 ml. of reagent B
within the same fluoride concentration range would increase to 0.346 (Figure IC). Therefore, for analyzing samples containing about 1.4 p.p.m. of fluoride or less a modified reagent containing larger amounts of reagent h is preferred because of its greater sensitivity. However, a high sensitivity and a linear absorbance-concentration relationship are not the only considerations in selecting the best experimental conditions. I n spectrophotometric determinations, the relative concentration errors are minimum when determinations are made a t concentrations which correspond to the greatest Ringbom slope (11)-that is, the slope in an absorptancelog concentrat,ion curve. The entire concentration range over which absorbance varies linearly wibh concentration may not be suitable for a spectrophotometric determination unless this concentration range corresponds also to the greatest Ringbom slope ( 2 ) . Megregian’s original reagent gave a negative Ringbom ..lope of 1.5 (Figure 2.4). If this reagent were modified to contain 5.00 ml. of Reagent A and 10.00 ml. of Reagent B, the slope would reduce to 1.0 (Figure 2B), and the relative concentration error would increase in proportion to the reduction in Ringbom slope. On the other hand, if the modified reagent, contained 10.00 ml. of Reagent A and 5.00 ml. of Reagent B, the relative concentration error would be much larger, since the negative Ringbom slope would then be 0.38 (Figure 2C). Therefore, modifications
of Megregian’s reagent containing larger amounts of eriochrome cyanine R are undesirable in spite of their higher sensitivities. Also, the increased sensitivities of these modified reagents may be caused by a reduction in their hydrochloric acid contents. Modifications containing smaller amounts of eriochrome cyanine R appear useful, since they make absorbance vary linearly with concentration over a wider range of fluoride concentrations with only a slight loss in sensitivity and accuracy. Considering both the minimum relative concentration error and the linear absorbance-concentration range, hlegregian’s original reagent appears suitable for use in the determination of about 1.0 to 1.4 p.p.m. of fluoride (Figures 1.4 and 2.4). From the qame considerations, a modified reagent containing 1 volume of reagent h and 2 volumes of reagent B will be suitable for determining about 2.0 to 3.0 p.p.m. of fluoride (Figures 1 B and 2B). Other modifications containing various proportions of reagents .\ and B should permit determinations of fluoride a t concentrations not originally recommended by Megregian. With both the original and modified Megregian‘ reagents, absorbance varied linearly with concentration over these optimum ranges of fluoride concentrations. Within these ranges, a fluoride could be determined using a factor instead of a calibration curve. Such a factor would correspond to a negative absorptivity and would be deduced from
the negative slope of an absorbanceconcentration curve. For example, if the absorbance of an unknown containing no interfering material were measured with a modified Megregian reagent consifting of l volume of reagent A and 2 volumes of reagent I3, the p.p.m, of fluoride would be known by multiplying the absorbance of the unknown with 8.06, which is the reciprocal of the negative slope of the absorbanceconcentration curve (Figure 1B). Similar factors could be obtained for determining fluoride with other modified reagents. LITERATURE CITED
(1) Adams, D. F., Koppe, R. K., Dana,
H. J., J . i l i r Pollution Control Assoc.
9, 160 (1959).
(2) Ayres, G. H., ANAL.CHEM.21, 652 (1949).
(3) Boonstra, J. P., Rec. Trav. Chim. 70, 325 (1951). (4) De Boer, J. H., Chem. Weekhlad 21, 404 (1924). ( 5 ) Icken, J. XI., Blank, &I. B., ANAL. CHEM.25. 1741 11953). (6) Megregian, S.,Zhid.; 26, 1161 (1054). ( 7 ) Megregian, S., “Colorimetric Iletermination of Nonmetals,” 11. F. Roltz, Ed., p. 239, Interscience, New Y o r k , 1958. (8) Monnier, D., T‘aucher, R., Wenger, P., Helv. Chim. Acta 33, 1 (1950). (9) Powell, W. A , , Saylor, J. H., ANAL. CHEM. 25, 960 (1953). (10) Richter, F., Chem. Tech. ( B e r l i n ) 1, 84 (1949). (11) Ringbom, A . , Z. Anal. Chem. 115, 332 (1939). P. L. S A R M A Department of Chemistry University of Sorth Dakota Grand Forks, N. D. 58202 THISinvestigation was part of n pn),ject supported by Public Health Service Grant DE-01918-01 from the Xatir,na1 Institute of Dental Research. VOL. 36, NO. 8, JULY 1964
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