1166
ANALYTICAL CHEMISTRY
be determined on stoichiometric considerations alone. Although the maximum color of the lake is developed a t a molar ratio of 1 to 3 in 0.1N hydrochloric acid, the reagent is very sensitive to phosphate interference a t this acidity. Also, it is weakly buffered, and slow to reach equilibrium. Increasing the acidity reduces phosphate interference and increases buffer capacity and reaction rate. However, the reaction is no longer stoichiometric, and some reduction in fluoride sensitivity occurs, along with a reduction of the total color produced by the reaction (Figure 4).
(IV) produce a color reaction. In a reaction medium containing 0.2N hydrochloric acid, the absorbance change per cm. of light path per p.p.m. of zirconium is about 0.400 unit when the zirconium concentration lies in the vicinity of 4.0 p.p.m Thus, by proper control of reaction acidity, a method could be developed, using Eriochrome Cyanine R, which would determine small amounts of zirconium in the presence of many cations which usually interfere in other procedures. SUJIiMARY
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A rapid spectrophotometric determination of fluoride ion concentration uses the reaction between zirconyl ions and Eriochrome Cyanine R for color formation and the subsequent decolorization of the lake by fluoride ions. The reaction is immediate stable, and follon-s Beer's law in the concentration range of 0.00 to 1.40 mg. of fluoride per liter. The standard deviation in this range is +0.0163 mg. of fluoride per liter using standard photometer cells with a 10-mm. light path Use of this method, in place of the familar thorium titration on distillates of biological samples, will save considerable time and increases precision. When the method is applied directly to water samples, the interference caused by many ions commonly present in water is largely eliminated. Only sulfate ion concentration must be known and a correction applied. ACKNOWLEDGMENT
The assistance provided by Robert Likins, Isadore Zipkin, and Anastasia Steere of the staff of the National Institute of Dental Research in the preparation and titration of the biological samples is acknowledged. LITERATURE CITED
Figure 4. Effect of Reaction Acidity on Zirconium-Eriochronie Cyanine R Reagent
Some of the color lost can be regained by increasing the Eriochrome Cyanine R concentration of the reagent above the 1 to 3 ratio, However, too great an excess cannot be tolerated, because the excess dye absorbs strongly a t the wave length of measurement. The zirconium concentration determines the amount of initial color which the reaction will produce, and the effective fluoride range. I t is limited by the ability of the spectrophotometer to measure deep colors accurately. The selection of the reaction conditions, as presented above, was based on these factors and is believed to be near the optimum with respect to the various considerations previously listed. When 5.0 ml. of each of the reagents are added to a 50-ml. sample, it contains 0.375 mg. of zirconium and 9.0 mg. of Eriochrome Cyanine R (molar ratio 1 to 4) and the solution is 0.7N in hydrochloric acid. Under these conditions 0.050 mg. of fluoride (1.00 p.p.m.) will produce a change in absorbance of 0.480 unit using a 1.0-cm. light path. The Beckman Model B spectrophotometer can usually be read with precision to 0.005 absorbance unit. Thus, it is possible to detect changes in fluoride concentration of 0.57 of fluoride in a 50-ml. sample and to measure with precision 0.050 i 0.016 mg. of fluoride per liter. By scaling down proportionately the amounts of the reagents added, it is possible to determine as little as 0.25 y of fluoride in a 5.0-ml. sample. OTHER APPLICATIONS
At low acidities (up to 0.1N) Eriochrome Cyanine R produces a color with many cations such as tin( IV), thorium(IV), titanium(IV), iron(III), aluminum(III), and lead(I1). As the acidity is increased, only zirconium(1V) and, to a lesser extent, hafnium-
hlegregian, S.,and Maier, F. J., J . Am. Water Works Assoc., 44,239-48 (1952). Milton, R. F., Liddell, H. F., and Chirers, J. E., Analyst, 72, 43 (1947). Revinson, D., and Hartley. J. H., ASAL. CHEM.,25, 794-7 (1953). Richter, F., Chem. Tech. (Berlin). 1 , 84-90 (1949). Rowe, F. S.,"Colour Index," Bradford, England, Society of Dyers and Colourists, 1924. Saylor, J. H., and Larkin, 11.E., A N ~ LCHEM., . 20, 194 (1948). Sheen, R. T., Kaler, H. L., and Ross, E. AI., IND. ENG.CHEY., ANAL.ED.,7,262 (1935). Sidgwick, K. V., "The Chemical Elements and Their Compounds," 5'01. I, p. 644, London, Oxford University Press, 1950. Thrun, W. E., ANAL.CHEM., 22,918-20 (1950). Willard, H. H., and Horton, C. A,, Zbid., 1190-4. Willard. H. H., and Winter, 0. B., IXD.ENG.CHEM., ANAL.ED., 5 , 7 (1933). RECEIVED for review Xovember 21, 1953. Accepted March 31, 1954.
Spectrochemical Analysis of Aluminum Alloys Using Molten Metal Electrodes-Correction In the article on "Spectrochemical Analysis of Aluminum Alloys Using Molten Metal Electrodes" [ASAL.CHEM.,26, 795 (1954)], the second line of the third paragraph on page 795 should read: "in the conventional point-to-plane and point-topointexcitation. . " Page 797, first column, first paragraph, third line from the end should read: "a distance of 3 mm." Page 798, second column, the second and third lines of the table should read:
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Inductance Capacitance
90 ph. 0.007 pfd.
LEOD. FREDERICKSOK, JR. J. R. CHURCHILL
