ANALYTICAL CHEMISTRY
558 Versenate as the titrating reagent. I n waters containing up to 1400 p.p.m. of calcium carbonate, the authors claimed an accuracy within 2 p.p.m. Betz and No11 (3), in a direct titration with Versenate, obtained an accuracy of 2y0 in the concentration range of 100 to 1200 p.p.m. Hahn ( 5 ) determined calcium by an alkalimetric titration with Versenate. The titration error for 100 ml. of water containing 50 to 400 mg. of calcium carbonate per liter was less than 5 mg. of calcium carbonate per liter when indicators were used and less than 1 mg. of calcium carbonate per liter in potentiometric titrations. Barreto ( 8 ) suggested a colorimetric method for calcium. Tyner (1.4) made a systematic study of this method and concluded thst there was a systematic error of +5% (relative) as compared with the stsndard Association of Official Agricultural Chemists volumetric procedure (1). Le Peintre (9),by rigorous control of pH, found no systematic error. West, Folse, and lllontgomery (15) determined calcium by flame spectrophotometry. For a concentration of 90 p,p,m. a standard deviation of 4.06 p.p.m. was obtained. At 50 p.p.ni. the standard deviation was 2.82 p.p.m. and a t 10 p.p.m. it was 3.28 p.p.m. Saifer and Clark ( I f ) describe a turbidimetric method for ralcium. I n the concentration range of 0.004 to 0.28 mg. of calcium per 10 ml., they obtained an average error of 2 ~ 4 % . The method described in this paper seems to be more accurate than the optical methods, but not so sensitive. Its accuracy compares favorably with that of most of the volumetric methods. PROCEDURE
Use 45 to 50 ml. of the sample solution containing not less than 2.2 mg. of calcium. Add to the sample solution 10 ml. of 0.01[11
zincate solution in 1 to 1.4M potassium hydroxide and 0.5X potassium chloride. If the sample was put in solution by adding excess acid, adjust the p H of the solution to nearly neutral before adding the zinc solution; or increase the hydroxyl ion concentration in the indicator solution in order to be sure the solution is sufficiently alkaline before the titration is begun. Remove oxygen from the solution by bubbling it with oxygen-free nitrogen. The solution is then ready to be titrated. The potential applied is - 1.700 volts va. the saturated calomel electrode. Take at least four readings both before and after the end point. Bubble the solution with nitrogen for at least 2 minutes after each addition from the buret. LITERATUKE CITED
(1) Assoc.
Offic.Agr. Chemists, “Officialand Tentative Methods of
Analysis,” 5th ed., p. 127, 1940. SOC. b r a d agron. (Aio de Janeiro), 8 , 351
(2) Barreto, A., Bol.
(1945). (3) Beta, J. D., and Noll, C. d.,J . Am. Water Works Assoc., 42, 49 (1950). (4) Diehl, H., Goeta, C. A., and Hach, C. C., Ibid., 42, 40 (1950). (5) Hahn. F. L., Anal. Chim. Acta. 4, 583 (1950). (6) Kolthoff, I. IM., and Lingane, J. J., “Polarography,” 2nd ed., p. 504, New York, Interscience Publishers, 1952. (7) Laitinen, H. A., and Burdett, L. W., ANAL. CHEM.,22, 833 (1950). (8) Latimer, W. Id.,“Oxidation Potentials,” 2nd ed., p. 170, New York, Prentice-Hall, 1952. (9) Le Peintre, M., Compt. rend., 231, 968 (1950). (IO) Ringbom, il., and Wilkman, B., Acta Chem. Scand., 3, 22 (1949). (11) Saifer, A,, and Clark, F. D., IXD.EKG.CHEM., ANAL.ED., 17, 757 (1945). (12) Schwarrenbaoh, G., and Ackermann, H., Helv. Chim. Acta, 30, 1798 (1947). (13) Schwaraenbach, G., and Freitag, Eisi, Ibid., 34, 1503 (1951). (14) Tyner, E. H., ANAL. CHEM., 20, 76 (1948). (15) West, P. W., Folse, P., and Montgomery, D., Ibid., 22, 667 (1950). RECEIVED for review June 29, 1953. rlccepted October 23, 1953
Determination of Fluoride Ion Using a Monohydroxy Azo Dye-Thorium lake JACK L. LAMBERT Kansas State College, Manhattan, Kan.
most successful colorimetric methods (1, 3) for the deterT mination of fluoride ion in small concentrations have been those using chelation compounds (lakes) of zirconium with 1,2HE
dihydroxyanthraquinone dyes. Fluoride ions, which form very stable complex ions with zirconium, react to disp1ac.e the dye molecules. Quantitative determinations are made by measuring the ratio of the displaced free dye to the color of the unrlianged lake. Other colorimetiic methods involve the bleaching of colored ferric complexes or pertitnnates by their reaction with fluoride ion to form stable, colorless complex ions (3). The work described here was undertaken to develop a colorimetric method in which the intensity of co101 developed in the treated sample would be dirertly proportional to the concentration of fluoride ion. -4n insoluble thorium-Amaranth lake supported on filter paper was found to exchange dye molecules rapidly for fluoride ions, producing solutions of Amaranth Tvhich closely approximated Beer’s law. I t is a justified assumption that a chelation compound in which ring closure with the zirconium atom was effected through two adjacent hydroxyl groups in the alizarin-type dyes would require either a stepwise replacement by two fluoride ions or (unlikely) a three-body collision of two fluoride ions and a zirconium-dye molecule. Either should result in a relatively slow reaction, which is the case in methods based on this type of reaction. The azo dyes studied in this work ran effect a ring closure with tho-
rium or zirconium through only one hydroxyl group and a coordination bond with a nitrogen atom of the azo linkage. The metalnitrogen (dye) bond should be much the weaker of the two bonds, and the breaking of the metal-oxygen (dye) bond should set free the dye molecule. Evidence supporting this idea is, first, the lack of known stable complex ions involving thorium-nitrogen or zirconium-nitrogen coordination bonds and, second, the very rapid reaction observed between fluoride ion and the thorium and zirconium lakes of the monohydroxy azo dye used in this work, x-hich indicates a bimolecular reaction. REAGERTS AND EQUIPMENT USED
Thorium nitrate, tetrahydrate, reagent grade, 1% solution. Amaranth, certified food color grade, 0 2% solution. Standard fluoride solution, 100 p.p.m., 0.221 gram of C.P. grade sodium fluoride per liter of solution. Filter paper, Whatman No. 42, 5.5-cm. diameter circles. Evaporating dishes, porcelain, Coors NO. 2 (90-mm. diameter). Interval timer. Spectrophotometer, Beckman Model DU, IO-mm. Corex cells.
Of the dyes a t hand, only Amaranth, Ponceau SX, and Acid Alizarin Red B contained just one hydroxyl group not adjacent to an oxygen-containing chelating group, and formed insoluble lakes with thorium and zirconium. The lakes of the other two dyes are much less satisfactory than the thorium and zirconium lakes of Amaranth, and the tinctorial powers of the free dyes are
V O L U M E 26, NO. 3, M A R C H 1 9 5 4
559
less. The thorium lake of Amaranth is more insoluble and more sensitive to fluoride ion than the zirconium lake, and so was chosen for this investigation. The structure of the Amaranth lake probably is:
_______
Table I.
_
_
~
Substances Causing Little or No Interference (Fluoride ion absent)
C1C1-
diihstance (SaC1) (UaC1)
Sa
(XaC1)
Concentration, P.P.X.
Observation Very faint color s o color N o color N o color Very faint color
I(Y$) Koa - (IIaNOa) Na: (NaCI)
To
+
Very faint color
No color Tery faint color S o color Very f a i n t color S o color N o color N Noo colora color
I
J
f
50 500 100 10-3~
+ +
:I
COlOl
N o color
I
