at this laboratory was described in a paper on continuous analysis of column effluents in ion exchange chromatography (6). ACKNOWLEDGMENT
The author thanks R. J. Houle of Lever Brothers Co. for the encouragement he has given t o this project. LITERATURE CITED
(1) Bolta, D. F., Mellon,
M. G., IND.
Exo. Cmhf., ANAL. ED. 19, 873-7 (1947).
(2) Davies, D. R., Davies, W. C., Biochem. ( 9 ) Muller, R. H., Ibid., 30, No. 1, 53 A, J. 26,2046-55 (1932). 54 A, 56.4 (1958). (3) Ferrari, A., . Russo-Alesi, F. ill., (10) Quimby, 0. T., Mabis, A. J., Lampe, Kellv. J. M.. ANAL.CHEW31. 1710-17 H. W., Ibid., 26, 661-7 (1954). ( 1959j. (11) Skeggs, L. T., Am. J . Clin. Pathol. (4) Fiske, C. H., Subbarow, Y., J . Biol. 28,311-22 (1957). Chem. 66, 375-400 (1925). (12) Van Wazer, J. R., in “Encyclopedia (5) Jones, R. T., Swift, E. H., ANAL. of Chemical Technology,” vol. X, pp. CHEM.25, 1272-4 (1953). (6) Lundgren, D. P., Loeb, K. P., Division 403-510, Interscience Encyclopedia, of Bnalytical Chemistry, 136th Meeting, New York, 1953.
ACS. Atlantic Citv. ” , N. J.., SeDtember I
1959: (7) Marsh, W. H., Fingerhut, B., Kirsch, E., A ~ J ,, clin. pathol. 28, 681-8 (1957). (8) Martin, J. B., Doty, D. hI., ASAL. CHEM. 2 1,965-7 (1949).
Spectrophotometric Determination of
RECEIVEDfor review April 27, 1959. Accepted February 9, 1960. Division of Analytical Chemistry, 135th hleeting, iiCS, Boston, Mass., April 1959.
FIuoride
with Thorium Chloranilate A. L. HENSLEY and J. E. BARNEY
Ill
Research and Development Deparfmenf, Standard O i l Co. (Indiana), Whifing, Ind.
,A new spectrophotometric method for determining fluoride i s based on the reaction with thorium chloranilate in aqueous methyl Cellosolve, buffered at pH 4.5. Sensitivity i s varied b y measuring absorbance a t either 540 or 330 mM, and b y varying the amount of methyl Cellosolve in the solvent. The method has been tested b y analyzing waters and catalysts. Most o f the common anions do not interfere, and many of those that do can be removed with ion exchange resins. The new method i s as sensitive as widely used colorimetric methods based on the bleaching of a complex b y fluoride ion; in addition, a wide concentration range may b e covered and frequent recalibration is not required.
D
the past f e r years many methods have been proposed for determining microgram amounts of fluoride, and several older methods have been improved. The reasons for this have been the increasing use of fluorides in water treatment t o reduce the incidence of dental caries, their use in nuclear chemistry research, and their use as catalysts in petroleum processing. Widely used classical methods for fluoride based on the distillation of fluoride as fluosilicic acid ( I S ) and titration with thorium nitrate in the presence of a suitable indicator (12) are tedious, URING
1 Present address, Spencer Chemical Co., Kansas City, Kan.
828
ANALYTICAL CHEMISTRY
and require frequent calibration and strict adherence to empirical conditions. Generally, they cover only a narrow concentration range or are not sensitive enough for some applications. New and improved colorimetric and spectrophotometric methods have been reviewed ( 9 , l I ) ; less widely adopted techniques are turbidimetric titration (4), aniperometry (6, ?’), and electron-transfer catalysis ( 1 ) . Coloiimetric and ultraviolet spectrophotometric methods, based on the reaction of metal chloranilates with the anion to form the strongly absorbing acid chloranilate ion, should permit simpler calibration procedures, require less skill, and be free of most empirical factors. Selected metal chloranilates have been used for determining sulfate, 5,8). Methchloride, and fluoride (W,S, ods for fluoride have employed strontium cliloranilate (3) and lan-
thanum chloranilate (6); the former did not give enough sensitivity, and the latter, although more sensitive, n as still not as sensitive as methods based on bleaching of a colored complex. Thorium chloranilate, a new reagent developed in the authors, laboratories and available from Fisher Scientific Co., has been made the basis of a sensitive spectrophotometric method for determining fluoride. Selection of the thorium salt was suggested by the known great stability of thorium fluoride complexes. I n this method, fluoride reacts with thorium cliloranilate in buffered aqueous methyl Cellosolve, probably t o form both thorium fluorochloranilate and acid chloranilate ion ; absorbance is measured a t 540 mfi in 1 to 3 methyl Cellosolve-water for high concentrations of fluoride, and a t 330 mfi in 1 to 9 methyl Cellosolve-water for low concentrations. Most interfering cations can
F i g u r e 1. E f f e c t o f methyl Cellosolve on development of color
c-
-2 3
I
ME.*O’JRS
4
be removed with ion exchange resins. Sensitivity is controlled by proper combination of wave length and methyl Cellosolve concentration. The limit of detection (based on a 0.005 absorbance unit increment) is 0.01 pap.m. of fluoride in the original sample. METHOD
Two calibration curves are necessary to cover the concentration range from 0 to 10 mg. of fluoride per 60 ml. of water. TO make the curve covering the range from 0.2 to 10 mg. of fluoride, add the appropriate amounts of standard sodium fluoride solution, 25 ml. of methyl Cellosolve (Union Carbide Chemical Go.), and 10 ml. of a buffer 0.1M in both sodium acetate and acetic acid to a 100ml. volumetric flask. Dilute to volume with distilled water and add about 0.05 gram of thorium chloranilate. 2 4 I
,
1
r I
1
Shake the flask intermittently for 30 minutes and filter about 10 ml. of the solution through a dry Whatman KO.42 filter paper. Read the absorbance of the filtrate in 1-cm. cells a t 540 mp versus a blank, prepared in the same manner, using a suitable spectrophotometer. Make the calibration curve for the concentration range from 0.0 to 0.2 mg. of fluoride in the same way, but add only 10.0 ml. of methyl Cellosolve and measure the absorbance of the filtrate in a 1-em. silica cell a t 330 mp. Treat fluoride samples in the same manner as described for making the calibration curve after removing interfering ions and adjusting the pH of the solution t o between 4 and 7 with dilute nitric acid or sodium hydroxide. RESULTS
Precision and accuracy were established by analyses of standard sodium
' I /
1
1
'
I
~
I
'
I
I
METHYL CELLOSOLVE WATER
'=LUORIDE,mg / I O O m l
Figure 2.
Effect of methyl Cellosolve on sensitivity
- i t
L-*AVE
-Eb
C'ri
m u
Figure 3. Absorption spectra of chloranilate complexes
fluoride solutions. The method was applied to n-aters and to catalysts. All measurements were made with a Beckman DU spectrophotometer. Table I shows replicate analyses of six qtandard solutions containing 0.5 to 50 p.p.m. of fluoride as sodium fluoride. The results show a coefficient of variation of 1.5% and an error of 1.3%; no significant difference in precision or accuracy mas observed a t either wave length. Samples of two fluoride-treated municipal waters and one natural well water were anaIyzed by the spectrophotometric method under three different conditions; results were compared with those obtained using a referee method ( l a , IS) (Table 11). Agreement among the methods is as good as the precision of the methods, and these municipal waters can be analyzed for fluoride without removing interfering ions. Results of the analyses of six cata-
Table
I.
Added 0.50 1.00 1.50 2.00 20.0 50.0
Analyses of Standard Sodium Fluoride Solutions (Fluoride, p.p.m.)
Founda 0.50, 0.49, 0.49, 0.51 1.00, 0.99, 1.00, 1.02 1.48, 1.50, 1.49 2.00, 1.97, 2 . 0 2 20.2, 20.0, 20.5, 21.0 49.5, 49.0, 49.6, 51.0
Absorption maximum at 330 rnw used for first four samples; 540 mp for last two.
lysts for fluoride by this method and by the referee method are shown in Table 111. I n both methods fluoride n-as separated from the catalysts by distillation ( I S ) . Agreement between the two methods is as good as the precision of the methods. DISCUSSION
Methyl Cellosolve serves t v o purposes in the method. It accelerates the reaction of fluoride with thorium chloranilate. Figure 1 shows that, a t 330 mp, about 5 hours are required t o develop the color fully if methyl Cellosolve is not present, and 1 hour if it is. Furthermore, a reaction time of 30 minutes may be used because the color is 90% developed in this interval. Methyl Cellosolve greatly increases the sensitivity of the method m-ithout greatly increasing the absorption of the blank. Figure 2 shows that methyl Cellosolve increases sensitivity a t 330 and a t 540 mp Maximum sensitivity is obtained at both wave lengths in 1to 1 methyl Cellosol1.e-wter. Hon-ever, to cover the range of 0 t o 10 nig. of fluoride without aliquoting and to minimize high absorbance readings, 1 to 3 methyl Cellosolve-n-ater was selected for use a t 540 mp, and 1 to 9 methyl Cellosolven-ater, a t 330 mp. Sensitivity of the method a t 330 mp could be increased twofold if required by using 1 to 1 methyl Cellosolve-water. I n the ultraviolet region, the absorption spectra are the same in water and aqueous methyl Cellosolve. I n the visible region the maximum occurs a t 530 mp in aqueous solutions (2, IO), but it shifts to 540 mp in aqueous methyl Cellosolve solutions. This wave-length shift can be esplained by postulating a different reaction in aqueous methyl Cellosolve. This solvent is less polar than water, and it may promote the formation of soluble thorium fluorochloranilate rather than a thorium fluoride compleu. Thorium chloranilate may ionize in stepwise fashion:
+
+
Th(CsClzO4),(~) H + ThCsClz04+'
+ HCeC1204-
VOL. 32, NO. 7, JUNE 1960
829
Table 11.
Analyses of Treated and Natural Waters
Target Level 0.95
Source Chicago, 111. Gary, Ind.
0.80
Western Springs, Ill. (well)
...
(Fluoride, p.p.m.) Colorimetric .IS Ion received exchanged 0.96 1.02 0.95 1.03 0.95 1.01 0.98 1.01 0.98 0.99 1 .00 1.00 0.46 0.49 0.47 0.49 0.47 0.50
Distilled 1.02 0.98
Referee l\lethoda 0.94 1.03
0.96 0.98
0.97 0.97
0.49 0.50
0.54 0.52
lated for water and methyl Cellosolvewater, or, less likely, formation of other thorium-fluorochloranilate complexes. The p H of the solution must be belo\v 5.0 because a t this or higher values hydroxide ion reacts with thorium chloranilate to form insoluble thorium hydroxide and chloranilate ion. Below pH 3 a considerable amount of unionized chloranilic acid is formed and the molar absorptivity decreases. The buffer 0.1.11 in both sodium acetate and acetic acid was chosen because it has good buffering capacity in the desired pH range and does not interfere.
a Distillation and titration with thorium nitrate (13) and Eriochrome Azurole S as indicator (12).
INTERFERENCES
ThFzCsClz04 Analyses of Catalysts (Fluoride, per cent) SpectroType of photometric Referee Catalyst hlethod Method Silica-alumina 3,6T, 3.70 3.67, 3.69 3.98, 4.03 4.03, 4.12 Alumina 4 47, 4 42 4 42, 4 45 1 04, 1 06 1 03, 1 10 0 68, 0 69 0 69, 0 70 0 32, 0 32 0 32, 0 31
+ 4F- + H +
Table Ill.
ThFs-'
+ Hc6Cl?Oa-
Addition of thorium ion causes the maximum a t 540 to shift t o 550 mM, nhere a maximum occurs in the visible region for the thorium chloranilate complex ion (ThCGC1:04"2). The propoqed reaction between thorium ion and the complex is : HCsClzO4-
+ Th+,
+
ThCsC1~04+~ H C
The visible absorption spectrum should be the same for ThFzC6Cl2O4and ThC6Clz04+2because in both caqes the and chloranilate ion causes the absorbance. ThCsCl~04+~H + * Th-4 HCsClz0,Thus, in the reaction of fluoride n i t h thorium chloranilate in aqueous methyl If water is the solvent, the reaction Cellosolve solutions, the absorption peak a t 540 mp is produced by ThF2C&1204, Tht4 6F- e ThFs-' absorbing a t 550 m p , and HCGC1?04-, may predominate. I n aqueous methyl absorbing a t 530 mp. Cellosolve the reaction I n the second euperiment, the proposed reaction mechaniqm and the strucThCeClzOa+* 2F- e ThFzC6CIzOr tures of the complex species nere confirmed by measuring molar absorptiyimay predominate. Thus, in water the ties. Table IT' shows molar absorptiviover-all reaction of thorium chloranilate ties of the acid chloranilate ion, the with fluoride would be : thorium chloranilate complex, and the Th(CsClzO4)n(~) 6F2HT e reaction produced of fluoride n ith solid ThFs-' 2HCsC1204thorium chloranilate, in several solvent mixtures. The absorptivity for the I n aqueous methyl Cellosolve, the preacid chloranilate ion is three times that dominant reaction with fluoride would for the reaction product of fluoride and be : thorium chloranilate. Thus fluoride must react with thorium chloranilate in T ~ ( C G C I Z O ~ ) Z 2F(S) H+ + ThFzC6CLO4 HC~CIz0~- water to form the thorium fluoride complex, ThF6-2, and the acid chloranilate ion, HCsCl2O4-, because three fluoride I n the first of two experiments perions produce one acid chloranilate ion. formed to test this hypothesis, a soluI n 1 to 1 methyl Cellosolve-water, the tion containing 50 p.p.m. of fluoride was absorptivity for the reaction product is reacted with thorium chloranilate in intermediate betn-een the values for buffered 1 to 3 methyl Cellosolre-water. the acid chloranilate ion and for the To one third of the filtrate \vas added thorium chloranilate complex ion, indisolid thorium nitrate, to another third, cating that both thorium fluorochloranisolid sodium fluoride, and to the other late (ThFzCsC1204)and the acid chlorthird, nothing. Absorption spectra obanilate ion are present, as predicted. tained near 540 mp with these solutions The molar absorptivities of the reaction are shown in Figure 3. Addition of product in 1 t o 3 and 1 to 9 methyl fluoride ion causes the maximum a t 540 Cellosolve-water indicate either comto shift to 530 nip; the proposed reaction is : petition between the reactions postu-
+
+
+
+
830
+
+
+
+
+
+
ANALYTICAL CHEMISTRY
Cations may interfere n i t h the reaction of fluoride n i t h thorium chloranilate by forming insoluble chloranilates, by forming strongly absorbing complex ions with chloranilic acid, or by forming complexes with fluoride. To study these interferences, 20 p.p.m. of 18 cations were added to separate solutions containing 1 p.p.m. of fluoride, and the fluoride was determined. Silver, calcium, barium, magnesium, sodium, potassium, and ammonium ions did not interfere. Interferences produced by other cations Ivere: Cation Cd Sn Sr Fe Zr
Fluoride Present, % f 7
f
7
- 7 10 - 33 38
Xi Zn
+ +- 46 ++ 6082
A1
+210
co Pb
cu
+ 105
The ion exchange resin used in the method will remove all except aluminum and zirconium. If they are present, fluoride should be separated by distillation. Anions may interfere by reacting Ti-ith thorium ion to form slightly dissociated compounds or stable complex ions. Anions a t a concentration of 50 p.p.m. were studied for interference a t 540 nip in the determination of 50 p.p.m. of fluoride, and a t a concentration of 1 p.p.m. a t 330 mp in the determination of 1 p.p.m. of fluoride. Those m-hich did not interfere nere studied a t a tn-entyfold increase in anion concentration and those which interfered seriously mere also studied a t either wave length a t one-twentieth concentrations of anion. Chloride, bromide, iodide, borate, sulfite, sulfate, nitrate, and acetate did not interfere under these conditions. Phosphate, molybdate, citrate, and tartrate interfered seriously TT hen they were present in amounts equal to the fluoride concentration. Kevertheless,
distillation ( I S ) can be used to separate fluoride from these anions.
Table IV.
Molar Absorptivities a t
Species -4cid chloranilate ion
CONCLUSION
Fluoride in aqueous solutions can be determined by this method in one hour. Sensitivity can be increased over twofold by using the higher proportion of methyl Cellosolve. The method compares favorably in sensitivity and accuracy m-ith the classical distillationtitration method. It is superior to it because it is more rapid, a wide concentration range may be covered n-ithout aliquotting, and frequent recalibration is not required. LITERATURE CITED
(1) .irmstrong, TV. D., Singer, L., A N ~ L . CHEY.26, 1047 (1954). 1 2 ) Bertolacini, R. J., Barney, J. E., Zbid., 29, 281'(1957). ' (3) Ibid., 30, 202 (1998).
__
Solvent Kater, 1:9, 1: 3, or 1: 1 methyl Cellosolve-water Water 1: 1 methyl Cellosolve-water Water 1:9 methyl Cellosolve-water 1: 3 methyl Cellosolve-water 1: 1 methyl Cellosolve-water
Thorium chloranilate complex Reaction product of fluoride ion with thorium chloranilate
(4) Brandt, W.TV., Dusmalt, A. A, Ibid., 30, 1120 (1958). (5) Fine, L., Rynne, E. A, Microchem. J., in press. (6) Harris, IT. E., ASAL. CHEM.30, 1000 (1908). (7) Johannesson, J. K., Chem. R. Ind. (London)7,81 (1957). (8) Klipp, R' (8jpKlipp,' R.. -IT., IT:,Bahey, Barney, J. E., ANAL. CHEW31,596 (1999). (9) M$on, AI. G., Boltz, D. F., Ibid., 30,394 (1958). (10) Schlvarzenbach. Schlvarzenbach, G.. G., Suter. Suter, €1.. €I., Helv. ' dhim. Acta 24, 617 (1941). Chzm. I
330 Mp
,
dbsorptivity 24,900 27,500 31,000 8,500 12,500 16,000
29 200 ~
(11) Thatcher, cher, L. L., Xiser, R. T., A s . 4 ~ . CHEM.31, $1, 776 (1959). trd, H. H., Horton, Horton, C. A , , Ibid., (12) Killard, 22, 11001 (1950). trd, H. H., Winter, 0. B., (13) Willard, B , ISD.
