megregian1954

V O L U M E 26, NO. 7, J U L Y 1 9 5 4 1161

The final correction curve may now be constructed by plotting the values of function g from Table I for the most appropriate geometry against the values of x obtained by dividing the cor- responding values of X by the value of k determined in c. From the curve thus obtained the proper self-absorption correction factor mav be read for any thickness. Dividing the specific activity o f the sample by this factor, one obtains a fairly reliable estimate for the specific activity a t infinite thinness.

DISCUSSION

Several authors (1) obtain satisfactory agreement bebeen their experimental data and self-absorption curves of the type ( 1 - e-kz)/kx in both narrow- and large-angle geometries. Ac- cording to the authors’ results, this is due to the circumstance that curves similar in shape are obtained in all geometries, but different k-values would be required if this simple formula were used in different geometries. Figure 1 shows that k cannot be uniquely defined without taking the geometry into account. This is confirmed by Figure 2, which shows that almost identical curves may be obtained with two different geometries by merelv doubling the value of k . The zero geometry curve, (1 - e+’.)/ k’z, with k‘ = 212 and the 50% geometry curve, F(kx) = g(0.5, kz), are practically coincident for kx > 3, and do not differ any- where by more than 10%. Thus, if the above self-absorption curve is used, which is the authors’ zero geometry curve, values of k up to twice as large as the actual k-values had to be used. Using this procedure the same value, k = (0.25 zk 0.01) sq. cm. per mg., is obtained from both the narrow- and large-angle geometry data. This value is in satisfactory agreement with the value k = 0.26 sq. cm. per mg. obtained from the empirical formula for light absorber elements, k = 0.022/Em1.33, as quoted by Siri (3 ) ( E , = 0.156 mev for carbon-14). Figure 1 illus- trates the fit between the experimental data and the theoretical curves with the above choice of k .

The procedure proposed here may seem somewhat more com- plicated than those previously employed, but it appears to be more consistent and, in the authors’ experience, more precise.

The error in the extrapolation to zero thickness which is inherent in all such procedures is still present but, to a certain extent, reduced by the possibility of adjusting experimental curves obtained in different geometries to the theoretical curves through the proper choice of the absorption coefficient, k . Standard deviations from 1.2 to 6.8% were obtained in three entirely different series of experiments in which different samples, mountings, and instruments were used. I t would be desirable to take back- scattering and self-scattering into consideration. However, the method appears practicable even without these corrections, and preferable to previously used methods for carbon-14 counting in organic substances.

ACKNOWLEDGMENT

The authors are indebted to William M. Stokes of this laboratory for the experimental data on cholesterol digitonide.

LITERATURE CITED

(1) Baker, R. G., and Katz, L., Arl’ucZeonics. 11, No. 2, 14 (1953). (2) Jahnke, E., and Emde, F., “Tables of Functions,” pp. 6-9,

(3) Siri, W. E., “Isotopic Tracers and Xuclear Radiations,” p. 58,

(4) Stokes, W. IM., Fish, W. A., and Hickey, F. C., J . Biol. Chem.,

(5) Yankwich. P. E., Norris. T. H. and Huston. J., A x . 4 ~ . CHEM..

New York, Dover Publications, 1945.

New York, hlcGraw-Hill Book Co., 1949.

200, 683 (1953). . ,

19, 439 (1947). (6 ) Yankwich, P. E., and Weigl, J. W., Science, 107, 651 (1948) (7) Yankwich, P. E., and Weigl, J. W., as quoted by Calvin, M.,

Heidelberger, C., Reid, J. C., Tolbert, B. &I., and Yankwich, P. E., “Isotropic Carbon,” p. 305, New York, John Wiley 8: Sons, 1949.

(8) Ibid., p. 318.

RECEIVED far review December 11, 1953. Accepted Bpril 22, 1954. Study supported by grants-in-aid from the American Heart Association and the American Cancer Society upon recommendation of the Committee on Growth of the National Research Council.

Rapid Spectrophotometric Determination of Fluoride With Zirconium-Eriochrome Cyanine R lake STEPHEN MEGREGIAN Division o f Dental Public Health, Public Health Service, Washington, D. C.

A simple, rapid technique which can be applied directly to water samples or to fluoride distillates usually gives results within 5 minutes with an accuracy within 0.02 mg. per liter of fluoride in the fluoride concentration range of 0.00 to 1.40 mg. per liter. It can replace the thorium titration for distillates in which the recogni- tion of the thorium-alizarinend point is often very diffi- cult. With the exception of sulfate, interference due to ions normally present in water is largely eliminated. A simple procedure for the correction of sulfate interference is included.

IOLOGICAL samples, which are analyzed for fluoride con- B tent, are usually ashed, and the ash is subjected to the Willard and Winter (11 ) distillation in order to remove the fluoride from interfering substances. The fluoride in the distillate is usually determined by thorium titration. This requires concentration of the distillate, careful buffering, and a tedious titration in which

the thorium-alizarin end point must be approached with great care.

Water samples are usually analyzed directly for fluoride by one of many colorimetric methods, all of which are based on the complex-forming property of the fluoride ion with various cations. These colorimetric methods are all subject, in varying degree, to the many interferences normally present in water and lack either rapidity or accuracy, or both, when applied directly to water samples.

The method proposed, when applied to distillates, eliminates the necessity for further manipulation of the distillate once it has been collected. The fluoride value of the distillate can be obtained with great precision and accuracy within 5 minutes following the completion of the distillation. If used directly on a water sample, only the sulfate concentration need be known and a correction applied. The total time required to obtain both fluoride and sulfate readings can usually be limited to 10 minutes.

Eriochrome Cyanine R is listed as No, 722 in “Colour Index”

1162 A N A L Y T I C A L C H E M I S T R Y

(6) and is the sodium salt of o-sulfohydroxydimethylfuchson di- carboxylic acid. Its structural formula is given as:

,COONa

'COONa

Thrun (9) developed a colorimetric method for fluoride using the aluminum lake of Eriochrome Cyanine R at controlled pH. Richter ( 4 ) applied the aluminum lake to photometric measurement. Saylor and Larkin (6) used it as an internal indicator in the titration of fluoride with aluminum. Milton et al. ( 2 ) , used a closely related dye, Chromazurol-S, C.I. No. 723, as an indicator in the titration of fluoride with thorium. Revinson and Hartley (3) worked out a photometric method for fluoride using thorium and Chromazurol-S, and Willard and Horton (IO) compared many indicators for use in the titration of fluoride with thorium. They judged Eriochrome Cyanine R and Chrome- azurol-S to be inferior to Purpurin sulfonate and Alizarin Red S for this purpose. A search of the literature revealed no previous record of a reaction involving zirconium with Eriochrome Cya- nine R.

PRELIMINARY STUDIES

The Megregian and Maier ( I ) photometric reagent for determination of fluoride ion concentration was studied to determine the optimum conditions under which this reaction should be carried out. The results of this study indicated that the reaction between zirconium, alizarin sulfonate, and fluoride ions could best be carried out in the region of 0.9N hydrochloric acid. At this acidity, the reaction rate between zirconium and alizarin was increased, and the fluoride effect on the reaction was more pro- nounced. Also, the interferences of alkalinity, chloride, and ferric iron were largely eliminated, and the degree of sulfate interference was decreased. On the other hand, the reaction became very sensitive to slight temperature variation, and low concentrations (less than 1.0 p.p.m.) of phosphate increased the reaction rate between zirconium and alizarin. When both phosphate and sulfate were present, the interference due to a given concentration of sulfate was increased.

This study indicated that i t was possible to prepare a reagent which was more sensitive to slight differences in fluoride concentration, less subject to interferences, and required less time for reaction equilibrium to be reached. However, the advantages gained with this reagent were outweighed by the uncertainty imposed by the possible presence of trace quantities of phosphates in water samples and the necessity for close temperature control.

Using the knowledge gained from this study, a search was ini-

tiated for a compound other than alizarin which might undergo a color reaction with zirconium and fluoride ions without introduc- ing errors due to minor constituents normally present in water. Over 200 dyes and other organic compounds were screened. Table I lists the compounds which were found to produce a color with zirconium and indicates the influence of fluoride on the color produced.

The choice was narrowed doivn to Eriochrome Cyanine R and Chromazurol-S. The former was selected for further study because: the zirconium lake is soluble and stable, equilibrium between zirconium, Eriochrome Cyanine R, and fluoride ions is reached very quickly a t room temperatures, the dye, as purchased, can be used immediately without further treatment, and the change in absorbance per unit fluoride per centimeter of light path is very large

Reagents. All of the colorimetric methods proposed for the measurement of fluoride ion concentration have a common de- fect-namely, the fluoride ion is not directly involved in the primary color-producing reaction. In every method proposed to date, fluoride ion concentration is measured by the amount of decolorization of the product of two reactants, one of which can form a complex with fluoride ions and can thus be restrained from entering into the color-forming reaction. A satisfactory reagent for this type of reaction should have:

Maximum sensitivity to fluoride ions in the desired concentration range

A color iange which can he adequately measured by avail- able colorimetric or photometric instruments

Nonsensitivity to other ions or substances commonly occurring with fluoride in the test medium

A.

B.

C.

D. Conformity to Beer's law E. A low temperature coefficient F. G. Reagent stability H. Reaction stability a t equilibrium I.

rium J. A simple procedure

Sone of the published methods in current use conform to all of the above ideals. The propofied method has advantages A, B, C (except for sulfate), D, E, F, G, H, I and J.

Apparatus. Beckman Model B spectrophotometer or equivalent and 10-mm. matched cuvettes.

Preparation of the Reagents. REAGENT A. Eriochrome Cyanine R (1.800 grams) is dissolved in distilled water and di-

High buffer capacity a t the optimum pH of the reaction

A minimum time interval for the reaction to reach equilib-

luted to 1 liter. REAQENT B. Zirconyl chloride octahydrate (0.265 gram) or

zirconyl nitrate dihydrate (0.220 gram) is dissolved in about 50 ml. of distilled water. Seven hundred milliliters of concentrated hydrochloric acid (reagent grade, specific gravity 1.19) are added to the zirconium solution, and the mixture is diluted to 1 liter with distilled water.

Reference Solution. Ten milliliters of Reagent A are added to 100 ml. of distilled water, and then 10.0 ml. of a solution prepared by diluting 7.0 ml. of concentrated hydrochloric acid to 10.0 ml. with distilled water are added. This solution is used for setting the reference point (zero) of the photometer.

Reagents A and B cannot be combined as a single reagent, because prior mixing produces precipitation on prolonged storage.

Procedure for Willard and Winter Distillates. A 50.0-ml. portion of distillate, or aliquot diluted t o 50.0 ml., and containing no more than 0.070 mg. of fluoride (1.40 p.p.m.) is taken and adjusted to a standard temperature (&2O C.). Five milliliters

Blue intensi%es in pres- of Reagent A followed by 5.0 ml. of Reagent B are added to the sample and mixed well. The photom-

Alizarol Azurine ECAa Orange Red Marked Ppt . eter is set a t zero absorbance a t 527.5 mp. (range 525 to 530 mp.) with the reference solution and the

Chloranilic acid Pink Purple Very weak . . . . . . . . . . . . . absorbance of the sample recorded. C hromazurol-S b Orange Lavender Weak Marked Rapid Slow reaction reaction, ppt. The fluoride value of the sample aliquot is read

from a curve prepared by subjecting standard solu- Quinalizarin Orange Violet Marked Slow reaction, ppt. tions of fluoride to the procedure and plotting the Quinizarin-2-sulfonate Orange Red Marked Rapid reaction, ppt. absorbance values a t the standard temperature.

Standards should be selected in the range of 0.00 to 0.070 mg. of fluoride (0.00 to 1.40 p.p.m.). The resultant curve should be a straight line of

Table I. Compounds Producing Color Change With Zirconium Color in Effect of

Compound, (1 M1. 0.5% Color in 0.1N HC1 + Fluoride,

Alizarin Blue Sa Red Brown Marked Zr lake preci itates Alizarin Sapphire BLN" Blue

Benzopurpurin 4B Blue-violet Red Weak Ppt . Carmine Pink Lavender Marked Slow reaction, ppt.

Curcumin Yellow Orange Eriochrome Cyanine Rb Orange Lavender Marked Rapid reaction, ppt.

per 50 MI. S o h ) 0.1N HC1 9 Mg. Zr/L. 5 Mg./L. Remarks

Lighter blue Weak ence of F ions

a General Aniline & Film Corp., 435 Hudson St., New York, N. Y. b Geigy Co., Inc., 89 Barclay St., New York, N. Y.

V O L U M E 26, NO. 7, J U L Y 1954

negative slope. As stated previously, the color reaction is im- mediate and stable. There is no waiting period for color to de- velop, and readings can be taken immediately, or a t any other desired time without significant change in absorbance, provided the temperature is within the limits set forth. A new curve must be prepared for each fresh batch of Reagent A or B.

A 50.0-ml. water sample, or aliquot diluted to 50.0 ml. and containing no more than 0.070 mg. of fluoride (1.40 p.p.m.), is adjusted to the temperature of the standard curve (&2O C.). If the sample contains free chlorine, two drops of 0.1-V arsenite solution are added for each part per million of chlorine present. Five milliliters of Reagent A followed by 5.0 ml. of Reagent B are added to the sample and mixed well.

The photometer is set at zero absorbance a t 527.5 mp. (range 525 to530 mp.) with the reference solution and the absorbance reading of the sample is taken within 5 minutes after the reagents have been added. If the absorbance reading of the sample falls beyond the range of the standard curve, the combined sulfate and fluoride Concentrations of the sample are too high. In this case the procedure must be repeated with a smaller sample aliquot.

If the presence of aluminum ion is suspected in the water sample, the reaction is permitted to continue an additional 15 minutes and another reading taken. An appreciable drop in absorbance indicates the presence of aluminum ions as an interference. At this point, holding the reacted sample for 2 hours bpfore making the absorbance reading will eliminate the interference effect of up to 5.0 p.p.m. of aluminum. Or the water sample can be subjected to the Willard and Winter distillation and the distillate analyzed according to the procedure for distillates given.

The correction for sulfate concentration in the sample aliquot is made as follows: The fluoride value of the absorbance reading is obtained from the standard curve; this value is applied to Figure 3 and the sulfate error determined; the sulfate error is then subtracted from the first value to obtain the corrected fluoride value of the aliquot; and this value is multiplied by the aliquot factor to obtain the fluoride concentration of the original sample.

Procedure for Water Samples.

1163

L

.oo ,D? 6 5 0 20 0 "3 0 ( 3 0 B , " ~ , 4 I 1 I 8

' J \&.j wl": L E b 6 - H ,wJ> M C R 3 5 R A M S F h 50.0 "L

Figure 1. Absorption Spectra and Standard Curve for Zirconium-Eriochrome Cyanine R Reaction

Absorption Spectrum. The absorption spectra and a standard curve of the zirconium-Eriochrome Cyanine R reaction are presented in Figure 1. The spectral curve for the unreacted Erio- chrome Cyanine R (reference solution) is very near that of the zirconium lake. Therefore, this color system requires the inci- dent light to be of high purity a t the wave length of measurement. If a filter photometer is used for absorbance measurement, the filter should have a band pass no greater than 20 mp. a t 527.5 mp. The wave length of 527.5 mp. was chosen, because, as illustrated in Figure 1, the standard curve measured at that value is a straight line in the range of 0.00 to 1.20 p.p.m. of fluoride with only a slight deviation from 1.20 to 1.40 p.p.m. Also, the maximum differential exists at, this wave length.

The best temperature a t which the reaction should be carried out is in the range of 22 O to 28" C. Within this range a 3" C. differential between the temperature of the standard curve and that of a sample is equivalent to 0.01 p.p.m.

Temperature Effect.

of fluoride. Thus, readings which are made a t temperatures f 2 " C. from that of the standard curve are well within the standard deviation of the method.

The Willard and Winter distillates of ashed biological samples can be analyzed directly by this procedure. The complete analysis of a distillate need not take longer than 5 minutes. The distillate need not be neutralized or concentrated, and buffer must not be added. The only possible interference would be the presence of large quantities of volatile anions, such as nitrate or unprecipitated chloride, which would come over during the distillation and produce high results in the subsequent analysis. If this is suspected, a portion of the distillate can be neutralized to the phenolphthalein end point with LON sodium hydroxide and the neutralized aliquot taken for the fluoride analysis. The effect of large quantities of volatile anions on the zirconium-Eriochrome Cyanine R fluoride reaction is to in- crease the acidity a t which the reaction takes place, depressing the total color produced.

For water samples which are subjected to direct analysis, the tolerance to various interferences is shown in Table 11.

Interferences.

Table 11. Interferences Normally Present in Potable Waters

Concentration, %./Liter, Necessary t o Produce Fluoride Error

Equivalent to Interfering Substance 0.02 mg./liter F 0.10 mg./liter F Alkalinity (as CaCOd 3300 .0 Undetermined

3500 .0 Undetermined g:;++ i n n All n

Free chlorine Turbidity, color

_"..I

2 . 5 6 . 0 1 . 0 2 . 0

Reduce with arsenite ' Compensate

5 . 0 5

a When reaction is permitted to continue for 2 hours.

KO interference was noted with calcium ion concentrations of 200 p.p.m. or magnesium ion concentrations of 100 p.p.m. The effect of very large concentrations of bicarbonate or hydroxide ions (alkalinity) is to reduce the acidity of the reaction, thereby increasing the total color produced by the zirconium-Eriochrome Cyanine R system and resulting in low fluoride values. Very large concentrations of chloride ion influence the reaction in the same direction. Ferric iron in high concentration produces a color reaction with Eriochrome Cyanine R, thus increasing the apparent color of the system and resulting in low fluoride values. Sone of the above interferences is normally present in sufficient quantity in water supplies to interfere with the fluoride determination.

Phosphate and hexametaphosphate (also other polyphosphates) act on the zirconium-Eriochrome Cyanine R reaction in a way similar to fluoride, in that they complex the zirconium and inhibit the color forming reaction. Water samples containing phosphates in excess of that listed in Table I1 should be distilled prior to fluoride analysis.

Aluminum does not affect the primary color reaction a t the acidity specified. I t reacts with fluoride to form the complex, anion (A1 F6)--- similar to (Zr Fa)--. Therefore, if no fluoride is present, aluminum does not interfere. When both fluoride and aluminum are present in a sample which has been treated with the reagents, the fluoride is distributed between the zirconium and aluminum ions. Under the reaction conditions imposed, the zirconium slowly removes the fluoride from the aluminum complex and after a sufficient lapse of time (about 2 hours), the aluminum effect is no longer evident.

An interesting phenomenon occurs when aluminum is present in the sample as the metaaluminate ion-Le., when hydroxide alkalinity is present in the original water sample. Under these conditions. the interference effect of a given concentration of


aluminum on the zirconium-Eriochrome Cyanine R fluoride reaction is greatly reduced, and it disappears in 10 to 15 minutes. A logical explanation for this phenomenon would be that in solutions containing aluminum, fluoride, and hydroxide ions, the aldminum exists as the metaaluminate anion and does not complex with the fluoride present. When the reagent is added to this system, the fluoride, being in the ionic form, can immediately complex with the zirconium which has been added. By the time the aluminum ion has been released from the metaaluminate anion there is very little fluoride left; to form the aluminum complex. Therefore, it exerts much less interference on the zirconium- Eriochrome Cyanine R fluoride reaction. Advantage can be taken of this phenomenon when the analyst has many samples containing both aluminum and fluoride. Simply by treating the water sample with a few drops of 1.0s sodium hydroxide until the solution is alkaline and adding the reagents as in the regular procedure, the waiting time for elimination of aluminum interference can be reduced from 2 hours to 15 minutes,

’ I D O 0 100 200 300 100 $00

000

P P M 50,

Figure 2. Effect of Sulfate Interference

Free chlorine and other strong oxidizing agents attack Erio- chrome Cyanine R thereby reducing the total color of the system. They can be easily reduced with sodium arsenite. -4 slight excess of arsenite does not affect the fluoride reaction. If desired, arsenite can be incorporated in Reagent A.

Turbidity and color in the original sample can be compensated by the following technique: A 50-ml. duplicate of the water sample is taken and treated with 5.0 ml. of Reagent A. This is followed by 5.0 ml. of a hydrochloric acid solution of the same acidity as in Reagent B. The absorbance of this solution is subtracted from the absorbance of the original water sample, which had been treated with both reagents. The net absorbance is then used to determine the fluoride concentration.

SULFATE ERROR AND CORRECTION

Zirconium ions in acid solution complex with sulfate ions to produce the complex (ZrO(SO&-- anion (8) . iilthough the reaction conditions have been adjusted to reduce this effect, the zirconium sulfate complex is strong enough to prevent some of the zirconium from reacting with Eriochrome Cyanine R to produce the full color of the reaction. Therefore, the total color reduction produced in a water sample containing both sulfate and fluoride ions is the result of the sum of the two complex-forming reactions. Figure 2 shows how the presence of both fluoride and sulfate af- fects the fluoride reading. The data obtained in this figure were used to construct the nomograph in Figure 3, which can be used to determine the sulfate error in a given sample.

Figure 3 is used as follows: Obtain the fluoride reading of the Determine the sulfate concentration of the

Determine the fluoride error of the sulfate concen- sample aliquot, A . aliquot, B.

tration by running a line through A and B to C. Subtract the value obtained on line C from A to give the fluoride value of the aliquot. Multiply this value by the aliquot factor to obtain the fluoride content of the original sample. Lines B’ and C’ are ex- pansions of A and B and should be used when the sulfate concentration is in the range of zero to 100 p.p.m.

Any procedure requiring added manipulations to remove interferences would nullify the advantages gained by using a rapid method for determining fluoride concentration. Thus, sulfate might be removed from the sample by barium precipitation or by the usual distillation procedure, but the advantages gained in using the rapid method would be counterbalanced by the time required to effect a separation by these conventional procedures. Another possibility would be to diminish the sulfate effect on the reagent by adding a known amount of sulfate to the reagent. Khen this is done, the reagent becomes less sensitive to fluoride, and the measurable fluoride range is reduced. Large quantities of sulfate in the reagent render it practically insensitive to fluoride, and this cannot be counterbalanced by increasing the zirconium concentration of the reagent.

Fortunately a method exists whereby the sulfate concentration of a water sample can be quickly and accurately determined, and by use of Figure 3 a correction can be applied, which eliminates the necessity for time-consuming separations without materially sacrificing the accuracy of the rapid method. A slightly modified procedure based on Sheen’s ( 7 ) turbidimetric method for sulfate is described below.

Reagents. Barium chloride dihydrate crystals, reagent grade, 20 to 30 mesh.

Acid salt solution. Sodium chloride (240 grams) is dissolved in 900 ml. of water.

Twenty milliliters of concentrated hydrochloric acid are added and the solution is diluted to 1 liter.

Procedure. A 50-ml. water sample or aliquot which, when diluted to 50 ml., will contain no more than 5 0 mg. of sulfate (100 p.p.m.) is transferred to a 250-ml. beaker or Erlenmeyer flask. Ten milliliters of the acid salt solution are added and mixed. Approximately 0.4 ml. (measured dry with a small spoon) of barium chloride crystals are added. Rapid 81% irling of the container is begun immediately and continued for 30 seconds. The sample is allowed to stand for 5 minutes, after which the re-

A FLUORIDE R E A D I N G (ERM.)

C C’

t e

Figure 3. Sulfate Correction Nomograph

V O L U M E 26, NO. 7, J U L Y 1954 1165

Table 111. Zirconium-Eriochrome Cyanine R Reagent Stability

Fluoride Concentration of Standards, P.P.X. 0.00 0.50 1.00 1.40

Deviation from Standard Curve. P.P.11. Reagent Days Age'

1 hour 0.013 0,031 0.019 0.008 1 0.008 0.002 0.013 0 .027 3 0.008 0,010 0 006 0.004 w 0.008 0.004 0 015 0.000

42 0 008 0.023 0,019 0.004 67 0.013 0.038 0.008 0.036

16 o 002 0.002 0.013 n 004

Blank

AV. Composite urine

Av.

Table IV. Biological Samples Thorium Titration,

Sample P.P.R.1. 0 30 0.28 0.29 0.97 0.80 1.03 0.93

Composite urine, 0.50 p.p.m. F -added 1.45 1.39

Av. 1.36 1.40

Composite urine, +l.OO p.p.m. F- added 1.79

1.81 1.91

Av. 1.84

0.086 0.082 0,091 0.086

E.C.R. Method, P. P. RI . 0.05 0.04 0.045 0.91 0.91 0.95 0.92

1.13 1.13 1.13 1.13 1.41 1.36 1.38 1.38

1.83 1.81 1.83 1.82

0.085 0.086

0,085 0.083

sulting suspension is transferred to a photometer cuvette and the absorbance read at 525 mp.

The sulfate concentration of the aliquot is obtained from a standard curve, which is prepared by subjecting solutions of known sulfate concentration in the range of 0.0 to 5.0 mg. (0.0 to 100 p.p.m.) to the above procedure.

ANALYTICAL DAT4

Reagent Stability and Precision. Replicate portions of four standards containing 0.00, 0.50, 1.00, and 1.40 mg. of fluoride per liter were taken on different days and treated with the same batch of zirconium and Eriochrome Cyanine R reagents. The daily absorbanre readings obtained for each standard were aver- aged, and the individual deviations from the averages were determined in terms of fluoride concentration without regard to sign. Table I11 is a summary of these results. Temperatures varied betneen 25" and 30" C. Using the data from Table 111. the

standard deviation throughout the range of fluoride concentrations taken was calculated to be 10.0163 mg. per liter of fluoride.

Urine and bone samples taken in tripli- cate were distilled, and the fluoride content of the distillates was determined according to the following procedure: Each distillate collected A Z ~ S diluted to 200 ml. One hundred milliliters were taken for analysis by the thorium titration procedure, and the remaining 100 ml. were analyzed by the zirconium-Erio- chrome Cyanine R method. The results for urine are expressed as the net fluoride concentration in mg. per liter after subtraction of the blank for the method used. The bone results are expressed as per cent of dry bone. The blank obtained in the zirconium- Eriochrome Cyanine R procedure includes the calcium oxide used in ashing the samples and the perchloric acid and silver per- chlorate used in the distillation; the blank obtained by the thorium procedure includes, in addition, that contributed by the buffer and the titration blank. A comparison of the two methods is shoa n in Table IS'.

Esamination of the individual results discloses the greater precision of the zirconium-Eriochrome Cyanine R method, and the close agreement between the two methods used. These results should not be interpreted in terms of fluoride recovery, since the distillation procedure is not a subject of this study.

I n order to test the applicability of the direct method to water samples, the following waters were analyzed in duplicate by distillation and by the zirconium-Erio- chrome Cyanine R direct method. The Britton, S. Dak., and Bartlett, Tex., samples were analyzed as submitted. The synthetic waters A and B were prepared in the laboratory from distilled water and standard solutions of the various ions. The mineral analysis of these waters is shown below in parts per million.

Britton, S. Dak. Bartlett, Tex. Synthetic A Synthetic B

Biological Samples.

Water Samples.

Total solids 2550 1660 . . . . . . Total alkalinity 260 390 32 32

so4 - - 1100 480-000 60 60 330 320 56 56

Total hardness 100 !SO 75 75

& + + 0.1 1.0 1.0 0.1-1.0 A I + + + . . . . . . 0.2 0.2 Si02 . . . 20 20 20 POa--- 3.0 3.0

Na + 860 530 79 79 F- 6 .0L6.8 7.618.5 0.45 1.15

Table V is a comparison of the fluoride values obtained by the two methods. The sulfate correction used in the direct procedure was determined by applying the sulfate concentration of the sample aliquot, obtained by the turbidimetric procedure, to the nomogram in Figure 3.

FACTORS INFLUENCING THE SELECTION OF REACTION CONDITIONS

The optimum concentrations of zirconium, Eriochrome Cya- nine R, and hydrochloric acid, for use as a fluoride reagent, cannot

Table V. Water Samples - E.C.R., Direct=

SO1-- concn. of aliquot

Distillation hll. sample F- concn.

distilled of hI1. sample (turbidi- Apparent Sod-- corr. F-, in Net (200 ml. distillate, Aliquot Net F- , (diluted metric), F-, (Figure 3), aliquot Aliquot F-,

Source collected) p.p.m. factor p.p.m. to 00 ml.) p.p.m. p.p.m. p.p.m. p.p.m. factor p.p.m. Britton. S. Dak. 25 0.81 8 6.48 5 109 0.92 -0.27 0.65 10 6.50

0.80 6.40 0.94 0.67 6.70 .4v. 6.44 6.60

Bartlett, Tex. 25 1.01 8.08 5 48 0.96 -0.13 0.83 10 8.30 1.03 8.24 0.97 0.84 8.40

Av. 8.16 8.35 Synthetic A ( F -

added = 0.45 p.p.m.) 100 0.24 2 0.48 50 59 0.58 -0.18 0.40 1 0.40

0.19 0.38 0.60 0.42 0.42 .4v. 0.43 0.41 Synthetic B (F-

added = 1.15 P.P.m.) 50 0.29 4 1.16 25 30 0.69 -0.10 0.59 2 1.18

0.30 1.20 0.67 0.57 1.14 Av. 1.18 1.16 a Corrected for sulfate.


(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.

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, i t 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).

I i '.\ I s

1 :

Figure 4. Effect of Reaction Acidity on Zir- conium-Eriochronie Cyanine R Reagent

Some of the color lost can be regained by increasing the Erio- chrome 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-

SUJIiMARY

A rapid spectrophotometric determination of fluoride ion concentration uses the reaction between zirconyl ions and Erio- chrome Cyanine R for color formation and the subsequent decolorization of the lake by fluoride ions. The reaction is immedi- ate 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

hlegregian, S., and Maier, F. J., J . Am. Water Works Assoc.,

Milton, R. F., Liddell, H. F., and Chirers, J. E., Analyst, 72,

Revinson, D., and Hartley. J. H., ASAL. CHEM., 25, 794-7

Richter, F., Chem. Tech. (Berlin). 1 , 84-90 (1949). Rowe, F. S., "Colour Index," Bradford, England, Society of

Saylor, J. H., and Larkin, 11. E., A N ~ L . CHEM., 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 Com-

pounds," 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.,

44,239-48 (1952).

43 (1947).

(1953).

Dyers and Colourists, 1924.

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-to- pointexcitation. . . "

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:

Inductance 90 ph. Capacitance 0.007 pfd.

LEO D. FREDERICKSOK, JR. J. R. CHURCHILL

megregian1954

Documents