ature to make the solution more nearly ideal. I n Figure 1 the curves at 40" and 60" C. illustrate what is involved. With dry helium a t 40" C. leading is negligible but tailing is severe. When water is added to reduce tailing, the peak is unsymmetrical in the other direction. At 60" C. with wet helium the peak is fairly symmetrical. I n general, 60" C. is a good operating temperature for attaining symmetrical peaks. At higher temperatures the equilibrium amount of water on the column is too small to prevent tailing. At lower temperatures leading is common. The following solvents were found to give Gaussian peaks for alcohols and water a t 60" C. with wet helium: diisodecylphthalate, tricresyl phosphate, triethylene glycol, silicone fluid DC-550, 2,2'-oxydipropionitrile, and Ucon fluid DLB-19Ob. The solvent concentration was kept a t 10% except for silicone fluid, 40% of which was used. Under these conditions the time for 1-butanol (boiling point 118" C.) was a maximum of 40 minutes n-ith triethylene glycol. The 10% diisodecyl phthalate column was compared with one containing 30% solvent, but the latter offered no advantage to offset the disadvantage of longer time per analysis.
Certain solvents will still cause water and alcohol peak distortions at 60" C. with water in the helium. For example, leading occurred with dimethylsulfolane. When the temperature was raised the leading was improved, but the column no longer retained enough mater to prevent tailing. Presumably the relative humidity of the carrier could be increased but this involves another bath whose temperature must be closely controlled, or some other change to introduce water uniformly. If too much water is added, it will affect the over-all thermal conductivity detector sensitivity and also the relation between area per cent and weight per cent. Hydrocarbon and methyl silicone (SF96) solvents also require further consideration. It is difficult to balance the conditions to get symmetrical alcohol peaks with these solvents, as with dimethylsulfolane. I n addition, even ketones and aldehydes tail on these solvents. They are probably best avoided for polar samples, in view of the variety of solvents that are readily adaptable for these samples. CONCLUSION
Peak symmetry of polar materials
analyzed by gas-liquid chromatography may be markedly improved by adding a similar polar material continuously with the carrier gas and operating the column at the proper temperature. For lower alcohols and water, water can be added t o the carrier and the column temperature should be about 60" C. The amount of gas-liquid chromatography solvent can be kept low to reduce the emergence time a t 60" C. without affecting efficiency. LITERATURE CITED
(1) Dimbat,
Martin, Porter, P. E., Stross, F. H., ANAL.CHEM.28, 290 (1956). (2) Dintenfass, H. T., Kolloid 2. 151, 154 (1997). (3) Eggertsen, F. T., Knight, H. S., ANAL. CHEW30, 15 (1958). (4) Eggertsen, F. T., Knight, H. S., Groennings, Sigurd, Ibid., 28, 303 (1956). (5) Johns, Theron, Symposium on Gas Chromatography, Lansing, Mich., 1957. (6) Knight, H. S., ANAL.CHEM.30, 9 (1958). ( 7 ) Pierotti, G. J., Deal, C. H., Derr, E. L., Porter, P. E., J . Am. Chem. SOC.78,2989 (1956). (8) Tenney, H. M., AKAL. CHEW.28, 1 (1958). RECEIVEDfor review March 28, 1958. Accepted August 4, 1958.
Rapid Photometric Determination of Fluoride in Water Use of Sodium 2-(p-SuIfophenylazo)-1,8-dihydroxynaphthalene3,6-disulfonate-Zirconium Lake P. J. SCHOUBOE' Dental Public Health, U. S.
ERVIN BELLACK and Division of
A simple and rapid technique for determining fluoride in water samples gives an accuracy within 0.02 mg. per liter in the fluoride concentration range of 0.00 to 1.40 mg. per liter. Its speed and relative tolerance to interfering ions make it a potential substitute for present alizarin-zirconium photometric methods, as well as for spectrophotometric methods requiring special devices for handling interfering ions.
W
samples are usually analyzed for fluoride by one of the several alizarin-zirconium methods, either visuATER
1 Present address Division of Pharmacology, Food and brug Administration De artment of Health, Education, and WeTfare, Washington, D. C.
2032
ANALYTICAL CHEMISTRY
Public Health Service, Washington,
D. C.
ally or by using a photoelectric colorimeter (1). These methods have one serious drawback in that color development is progressive, requiring accurate timing to achieve consistent results. The waiting period is also a source of constant annoyance. Recently, more rapid methods have been developed, but these usually require the use of a spectrophotometer and are particularly sensitive to certain interfering ions (7, 9 ) . This sensitivity necessitates distillation for most determinations, or, as an alternative, requires an extra step for precipitation of, or compensation for, the interference. The method proposed can be applied directly to most water samples without prior distillation or other pretreatment, giving fluoride values with precision
and accuracy with no waiting period. Because the reagent has high sensitivity to fluoride ions over a comparatively broad range of wave lengths, either a spectrophotometer or a filter photometer can be used for the determination. Sodium 2-(p-sulfophenylazo)-1,8-dihydroxynaphthalene - 3,6 - disulfonate (SPADKS) has appeared in the literature as a zirconium and thorium reagent, and also as an indicator in the thorium titration of fluorides (2-4, 6). A search of the literature revealed no previous record of the use of the SPADKS-zirconium lake in the direct photometric determination of fluoride in water samples. The structural formula of SPADXS is given as:
.
7sori I,
Figure 1. Absorption spectra and standard curve for zirconium SPADNS reaction
-
~
,> I ! I
DIEFEREHTlAL
~~~~~~~
~~~
T
~
~
50:
I I B S O R B I N C E OF
;
, ‘
,400-
c e 2 Y
2
-1200
5 Y
0
L 4
e m ,200-
. l o o - IO0 M G / i
FLUORIDE A G I I N S T
R E F E R E N C E SOLUTION
l
,000
500
525
550 WAVE
575
600
LENGTH I M P 1
625
0
20.0
40.0
MICROGRAMS
600
80.0
F- IN 50.0 M L .
100.0
o
Standards should be selected in the range of 0.000 to 0.070 mg. of fluoride in 50.0-ml. aliquots (0.00 to 1.40 mg. per liter). As the resultant curve is a straight line of negative slope between these figures, fluoride value can be calculated on the basis of a pair of standards run concurrently with the unknown sample. Where A. is the absorbance of a 0.00 fluoride standard, Ai is the absorbance of a 1.00 mg. per liter standard, and A , is the absorbance of the unknown sample, the fluoride value in milligrams per liter of the sample is:
- Az x = A0 ___ Aa - A I
Calculating fluoride values on the basis Apparatus. Beckman Model B of a t least one pair of standards per day spectrophotometer, or equivalent, and eliminates, to some degree, the necessity 10-mm. matched cuvettes, or KlettIYaOsSC b ; t a - o o 8 N a for adjusting the temperature of the Summerson photoelectric colorimeter, sample to that of the standard curve, as or equivalent, with rectangular cunormally both standards and sample vette having 2-em. light path. will be at room temperature. Also, Preparation of Reagent. REAGENT It is also referred to as 4,5-dihydroxyany slight aging effect of the reagent 3-(p-sulfophenylazo)-2,7-naphthalenedi- A. SPADNS (0.958 gram) is diswill be taken into account. solved in distilled water and diluted sulfonic acid trisodium salt (Eastman Procedure for Water Sample. If t o 500 ml. the sample contains free chlorine, two Organic Chemicals, No. 7309) : REAGENT B. Zirconvl chloride octadrops of 0.1N arsenite solution are hydrate (0.133 gram) “is dissolved in added for each milligram per liter of about 25 ml. of distilled water. To the chlorine present. If other interfering zirconium solution are added 350 ml. of ions are present in sufficient quantity concentrated hydrochloric acid (reagent to produce an error of 0.1 mg. per liter grade, specific gravity 1.19), and the or more, or if analysis is unknown, the mixture is diluted to 500 ml. with dissample should be distilled. A 50.0-ml. tilled water. Equal volumes of Reawater sample, or aliquot diluted to 50.0 gents A and B are combined to produce PRELIMINARY STUDIES ml., containing no more than 0.070 mg. a single reagent. of fluoride (1.40 mg. per liter), is adReference Solution. Ten milliliThe objectives for the proposed justed to the temperature of the standters of Reagent A are added to 100 ml. reagent were the following: ard curve or standards (+2O C.), Ten of distilled water, and t o this are milliliters of mixed reagent, or 5 ml. Rapid reaction with fluoride ions plus added 10.0 ml. of a solution containing each of Reagent A and B, are added stability a t equilibrium. 7.0 ml. of concentrated hydrochloric to the sample and mixed well. Maximum sensitivity to fluoride ions acid diluted to 10.0 ml. with distilled The spectrophotometer is set a t zero nithin a broad range of fluoride conwater. This solution is used for setting absorbance a t 570 mr with the reference centration. the reference point (zero) of the specsolution, or the colorimeter is zeroed Minimum sensitivity to other ions trophotometer or colorimeter; it is similarly using a filter having a transnormally found in potable water, so that stable and may be re-used indefinitely. mittance range of 540 to 590 m p (Klett most water samples can be analyzed Preparation of Standard Curve. Xo. KS-56 or equal). The absorbance without pretreatment. The standard curve is prepared by reading of the sample is taken immediA color range which can be accurately subjecting standard solutions of fluoately or at any subsequent time. If the measured with a filter photometer, as ride to the following procedure and absorbance falls beyond the range of the well as with a spectrophotometer-i.e., plotting the absorbance values a t a standard curve or calculates to be higher sensitivity to fluoride extending over a standard temperature. than 1.4 mg. per liter, the fluoride concomparatively broad range of wave lengths. A simple procedure. Table 1. Effect of Interfering Substances on Fluoride Determination Some compromise regarding the range Interfering Concentration, Effect on F Reading at 1.00 Mg./L. of fluoride sensitivity had to be made Substance Mg./L. Increase, mg./l. Decrease, mg./l. in order to confine the range to that Acidity 3000 0.1 Alkalinity (as CaCOa) 5000 0.1 portion obeying Beer’s law, but otherAl+++ 0.1 0.15 wise the proposed method satisfactorily Arsenite 1300 0.1 conformed to the above objectives. Ca++ 800 0.1 Previous studies revealed that reacc17000 0.1 C 1 , (should be completely . . . If not removed* tion rate between fluoride ion and zireliminated) conium-dye complexes is influenced Color (should be low or ... May do either greatly by acid concentration, the reaccompensated for) tion being almost instantaneous in the Fe+++ 10.0 0.1 Mg++ 1250 0.1 region of 0.7N hydrochloric acid. Also, Mn++ 40 0.1 a t this acidity the interferences of P01--16 0.1 alkalinity, chloride, and ferric iron are (NaP03)6 1.0 0.1 largely eliminated and the sensitivity so,-; 200 0.1 Turbidity (should be t o fluoride ion is increased. On the clear) ... basis of Megregian’s experiments with Figure for immediate reading. Allowed to stand 2 hours, tolerance is 3.0 p. p. m.; Eriochrome Cyanine R, a molar ratio 4-hour tolerance is 30.0 p. p. m. of 1 zirconium to 4 of dye was selected * Reduce with arsenite. for preliminary studies (7). H
H
VOL. 30, NO. 12, DECEMBER 1958
2033
Table II. Sample Synthetic 1
Fluoride Determination in Water Samples (Values given in milligrams per liter) SPADXS Megregian-Maier Composition
1.20) Fdlkalinity (as CaCOJ) 150 (
1.22 1.20 501 1 19 951 Av. 1 20 0.75) 0.76 1001 0 75 0 76 Av. 0 76 0.50 0.05 2.0 0.49
so,-B
Sa+ F-
c1-
PO,--Na + C
FAl+T+
Fe+++
c1-
Unknown
Sa+
A
B C
...
1)
...
E F
...
centration is too high. I n this case, the procedure must be repeated using a smaller sample aliquot. If the presence of aluminum ion is suspected, the reaction is permitted to continue for an additional 15 minutes and another reading is taken. An appreciable drop in absorbance indicates the presence of aluminum ion as a n interference. At this point, holding the reacted sample for 2 hours before making the absorbance reading will eliminate the interference effect of up to 3.0 mg. per liter of aluminum; for 4 hours, up to 30.0 mg. per liter. Interference. Water samples subjected t o distillation by either t h e TTillard and Winter (IO) method or Bellack (5) modification can be analyzed directly by this procedure, with no neutralization, concentration, or buffering required. For water samples which are subjected to direct analysis the tolerance to various interferences is shovn in Table I. The error produced by a given quantity of interfering ion is not proportional to the quantity, so no mathematical compensation for interference may be made. Whenever the amount of interfering ion present is unknown, or any single ion or combination of ions is present in quantity great enough to produce an error of 0.1 mg. per liter or more, the sample must be distilled. Temperature Effect. The determinations may be carried out at any convenient temperature within t h e range of 15' t o 30" C. The important consideration here is t h a t standards and sample be a t nearly identical temperature, because a n error of approximately 0.01 mg. per liter of fluoride is caused by each degree difference in temperature. ANALYTICAL DATA
Reagent Stability. 2034
Batches of re-
ANALYTICAL CHEMISTRY
1.08 1.10 0.99 1.09 0.95 1.15
1.05 1.09 1.01 1.14 0.96 1.11
agents including samples of A alone and B alone, as well as t h e two mixed, were subdivided into several lots and stored under various conditions of light and temperature. Temperature varied from 14" to 30" C. and light varied from total darkness to normal room lighting. After 3 months of storage, no perceptible changes in absorbancy or sensitivity were evident under any of these conditions. Water Samples. T o test the applicability of t h e method t o water samples, a number of synthetic waters, prepared from distilled water and analytical grade chemicals, wew analyzed in triplicate. I n addition, unknown water samples received for routine analyses were subjected to parallel testing by both the SPADES method and that currently in use in the laboratory (Megregian-Maier method) (8). Results of these tests (Table 11) showed excellent accuracy and precision on the synthetic waters, and good agreement between the methods on the unknown samples.
SELECTION OF REACTION CONDITIONS
The reaction conditions investigated were acidity, ratio of dye to zirconium, quantities of dye and zirconium, and the optimum wave length for use. As mentioned previously, 0.7N hydrochloric acid mas chosen as the reaction acidity for preliminary studies. Varying the acidity from 0.2,V to 0.9N revealed that the greatest sensitivity for fluoride was attained at 0.5N. However, a t this point the interference caused by certain ions, particularly sulfate, was enhanced. Therefore, 0.7N was selected as the acidi%y giving the best compromise between sensitivity and tolerance to interferences.
Over the considerable range of dye to zirconium ratios studied, 4 to 1 provided the greatest fluoride sensitivity. Quantities of dye and zirconium, in a n acid medium of 0.7iYhydrochloric, a t the ratio described above, were also varied in an effort to extend the range of measurement and to provide the maximum sensitivity to fluoride. The optimum quantities yielded a reagent which followed Beer's law from 0.00 to 1.40 mg. per liter of fluoride. Khen quantities were varied to extend this range, sensitivity to fluoride or tolerance to interference decreased. In Figure 1 are presented curves for the differential absorbance of the zirconium-SPADXS reaction a ith 0.00 and 1.00 mg. per liter of fluoride. The maximum differential occurs a t 570 nip; therefore, this wave length was chosen for the spectrophotometer setting. As the sensitivity to fluoride extends over a comparatively broad range of nave lengths, most glass filters having peak transmittancy near the optimum wave length can also be used. One of these, the Klett KS-56, was used in these experiments. SUMMARY
d rapid photometric determination of fluoride ion concentration uses the reactions between zirconyl ions and SPADSS for color formation, followed by decolorization of the lake by fluoride ions, The reaction is immediate, stable, and follows Beer's lam in the concentration range of 0.00 to 1.40 mg. of fluoride per liter. This method, used in place of the usual alizarin-zirconium colorimetric methods, will save Considerable time and increase precision. It may be applied directly to most lvater samples without any pretreatment. LITERATURE CITED
(1) Ani. Public Health ASSOC., Am. Water Works Assoc., and iim. Federation of
Sewage and Industrial Wastes, New York, "Standard Methods for Examination of Water, Sewage, and Industrial Wastes," 10th ed., pp. 98-
107, 1955. (2) Banerjee, G., Anal. Chirn. Acta 13, 409-14 (1955). (3) Zbzd., 16, 56-61 (1957). (4)Ibzd., pp. 62-6. ( 5 ) Eellack, Ervin, J . Am. Water' Works A ~ S O50, C . 4, 530-6 (1958). (6) Hollingsworth, R. P., ANAL. CHEM. 29, 1130-3 (1957). ( 7 ) Ltegregian, Stephen, Zbzd., 26, 1161-6 (1934). (8'1 Megregian, Stephen, hlaier, F. J., J . Am. Wuter Works Assoc. 44, 239-48 (1952). (9) Thatcher, L. L., AKAL. CHEM. 29, 1709-12 (1957). (10) Killard, H. H., Winter, 0. B., IND. ENG.CHEX, ANAL.ED. 5 , 7-10 (1933).
RECEIVEDfor review March 19, 1958. Accepted August I , 1958.
