Modified Zirconium-Eriochrome Cyanine R Determination of Fluoride L. L. THATCHER
U. S. Geological Survey, Washington, D. C. The Eriochrome Cyanine R method for determining fluoride in natural water has been modified to provide a single, stable reagent solution, eliminate interference from oxidizing agents, extend the concentration range to 3 p.p.m., and extend the phosphate tolerance. Temperature effect was minimized; sulfate error was eliminated b y precipitation. The procedure i s sufficiently tolerant t o interferences found in natural and polluted waters to permit the elimination of prior distillation for most samples. The method has been applied to 500 samples.
F
in natural water has been studied extensively in recent years. Several colorinietric procedures, sonic adaptable to spectrophotometric measurement (4! E ) , have been published. For many years the Quality of Tl-ater Branch of the U. S. Geological Survey has been using the Lamar modification (1) of the zirconiumalizarin method introduced by Sanchis ( 5 ) . Khile this method is reasonably convenient and reliable for routine analytical purposes, it fails in the prescwce of scvernl cation and anion interferences, espccially that of phosphate. This necessitates frequent application of the Willard and \Tinter distillation ( 7 ) when brines, mine drainage, and industrially polluted n-aters are concerned. Furthermore, it has been difficult to adapt the zirconium-alizarin niethods successfull>- to spectrophotometric measurement. The photometric procedure of LIegreginn and Maier (3) requires careful adjustment of experimental conditions for consistent results. The Eriochrome C!anine R photometric method of llcgregian ( 2 ) seemed to be a consideral)lc improvement over the previous niethods reported in the literature; it \vas inwstigated for general suitability in quality-of-mtcr investigations, particularly with respect to reliability under adverse analytical conditions as encountered with polluted waters. One phase of the test program for the 1:riochrome Cyanine R method included annlysis by 12 Geological Survey laboratorics of standard fluoride solutions, using both the Lamar and the Megregian Eriochrome Cyanine R methods. Two LUORIDI: D L i E R x i x A n o i x
of the known solutions contained 5 and 25 p.pm., respectively, of phosphate. K i t h the standards containing no phosphate and 5 p.p.m. of phosphate, acceptable replication of results by both methods was obtained. For the standard containing 25 p.p.m. of phosphate, the results were widely scattered in both cases. Departures were both plus and minus, with a small positive bias for the mean value bJ- the Eriochrome Cyanine R method and no significant bias for the mean by the Lamar method. These results indicated the desirability of further improvement in the Eriochronie Cyanine R method. Investigative study indicated t h a t this mas possible. Initial tests showed that the quality of the Eriochronie Cyanine R rtagcnt was a significant factor in the performance of the procedure. The study of this effect and others led to the development of a procedure which provides the advantage of a single, stable reagent mixture, minimization of chromate and chlorine interference, improl-ed phosphate tolcrance, and extended concentration range. REAGENTS AND APPARATUS
Erioclironie Cyanine R stock solution. Dissolve 1.80 grams of Eriochrome Cyanine R (sodium salt of o-sulfohydroxydimethylfuchson dicarboxylic acid) in 200 ml. of distilled water. Zirconyl nitrate stock solution. Dissolve 0.40 gram of zirconyl nitrate dihydrate in 100 nil. of concentrated hydrochloric acid. Dilute to 200 ml. Fluoride indicator solution. To approximately 500 nil. of distilled water add 20.0 ml. of Eriochrome Cyanine R solution. With stirring add 10.0 ml. of the zircoiiyl nitrate solution. Add 75 ml. of concentrated hydrochloric acid (specific gravity, 1.18) and 4 grams of barium chloride. Dissolve and dilute to 1 liter. The reagent miyture is stable for 4 to 6 months. Thiosemicarbazide, powdered solid. Color correction solution. Dissolve 4 granis of barium chloride in 500 ml. of distilled water. Add 75 ml. of concentrated hydrorhloric acid and dilute to 1 liter. Beckman Model B spectrophotometer equipped with 40- or 50-mm. rectangular cells. PROCEDURE
Color
Development.
Measure
a
sample containing less t h a n 0.03 iiig. of fluoride into a 50-ml. graduate or cylinder. (Kormally a 10.00-ml. sample is used. This permits a maximum fluoride concentration of 3 p.p.m. I n case i t is necessary to take a smaller sample, make t h e volunie u p t o 10.00 ml.) Add a fen- milligrams of solid thiosemicarbazide from the tip of a niicrospatula and stir. 4 d d 25.0 nil. of the fluoride indicator solution. If the water contains sulfate a precipitate will develop. Place the samples near the spectrophotometer so that the precipitate will not be later dist'urbed by transport of the sample, and allow t o stand several hours or overnight. The samples may be clarified by centrifuging if an immediate answer is desired. For each significantly colored sample (see Interferences) make u p a correction sample by adding 25 nil. of the color correction solution to the same volume of sample as used in the analysis. These samples do not require volumetric accuracy. Prepare them and the blank a t the same time as the samples and allow to stand for the same length of time. Color Measurement. Determine t h e absorbance of t h e samples in t h e spectrophotometer using 40- t o 50mm. cells. T h e Photovolt or Colenian 40 X 20 mm. rectangular cells are also satisfacbory. They require approximately 25 ml. of solution t o intercept t h e light beam satisfactorily in t h e Beckman instrument. This volume can be easily decanted from the 35 ml. of sample solution ivithout disturbing the precipitate. Set the spectrophotometer to sensitivity 3 and position the blank (10 ml. of distilled water plus 25 ml. of reagent). Adjust the slit to give an absorbance reading of 0.50. This arbitrarily adjusts the absorbance of the zero fluoride standard t'o 1.50, which becomes the referenre point of the determination. Typical absorbance x-alues obtained a t 540 mu with 40mm. are: P.P.M. Fluoride Blank
Absorbance Reading
1.00 2.00 3.00
1.10 0 7.5
1.50 0 45
TT'hen color correction is necessary, measure the absorbance of the color correction sample against distilled water and subtract this value from the measured absorbance of the fluoride test sample. VOL. 29, NO. 11, NOVEMBER 1957
1709
DISCUSSION
Indicator Composition. Samples of indicator from different sources varied considerably in color. To investigate the effect of variation in composition on the reaction, t x o samples of indicator were selected for comparative tests. Reagent I was dark red and reagent I1 was reddish tan. The absorbances of the dyes and their zirconium complexes mere plotted from 350 to 650 mp and the results are shown in Figure 1. Reagent I gave the maximum absorbance difference a t 540 mp and reagent I1 gave the maximum difference a t 520 mp. Reagent I iTas used in all the folloxing tests except where noted and 540 mp was selected as the operating wave length. I n selection of the optimum ware length, the color density of the solution must be considered as \vel1 as the sensitivity. -4s the wave length moves toward the red, the color density of both sample and blank is less and lower sensitivity settings for the measuring instrument are possible. For example, r i t h the Beckman Model B spectrophotometer, a slit width (sensitivity 4) of 0.25 mni. was required a t 540 mp, whereas a t 560 mp the slit width mas only 0.07 mm. There Tvas a 15% loss in sensitivity, however. At 520 mp a slit width of 0.7 mm. is required. I n order to adapt the determination to filter photometers, it will be necessary in most cases to accept some loss of sensitivity and operate a t a longer wave length. Also, with most filter photometers the concentration range is limited to approximately 1 p.p.m. because it is necessary to restrict the blank setting to the relatively low absorbance readings of 0.50 or 0.75 for the sake of accuracy. Temperature Effect. T h e temperature of the solution is controlled essentially by the reagent temperature, inasmuch as the volume of reagent is 2.5 times greater than t h a t of the sample. T h a t the effect of variation in sample temperature is small a t concentrations of 1 p.p.m. and less is shown by the following results where t w o sets of standards mere adjusted to 32' C. and 11' C., respectively, and were then compared against a graph made a t 20' C. The determinations gave the following values.
1.00 2.00 3.00
1.00 1.95 2.85
1.00 2.05 3.07
After the samples reached room temperature the absorbance values were identical for the two sets. The simplest and most reliable method of correcting 1710
ANALYTICAL CHEMISTRY
t
350
t
I
I
I
I
I
I
I
400 450 500 400 450 500 550 WAVE LENGTH IN MILLIMICRONS
Figure 1. Absorbance curves of two Eriochrome Cyanine R reagents
-_ ., .
Table I.
Reagent A B C
D E F
ECR, Zr ECR, Zr, F
Effect of 20 P.P.M. of Aluminum with 2 P.P.M. of Fluoride Ml. of Acid Fluoride Found, P.P.M. per Liter of Reagent 2 hours 6 hou:J PH 8 1.3 0.6 20 0.9 1.5 40 0.6 1.5 1.8 60 0.35 1.5 1.9 0.15 100 1.7 1.95 200
