Automated Ion-Selective Electrode Method for Determining Fluoride in Natural Waters David E. Erdmann, U.S. Geological Survey, Lakewood, Colo. 80225
An automated fluoride method which uses AutoAnalyzer modules in conjunction with a fluoride ion-selective electrode was evaluated. The results obtained on 38 natural water samples are in excellent agreement with hhose determined by a similar manual method (average difference = 0.026 mg/l). An average fluoride concentration of 0.496 mg/l was found when several natural water samples were spiked with 0.50 mg/l fluoride. Aluminum is the only significant interfering substance, and it can be easily tolerated if its concentration does not exceed 2 mg/l. Thirty samples were analyzed per hour over a concentration range of 0-2 mg/l. Fluoride is one of the more commonly determined constituents in natural water; therefore, it is desirable to determine fluoride by a method which is not only accurate, simple, and relatively free from interference, but also rapid. Frant and Ross ( I , 2) introduced a fluoride-ion-selective electrode which exhibited a Nernstian response over a wide concentration range. A total ionic strength adjustment buffer (TISAB) was used to regulate the pH of the samples and also to provide an ionic strength which remains very nearly constant for the samples analyzed. The chemical interference from aluminum was greatly reduced when Hanvood ( 3 ) replaced titrate with cyclohexanediaminetetraacetic acid (CDTA) as a chelating agent. This paper describes an automated fluoride method which utilizes an ion-selective electrode. Carryover, reproducibility, concentrations found for known additions, and interferences are considered.
Experimental Apparatus. A Technicon sampler, proportioning pump, analytical cartridge, and recorder were used. The potentiometer incorporated into the above system was a prototype model supplied by Technicon Instruments Corp. It utilizes an Orion fluoride electrode (No. 94-09) and a Corning reference electrode (No. 476001). The small amount of sample required by the fluoride electrode assembly facilitates rapid response with a minimum of carryover from the previous sample. The assembly for the reference electrode is similar although the liquid reservoir is larger. Reagents. All chemicals were reagent grade unless otherwise specified. A stock fluoride solution (100 mg/l) was prepared by dissolving 0.2210 gram of NaF in distilled deionized water and diluting to 1 liter. Working standards were prepared by appropriate dilution of this stock standard. The TISAB solution was prepared by adding 58 grams of NaCl, 57 ml of glacial HCzH302, and 4.5 grams of CDTA to approximately 500 ml of distilled deionized water, and then slowly adding 5M NaOH with stirring and cooling until the pH of the solution was 5.0-5.5. When the solution reached room temperature, it was diluted to 1 liter with distilled deionized water and 0.5 ml of BRIJR-35surfactant was added. Results and Discussions The water sample and the TISAB solution, both of which are carried in a 0.051-in. pump tubing, are mixed, 252
Environmental Science & Technology
and the resulting solution stream is heated to 55°C before it enters the potentiometer. When this determination was considered, two approaches were taken. In the first, distilled water containing 0.2 mg/l of fluoride was used as the wash solution; consequently, 0.2 mg/l of fluoride was added through the sample tube during the rinse cycle. In the second approach the TISAE solution was spiked 0.2 mg/l of fluoride and, as a result, fluoride was continuously introduced into the system. A concentration of 0.2 mg/l of fluoride was added in both of these approaches with the intention of avoiding the slow electrode response normally encountered for low-level concentrations of fluoride. The potentiometer was adjusted in both procedures to give a full-scale reading for a 2-mg/l standard. The potentiometer output is very nearly linear over the 0-2-mg/l range. If conditions dictate, this range can be either expanded or contracted somewhat by adjusting the calibration control on the potentiometer. The spiked TISAB method proved to be superior, as shown in Tables I and 11, when considering reproducibility, low-level fluoride samples, and results obtained when consecutive samples differed greatly in concentration. Six replicates of each of 0.50- and 1.50-mg/l standards were analyzed with the first sample in each group preceded by a distilled water blank. As given in Table I, the results from the 0.50-mg/l standard were identical for both methods but they were considerably poorer for the unspikedTISAB method when the 1.5-mg/l standards were analyzed.
Table I. Fluoride Reproducibility Involving Six Replicate Samples (All concentrations are in mg/l) Standard concn
Range
Mean
Std dev
0.497 1.492
0.005 0.012
Spiked T I S A B 0.5 1.5
0.49-0.50 1.47-1.50
0.5 1.5
0.49-0.50 1.44-1.52
Unspiked T I S A B 0.497 1.488
0.005 0.027
Table 11. Effect of Electrode Response and Carryover on Two Fluoride Methods Apparent concn, mg/l Sample concn, mg/l
Spiked TlSAB
Unspiked TISAB
2.00 0.00 0.00 0.00 1.00 0.00 2.00 2.00 0.20 0.00 2.00 0.10
2.01 0.04 0.01 0.01 0.98 0.02 1.98 2.01 0.22 0.02 1.98 0.12
2.00 0.09 0.05 0.04 1.00 0.06 1.93 s2.00 0.25 0.06 1.94 0.17
Table V. Interference Study in Which All Samples Contained 1.00 mg/l of Fluoride Possible Interfering Substance
Table 111. Comparison of Manual and Automated Fluoride Methods Manual method,
mgll
0.24 0.14 0.25 0.32 0.31 0.28 0.44
Automated method, mg/W
0.26, 0.16, 0.28, 0.34, 0.30, 0.30, 0.44,
0.26 0.14 0.28 0.36 0.27 0.29 0.44
Manual method,
mgll
0.08 0.14 0.15 1.20 1.10 1.01
+
Automated method, mg/l
0*11,0.10 0.16, 0.14 0.20, 0.20 1.20,1.20 1.13, 1.17 0.98, 0.98
Duplicate analyses.
Table IV. Recovery of Fluoride Added to Natural Water
+
Concn of sample known addition, mg/l
Fluoride foundfluoride originally present, mg/la
0.75 0.65 0.77 0.84 0.78 0.79 0.94 0.60 0.64 0.69 1.68 1.63 1.46
Mean values Std dev
0.50 0.50 0.51 0.49 0.49 0.51 0.51 0.48 0.50 0.48 0.49 0.49 0.50 0.496 0.0103
% Of
Substance added
Fe Fe Fe Fe AI AI AI AI AI Pod-P
SiOn " V a l u e s are average of
Concn, mg/l
0.05
1.0 5.0 10.0 0.5 1.0 2.0 5.0 10.0 25 100
Apparent fluoride concn, mg/lu
1.00 1.00 0.99 0.99 0.98 0.99 0.98 0.94 0.89 1.01 1.01
determinations.
,
known addltlon, %
100 100 102 98 98 102 102 96 100 96 98 98 100 99.2
a Corrected for dilution factor.
Part of the results of a study involving possible errors due to sample carryover and electrode response is given in Table 11. The samples are listed in the same order as they were analyzed. When using unspiked TISAB, it is apparent that deviations from the actual concentrations are largely due to slow electrode response. This is especially true at low concentrations and when a sample of high concentration immediately follows one of low concentration or vice versa. The ability to overcome this problem by using spiked TISAB, basically a known addition method, was the reason that this automated procedure was adopted as the method of choice for determining fluoride. The carryover problem is not highly significant if fluoride concentrations are less than 2 nig/l. Thirty-eight natural water samples were provided by the U S . Geological Survey Laboratory in Salt Lake City. The specific cclnductances of these samples ranged from 24-45,000 pmho/cm at 25°C. The fluoride results obtained on these samples by the above automated method were compared with those from a manual ion-selective electrode method. I-Iarwood's ( 3 ) manual method was followed with the exception that the CDTA concentration in the TISAB was increased to 4.5 g/l. The fluoride concentrations ranged from 0.04-7.75 mg/l. The average difference between the two methods was 0.026 mg/l for these samples. The differences encountered between methods were comparable over the concentration range studied. The average difference between the duplicate runs for the automated method was 0.011 mg/l. A random sampling of these results is listed in Table 111.
These same 13 samples were also spiked with 0.50 mg/l of fluoride by adding two ml of 25 mg/l of standard to 98 ml of sample. The final concentration of fluoride in these solutions is therefore equal to 0.50 mg/l from the known addition plus 98% of' the original sample concentrations. Table IV lists the amount of fluoride found after correction for the original sample concentration. Silica, iron, and aluminum form complexes with fluoride and will interfere with this ion-selective method if CDTA is incapable of destroying these complexes. Consequently, standards containing 1.00 mg/l of fluoride were spiked with varying concentrations of aluminum, iron, and silica to determine if any of these would interfere with this determination. A standard containing 25 mg/l of POI-P was used to check for anionic interference. The results are given in Table v. Aluminum is the only ion which interfered and its effect is small if the aluminum concentration is less than 2 mg/l. A concentration of 5 mg/l of aluminum produces an error of approximately 6% a t the 1-mg/l fluoride level. This will not usually be a factor when analyzing ntstural waters because aluminum concentrations of this magnitude are rarely found. A 30-(6/1) cam was used with the Technicon sampler throughout most of i;his study. However, 40 samples per hour can be analyzed without substantial loss of accuracy if the concentration of fluoride does not vary greatly from sample to sample. 11, is necessary to add continuously a constant concentraticln of fluoride to the system to achieve reproducible and accurate results. In this way, low concentrations which produce much slower electrode response are avoided. The accuracy, reproducibility, and recovery of this method are excellent. Acknowledgment The author thanks Technicon Corp. for supplying the potentiometer prototype, and Martin Topf, Ralph Pirritano, Marvin Fishman, and Oliver Feist, Jr., for their help and suggestions. Literature Cited (1) Frant, M. S., Ross, J.W., Jr., Science, 154, 1553-5 (1966). (2) Frant, M . S., Ross, J. W., Jr., Anal Chem., 40, 1169-71 (1968). (3) Hanvood, J. E., Water Res., 3,273-80 (1969).
Received for review July 12, 1974. Accepted Nou. 21, 1974. Mention of commercial products does not constitute endorsement by the U.S. Geological Survey'or any other branch of the U.S. Gouemment.
Volume 9, Number 3, March 1975
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