group, The results were shown to be in the region of quantitative conversion of the tetramethylammonium salts to methyl esters by direct comparison of the peak areas obtained for caprylic and. pelargonic acids with those obtained for the corresponding methyl esters. These are shown in Table I. The convenjion figures for the remaining fatty acids are shown in Table I in parentheses. Separation of Mixtures of Mono- and Dicarboxylic Acids. In many potential appiications the mixtures of organic acids to be examined may contain both mono- and dicarboxylic acids. When chromatographed on a silicone column, individual members of these series overlap, so that C4-diester is eluted together with Crmono, C5-diester with Cs-mono, and so on. Chromatography on a polar column, while effecting a separation of most of these pairs, leads to inconveniently long retention times for the diesters compared with the monoesters. This (effectwas minimized, and a separation achieved, by use of a main column of silicone gum, followed by a polyester column just long enough to retard the diesters sufficiently to separate the overlapping peaks. Figure 2 shows a chromatogram of a mixture containing the Ca to C9monoesters and the C4 to Cs diesters.
CONCLUSION
The present technique has the same disadvantage as that of Robb and Westbrook (4) in that it is inapplicable to oxalic and malonic acids and probably to most hydroxyacids. It does, however, offer a very convenient technique for the study of aqueous solutions of the simpler organic acids and has the advantage that the efficiency of the conversion to methyl esters is unaffected by sample size or concentration, the presence of moderate amounts of monovalent inorganic cations, or the time of drying of the sample for injection. The yields are also largely insensitive to reagent concentration or to variation, within wide limits, or injection port temperature. For quantitative work it would always be advisable to calibrate both the method and the gas cmomatographic system, simultaneously, by chromatography of a standard mixture of all of the acids present in the solution to be analyzed. RECEIVED for review May 27, 1966. Accepted November 22, 1966.
Rapid Potentiometric Determination of Fluoride at High Coricentrations Joseph M. Hogan and Frank Tortorici IBM Components DiGiJion, East Fishkill Facility, Hopewell Junction, N . Y . 12533
THE DEVELOPMENT of a rapid and reliable method for the determination of fluoride has been the objective of numerous investigations. Several volumetric (1, 2), conductometric (3), potentiometric (4, complexometric (5, a), gravimetric (2), and colorimetric (7-10)techniques have been established. None of these completdy satisfied the present need for speed, range, and reliability. The thorium nitrate procedure ( 2 ) , probably the most commonly used volumetric technique, has numerous disadvantages -e.g., difficult detection of the equivalence point and the time needed to acquire proficiency. Fluoride has been determined gravimetrically by the lead chlorofluoride method since 1911 (11). This precipitation
(1) M. J. Gilbert and J . H. Saylor, ANAL. CHEM.,22, 196-7 (1950). (2) W. F. Hillebrand, G . E. F. Lundell, H. A. Bright, and J. I. Hoffman, “Applied Inorganic Analysis,” 2nd ed., pp. 742-6, Wiley, New York, 19621. (3) H. Kubota and J. G. Surak, ANAL.CHEM., 31, 283-6 (1959). (4) T. A. O’Donnell and D. F. Stewart, Ibid.,33, 337-41 (1961). (5) M. A. Leonard, Analysf, 88,404-6 (1963). (6) J. VIeS%l, J. Havii, J. BrandStetr, S. Kotrljl, Chem. Listy, 51, 1677-9 (1957); Anal. Abstr., 5, 2199 (1958). (7) A. L. Hensley and J. E. Barney, 11, ANAL. CHEM., 32, 828-31 (1960). (8) R. S. Ingols, E. H. Shaw, W. H. Eberhardt, and J. C. Hildebrand, Ibid.,22, 799-803 (1950). (9) M. A. Leonard and T. S. West, J . Chem. Soc., 1960, 447786. (10) S. S. Yamamura, M. A. Wade, and J. H. Sikes, ANAL.CHEM., 34, 1308-12 (1962). (11) Starch, Z . Anorg. Allgem. Chem., 70,173 (1911).
technique has since been subjected to detailed studies by several investigators ( 2 , 1 2 , 1 3 ) . Although it has been proved to be a relatively accurate method, it is hindered by comparatively high solubility of lead chlorofluoride in waterLe., 325 mg/liter at 25” C ; by many interfering cations and anions; and by lengthy time required to complete an analysis. The analysis time was reduced by dissolving the precipitate in nitric acid and volumetrically determining the resulting chloride content. However, this did not eliminate the long digestion periods. VfeBtB1, HavfE, BrandBtetr, and Kotrl? (6) further reduced analysis time by precipitating the sample with a known amount of lead, filtering the PbClF formed, and complexometrically determining the quantity of excess lead in the filtrate. Leonard (5) improved this technique and made a detailed study of the variables encountered during analysis. Although these approaches are accurate, they are limited by ions that precipitate lead--e.g., acetate, sulfate, and phosphate. In the proposed method, fluoride is precipitated with lead nitrate and a known concentration of HCl. The liquid portion is separated by filtration, and the excess chloride determined potentiometrically. With this technique, the sample solution may contain up to 250 mg of fluoride before the accuracy decreases appreciably. Most of the ions causing precipitation of lead do not interfere. Should these ions be present, additional lead nitrate is added. (12) D. S. Reynolds and K. D. Jacob, ANAL.CHEM.,3, 366-70 (1931). (13) J. H. Saylor, C. H. Deal, Jr., M. E. Larkin, M. E. Tavenner, and W. C. Vosburgh, Anal. Chim. Acta, 5 , 157-9 (1951). VOL 39, NO. 2, FEBRUARY 1967
221
(N Table I.
Fluoride, mg 20.0 50.0
100.0 200.0 250.0 0
0.3N HCI;
Pb(NO&, Methanol, ml gram 2 10 2 10 3 15 5 25 6 25
ml
15 15 25 50 75
Total volume, ml 90-100 90-100 -150 -200 -250
Add ~0.25-0.30ml of 0.3N HCI per mg F in sample.
Table 11. Analysis of Fluoride in 20-250 mg Range Av.
Theoretical
recovered
F, mg
F,mg 19.9 49.9 99.7 200.2 253.6
20.00 50.0
100.0 200.0 250.0
Range F, mg 1.4 1.3 1.7 3.7 3.0
No. of Av. determinations error, % 10 0.5 14 0.2 10 0.3 10 0.1 10 1.4
a If samples are to be determined regularly at the 20-mg F level, it is recommended that a more dilute solution of silver nitrate (less than 0.1N) be used and that a curve be plotted.
EXPERIMENTAL Procedure and Apparatus. To a beaker containing an aqueous solution of the fluoride (20 to 250 mg), was added an excess amount of standardized 0.3N HC1 (Table I). After the addition of 5 or 6 drops of bromophenol blue indicator the pH of the solution was adjusted to a definite blue with IN NaOH (pH -4.5) and then to a greenish blue with 1 :2 "0,. The solution was diluted to approximately 90% of the suggested total volume with water. While stirring, the weight of lead nitrate and the volume of methanol given in Table I were added. Stirring was continued for 15 minutes, then the precipitate was allowed to settle (10 -15 minutes). The liquid portion was filtered by decantation through a medium porosity fritted crucible leaving the major portion of the precipitate in the beaker. The beaker walls and precipitate were washed twice with 2-4 ml of a 10% (v/v) methanol-water solution. The walls of the fritted crucible were washed once with a 2-4 ml portion of the methanolwater solution. The filtrate was transferred to a beaker, and titrated potentiometrically with standardized 0.32N AgN03. For the potentiometric titration, a Fisher Model 36 automatic titrimeter, or a Beckman Zeromatic I1 pH meter, was used, For both instruments, the same electrode combination was used: Beckman No. 39261 silver billet electrode and Corning No. 476022 triple-purpose glass electrode. The silver billet electrode maintains its sensitivity with prolonged use. In time, an oxide coating appears on the electrode, which does not seem to affect its sensitivity. Other volumetric techniques for the determination of chloride, such as the Volhard Method (Ref. 2, p. 207), are readily adapted to this procedure. Per cent fluoride may be calculated by
( N X V)HCI- ( N X V)A~NO$ (0.019) (100) Weight of Sample
x
0.019
Analytical Parameters
=
ZF
V ) A ~ ~=ONormality , times volume of AgN03 = meq wt. of fluoride
If the sample is not soluble in nitric acid or sodium hydroxide, the weighed sample may be fused with either sodium or potassium carbonate. The fused material is subjected to a water leach, and the soluble fluoride is separated from the insoluble portion by filtration. The filtrate is made slightly acid with 1 :2 HNO, and is subjected to the fluoride analysis outlined above. For samples (e. g. refractory materials) that do not yield a satisfactory fusion with either sodium or potassium carbonate in releasing all of the fluoride, pyrolytic separations of fluorine similar to those employed either by Powell and Menis (14) or Warf et ai. (15)are recommended. Interfering ions (chlorides, bromides, or thiocyanates) should be removed from the distillate solution by precipitation with silver nitrate, This can be accomplished by titrating the solution just to the equivalence point with silver nitrate (0.32 N ) before the addition of the 0.3N HCI, prior to the precipitation of the PbClF. About five minutes is required to filter (decant) the liquid portion. The PbClF precipitate should be considered merely an entrapper of the free chloride and, as such, does not require the careful handling given it in the gravimetric procedure. Occasionally, after filtration, a slight turbidity appeared in the filtrate because fine PbClF particles passed through the filter. Each time the turbidity appeared, the filtrate was titrated, and no deviation from the calculated precision and accuracy was found. However, our studies showed that a large quantity of PbClF present during the chloride titration gave rise to severe voltage drifts ( 4 0 mv) near the theoretical equivalence point. Such a phenomenon indicates that a portion of the PbClF dissolves each time the free chloride concentration of the liquid approaches zero. To assure accurate analysis, no voltage drift should occur during the titration. During the determination of reagent blanks, it is not uncommon for lead chloride to precipitate from solution. This precipitate has no effect on the titration with AgN03, and is not necessary to dissolve the lead chloride by warming prior to titration. For this reason, the blank should never be filtered. RESULTS Precision and Accuracy. Table I1 summarizes the precision and accuracy obtained by this method. Precision at the 95 % confidence level (2u) for single determinations based on ten values in the 200-mg fluoride region was *2.7 mg F. The relative standard deviation at the 200-mg level was *0.59Z. Five analysts participated in the study over a period of 5 days. During this period, at least two preparations ofeach reagent were used in the analyses. If the values obtained by Saylor et al. (13) are representative of the gravimetric and the original volumetric procedure (dissolving PbClF), the accuracy of the proposed method is comparable to that obtained by the gravimetric method and considerably better than that obtained by the original volumetric method. No increase in precision and accuracy can be obtained by plotting a curve to detect the equivalence point. If 0.1-ml increments are added near the end point, the equivalent volume of silver nitrate is easily determined within 0.03 ml.
(1)
where
( N X V)HCL = Normality times volume of HC1 solution added 222
ANALYTICAL CHEMISTRY
(14) R. H. Powell and 0. Menis, ANAL.CHEM., 30, 1546-9 (1958). (15) J. C. Warf, W. D. Cline, and R. D. Tevebaugh, Zbid., 26, 342-6 (1954).
The 0.1-ml increment in the end point region causes a potential change of 40 to 80 mv. Chloride Effect. It has been reported that the concentration of chloride in the precipitation medium should be equivalent to approxirnately a three-fold stoichiometric excess over the fluoride. Smaller quantities yield low results; larger concentrations lead to high results because of coprecipitation of lead chloride. Precision and accuracy studies using the proposed method indicate that the ratio is not completely true. Exceeding the excess might explain why lead chloride is sometimes precipitated in the blank. Table I11 indicates an appreciable decrease in accuracy compared to Table I1 when the quantities of HCI, Pb(NOJ2, and methanol and the total volume are constant over the 20-250 mg F range. The relatively high values obtained for the 20-mg standard and the low results for the 250-mg standard are evidently due to too high a concentration of chloride in the former and too low a concentration in the latter. Interferences. STAELECOMPLEXES. Iron and aluminum form stable complexes with fluoride. These complexes have been used to quantitatively determine fluoride [iron (8) and aluminum ( I ) ] . Because of this they must be removed or low fluoride results will be obtained. The interference of iron can be eliminated by precipitating iron as the hydroxide. The fluoride can then be precipitated as PbClF from the solution containing the iron hydroxide precipitate (pH 8.4; phenol red); or the hydroxide precipitate can be filtered, and the filtrate subjected to the procedure as listed. Both techniques have yielded acceptable results. However, because thz combined precipitate is so bulky, filtering the hydroxide prior to the precipitation of PbClF is recommended. Limited success ha:; been achieved with coprecipitation of aluminum as the hydroxide and fluoride as PbClF. The precision of this method has not been good, and recovery of the fluoride has been low-e.g., in a solution containing 200 mg F and 100 mg Al, recovery of 187 mg F av., was obtained. The low recovery is due in part to the bulky precipitate obtained at these quantities of aluminum and fluoride. The following coprecipitation technique was used: To an acid solution of fluoride containing aluminum, add 50 ml of 0.3N HC1 and 5 grams of lead nitrate; add 5-10 drops of phenol red followed hy 1 N NaOH until the mixture turns pink; stir for 15 minutes; filter through a rapid paper (Whatman 41); titrate the filtrate for chloride. At lower concentrations of fluoride or aluminum, this method should present less difficulty. If the above listed techniques are unsatisfactory, fluorine can easily be separated from solutions containing iron and aluminum by a Willard-Winter distillation (16). Grimaidi et al. (17) have described the separation of fluorine in the presence of much aluminum using phosphoric acid in the distilling flask. Kubota and Surak (3) describe a still that would make the distillation quite simple and fast. The distillate would then be carried through the normal procedure, Pyrolytic or pyiohyclrolytic (14, 15) techniques are also satisfactory for the sel3aration of fluoride from metal ions forming stable complexes with fluoride. Such techniques are a necessity when the Willard-Winter type of distillation does
(16) H. H. Willard and 0. B. Winter, ANAL.CHEM., 5, 7-10 (1933). (17) F. S . Grimaldi, B. Iiigram, and F. Cuttitta, Ibid., 27, 918-21 (1955).
Table 111. Fluoride Recovery at Constant Reagent Concentrations" Theoretical Av. recovered F,mg F, mg 20 21.5 50 52.9 100 102.1 200 200.2 250 245.2
Range F, mg
No. of determinations
Av. error,
0.9 1.3 0.9
2 5
7.5 5.8 2.1
3.7
10
0.1
2.8
2
1.9
3
a 50 ml of 0.3N HCI, 5 grams of Pb(NO&, 25 ml of methanol, and 200-ml total volume.
not completely dissolve the sample or when fusions are not totally effective in rendering all the fluoride soluble. Such distillates will also be free from phosphates. PRECIPITATES. The interference of large concentrations of sulfate and acetate, which form precipitates with lead, can be eliminated by increasing the amount of lead nitrate. Samples were prepared by adding 1 gram of sulfate to a solution containing 200 mg of fluoride and 1 gram of acetate to a similar solution. The normal procedure was followed except for an increase in the amount of lead nitrate from 5 to 8.5 grams for sulfate and from 5 to 10.5 grams for acetate. Recoveries were obtained of 198.1 mg F for the sulfate sample and 202.6 mg F for the acetate sample. These values are within the calculated precision range of the method. Reynolds and Jacob (12) point out that high results obtained with phosphates, which interfere, are probably caused by the precipitation in the presence of chloride of lead phosphate which then carries with it some lead chloride. Precipitating the phosphate as lead phosphate prior to the addition of the chloride (as sodium chloride) yielded low results,-e.g., starting with 200 mg of fluoride in solution with 100 mg of phosphate, the average fluoride recovery was 181 mg F. Lead fluoride is apparently coprecipitated with the lead phosphate thus giving low results. If phosphates are present, a Willard-Winter distillation (16) is recommended. Ammonium, considered to be an interfering ion in the gravimetric procedure, had no effect in quantities up to one gram. TITRATION INTERFERENCE. Interferences with the silver nitrate titration due to the presence of chloride, bromide, or thiocyanate in the sample solution are easily eliminated. This can be accomplished by titrating the solution just to the equivalence point with silver nitrate (0.32N) before the addition of the 0.3N HCI. The insoluble silver salt does not have to be removed prior to the precipitation of the PbClF. Recoveries within the calculated precision have been obtained by this technique. ACKNOWLEDGMENT
The authors express their thanks to E. P. Cocozza for his encouragement and critical appraisal of the work, to R. J. Lee for his cooperation, and to M. F. Varano, J. V. Zmudzinski, and D. R. Kitchen for their assistance in the precision and accuracy studies. RECEIVED for review September 7,1966. Accepted November 29,1966. VOL. 39, NO. 2, FEBRUARY 1967
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