by a plot of distribution coefficient us. per cent acetone in Figure 1, will be eluted more slowly by eluents of 85% or less acetone. A list of the successful separations obtained is given in Table 11. Only cadmium, iron(III), and zinc were used as major constituents. Some separations involved a ratio of 10,000 to 1 of major constituent to minor constituent, but more were done in the 1000 to 1 range. From the Cd+2-Cu+2separation where C U +has ~ a distribution coefficient of about 190 a t 50% acetone-0.5M HC1, separations of Cd+2 from C O + ~ , Fe+3, Ga+3, Xln+2, or COz+2as minor constituent are suggested. Likewise, the Zn+2-Fe+3 separations, where Fe+3 has a distribution coefficient of 134 in 55% acetone, suggests separations of large amounts of Zn+2from C O + ~Gat3, , Mn+*, U O t 2 , or VO+2 as minor constituent. However, the problem of tailing when iron is the major constituent requires the minor constituent to have a
distribution coefficient of around 300 or more. An attempted separation of Fe+3 from VO+2 as minor constituent failed using 75y0 acetone-0.5M HCl, where VO+2has a distribution coefficient of 163. Lowering the eluent composition much below 75% acetone-0.5M HCl was impossible, because tailing then occurred even at 500 ml. of eluent. At 50% acetone-0.5M HC1 or higher concentrations of acetone, Bi+3 and In+3 are other possibilities for major constituents, because their distribution coefficients are essentially zero. At 85% acetone-0.5M HCl, Ga+3, C U + ~ , or UOz+2may be used as a major constituent to be separated from either Ni+2 or Mn+2. Only the minor constituent was determined quantitatively. It was felt that qualitative tests were sufficient for the major constituent. Eluent fractions were analyzed qualitatively in the manner previously described and, when two succeeding negative tests were
obtained, i t was assumed that the major constituent was completely eluted. The average recovery for the minor constituent was 99.9%. The relative standard deviation for all 26 results was *0.9%. LITERATURE CITED
(1) Fritz, J. S., Karraker, S. K., ANAL.
CHEM.32, 957 (1960). (2) Fritz, J. S.. Rettia, T. A., Ibid.. 34. 1562 (1962). (3) ~, Kember. N. F.. Macdonald. P. S.. Wells, R.'A., J . Chem. SOC.1955, 2273: (4) Kojima, M.,Bunseki Kagaku 7, 177 (1958). (5) Van Erkelens, P. C., Anal. Chim. Acta 25, 42 (1961). Division of Analytical Chemistry, 148th Meeting, ACS, Chicago, September 1964. Work performed in Ames Laboratory of U. S. Atomic Energy Commission. JAMES S. FRITZ JANET E. ABBINK Institute for Atomic Research and Dept. of Chem. Iowa State University Ames, Iowa I
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Microdetermination of Fluoride Using an Improved Distillation Procedure SIR: The interference of many cations and anions in available colorimetric methods for fluoride necessitates a preliminary separation of the fluoride. The classical Willard-Winter distillation procedure (6) has been in use since 1933, but its usefulness has been limited because the large volume of distillate required for the quantitative recovery of fluoride complicates the subsequent measurement of small amounts of fluoride and because quantitative distillation of fluoride is not obtained readily in the presence of metal ions that complex fluoride. These shortcomings were remedied recently by Singer and Armstrong ( 5 ) who developed a microstill that gives quantitative distillation with only 20 ml. of distillate, and by Blake (3)and Grimaldi, Ingram, and Cuttitta (4) who overcame the adverse effect of fluoride-complexing metal ions by the use of metal-complexing phosphoric acid in the distillation medium. This paper describes a versatile microdistillation procedure that is applicable to the separation of microgram levels of fluoride from anions and cations that interfere in colorimetric fluoride 1x0cedures. The distillation is carried out from a phosphoric acid medium with a micro still, similar to that of Singer and Armstrong, which has been improved to minimize the carryover of phosphate and sulfate. The fluoride in the distillate is determinable by a number of colorimetric methods. I n our study, the 1276
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INNER TUBING
Figure 1,
Modified distillation flask
improved distillation-separation procedure was coupled with an Alizarin Complexone spectrophotometric measurement of the fluoride ( I , 2, 7 ) . This resulted in a versatile, reliable method that appears to be suitable for a wide variety of samples. I n fact, of 42 diverse ions tested, most of them at the 500 to 1 diverse ion to fluoride molar ratio, only borate and silicate interfered seriously. EXPERIMENTAL
Apparatus. T h e distillation apparatus is similar to t h a t of Singer and ilrmstrong with t h e exception of t h e distillation flask which has been modified as shown in Figure 1. Teflon standard taper 14/35 sleeves (Arthur E. Smith, Inc., Pompano
Beach, Fla.) were used on all j o i n s to prevent freezing. Polyethylene 50ml. graduated cylinders were used to collect the distillate and a Cary Model 14 spectrophotometer with 1-em. optical cells was used for the absorbance measurements. Reagents. Concentrated perchloric acid and 850/, phosphoric acid were both purged of lower boiling point components including fluoride b y heating t h e acid to 160' C. in a roundbottom flask and bubbling steam through the acid for 4 hours. T h e fluoride solution was prepared from reagent grade sodium fluoride. T h e composite lanthanum(II1) - Alizarin Complexone (La-.1C) reagent was prepared as described earlier ( 7 ) . Lanthanum was substituted for cerium used in the earlier procedure because the sensitivity of the lanthanum reagent is constant, whereas the sensitivity of the cerium reagent tends to vary with different lots of cerium nitrate reagent. Also, the La-AC reagent seems to be slightly more sensitive to fluoride than the corresponding cerium reagent. Procedure. Clean all glassware in hot sulfuric acid and rinse thoroughly with distilled water. Heat a glycerol bath to 165' C. Add 0.5 ml. of 70% perchloric acid to the t r a p through t h e arm of t h e distillation flask and assemble t h e apparatus. By means of a polyethylene pipet, transfer 4 ml. of 0.5,11 sodium hydroxide to t h e 50-ml. polyethylene graduated cylinder and immerse the polyethylene tip of the condenser in the sodium hydroxide. Remove the bubbler tube and add an aliquot (5 ml. or less) of the
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sample, containing between 2 and 25 p,g. of fluoride, to the distillation flask. Replace the bubbler tube and add 3 ml. of phosphoric acid to the distillation flask through this tube. Attach the reservoir and pass air, purified by a sodium hydroxide scrub solution, through the sample a t the rate of 2 to 3 bubbles per second. Raise the heating bath to immerse the distillation flask in the hot glycerol. Add water from the reservoir to the sample a t the rate of 10 drops per minute and collect 20 ml. of distillate while maintaining the temperature of the glycerol bath at 160' to 170" C. Disconnect and rinse the condenser with 3 ml. of water and add the rinse to the distillate. Add 1 ml. of 2J1 hydrosylamine hydrochloride and 1 drop of alcoholic 0.2% phenolphthalein to the distillate. Add l d l hydrochloric acid until neutral, then add 1 drop more. Swirl the solution intermittently for 2 to 3 minutes to espel any gas formed. Add 1M carbonate-free sodium hydroxide dropwise until basic, then add 1M hydrochloric acid dropwise until the solution just turns acid. Add 15 ml. of t,he composite La-XC reagent and transfer the solution quantitatively with water rinses to a 50-ml. volumetric flask. Dilute to 50 ml., mix, and, after 20 minutes, measure the absorbance of the sample a t 617 mp. in a 1-em. cell. Compare the sample against a blank carried through the entire procedure. Process two standards containing 10 and 20 pg. of fluoride, respectively, through the entire procedure with each series of samples. The lanthanum(II1)Alizarin Complesonc-fluoride standard curve is characterized by a negative intercept. Therefore, add the absolute intercept, value to the net absorbance of each of the two standards and t'he samiiles and calculate the concentration of the samples from the average absorptivity of the two standards. The intercept, with a value of about -0.007 absorbance unit, should be obtained from a regression equation calculated from a series of undistilled fluoride standards. RESULTS A N D DISCUSSION
Microdistillation of Fluoride. T h e improved version of the microdistillation flask of Singer and Armstrong (5) is illustrated in Figure 1. The unique feature of this apparatus is the trap which reduces the carry-over of phosphate to less than 50 pg., a t which level no interference is encountered in the La-AC colorimetric procedure. The phosphoric acid is necessary, as noted above, to quantitatively distill fluoride from fluoride-complexing metal ions (3, 4 ) . The modified apparatus retains the desirable feature of the Singer and Armstrong apparatus in that quantitative fluoride distillation is obtained with only 20 ml. of distillate. The standard taper joints of the distillation apparatus tend to "freeze" and
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ANALYTICAL CHEMISTRY
to avoid this, Teflon sleeves are used on the joints. The introduction of fluoride from the Teflon is nil as shown by the similarity of the absorbances of distilled blanks with and without the sleeves. On the basis of data collected over a Cmonth period, there appears to be a 3% negative bias inherent in the distillation-colorimetric measurement procedure. However, this is easily corrected by comparison of the samples with standards also carried through the entire procedure. The negative bias is thought to be caused by the codistillation of an interfering substance such as silicate which is known to interfere (7) rather than by incomplete fluoride distillation. Enough silicate is introduced by the action of the sodium hydroxide absorber solution on glass to cause low results. Consequently, polyethylene tubing is used to extend the condenser and a polyethylene cylinder is used to collect the distillate. Tygon with extractable plasticizers is not a suitable substitute. Although a range of 2 to 25 pg. of fluoride is specified for the procedure, the microstill has been used successfully for the distillation of 1-mg. levels of fluoride. With larger amounts of fluoride, the glass is etched and a considerable amount of fluoride is adsorbed so that the apparatus cannot be cleaned adequately for microgram distillations. Effects of Diverse Ions. T h e effects of diverse ions on the combined distillation-spectrophotometric measurement procedure were determined b y analyzing synthetic samples which contained 20-pg. (0.00108 mmole) amounts of fluoride and high concentrations of selected ions. Generally, the cations were added as nitrate or chloride salts, and the anions were added as sodium or potassium salts except for nitrate and sulfate which were added as the acid. Each sample was measured against a similar sample blank without added fluoride to correct for the small amount of fluoride in the diverse ion solutions. A "t" test a t the 95% confidence level was used to establish interference. For a single determination at the 20-pg. fluoride level, the allowable limits were & I 2 pg. Figure 2 gives the known tolerance levels for many elements. Only a few ions interfere. Borate and silicate, even at a 100: 1 molar ratio, interfere seriously by retarding the distillation of fluoride. Thus, in the presence of these two ions, the collection of a larger volume of distillate is necessary (6'). A t a 500:l diverse ion to fluoride molar ratio, Al(IIIj, Cd(II), Ce(lII), and bln(I1) cause a 5 to 10% negative
bias. However, these metal ions do not interfere a t the 100: 1 ratio. A precipitate forms when the ions Ce(III), La(IIIj, Pb(II), Th(lV), Zr (IV), dichromate, and silicate are admitted into phosphoric acid, but on heating only silica and the phosphates of cerium(II1) and (IV) and zirconium (IV) remain undissolved. I n the presence of nitrate, manganese(I1) is oxidized partially to insoluble manganese dioxide. With samples containing nitrate, nitrite is formed in the presence of oxidizable substances such as chloride, cerium(III), and manganese(I1). The nitrite formed must be reduced with hydroxylamine in acid medium prior to color development. Reliability. Based on 27 determinations at the 20-pg. fluoride level, the relative standard deviation of the La-AC spectrophotometric procedure is 1.8%. For the combined distillationspectrophotometric procedure, the relative standard deviation is 3.0y0 based on 22 determinations. I n both cases, all data collected over a 4-month period were used without deletions. Fluoride Fixing with Zirconium. Because of t h e strong affinity of zirconium for fluoride, zirconium can be used effectively to retain the fluoride during the removal of large interfering amounts of other volatile ions such as carbonate, chloride, and nitrate. For example, in the presence of zirconium, 2 ml. of concentrated nitric acid can be distilled from perchloric acid medium without any loss of fluoride. The volatiles are steam-distilled a t 150' to 160" C. from 1 ml. of perchloric acid containing 0.5 mmole of zirconium. Phosphoric acid is then added and the fluoride is distilled and determined as described in the procedure. LITERATURE CITED
(1) Belcher, R., Leonard, 31. A., West, T. S.,Talanta 2, 92 (1959). (2) Belcher, R., West, T. S., I b i d . , 8, 863 (1961). (3) Blake, H. E., U . S. Bur. Mines, Rept. Invest. 6314, 1963.
( 4 ) Grimaldi, F. S., Ingram, B., Cuttitta, F., ANAL.CHEM.27, 918 (1955).
(5) Singer, L., Armstrong, W. D., Ibid., 31, 105 (1959).
( 6 ) Willard, H. H., Winter, 0 . B., IND. ENG.CHEM.,ANAL.ED. 5, 7 (1933).
( 7 ) Yamamura, S. S., Wade, M. *A,,Sikes, J. H., Ibid., 34, 1308 (1963).
MARVEN A. WADE S. YAYAMURA STANLEY Phillips Petroleum Co. Atomic Energy Division Idaho Falls, Idaho WORKdone under Contract AT(10-1)-205 to Idaho Operations Office, U. S.Atomic Energy Commission.
