distribution of the hydrocarbons present paralleled the results of other workers (4, 5, 19) but direct comparison cannot be made because of the different types of tobacco used in the sample preparation. It is apparent, however, that the method described can be used effectively as a rapid qualitative technique for the analysis of straight chain hydrocarbons present in complex mixtures.
RECEIVED for review April 12, 1968. Accepted June 13, 1968. Abstracted in part from the M.S. Thesis of R. L. J., North Carolina State University, June 1965. Presented at the Eighteenth Tobacco Chemists' Conference, October 20, 1964, North Carolina State University, Raleigh, N. C. The authors are indebted to the R. J. Reynolds Tobacco Co. for support of this work.
Determination of Lithium by Potentiometric Titration with Fluoride Elizabeth W. Baumann Savannah River Laboratory, E . I . du Pont de Nemours and Co., Aiken, S . C. 29801
THEFEW published volumetric methods for the determination of lithium are either complicated or nonspecific; consequently, none of the suggested procedures have gained general acceptance. For example, one determination based on iodometric titration of the precipitated complex lithium periodate ( I ) distinguishes lithium from the other alkali metals, but requires that all metals except those of the alkali group be removed and that ammonia be removed if more than a few milligrams are present. Lithium determination by titration of lithium acetate as a base in an acetic acid-chloroform solvent (2) cannot distinguish quantitatively between lithium and sodium, but can distinguish lithium from potassium. A method was developed for analysis of concentrated solutions of LiCl and LiNOa that are used in the Tramex process for separation of lanthanides from actinides (3). This method utilizes quantitative precipitation of LiF in alcohol (4) as the basis for a potentiometric titration with the lanthanum fluoride membrane electrode (5)serving as an indicator electrode. Moderate amounts of sodium can be tolerated. Interference from free acid and some metal ions is eliminated by adding N H 4 0 H and (NH4)2S,respectively. EXPERIMENTAL
Equipment. An expanded scale pH meter was used with a saturated calomel reference electrode and a specific fluoride ion indicator electrode (Model 94-09, Orion Research Inc., Cambridge, Mass.). A 2-ml microburet with a micrometer control and a precision Teflon (Du Pont) plunger (Cat. No. 7846, Cole-Parmer Instrument and Equipment Co., Chicago, Ill.) was modified by replacing the glass reservoir with one of polyethylene or Teflon, so that the standard NHIF titrant does not contact glass. Reagents. Reagent grade chemicals were used. LiCl was not further purified; maximum limits of major impurities were: K, Na, Ca, sulfate, 0.01 % each; alkalinity as Li2C03, 0.02 %. Standard 0.5M NH4F was prepared by neutralizing HF, previously standardized against NaOH, with N H 4 0 H to pH -8. Procedure. A 100-p1 sample was placed in -30 ml of 95 % ethyl alcohol. If the sample contained free acid, 1 drop of concentrated NH40H was added. If the sample contained interfering metal ions, 250 to 500 p1 of 20% (NH4)*Sreagent solution was added. (1) ANAL.ED., . . IND. ENG.CHEM., . , L. B. Rogers and E. R. Caley,
15, 209 (1943). (2) C. W. Pifer. E. G. Wollish. and M. Schmall. ANAL.CHEM.. 26. 215 (1954). (3) H. J. Groh, C. S. Schlea, J. A. Smith, R. T. Huntoon, and F. H. Springer, Nuclear Applicatiorzs, 1 (4), 327 (1965). (4) E. R. Caley and G. R. Kahle, ANAL.CHEM., 31, 1880 (1959). (5) M. S. Frant and J. W. Ross, Jr., Science, 154, 1553 (1966).
.,
I
,
The stirred solution was titrated potentiometrically with "IF. Near the equivalence point, the potential was measured after each addition of 100 pl. The volume of titrant at the equivalence point was calculated conventionally as the point of inflection in the curve. RESULTS AND DISCUSSION
Accuracy and Precision. The accuracy of the potentiometric titration method was assessed by determining both the lithium and chloride content of the nominal 11M LiCl solution used throughout the investigation and of a dilute LiCl solution. The latter was selected to eliminate errors inherent in pipetting the viscous concentrated solution; aliquots were concentrated by evaporation for the lithium determination. Chloride was determined by potentiometric titration with 0.1M AgNOa, with a silver indicator electrode. Table I shows that the lithium and chloride determinations agreed well. The precision of the method was calculated for each of the two LiCl solutions (Table I). The higher relative standard deviation for the 11M LiCl (0.4% compared with 0.3% for the dilute solution) probably reflects pipetting error, which is also evident in the chloride values.
Table I. Analysis of LiCl Solutions by Potentiometric Titration
Mean concn, M Standard deviation No. of detns Sample size, ml ~~
~
11MLiCl Lif c110.71 10.67 0.047 0.034 6 3 0.100 0.250
Dilute LiCl Li+ c10.08540 0.08549 O.OOO27 0. oooO3 6
5
10
10
~
Table 11. Comparison of Titration and Ion Exchange Methods Li concentration Nominal found, M Ion composition of solution Titration exchange 11M LiCl 10.71 10.80 11M LiCl 10.82 10.94 9M LiCl 9.07 8.96 11. 16a 11.28 11MLiC1,0.3MHCl 1M LiCI, 6.9M LiN03, 0.215M NzH4,HNOs 8.07 7.93 a 1 drop of NH40H added.
VOL 40, NO. 1 1 , SEPTEMBER 1968
Difference +O. 09
+o. 12 -0.11 +o. 12
-0.14
1731
Table III. Investigation of Interferences Sample composition, in -30 ml ethanol Li+ concentration found, M 10.71 100 p1 LiCl 100 91 LiCl, 50 pl 1M HCl 11.06 10.68 Above 1 drop “,OH 100 p1 LiCl, 100 pl 2M SnCll No value; response sluggish 10.71 Above 1 drop (NH&S 100 pl LiCl, 50 pl 0.2M kcla 10.88 10.75 Above 250 or 500 pl (NH&S 11.05 100 pl LiCl, 100 p1 5M NaCl 100 p1 LiCl, 20 p1 5M NaCl 10.80
+ + +
No bias existed between the potentiometric titration results and those obtained by an ion exchange procedure. Table I1 shows that the two methods generally agreed within 1%; the ion exchange results scatter above and below the titration values, In the ion exchange procedure, which has been our routine determination of lithium in the Tramex solutions, the solution is passed through a column of cation exchange resin in the acid form, and the acid displaced is titrated. Suitable corrections are made for other cations. Effect of Other Ions. Ions investigated as interferences were primarily those encountered in samples from the Tramex system: H+, Sn*+, and La3+ (a stand-in for lanthanides and actinides). As shown in Table 111, these ions are adequately masked by adding NHdOH or (NI-I&S. Because LaF, is very insoluble, more than fifty times as much sulfide as lanthanum was required to mask it. These reagents would also mask other elements that form insoluble hydroxides or sulfides. Although (NH&S masked Laa+ in these nonradioactive experiments, it did not mask the lanthanide fission products and actinides in a highly radioactive Tramex solution. The potential drifted slowly after each addition of NH4F. Steady readings were obtained when the sulfide was omitted, and titrations in the radioactive solution are corrected for the contribution of the lanthanides and actinides, which are determined separately. The specificity of this method for lithium in the presence of other alkali metals was also considered. The potential changes with NaCl were smaller than those with LiCl: NaF is more soluble than LiF. Figure 1 shows that the presence of NaCl in a LiCl solution alters the shape of the titration curve and produces a plateau below the inflection point. As shown in Table 111, the results calculated from the inflection point are slightly high, probably because NaF coprecipitates with LiF. Potassium and the heavier alkali metals will probably interfere less than sodium, because the heavier alkali fluorides are increasingly soluble in water and K F is more soluble in alcohol than NaF (6). pH. Titrations were made in slightly alkaline solution, even though hydroxyl ion is reported to interfere with the fluoride electrode response (5). In the reverse titration (fluo-
(6) I. G. Ryss, “The Chemistry of Fluorine and Its Inorganic
Compounds, Part 2,” State Publishing House for Scientific, Technical, and Chemical Literature, Moscow, 1956. AEC Translation Series, AEC-tr-3927.
1732
ANALYTICAL CHEMISTRY
mi of 0.5M NH4F
Figure 1. Titration curves of lithium and sodium chlorides in ethyl alcohol A Licl only 0 NaCl only
+
0 LiCl NaCl (Na+/Li+-0.5) J Calculated equivalence (inflection) points
ride by metal ion) investigated by Lingane (7), the medium was maintained neutral or slightly acid, which avoided hydroxyl interference. However, acid in the lithium sample causes high results; the added fluoride is partially complexed by the hydrogen ion. Hydroxyl interference is not of concern because ammonium hydroxide only slightly ionizes in alcohol solution and because the fluoride concentration at the end point is relatively high. Application. The range of usefulness of the method is limited by the solubility of LiF in alcohol. Fifteen to twenty mg of lithium, present as the nitrate or chloride salt, has been determined in 95% alcohol. Determination of 5 mg of lithium (25 p l of the 10.71MLiCl in Table 11) resulted in the slightly lower value of 10.64M, probably because a larger fraction of the precipitated LiF was in solution. Samples that contain >20 mg of lithium can be analyzed, if the total aqueous concentration in the titration medium is maintained low, and if the sample is soluble in the titration medium. If absolute alcohol were used instead of 95% ethyl alcohol, as much as 5 % of the volume could be aqueous without appreciably affecting the titration curve. In summary, the method is applicable to aqueous solutions that are predominantly lithium chloride, lithium nitrate, or other lithium salt soluble in ethanol. Although the method can be used to determine as little as 5 mg of lithium, more satisfactory results are expected above 15 mg. The amount of water in the titration medium must be minimized to avoid excessive solubility of LiF. Sodium causes < 3 % error, when present in amounts up to 50 mole % of the lithium; other alkali metals are expected to interfere less than sodium. RECEIVED for review April 29, 1968. Accepted June 17, 1968. Information contained in this article was developed during the course of work under Contract AT(07-2)-1 with the U. S. Atomic Energy Commission. (7) J. J. Lingane, ANAL.CHEM., 39, 883 (1967).
