an alkaline hydrolysis are presented in Table I. The average value of nine quantitative chromatograms was 1.54 mg. of tryptophan per 100 mg. of CYcasein, &-hich compares well with the value of 1.6 mg. reported previously (4). It is appreciably lower, however, than the value of 2.2% reported b y Gordon and conorkers (3’) employing the method of Spies and Chambers (10)on unliydrolyzed a-casein. The accuracy and precision of the method were established by determining the tryptophan content of a solution containing 18 common amino acids (40 mg. of each amino acid per 100 ml. total volume, pH 6.5). The average value of 39 2 nig. obtained from nine yiiantitatiT-e chromatograms (Table 11) differed from the theoretical value by 2 0%. The standard deviation, 1.14 rng. per 100 ml., indicates that the assay can be repeated with good precision. TThile the accuracy of the procedure is about the same as that previously reported for descending paper chromatography and the maximum density method (9). the precision of the horizontal method. 1.14 mg. per 100 nil., is an improvement over the previous value of 2.34 mg per 100 ml. ACKNOWLEDGMENT
The authors n-ish to thank Elizabeth Stanton and George Kissel for the sample of a-caspin and Margaret &I.
Table 1. Tryptophan Content of QCasein as Determined b y Horizontal Paper Chromatography
Table ll. Accuracy and Precision of Horizontal Paper Chromatographic Determination of Tryptophan
Calcd. Concn., Mg./100 Mg. a-Casein 1.54 1.36 1.54 1.48 1.60
Chromatogram 1 2
3 4 5
Chromatogram 1 2 3
4
5
1 67
6 7
_ .
1.54
9 Average Standard deviation (9) Chemical determination ( 4 ) Chemical determination on unhydrolyzed a-casein ( 3 )
Fitzpatrick analysis.
for
the
1.54 1.60 1 54 0 087 16 2 2
electrophoretic
LITERATURE CITED
(1) Block, R. J., ANAL. CHEX 22, 1327 (I950). (2) Bliqck, R. J., Durrum, E. L., Zweig, G., Manual of Paper Chromatography and Paper Electrophoresis,” 2nd ed., Academic Press, Xew York, 1958. (3) Gordon, W. G., Semmett, W. F., Bender, hf.. J. Am. Chem. SOC. 75, 1678 (1953).’ (4) Gordon, W. G., Semmett, IT. F., Cable, R. S., Morris, M., I b i d . , 71, 3293 (1949).
8
9
Average Standard deviation (9) Deviation from theoretical value, 40 mg./100 ml., yo
Calcd. Concn., Mg./lOO Wl. 39.4 37.9 37.9 39 4 39.4 40.8 39.4 37.9 40.8 39.2 1.14 2.0
(5) Hipp, Y. J., Groves, AI. L., Custer J. H., McMeekin, T. L., J . D a i r y Sci
35, 272 (1952). (6) McFarren, E. F., AKAI,.CHEJI. 23, 168 (1981). ( 7 ) McFarren, E. F., Mills, J. A,, Ibid., 24, 650 (1952)). (8) Roberts, H. R., Zbid., 29, 1443 (1957). (9) Roberts, H. R.: Kolor, bl. G., Zbid., 29, 1800 (1957). (10) Spies, J. R., Chambers, L). C., Zbid., 21, 1249 (1949). (11) Warner, R. C., J . i i m . Chem. SOC. 66, 1725 (1944).
RhCE1T.m for review February 27, 1958. Accepted &la>-31, 1958.
Determination of Lithium and Sodium Chlorides by Potentiometric Titration Following 2-Ethyl-1-hexanol Separation GLENN R. WATERBURY, EDWARD H. VAN KOOTEN, and BRUNO MOROSIN’ University o f California, 10s Alamos Scientific Laboratory, Los Alamos, N. M.
b Sodium and lithium chlorides are titrated potentiometrically with silver nitrate following two extractions of the lithium chloride with 2-ethyl-1 -hexanol. Using glass and silver-silver chloride electrodes, the detection of the end point i s enhanced in the organic medium. For 17 determinations, an average of 99.99y0 was obtained for lithium, with a standard deviation of 0.1670, and an average of 99.9% for sodium, with a standard deviation of o.4y0. A study of the effect of 20 other metals on the analysis showed that lithium i s also separated b y the extraction from aluminum, barium, and tungsten; so-
dium may b e determined in the ence of lithium and plutonium, mium, cobalt, iron, magnesium, dymium, nickel, tin, uranium, or
T
prescadneozinc.
purpose of this investigation was twofold: to develop a precise procedure for the analysis of lithiumrich mixtures of sodium and lithium using the 2-ethyl-1-hexanol separation, and to determine the applicability of the method for the separation and determination of lithium or sodium in other metals, especially plutonium or uranium. I n the original method described by Caley and Axilrod ( I ) , a solution of the alkali chlorides is deHE
hydrated in contact with 2-ethyl-lhexanol which selectively dissolves the lithium chloride. The insoluble sodium or potassium chloride is removed by filtration, and the separated alkali metals are determined as the chlorides or sulfates by time-consuming gravimetriC methods. White and Goldberg ( 5 ) applied the Volhard method (3) for the titration of chloride to the determination of lithium chloride in alcoholic solution following the dehydration. This method requires a back1 Present address, Chemistry Department, University of Washington, Seattle, Wash.
VOL.
30, NO. 10, OCTOBER 1958
1627
titration, and the end point is determined visually in the presence of the silver chloride precipitate. By using glass and silver-silver chloride electrodes, direct potentiometric titration of the lithium chloride in alcoholic solution, and of the sodium chloride in aqueous solution, may be carried out quickly and precisely. The effect of several metals on the deterniination of lithium and sodium is described. This method may be used to determine sodium or potassium in chloride solutions containing lithium and noniiiterfering metals, such as cadmium, cobalt, iron, magnesium, neodymium, nickel, plutonium, tin, uranium, or zinc. One fruitful application of this method has been in the deterniination of sodium in uranium samples. Because only one dehydration is required to remove the uranium, the sodium can be determined quickly and easily. For simplicity, the proccdure is written specifically for the determination of lithium and sodium in the absence of other metals, but applications for the separation of sodium or lithium from other metals may he deduced from the effects of diverse ions. REAGENTS
2-Ethyl-1-hexanol, practical grade. Although this grade reagent is not of high purity, no interference \vas caused by any of t h e contaminants n hen sodium and lithium were determined using this potentiometric titiation method. Lithium Chloride, standard solution, 0 . l M . Weigh accurately about 7.4 grams of pure dried lithium carbonate, dissolve t h e salt in 1 t o 6 hydrochloric acid, boil to remove carbon dioxide, and dilute to 1 liter with water. The lithium carbonate \vas spectrographic standard grade reagent but the pure salt may also be prepared by the method of Caley and Elving (2). Silver Nitrate, standard solution, 0.4M. Weigh accurately about 68 grams of t h e dried salt; dissolve in nater to make 1 liter. Standardize against pure sodium chloride. Dilute 250 ml. of this solution t o 1 liter to prepare a 0.1M silrer nitrate solution. Sodium Chloride, standard solution, 0.1M. Weigh accurately about 6 grams of dried sodium chloride, dissolve in n a t e r , and dilute t o 1 liter. PROCEDURE
Dissolve a sample containing a t least 10 mg. of sodium, and no other p e t a l s except lithium, in 1 to 6 hydrochloric acid in a 250-ml. beaker. Cover the beaker lvith a ribbed witch glass and evaporate the solution to dryness on a hot plate. Dissolve the residue in about 3 ml. of 1 to 6 hydrochloric acid, add 50 nil. of 2-ethyl-1-hexanol, and place the beaker on the hot plate maintained a t 130" to 140" C. until all of the aqueous phase has evaporated and the salt 1628
ANALYTICAL CHEMISTRY
crystals do not cling to the beaker. If too much dilute hydrochloric acid is added prior to the dehydration, bumping and sample loss may result unless glass beads are added or the sample is agitated. After dehydration, allow the solution to cool to room temperature and decant the supernatant liquid into the filter funnel, leaving the salt crystals in the beaker. Filter the supernate into a 400-ml. receiver beaker. Wash doivn the walls of the 250-ml. beaker containing the sodium chloride with 1 to 2 ml. of 1 to 6 hydrochloric acid, add 25 nil. of 2-ethyl-1-hexanol, and repeat the dehydration and filtering operations. Wash the beaker and funnel three or four times with small amounts of 2ethyl-1-hexanol, and filter the washings into the 400-ml. receiver beaker. Add 200 to 300 ml. of acetone (or dioxane) to the filtrate, and titrate the chloride with 0.4M silver nitrate standard solution using a 50-ml. buret. A reading of about 11.3 is obtained a t the end point on the Model H-2 p H meter, but the titration curve should be plotted to determine the correct value to use under the conditions employed in the analysis. If the major conPtituent of the sample is sodium, take a sample containing at least 5 mg. of lithium, perform only one dehydration, and titrate the alcoholic solution with 0.1-M silver nitrate from a 5-ml. buret. T o calculate the per cent lithium, niultipIy the niilliequivalents of silver nitrate used by 694 and divide by the sample Iveight in milligrams. Place a 250-ml. beaker used in the dehydrations in the bell jar as a receiver, add water to the filter funnel to dissolve the sodium chloride, and wash the funnel with several small portions of \va.ter. Using a 5-ml. microburet, titrate the chloride in the filtrate with O . 1 M silver nitrate solution. The slope of the titration curve near the end point can be increased and the titration improved by evaporating the sodium chloride solution until crystals start to form and then adding acetone before the titration. To calculate the per cent sodium, multiply the milliequivalents of silver nitrate used by 22.991, and divide by the sample weight in milligrams. EXPERIMENTAL RESULTS A N D DISCUSSION
Dehydration or Extraction.
For samples with low sodium content, 10% or less, the total sample size necessarily had t o be large t o obtain acceptable precision, and two dehydrations or extractions were required. T h e poor results obtained by Caley and Axilrod ( 1 ) were verified under the conditions of a single dehydration of a large sample. For samples in which the sodium and lithium concentrations were more nearly equal, only one dehydration was required. I n performing two dehydrations, sodium loss was minimized by avoiding transfers of the salt and using the same beaker for the extraction and the titration. S o hydrolysis of lithium chloride
I3I I3O
tt
/
/
11
12 I 20' l
I
Figure 1.
Titration curves of 424 mg.
of lithium chloride with 0.4M silver nitrate 1. 2.
Aqueous solution Acetone-2-ethyl-1 -hexanol solution
was expected in the dehydrations because of the carefully rontrolled temperature. Homver! the salt reniaining froni the first extraction \vas treated with hydrochloric acid to convert any lithium hydroxide to the chloride. During the second dehydration, the lithium concentration i n s low and no hydrolysis, as evidenced by lo^ lithium results, was obseri-ed. Tahle I s h o w a n average of 99.99% for the lithium, with a standard der.iation of 0,16'j& obtained for 17 deteririiiations of 42 to 85 mg. of lithium in rhe presence of 0 to 12 mg. of sodium. The average value for the sodium of 99.9% \\-it,h a standard deviat,ion of 0.4%. indicates that washing the sodium rhloride residue lvith 2-ethyl-1-hexanol rffectively removes lithium chloride \vithout sodium loss. Small relative errors in the separation of the major c o n s t i t u d , lithium, would cause large relative errors in t'he sodium determination.
Potentiometric Titration of Chloride. Titration of the lithium chloride in the alcoholic phase by the Volhard method, as described by TThite and Goldberg ( 5 ) , permitted rapid and precise determinations. By performing this titration potentiometrically-. the determination was further improved in speed and precision by elimination of the back-titration of excess silrer nitrate with potassium thiocyanate and the visual detection a t the end point of a pink color appearing in the prescnce of the silver chloride precipitate. The addition of 200 to 300 nil. of acptone or dioxane to the 2-ethyl-1-hexanol solution of the lithium chloride prevented the formation of two phases during the titration. and titration curves with very sharp breaks n-ere ob-
taincd in the organic mt,dium. With a glass reference electrode and a silver TI ire (silver-silver chloride) indicating electrode, the pH scale on the Beckman Model H-2 p H meter was used t o keep the reading on scale; no significance I\ as attached t o the individual p H readings, however. The decreased soluhility of silver chloride in alcohols ( 4 ) apparently enhanws the potentiometric measurement of the end point and causes the yery sharp break. Typical titration curves for 424 mg. of Iithiun~ chloride with 0.4N silver nitrate in aqueous and in the alcoholawtone media are shovn in Figure 1. Rwause initial p H values, and therefore the pH a t the end point, are dep w d e n t upon the experimental conditions, several titration curves should he plotted to determine the p H a t the rid point. K i t h the equipment used in this investigation, 11.3 T T H S taken the end point reading after the shape o the titration curve \vas established. When dilute silver nitrate solution n a q used to titrate small amounts of sodium chloride in the aqueous phases, the breaks in the titration curves were less sharp. Evaporation of the sodium chloride solutions until crj stal forniation started and addition of acetone improved the titration.. Effect of Diverse Ions. Hydrochloiic acid solutions, each containing 4.4 mg. of lithium and 44 mg. of one of 20 foreign metals, were analyzed foi lithium b y t h e described procedure and t h e total amount of chloride in t h e alcoholic solution n-as considered to be lithium chloride in t h e calculations. As expected, several of t h e metal chlorides mere soluble in 2-ethyl1-hcuanol and high results for the lithium nere obtained (Table 11). Alumin ~ u i i ,barium, and tungsten caused no interference; strontium and zirconium mused slight interference ; b u t plutonium, i wium, cadmium, chromium, cobalt, iron, magnesium, manganese, molybdenum, neodymium, nickel, rhodium, ruthenium, tin, and zinc caused serious interference. The separation of sodium from the metals was more effective. Yo interference in the determination ot 8.2 mg. of sodium was caused by 8 iiig. of plutonium or 10 mg. of cadmium. cobalt, iron, magnesium, neodymium, nickel, tin, uranium, or zinc, or 50 nig. of uranium. Honeier, 10 mg. of niolybdenum caused slight interference, and 44 mg. of aluminum, barium, cesium, strontium, tungsten, or zirconium, or 10 mg. of chromium, manganese, or rutheniurn caused serious interference. Thus, the dehj dration separation viould be of value in the determination of sodium in the presence of lithium and plutonium or the other noninterfering metals. Based upon the solubility data for potassium chloride in 2-ethyl-lhexanol ( I ) , it was assumed that potas-
Table I. Analytical Results for Known Samples of Lithium and Sodium
Found, yo Li Sa
Taken, hlg. Li Na 11.52
42.26
11.62
63.39
11.67
84.52
84.52
100.2 99.8 99.4 100.0 100.09 100.17 ~ . . 99.91 100.17
99.6
11.72
100.17 99.74 99.80 99.80
100.4 100.4 100.1 100.3
11.72
100.13 99.97 99.94 100.08 100.11 99.99
99.8 99.3 99.3 99.4 100.5 99.9
0.16
0.4
Average, % Standard deviation, %
99.7
99.5 99.8
Table It. Effect of Diverse Ions on Determination of Sodium and Lithium
Foreign Metal Taken, 3Ig. Found, % Li Pia RIg. Li ?;a 4.424 8 244 AI, 44 100 3 >ZOO Ba, 44 100 7 >ZOO Cd, 44 >ZOO Cs, 44 T i 3 >ZOO Cr, 44 >ZOO 4.424
4.424
co, 44 >zoo Fe. 44 >ZOO Jig, 44 >zoo Rln, 44 >ZOO 310, 44 >zoo Kd, Xi, Pu, Rh, Ru,
44 >ZOO 44 200 44 >zoo 44 >ZOO 44 >ZOO
8.244 Cd. 10 co, 10 Fe, 10 Rlg, 10
100 1 149.0 99.6 98.8 99.2
9.0
8.244 Rln, 10 Rlo, 10 S d , 10 Xi, 10 Pu, 8
180. 105.9 100.2 100.6 100.4
9.0
8.244 Ru, 10 Sn, 10 Zn, 10
Cr,’ 10
u, U,
10 50
APPLICATION
This method has been used for mort’ than 200 determinations of d i u m and lithium in hydrochloric acid solutions, and the results for these determinations were n ithin the precision limits d a t e d above. I n addition, the method a a s applied to the determination oi sodium in uranium chloride solutions containing other noninterfering ions. An aveiagt’ precision of about 0.5 relatiw yG \\as obtained for these determinations in the absence of interfering ions. ACKNOWLEDGMENl
4.424 8.244 Sr, 44 104.4 >200 Sn, 44 >ZOO IT, 44 100.9 >200 Zn, 44 >200 Zr, 44 95 3 >200 9.0
sium could be separated and determined similarly to sodium. Reliability. In t h e absence of standard samples or alloys of lithium and sodium, a lithium-rich mixture of t h e po\T-dered salts of t h e two alkali metals was analyzed b y the recommended procedure. Six samples of the mixture were dissolved in hydrochloric acid, and four aliquots of each solution were analyzed for lithium. The standard deviation betlveen samples was 0.6i%, but the standard deviation bet’ween aliquots was only 0.32%,. Thus, t,he anal>-tical method proved better than the mechanical mixing of the salts, and the reliability of thr method is indicated best 11y the standard deviat,ions of 0.16%, for the lithium and 0.4% for sodium for 17 dcterminations (Table I). Because of the two dehydrations in t’his procedure, the time per analysis is longer than required by the method of Khite and Goldberg ( 5 ) . K i t h adrquate equipment for handling 12 samples simultaneously. 24 determinations can be p‘rformed in about S hours.
>200
100.1 100.1 98.4 98.8
The authors xish to acknowledgc th(. assistance of C. F. Mete, under ~hosc. supervision this xvork was coniplrtrd, and H. H. TTillard for their sugystions and advice. The determinations of sodium in uranium were performed hy Ross D. Gardner of this laboratory. LITERATURE CITED (1) Caley, E. R., ilxilrod, H. D., 1x11. ESG.CHEII.,ANAL.ED.7 , 38 (1935).
(2) Caley, E. R., Elving, P., ”Inorganic Svntheses,” Vol. I, p. 1, McGraw-Hill, Sew York, 1939. (3) Hillebrand, V.F., Lundell, G. E. F., Bright, H. A,, Hoffman, J. I., “Applied Inorganic .4nalysis,” 2nd ed., p. 732, \.T iley, Xew York, 1953. (4) Seidell, A4.,“Solubilities of Inorganic and Metal Organic Compounds,” Vol. I, p. 4i, Van Sostrand, Sew York, 1940. (5) White, J. C., Goldberg, G., AXAL. CHEM.27, 1188 (1955). RECEIVED for review February 27, 1958. Accepted RIay 22, 1958. \Tork done under the auspices of the Atomic Energy Commission. VOL. 30, NO. 10, OCTOBER 1958
1629
