women is comparatively rich in tyrosine and tryptophan. This difference may be related to the metabolism of the estrogenic hormones; more work will be required before this can be elucidated. KO other protein fraction displays significant sex variation m-ith respect to aromatic amino acid content. LITERATURE CITED
(1) Beaven, G. H., Holiday, E. R.,
Advances i n Protein Chem. 7, 319-82 (1952). (2) Bendixin, G., Nord. M e d . 5 8 , 1488 (1957). (3) Broughton, P. M. G., Biochem. J. 6 3 , 207-13 (1956). (4) Goodwin, T. W., Morton, R. A., Ibid., 40,628-32 (1946). ( 5 ) Hewitt, L. F., Ibid., 30, 2229-36 (1936). (6) Holiday, E. R., Ibid., 30, 1795-1803 (1936). ( 7 ) Strickland, R. D., Mack, P. -4.,
Gurule, F. T., Fodleski, T. R., Salome, O., Childs, W. -4.,-4iv.4~.CHEM.31, 1410-13 (1959). (8) Waddell, W, J., J . Lab. Clzn. M e d . 48, 311-14 (1956).
RECEIVEDfor review July 23, 1959. Accepted November 2, 1959. Investigation supported in part by research grant (H2100) from National Heart Institute, National Institutes of Health, Public Health Service, Department of Health, Education, and Kelfare.
Potassium Phosphate as a Reagent for the Gravimetric Esti mati o n of Lithium C. C. PATEL
and
K.
N. VISHWESHWARAIAH
Department of lnorganic and Physical Chemisfry, Indian Institute o f Science, Bangalore 12, India
b Previous aftempts for the quantitative estimation of lithium as orthophosphate, employing an alkali metal phosphate, have not been successful. A method is described for the estimation of lithium as trilithium phosphate from 60% ethyl alcohol solution a t 65' to 70' C., employing potassium phosphate reagent at p H 9.5. The method is applicable in the presence of varying amounts of sodium and/or potassium cations and chloride, sulfate, nitrate, and phosphate anions.
L
as trilithium phosphate was determined previously by Mayer ( 5 ) ,who used an aqueous alkaline medium and a reagent consisting of a mixture of sodium phosphate and sodium hydroxide; however, Rammelsberg (6) and Fresenius (3) found that coprecipitation of sodium phosphate invariably occurred with the precipitates of trilithium phosphate. Furthermore, lithium phosphate was found to be appreciably soluble in the aqueous ammoniacal solution used for washing the precipitates. Recently, Caley and Simmons (2) attempted to modify Mayer's method by employing a n alcoholic medium to precipitate lithium as phosphate at about 65" C., but the method did not succeed. However, choline (a quaternary ammonium base) phosphate was successfully used by Caley and Simmons for the precipitation of lithium as orthophosphate from 50% isopropyl alcohol, but these authors pointed out that it was difficult to get a pure quaternary ammonium base suitable for the preparation of the phosphate reagent. I n place of choline phosphate, Vishweshwaraiah and Pate1 (7) employed ITHIUM
202
ANALYTICAL CHEMISTRY
the phosphate reagent of 8-diethylaminoethyl alcohol and obtained good accuracy for the gravimetric estimation of lithium. The purity of the precipitate was very high, even when it was precipitated from 60% ethyl alcohol in the presence of tripotassium phosphate. This suggested the possibility of employing the common reagent potassium phosphate for the quantitative precipitation of lithium in place of other expensive organic phosphates. Potassium phosphate is particularly suitable for the estimation of lithium, because its solubility in 60% ethyl alcohol is more than that of sodium phosphate. d study was undertaken to find out the limitations of using potassium phosphate for the gravimetric estimation of lithium. Initially, potassium phosphate reagent from p H 8 to 10 was tried, but the reagent a t p H 9.5 gave the best results for the precipitation of trilithium phosphate and was therefore used for subsequent work. EXPERIMENTAL
Reagents. LITHIUM CHLORIDE. A neutral solution of lithium chloride was prepared from lithium carbonate, purified by t h e method of Caley and Elving (1). ORTHOPHOSPHORIC ACID. Guaranteed Dure Merck Droduct of specific gravitj- 1.75. ETHYL ALCOHOL. Absolute ethvl alcohol was further purified by the method of Lund and Bjerrum (4). POTASSIUM HYDROXIDE, Analytical reagent quality pellets were employed. TRILITHIUM PHOSPHATE.Prepared by reacting lithium chloride solution with potassium phosphate as described in the procedure. The precipitate was washed free from potassium ions by 60% alcohol and dried a t 110" C. for about 24 hours.
WASHLIQUOR. Clear saturated solution, obtained by shaking 60% ethyl alcohol (by volume) with pure trilithium phosphate for about 12 hours a t room temperature, was employed for mashing the precipitates of trilithium phosphate. PHOSPH.4TE REAGENT. One hundred milliliters of 8% aqueous solution of potassium hydroxide were mixed 11-ith the required amount of orthophosphoric acid till the p H of the resulting solution was 9.5. PROCEDURE
The procedure employed is practically the same as the one employed in the determination of lithium as phosphate, using the phosphate reagent prepared from 8-diethylaminoethyl alcohol (7). Lithium chloride solut,ion (10.0 nil.) containing 5 to 50 mg. of lithium was made to 14 ml. nith distilled vater and mixed with 6 nil. of the potassium phosphate reagent in a borosilicate glass beaker of 250-ml. capacity. The beaker n-as covered with B borosilicate glass dish containing cold water, xi-hich was frequently changed in order to minimize the losses of vapors during heating. The mixture was placed on a water bath maintained between 65" and 70" C. One hour after the first appearance of the precipitate, 30 ml. of absolute ethyl alcohol were added to t h e beaker, xhile stirring, to make the concentration of the alcohol 60%. The heating ivas continued for another 2 hours after the granulated precipitate was obtained. The hot solution was rapidly filtered through a medium porosity porcelain sintered crucible that was previously ignited, cooled, and weighed. The precipitate n a s washed successively with small quantities of wash liquor until about 50 ml. of it were used, after which air was sucked through the precipitate to remove most of the n-ash liquor.
The precipitate was then ignited between 650" and 700" C. for about hour in an electric muffle, cooled in a desiccator, and weighed. The weight of trilithium phosphate was multiplied by 0.1798 to obtain the amount of lithium. A solubility correction factor of 0.36 mg. of lithium was added to the lithium value for every 50 ml. of the precipitating medium. The same procedure was employed in presence of sodium and 'or potassium salts. When the amount of lithium exceeded 50 mg. in the unknown sample, the initial solution was adjusted to 24 ml and 16 mi. of the phosphate reagent were added. The final volume was made to 100 ml. by adding 60 mi. of absolute ethyl alcohol. The rest of the procedure was the same as described above. Effect of Total Volume of Precipitating Medium. I n Table I, after application of the necessary solubility correction factor, the estimated values of lithium are given when the amount of lithium in the solution is varied. The results in the table refer to the average of a minimum of two experiments and, in general, the difference between duplicates did not exceed 0.3%. As in the case of the organic phosphate reagents ( 2 , 7 ) ,the amount of potassium phosphate reagent and the volumc of the precipitating medium have to be adjusted according to the quantity of lithium present in the solution. Effect of Alkali Metal Chlorides and Some Anions on Estimation of Lithium. The results for the estimation of lithium, n h e n determined in the presence of sodium and/or potassium, are giT en in Table 11. The results of Table I1 show that the method is applicable when the ratio of lithium to potassium is 1 to 4, the amount of lithium being about 24 mg. As explained earlier (4), the greater negative error in the presence of higher amounts of potasssium chloride is a result of chloride ions beyond 85 mg. (when present as potassium chloride), which are responsible for the increased solubility of lithium phosphate. Trilithium phosphate precipitated in the presence of different amounts of potassium chloride was examined spectrographically by employing a Hilger medium quartz spectrograph. Potassium was detected in the phosphate samples, only when the ratio of potassium to lithium n-as higher than 5. Even in such cases, a faint line of potassium was observed, indicating only a trace (less than O.IpI,) of potassium. The results of Table I1 also show that potassium phosphate reagent can be successfully employed up to a ratio of lithium to sodium as 1 to 4. On examining the ignited precipitate by the spectrographic method, potassium
Table 1.
Effect of Quantity of Lithium Chloride on Estimation of Lithium Final Vol. of Pptg. Medium before Filtering, Lithium, Rlg. Difference, M1. Taken Estimated Mg. 0.0 50.0 5.9 5.9 0.0 11.9 11.9 0 .0 23.7 23.7 35.6 35.5 -.o. 1 0.0 47.4 47.4 64.8 61.8 0.0 100.0
Reagent Added 6.0
16.0
was found to be absent in all the experiments, but traces of sodium were found when the ratio of sodium t o lithium was higher than 5. When both sodium and potassium are present, the maximum tolerance Table 11. Effect of Alkali Metal Chlorides on Estimation of Lithium
(Lithium taken in solution = 23.7 mg.) Ratio by Lithium Weight of EstiDifferLlto mated, ence, Error, illkali(es) Mg. Mg. % POTASSIUX CHLORIDE l:o 1:l 1:2 1:3 1:4 1:5 1:6 1:8 1:10
23.7 23.7 23.7 23.7 23.7 23.5 23.5 22.6 22.5
0.0 0.0 0.0 0.0 0.0 -0.2 -0.2 -1.1 -1.2
0.0 0.0 0.0 0.0 0.0 -0.8 -0.8 -4.6 -5.0
SODICM CHLORIDE
1:5 1:6
1:s
1:lO
23.5 23.1 22.6 22.5
-0.2 -0.6 -1.1 -1.2
-0.8 -2.5 -4.6 -5.0
POTASSIUM AND SODIUM CHLORIDES 1:O:O 1:l:l 1:2:2 1:3:3 1:4:4 1:5:5
23.7 23.7 23.7 23.7 23.5 22.8
0.0 0.0 0.0 0.0 -0.2 -0.9
0.0 0.0 0.0 0.0 -0.8 -3.8
without sacrificing the accuracy of lithium is 3 parts of sodium and 3 parts of potassium for every part of lithium (Table 11). Sodium and potassium were found to be absent spectrographically in the samplesof ignited precipitate, when the ratio of sodium to potassium to lithium was 3 to 3 to 1 and below. The method is applicable when sodium and/or potassium arc present as chloride, sulfate, nitrate, and phosphate in moderate amounts. The details are given in Table 111. In some cases even larger amounts of the anions could be tolerated in the determination of lithium. No alkaline earth or any other metal salts should be present with lithium, as they coprecipitate as phosphate with lithium. DISCUSSION
The success of the present method over Mayer's method (5) for the quantitative estimation of lithium is due to three factors: using as the reagent a mixture of tripotassium phosphate and potassium hydroxide (pH 9.5) in place of the mixture of trisodium phosphate and sodium hydroxide, using 60% alcohol as the precipitating medium a t 65' to 70" C., and using 60% alcohol saturated with lithium phosphate monohydrate for washing the precipitates instead of ammoniacal solution. The solubility of lithium phosphate in 60% alcohol is very small, while the solubility of other alkali metal phosphates is appreciable. hgain the solubility of the former decreases with the
Table Ill.
Effect of Anions on Estimation of Lithium (Amount of lithium taken in solution = 23.7 mg.) Alkali Metal Anion, Li Estimated, Difference, Salt Added Mg. Mg. Rig.
KatSO, NaNOa
NaCl NaOAc NazHPOc &SO4 KXOa K,PO, KC1
55 80 100 25 50 50 50 50 60
23.6 23.7 23.6 23.6 23.6 23.7 23.7 23.8 23.7
-0.1 0.0 -0.1 -0.1 -0.1 0.0 0.0 0.1 0.0
Error,
96 -0.4 0.0 -0.4 -0.4 -0.4 0 .0
VOL. 32, NO. 2, FEBRUARY 1960
0.0 0.4 0.0
203
rise in temperature, nhile that of the latter phosphates increases. The eniployment of 60y0 alcohol medium a t 65’ to 70” C. has, therefore, the dual advantage of effecting almost complete precipitation of lithium phosphate and keeping other alkali metal phosphates in the solution for the limits of tolerance specified above. Washing the precipitates 1%ith 60% alcohol saturated with lithium phosphate enables the removal of any potassium or sodium salts adhering to the precipitates, thus providing lithium phosphate of very high purity, as indicated by the spectrographic method. Although the use of alcohol in concentrations greater than 60% promotes coniplete precipitation of lithium phosphate, it increases the contamination of potassium and/or sodiuni n-ith the precipitates. On the other hand, alcohol of less than 60% strength increasc‘s the solubility of lithium phosphate and affects the accuracj- of the method adversely. Therefore, alcohol of 60% strength is most suitable for the precipitation of lithium phosphate, even in the presence of other alkali metal salts. An increase in p H of the reagent to 10 to 10.5 did not materially alter the solubility data of the lithium phosphate,
but the adsorption errors in the presence of interfering radicals increased markedly. The p H of the reagent has, therefore, been maintained a t 9.5, mhich gives rise to a final p H of about 8 after precipitation of the lithium phosphate from the 60% alcohol medium. Lowering the p H below this value increases the solubility of lithium phosphate rapidly and the analytical results are not reliable. Addition of the potassium phosphate reagent of p H 9.5 to a neutral solution of lithium chloridc was preferred, because in the Lawrence Smith method of extraction of lithium from minerals, the solution obtained is neutral. If any volatile acid is present in the solution of lithium salts, it is desirable to remove it by evaporation. If a n alkali is present within the limits of tolerance of sodium and potassium given in this paper, it can be neutralized with hydrochloric acid. I n the present method, the precipitation of lithium phosphate does not occur immediately upon the addition of the potassium phosphate reagent. It takes place after the mixture is kept on the water bath for about 5 minutes, which shows that it results from a homogeneous medium. Such a method of precipitation has many advantages
over the conventional method, because the change of conditions such as pH, concentration of ions, etc., occurs gradually and uniformly throughout the precipitating medium. This prevents the coprecipitation of sodium and potassium salts, minimizes the adsorption of their ions, and provides granular precipitates. ACKNOWLEDGMENT
The authors thank 11.R. A. Rao for his encouragement and interest in this n-ork. LITERATURE CITED
(1) Booth, H. S., ed., “Inorganic Synthesis,” Vol. I, p. 1, McGraw-Hill, Yew York, 1939. ( 2 ) Caley, E. R., Simmons, G. A, ANAL. CHEW25,1386 (1953). (3) Fresenius, K. R., Z. anal. Chem. 1, 42 (1862). (4) Lund, H., Bjerrum, J., Ber. deut. chem. Ges. 64, No. 1, 210 (1931). (5) hlayer, W., Ann. Chem. Liebigs 98, 193 (1856). (6) Rammelsberg, C., iinn. Phys. Chem. 102,441 (1857); 7, 157 (1879). (7) Vishweshwaraiah, K. S . ,Patel, C. C., J . Indian. Inst. Sci. 41,16 (1959).
RECEIYEDfor review May 28, 1959. hccepted October 7, 1959.
Spectrophotometric Determination of Vanadium as Tungstovanadic Acid GERALD W. WALLACE with M. G. MELLON Purdue Universify, lafayette, Ind.
b An intensely colored complex, formed by condensation of the oxyacids of tungsten(V1) and vanadium(V) in a 3 to 1 ratio, i s used as the basis for a spectrophotometric method for vanadium. The complex has its maximum absorbance at 392 mp. The color i s stable for at least 24 hours and follows Beer’s law over the concentration range of 2 X to 8 X 10-4M vanadium. Factors affecting color formation and the analytical applications to the determination of vanadium are presented.
A
of chromogenic reagents has been used in spectrophotometric methods for vanadium, including hydrogen peroxide (5) quinolinol and its derivatives ( I ) , strychnine (8), benzohydroxamic acid ( I I ) , and thiocyanate (3). Among the most sensitive spectrophotometric methods for this element are those based on the mixed VARIETY
204
ANALYTICAL CHEMISTRY
vanadium-containing heteropoly acids ( 1 2 ) or their reduction products. Simple heteropoly acids of vanadium have been known for many years, hut the analytical applications of these complexes to the determination of this element have not been explored. This paper presents the results of a spectrophotometric study of the practical applications of tungstovanadate. EXPERIMENTAL
Apparatus. A Cary recording spectrophotometer, Model 10-11, was employed for all photometric measurements along with matched absorption cells of fused quartz having an optical p a t h length of 1.000 + 0.002 cni. All p H measurements n-ere made on a Leeds &- S o r t h r u p glass electrode p H meter. Reagents. A stock solution of sodium tungstate, approximately 270, was prepared by dissolving 20 grams of t h e dihydrate in water and diluting t o 1 liter in a volumetric flask.
-1solution containing approximately 0.1 mg. of vanadium per nil. was prepared by dissolving O.li85 gram of purified vanadium pentoside (la)in 300 nil. of 0.1S sodium hydroside. The solut,ion was neutralized and acidific,d t o p H 6 with sulfuric acid and n-as then diluted to 1000 nil. in a volumetric flask. The solution u-as standardized by the method of Killard and Young (10). Color Reaction. I t is non- well known, because of the work of Souchay (8) and others (4, 6, 7 ) , that tungstic
acid will condense with vanadic acid to yield simple heteropoly acids. These het,eropoly acids differ from those commonly employed in analytical chemistry in that the ratio of coordinating groups to the central atom is less than 12 to 1. The unsaturated complexes have not, had previous analytical applicat ion. Recommended General Procedure.
CURVE. Prepare a calibration curve b y transfering 0, 0.5, 1.0, 5.0, 10, 15, and 20 nil. of a standard solution of sodium PREP.4RATIOK O F CALIBR.1TIOS
