Determination of lithium in a Magnesium Alloy by the Flame Photometer E. E. STRANGE Metallurgical Branch, U. S . Bureau of Mines, Rolla, Mo. metallurgical research and development of the ternary I magnesium-lithium-aluminum alloys, a rapid and accurate
EXPERIMENTBL WORK
N THE
analytical procedure for determining lithium is particularly important. The classic methods described in the literature require prior separation of lithium from the other light metals, which involves several time-consuming operations. This paper presents a rapid procedure for determining lithium by the Beckman flame photometer, in which isolation of the lithium is not necessary. Similarly, minor amounts of sodium and potassium do not significantly influence the accuracy of the lithium determination. The classic gravimetric method of Gooch ( 6 ) , the volumetric method of Rogers and Caley ( 9 ) , and the colorimetric method of Sandell (IO)all require that the alkalies be separated from other elements before the lithium is determined. The separations required in these procedures are not only time-consuming but also a possible source of error. Since lithium has such a distinguishing and characteristic flame spectrum, consideration was given to the possibility of the use of the flame photometer for direct quantitative analysis. Several references on the use of this technique for determining sodium, potassium, and calcium ( I , d , 4 , 5 , 7 , 8 , I I , I 2 )appear in the literature. The characteristic intense red line of the lithium flame spectrum occurs a t 670.8 mp (S),x-hile the most intense line for magnesium is a t 295 mp (3). Aluminum has no sensitive line in its flame spectrum. The impurities most likely to be encountered in these alloys are sodium and potassium, which are distinguished by intense lines a t 589 nip and 767 mp, respectively (3). il series of experiments on synthetic mixtures was undertaken to determine the extent of possible interference of these elements in flame analysis.
To determine the effect of various concentrations of magnesium, aluminum, sodium, and potassium ions on the intensity of the lithium flame, a series of solutions was prepared, with the lithium concentration held constant, and the magnesium, aluminum, sodium, and potassium concentration progressively increased. The observed intensities and concentrations are given in Table I. The figure denoted as % ’ T is read on the scale normally used in measuring transmittance of light but in this case is an empirical reading, representing the light intensity of the flame. An error of approximately 3 ~ 2 %was obtained when synthetic standard solutions of lithium were analyzed. These solutions contained magnesium and aluminum in approximately the same concentration as the standards used for calibration. This is shown in Table 11. The method gave good reproducibility on duplicate determinations. -4standard curve was made for each test. This is shown in Table 111. PREPARATION OF STANDARD CURVE
Since the experimental work showed that magnesium and aluminum ions influence the intensity of the lithium flame, a standard lithium solution was prepared that contained magnesium and aluminum ions. This stock solution was prepared by dissolving 5.3235 grams of lithium carbonate, 5.8026 grams of magnesium oxide, and 0.5000 gram of aluminum metal in 100 ml. of 1 to 1 hydrochloric acid and diluting to 1 liter. The additions were calculated t o furnish a solution containing 1 gram per liter of lithium, 3.5 grams per liter of magnesium, and 0.5 gram per liter of aluminurn, which would correspond to an alloy consisting of 20% lithium, 70% magnesium, and 10% aluminum. This composition waE selected to assure maximum accuracy Table I. Effect of Concentration of Sodium, Potassium, Magnesium, a t the higher lithium concent.rations. and Aluminum on the Intensity of the Lithium Spectruma For alloys containing less than 2% Na K N a + Kb Mg AI Mg + Ala lit.hium, the standard solution should ’ T Gram/l. % T Gram/l. 70 T Gram/l. % T Gram/l. % T Gram/l. % T Gram/l. % have more magnesium and aluminum 0.02 25 0.02 25 25 0.02 25 0,002 0.002 25 2; 0.002 present. Aliquots of this stock solu0.2 23 0.2 21 25 0.2 23 0.02 0.02 25 0.02 25 0.2 25 0.2 25 0.2 25 2.0 17 2.0 19 2.0 14 tion were diluted to supply a series of A pure lithium solution of 0.02 grain p e t liter a Solutions contained 0.02 gram per liter of lithium. standard solutions of lithium concentraconcentration gave 26% T. All % T readings have had a distilled water blank subtracted. t.ion varying from 0.001 to 0.05 gram 6 Concentration of each. per liter in increments of 0.005 gram per liter. A standard graph was prepared by plotting on logarithm paper PROCEDURE a blank of distilled water) us. concentrat,ion of the %- T (less . Dissolve the entire sample of 1 to 5 grams as soon as received lithium in these solutions. A straight line graph was obtained in 20 to 100 ml. of 1 to 1 hydrochloric acid. (Some lithium-conin this range of concentration, s u c h a graph will have to be taining are~unstable.) Transfer the solution to a 1-liter . . . . ~allow ~~~ volumkric “flask, cool, dilute to volume, and mix thoroughly. (If the lithium content is more than 1%, further dilution will be necessary.) Dilute the sample to a concentration within the Table 11. Accuracy of Method range of t.he standard graph. Lithium, Gram per Liter Replace the 2000 megohm resistor in the amplifier circuit of the Beckman instrument with a 10,000 megohm resistor. Place Present Found Deviation % Error the Beckman Model 10300 flame photometer in operation 0,0396 0.0398 0.0001 -0.25 allowing a 15-minute stabilizing period. Set the controls as 0.0396 0.0390 0.0006 -1.51 0,01584 0.01620 0.00036 +2.27 follows: sensitivity, clockwise limit; switch, 0.1 ; wave length, 0.01584 0.01580 0.00004 -0.25 670.8 mp; gas pressure, 2.5 cm.; air pressure, 19 pounds per square inch; oxygen pressure, 60 inches of water; and slit opening, 0.1 mm. (These conditions will vary slightly with gas, air, Table 111. Reproducibility of Method and oxygen pressures which must be adjusted for the best re% ’ Lithium sults.) Obtain % T readings for a blank (distilled water), a Alloy Test 1 Test 2 Test 3 Test 4 Average series of standard lithium solutions containing 0.001 to 0.05 h-0. gram per liter of lithium, and the samples. Plot the yo T of the 1 1.37 1.39 1.41 1.39 1.39 9.48 9.48 standard lithium solutions (less the blank) us. the concentrations 2 9.47 14108 13.99 14.02 13:93 on logarithm paper. Determine the percentage of lithium in the 3 13.93 samples from the resulting graph.
650
V O L U M E 2 5 , NO. 4, A P R I L 1 9 5 3
651
Table IV.
Composition of Alloys % AIgb R Alb 9% Na5 % Ka
Alloy KO. 7%Lia 3960 1.39 84.13 14.57 0.009 3964 2.77 81.30 lL45 0.014 3969 6.58 84.12 9.28 0.029 3957 7.37 81.54 11.11 0.020 3950 90.00 7.83 1.97 0.024 3958 88.42 9.47 2.08 0,013 3956 10.34 84.30 5.37 0.013 Li Ka,and K were determined by flame photometer b Mg and AI were determined gravimetrically.
0,005 0,014 0,032 0,020
0.030 0.025 0.023 method.
% Total 100.10 99.55 100.04 100.06 99.85 100.01 100.05
plotted for each series of samples to compensate for variables such as gas, air, and oxygen pressures. DISCUSSION AND RESULTS
The data in Table I shorn- that sodium and/or potassium, in the concentration studied, have no effect on the intensity of the lithium flame. The data also show that high concentrations of magnesium and/or aluminum have a depressing effect. A ratio of magnesium and/or aluminum to lithium of 10 to 1 depresses the intensity only slightly, while a ratio of 100 to 1 has consider’able effect. This damping can be compensated by adding a p proximately the same concentration of alloying elements to the standard solution used in obtaining the reference curve. The nominal composition of the alloy is usually known and is sufficient t o prepare a working standard. In operating the flame photometer, the slit opening used must be adjusted for the concentrations encountered. In this investigation, a slit opening of 0.1 mm. was satisfactory. A larger slit, using the same concentrations, would have increased the readings but would also increase the instability or certainty of the meter readings for the blank and sample. The flame photometer method for determining lithium is much
faster than the gravimetric, volumetric, or colorimetric methods, since all tedious separations are eliminated. The readings on the flame photometer can be made easily a t the rate of 5 per minute. Using a series of 12 standard lithium solutions, about 12 lithium samples can be completed 2 hours after initial solution of the sample. This would mean that a set of 12 lithium samples would be completed in 1 day compared to an estimated 3 or 4 days for the various other methods. The method described has been used for a number of magnesium-lithium-aluminum alloys (Table IV) and has given good results. It is particularly recommended for its speed without loss of accuracy. LITERATURE CITED (1) Barnes, R. B., Berry, J. W,, and Hill, W , B., Eng M f n . J . , 149, No. 9, 92 (1948). (2) Barnes, R. B., Richardson, D., Berry, J. W., and Hood, R. L., IND. ENQ.CHEM..A k v a ~ED., . 17, GO5 (1945). (3) Beckman Instruments, Inc., South Pasadena, Calii , Beckman Bull. 193-A. (4) Berry, J. R . , Chappel, D. G., and Barnes, R. B., I s n . EVQ. CHEM.,ANAL.ED.,18, 19 (1946). (5) Fox, C. L., ANAL.CHEY.,23, 137 (1951). (6) Gooch, F. A , , Am. Chem. J . , 9, 33 (1887). (7) Mosher, R. E., Bird, E. J., and Boyle A . J., ANAL.CHEM.,22, 715 (1950). (8) hfyers, A. T., Dyal, R. S., and Borland, J. W., Soil Sci. SOC. A m , Proc., 12, 127 (1947). (9) Rogers, L. B., and Caley, E. R., ISD.ENG.CHEX.,IXAL. ED., 15, 209 (1943). (10) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” p. 301, 1st ed., Xew York. Interscience Publishers, 1944. (11) Standford, G., and English, L., A P Q O I L . J., 41,446 (1949). (12) West, P. W., Folse, P., and Uontgomery, D., ANAL.CHEM.,22, GG7 (1950). RECEIVED f o r review February 27,1952. Accepted December 13. 1952
Colorimetric Determination of Rhenium A Modi$ed Tribalat Method JOSEPH M. BEESTON AND JOHN R. LEWIS Department of Metallurgy, University of Ctah, Salt Lake City, L‘tah like other less well-known metals, has received considerable attention of late. One of the important problems in rhenium studies where routine analyses are to be made is to find an accurate and rapid analytical method for its determination. A slight modification of the Tribalat ( 4 ) method developed in this laboratory has resulted in an appreciable saving of time with increased precision. The Tribalat method is based on the separation of rhenium from molybdenum by the formation of tetraphenyl arsonium perrhenate in an aqueous solution of pH 8 to 9. The extraction of the tetraphenyl arsonium perrhenate is made with chloroform. while molybdenum remains in the aqueous phase. The chloroform containing the rhenium is evapwated to a small volume, and the perrhenate ion is freed from the chloroform with concentrated hydrochloric acid. The colored thiocyanate complex of rhenium is made by addition of thiocyanate and stannous chloride. The colored thiocyanate complex is then extracted by adding isoamyl alcohol to form a mixture with the chloroform, and the per cent transmission is determined. In the Tribalat method, as originally outlined, considerable care must be taken in the evaporation of the chloroform containing the tetraphenyl arsonium perrhenate and development of the colored thiocyanate complex in order that the precision of 5 to 10% claimed by Tribalat can be realized. A fading of the colored HENIUM,
thiocyanate complex with time as the hydrochloric acid strength is increased has been noted by Geilman, Krigge, and Weibke (I), by Snyder (S), and by others. Since a volume of 25 nil. is convenient for the transmittancy determination of the colored solution on a Coleman or Beckman spectrophotometer, it wy&5 necessary to dilute the extracted solution; hence, an elimination of the evaporation step would decrease the chance of error and result in a considerable saving of time. After a number of experiments with 4.5, 5, 6, 7, and 8 .V hydrochloric acid, it was found that the quantitative separation of the tetraphenyl arsonium perrhenate in the chloroform can he best made in 6 N hydrochloric acid aithout evaporation of the chloroform. The quantitative separation in 6 *L’hydrochloric acid is apparently due to the removal from the acid solution of the perrhenate ions by the passing of the perrhenate ions into a reduced state as the colored thiocyanate complex. The quantitative removal of the perrhenate from the chloroform is fairly rapid. Solutions containing 0.8 microgram of rhenium per ml. showed the same optical density whether the color was developed for 10, 20, or 30 minutes. The colored thiocyanate complex is apparently very stable in the extract solution, since the extract solutions checked after 14 hours shon ed no significant change in the optical density. Three checks of the modified proceduie were made. .1 com-
