Determination of Lithium in Aluminum by FIa me Photometry Utilization
of Organic Solvents for Submilligram Amounts
W. E. PILGRIM and W. R. FORD Reynolds Metals Co., Metallurgical Research Division, Richmond, Va.
b An improved flcime photometric method for the determination of submilligram amounts OF lithium in commercial purity aluminum i s described. The addition of isopropyl alcohol and acetone to acid solutions containing lithium and aluminum, permits the determination o f microgram levels of lithium without separations. The effects of various orcianic solvents and of some diverse ions on lithium emission are given. Background correction techniques, gas pressures, and solvent addition techniques are also described.
F
METHODS are often considerec when the determination of alkali metals is required. In spite of the inhei-ent sencitivity of these methods for alkalis, problems exist when aluminum is the base metal. It is desirable to have available an analytical procedure which eliminates the need for separations and incorporates the specificity and simplicity of flame methods. The improved flame photometric method t o be described accomplishes both t1ie.e objectivesanalysis without sepa -ation. The effects of aluminum on the flamp emission intensity for lithium ha1 e been investigated in detail (6, I O ) . Practically all observers noted a decrease in lithium emission whm aluminum was present. This effect, nhen only microgram quantities of lithium are present, make? qutntitatil e measIn urement a formidtble task. addition to Ion-er emic sion intensity, the presence of gross amounts of aluminum results in viscous so utions which are difficult to atomize 55th conventional burners unless thew ib considerable dilution. The minimum practical volume for a I-gram aluminum sample is 100 ml. With theze restrictions, the lowest lithium conceni ration measurable with confidence has bwn on the order of 10 pg. per gram, or O . O O l ~ o . It is apparent then that $1, more sensitive system is required if measurements are to be made a t O.OOOly& The ube of organic: solvent systems provides a coiiveiiieiit means for increasing the net emi:sion in the flame
LAME PHOTOMET IIC
for a number of elements. Methanol was used by Brewster and Claussen ( I ) , and Hourigan and Robinson (8, 9 ) , in their procedures for sodium in aluminum, which have been used extensively in the aluminum industry for several years. Other investigators (2, 3, 7 ) have utilized various solvents to enhance the emission of elements such as aluminum, boron, arsenic, antimony, bismuth, and lead, all notoriously difficult to excite. Solvents such as acetone, butanone, propanol, and ethanol are particularly effective for enhancement when employed singly, decreasing in effectiveness in the order named (3). Binary combinations of these solvents are even more effective. Organic solvents offer a twofold advantage in the analysis of aluminum. The enhancement effect is pronounced for lithium. In addition, the viscosity of the solution is considerably lowered, providing the proper solvents are chosen, resulting in a more regular and more rapid sample feed rate, with very little atomizer clogging. The utilization of certain organic solvent systems added to acid solutions of aluminum affords a means of increasing the sensitivity of flame photometric methods for lithium in aluminum down to a concentration of O . O O O l ~ e with reliability. It is felt that the method is equal in precision and accuracy to methods for other elements a t this level, and that the desired results may be obtained in a reasonable length of time. EXPERIMENTAL
Reagents. Standard lithium solutions were prepared by dissolving Johnson-Matthey “Spec-Pure” lithium carbonate, dried a t 110’ C., in a minimum amount of HCl, and diluting to volume with distilled water. The a h minum used in the preparation of standards was 99.9% XI, in which no lithium could be detected by emission spectroscopy; 99.57@aluminum may be used satisfactorily, provided i t is lithium free. All other reagents were reagent grade, and were used without further purification. Instrumentation. A Warren Spectracord, Model 3000, with Beckman
flame attachment and a 1P28 multiplier phototube was used. Procedure. For a sample containing from 0.0001 to 0.01% of lithium, a 1-gram sample is dissolved in 20 ml. of 1: 1hydrochloric acid. After solution is nearly complete, 1 ml. of 1: 1 nitric acid is added to oxidize the iron and aid in the solution of small amounts of copper. If the reaction proceeds slowly, 3 or 4 drops of a saturated solution of mercuric chloride may be added. After solution is completed, the volume is adjusted to 20 ml. and cooled. The solution is then transferred to a 100ml. volumetric flask. Twenty milliters of isopropyl alcohol, added incrementally, are used as a transfer solution. The volume is adjusted t o 100 ml. with acetone and mixed. The sample is excited with an oxygenhydrogen flame; the emission intensity is recorded and compared with calibration curves prepared from synthetic standards containing lithium-free aluminum. RESULTS
Precision and Accuracy. While the procedure described is simple, i t is capable of producing analytical data which are precise and accurate a t a level of less than 0.001%. Analytical working curves in the 0.0001 t o O.OOl’% range contain no points which vary from the curve by as much as 0.00005% Li. Any variation of these points from the curve is within the limits of instrumental reproducibility. It should be emphasized t h a t the standards used in the preparation of these curves are absolute. Aluminum in which no lithium is detectable is present in amounts equivalent to t h a t present in the samples. Known quantities of lithium are added. Repetitive analysis of numerous samples resulted in excellent precision. Comparative analytical data are given in Table I. All values shown were obtained at different times with different portions of the gross metallic samples being used. Working Curves. Analytical working curves prepared from samples containing 1 gram of aluminum, the organic solvents, and lithium ranging VOL 35, NO. 11, OCTOBER 1963
1735
/
U
/'ACETONEN
ISOPROPANOL O AI)
&AI)
0-100
MICROGRAMS
7 AC E T 0 N E
- I S 0 PR 0 P A N0 L
z z
-
0 v)
( I gr. ALUMINUM
Figure 1. Examples of calibration curves showing - effect of Li concentration on slope
from 10 to 100 pg. per 100 ml. exhibit the traditional shape of alkali metal curves, curving toward the concentration axis as self-absorption begins. However, on expanded curves covering the range of 1 to 10 pg. per 100 ml. and full instrumental sensitivity, a slight curvature toward the intensity axis occurs. Dean (6) mentions the possibility of this effect, and attributes it to a nonproportions1 increase in the number of neutral atoms entering the flame, in comparison to the number of ionized atoms. This type of curve has been obtained in every instance over the 1-to-10 pg, range. It is desirable to construct new fouror five-point calibration curves for every set of samples when determinations of less than 0.001% lithium are made since the instrument is operated a t maximum sensitivity. In the preparation of these curves, synthetic standards are made by dissolving lithium-free aluminum, adding the desired amount of lithium, and diluting with isopropyl alcohol and acetone. The exact reproduction of previously obtained curves is difficult a t this concentration. Examples of calibration curves are shown in Figure 1. DISCUSSION
Solvent System. I n choosing a solvent system for use in flame photometric measurements, the solubility of the salts in the organic material
Table 1.
-
-___. .I
(I
~~
0.0003-0.0004 0.0006-0.0006 0.0007-0.0009 0.0011-0.0013 0.0019-0.0022 0.0037-0).0040
Four determinations.
1736
ANALYTICAL CHEMISTRY
is of considerable importance. Attempts t o dissolve dehydrated salts in absolute solvents are generally unsuccessful, b u t 80% concentrations of many alcohols provide solutions usable with aluminum. Isopropyl alcohol solutions resulted in approximately threefold enhancement for lithium over aqueous solutions. Preliminary work had shown that in the absence of aluminum, acetone increased the intensity for lithium more than twice as much as isopropyl alcohol, indicating that a c e tone should provide an excellent medium for the determination. However, 1 gram of aluminum in a 80-ml. acetone20-ml. acid solution resulted in precipitation of salts, so a mixture of 20 ml. of isopropyl alcohol and 60 rnl. of acetone per 100 ml. was investigated. If the aluminum sample in acid solution is transferred to the volumetric flask with the isopropyl alcohol, acetone may be added, and a clear solution will result. The order of solvent addition must be maintained. This mixture gave a twofold increase in lithium intensity over isopropyl alcohol alone, and was used for all succeeding work. The relative intensities of lithium in the various solvent solutions investigated are illustrated in Figure 2. Excitation Conditions. The excitation conditions for the determination are quite critical, especially for values of less than O.OOlyo. Solutions containing organic solvents produce flame
lnterlaboratory Comparisons
Laboratorv A: 3- determins,t,inm . Mean Range
0.0003 0.0006 0.0008 0.0012 0.0020 0.0038
Figure 2.
% Li T .n.hnrn.t,nrvB; _"__
5 determinations
Mean
Range
0.0003
0.0003-0.0004
0,OOOG
0.0004-0.ooOG
0.0009 0.0012 0.0021 0.0041~
0.0008-0.0011 0.0010-0.0014 0.0021-0.0021 0.0040-0.0042
PRESENT)
Effect of various solvents
configurations t h a t are considerably different from those produced by aqueous samples, and the portion of the flame producing maximum emission intensity varies with the individual element to some extent (8). Flame heights were adjusted carefully, and the focusing mirror of the burner housing was profiled across the flame until maximum emission was obtained. Fuel and oxygen pressures were adjusted to give maximum sensitivity above background on samples containing 5 pg. of lithium per gram of aluminum in a 100-ml. volume. On the instrumentation employed, oxygen pressures of 12 to 13 p.s.i. and hydrogen pressures of 4 to 5 p.s.i. were satisfactory, Small variations in hydrogen pressure caused erratic results, so frequent monitoring of this setting was necessary. With these gas pressures, and a medium bore Beckman burner, the sample consumption rate was about 3 ml. per minute, allowing sufficient time for the spectrum to be scanned over the wavelength range of interest. Detectors. All samples were run by recording the spectrum from 700 to 650 mp, with the lithium line a t 671 mp being used for measurement. A 1P28 multiplier phototube was used as the detector. While this tube is not especially sensitive for measurements in this region of the spectrum, it has been reported (6) that the lithium response of this tube is approximately 20 times that of an ordinary red-sensitive phototube. Some 1P28 detectors were less responsive than others a t this wavelength. It therefore might be necessary to select a detector for this region. Background Correction. Considerable emission occurs in the 600to 700-mp. region when lithium-free aluminum solutions are examined. Pungor and Zapp (IO) have reported flame emission from aluminum oxide formed from aluminum chloride, being especially noticable at wide slit widths. While the slit widths used in our
experiments were mailer than those employed by Pungor and Zapp, the contribution of aluminum to total emission a t 671 mp had to be taken into account. The use of lithium-free metal as a blank caused erratic results when determinations a t low lithium levels, which required coiisiderable signal amplification, were attempted. However, the use of the bsse-line method of background correction, as described by Dean (4), gave much more reproducible measurements. I n tfis method, transmittance values are read first a t the emission peak, then a t wavelengths on either side of the penk, and the interpolated value is used as background. On recording instruments, a line is drawn through the nlinimum readings on either side of the peak. The background intensity is determined by the point of intersect of h i s baseline with a vertical line drawn through the transmittance peak (Figure 3). Interfering Elements. Cu, Mn, Cr, Ni, Zn, and T i in concentrations up t o 0.2% in aluminun: do not interfere from the standpoint of emission enhancement or suppression. However, if silicon is present in larger amounts, some atomizer clogging may occur. Filtration OIL a rapid paper is recommended in this case. Mag-
region are not usually found in aluminum. At the recommended slit width of 0.1 mm., resolution of the 671mp lithium line by the DU monochromator of the Spectracord is satisfactory.
1
ACKNOWLEDGMENT
The authors are indebted to C. A. Broyles, Reynolds Metals Co., Malvern, Ark., for his assistance in evaluating the method.
EMISSION INTENSITY
LITERATURE CITED
WAVE
Figure 3. rection
LENGTH
Method of background cor-
nesium suppresses lithium when present in amounts greater than 5%, but in pure metal, the effect is insignificant. Iron may be tolerated up t o 0.5%. Sodium has no measurable effect up t o 5 mg. per 100 ml. of sample. In the wavelength region under consideration, spectral interferences are at a minimum. Interference from the Sr lines a t 666 mp and 680 mp was not investigated, since this element and others having significant emission in this
(1) Brewster, D., Claussen, C., Iron Age 166, No. 18, 88 (1950). (2) Buell. B. E., ANAL. CHEM.34, 635
. (1962): (3) Dean, J. A., “Flame Photometry,” pp. 51-64, McGraw-Hill, New York, 1w,n. (4)bid., pp. 99-100. (5) Zbid., p. 103. (6) Zbid., pp. 155-60. ( 7 ) Dean, J. A., Carnes, W. J., Analyst 87, 743 (1962). (8) Hourigan, H. D., Robinson, I. W., Anal. Chim. Acta 13, 179 (1955). (9) Zbid., 16, 161 (1957). (10) Pungor, E., Zapp, E. Magy. Kern. Folvoirat. 66. 523 (1960): Anal. Abstr. . 8, $791 (1961). I ,
RECEIVED for review Bpril 22, 1063. Accepted July 25, 1963. Presented at 10th Detroit Anachem Conference, Detroit, Mich., Oct. 22, 1962.
Improved Spectrophotometric Method for the Determination of Small Amounts of Chloroform MARIANNA MANTEL, MAGDA MOLCO, and MARIANNA STILLER Israel Atomic Energy Commission, Rehovoth, Israel
b The reddish color obtained on addition of pyridine io a chloroform solution in the presence of sodium hydroxide (Fujiwara reaction) was used for the qualitative ond quantitative determination of chloroform. By measuring the absorbance at 366 mp instead of 525 mp, and working at optimum conditions established in this report, the sensitivity of the test was considerably increased. The method developed has a relative standard deviation of * 5 % and permits the detection of 0.2 p.p.m. crf chloroform. ADDITION of pyridine to an aqueous chloroform solution and heating for some minutes in presence of sodium hydroxide, a reddish color is obtained which depeids on the concentration of chloroform. The mechanism of the reaction :s not known, but the following has been proposed ( 3 ) .
N
0
-,,,OH;
N- C1 I
+ NaCl + H,O N=CH
CHONa
CHCI,
CHC12
Fujiwara (4) used this reaction for detecting chloroform in body fluids and tissues. ’ole ( I ) employed it for quantitative determinations. He compared the color produced by known concentrations of chloroform against standards obtained from acidified alcoholic fuchsin solutions. Though he claimed a limit of detection of 1 p.p.m., he reported that the accuracy of the method was practical only for higher chloroform concentrations. Later authors who determined chloroform in different media did not obtain a sensitivity better than 10 p.p.m. By establishing optimal working conditions, less than 1 p.p.m. of chloroform
in water was determined in the present work. A new absorbance peak, more intense than the maximum a t 528 mp used by previous groups (I-3), was discovered at 366 mp. The 525-mp peak and the red color faded on standing, while the 366-mp peak became more intense. Chloroform may be determined accurately by this method only in the absence of other polyhalogen compounds such as chloral and trichloroacetic acid, which react similarly. Under certain conditions, the interfering effect of these compounds can be allowed for. EXPERIMENTAL
All the chemicals used R-ere of analytical reagent quality. A Beckman DU spectrophotometer was used. Procedure. Pipet 5 ml. of a n aqueous solution containing from 2 VOL 35, NO. 11, OCTOBER 1963
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