V O L U M E 28, NO. 10, O C T O B E R 1 9 5 6 9.2 and the extraction was finished in the normal manner. The result shown is in exact agreement with the result obtained by Procedure I. Statistical Analysis. A statistical analysis was made of the results from 49 determinations in three percentage ranges0.0 to 0.5, 0.5 to 1.5,and 1.5 to 7.0%. Only results obtained by the application of Procedure I to certified standards from the National Bureau of Standards and the British Bureau of Analyzed Samples were used. For the 0.0 to 0.57, range, on the basis of 25 determinations, 957, of subsequent determinations will be within f 0 . 0 0 6 from the true value. For the 0.5 to 1.57, range, on the basis of 11 values, 95% of subsequent determinations will be within f0.028 from the true value. For the 1.5 to 7.0% range, on the basis of 13 values, 95% of subsequent determinations will be within f0.14 from the true value. A statistical analysis was not made of the limited number of results obtained by the application of Procedure 11. However, the degree of precision and accuracy appears to be comparable to that obtained with Procedure I.
1527 LITERATURE CITED
Center, E. J., Overbeck, R. C., Chase, D. L., ANAL. CHEM. 23. 1134-8 (1951). Classsen, A., ‘Bastings, L., Visser, J., A n d . Chim. Acta 10, 373-85 (1954). Divenpori, WT’H., Jr., ANAL.CHEM.21, 710-11 (1949). Gentry, C. H. R., Sherrington, L. G., Analust 71,432-8 (1946). Grandfield, J. hl., “Critical Study of Spectrophotometric Determination of Aluminum with Ferron (7-Iodo-8-hydroxyquinoline-5-sulfonic acid) ,” LIIT Engineering Laboratory, Watertown, Mass. Hillebrand, W. F., Lundell, G. E. F., Bright, H. A., Hoffman, J. I., “Applied Inorganic Analysis,” 2nd ed., p. 508, Wiley, Xew York, 1953. Kassner, J. L., Ozier, hI. A., A N A L . CHEM.23, 1453-5 (1951). hferritt, L. L., Jr., Walker, J. K., Zbid.. 16. 387-9 (1944). Talvitie, N. A , Ibid., 25, 604-6 (1953). Wiberley, S. E.,Bassett, L. G., Ibid., 23, 609-12 (1949). Youden, W. J., ”Statistical Methods for Chemists,” Chap. 5 , Wiley, New York. 1951. RECEIVED for review December 10, 1955. Accepted June 14, 1956. Division of Analytical Chemistry, 127th Meeting, ACE, Cincinnati, Ohio, March 1955.
Spectrophotometric Determination of lithium P. F. THOMASON Ana/ytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tenn.
Microgram amounts of lithium can be determined spectrophotometrically by using 0-(2-hydroxy-3,6disulfo-1-naphthy1aao)-benzenearsonic acid aa the chromogenic agent in a potassium hydroxide solution of water and acetone. Calcium and magnesium cause little interference in amounts less than 10 times the amount of lithium present. Sodium can be tolerated in amounts up to 50 times the amount of lithium. Results on determinations of lithium chloride solutions show an accuracy within the order of =k30/,.
M
ICROGRAM amounts of lithium are usually determined by flame photometry or by emission spectrography. A spectrophotometric determination was desired as a supplementary method, permitting laboratories that do not have a flame photometer or emission spectrograph to analyze samples for small amounts of lithium. A colorimetric procedure by Sandell (7 j depends upon the indirect estimation of lithium by the precipitation and isolation of the slightly soluble salt, LiKFeI06, followed by a thiocyanate colorimetric determination of the iron. This method requires the separation of lithium from other metals and is somewhat tedious. Kuznetsov ( 5 ) has described visual tests in which 12 organic compounds were used that give colored complexes with lithium. He states that 0-(2-hydroxy-3,6-disulfo-l-naphthylazo)-benzenearsonic acid (Thoron) forms an orange-colored complex with lithium in a strongly alkaline medium. He compared, visually in daylight, the color of solutions that contained approximately 2 y of lithium per ml., 3 drops of a O.lyOsolution of Thoron, and 2 to 3 drops of a 20% potassium hydroxide solution with the color of a solution that contained only Thoron. Nikolaev (6) extended the work of Kuznetsov to determine from 0.2to 25.0 mg. of lithium in a 50-ml. volume by visual comparison. He states that this method suffers from the difficulty of matching colors, “which can only be done in daylight.” Therefore, it seemed worth while to study this reagent with the
objective of developing a spectrophotometric procedure for lithium. As a result, two methods were developed for the determination of lithium. The method for aqueous medium, applicable in the range from 10 to 80 y per 10-ml. final volume, consists of the following steps: addition of potassium hydroxide solution to a 1ml. aliquot of sample, dilution to approximately 8 ml. with distilled water, addition of Thoron reagent, dilution to 10 ml., and measurement of the absorbance a t 458 mp us. a reagent blank. The procedure for the acetone-water medium, applicable from 1 to 10 y per 10-ml. volume of a 7070acetone-307, water (v./v.) is the same, except that 7 ml. of acetone is added after the addition of the potassium hydroxide solution, and the absorbance measurement is made a t 486 mfi us. a reagent blank. Thoron in an acid medium has been used as a chromogenic reagent to determine thorium (8), zirconium (S),and fluoride
(4). EXPERIMENTAL
Reagents. Acetone, reagent grade. Potassium hydroxide, reagent grade. o-(2-hydroxy-3,6-disulfo-l-naphthylazo)-benzenearsonicacid (Thoron), obtained from Fine Organics Inc., 211 East 19th St., S e w York 3,. K.Y. Standard lithium chloride solution. A stock solution was made by dissolving 42.4 grams of reagent grade lithium chloride in distilled water and diluting the solution to 1 liter. It was standardized gravimetrically by the sulfate method ( 2 ) . An aliquot of this solution was diluted to give a solution containing- 10 y of lithium per ml. Apparatus and Technique. A Beckman Model DU quartz spectrophotometer with Corex cells of 1-em. lengt,h was used for all absorbance measurements. The wave-length scale of the spectrophotometer was recently calibrated wit6 a hydrogen discharge lamp. All solutions were diluted to a final volume of 10 ml. before being transferred to the cells. Care was taken to keep the IO-ml. volumetric flasks stoppered after the addition of acetone in order to minimize losses from evaporation. Spectral band widths of 18.4and 12.4mp were used for the aqueous and acetonewater media, respectively, in the determination of standard calibration curves.
A N A L Y T I C A L C H EM I S T R Y
1528
Procedure. The following procedure was used for the analysis of aqueous samples for lithium. The sample solution was adjusted to contain from 1 to 10 y of lithium per ml. A 1-ml. aliquot was pipetted into a 10-ml. glass-stoppered volumetric flask. .4 0.2ml. aliquot of a 20% potassium hydroxide solution was added and 7.0 ml. of acetone carefully pipctted into the flask. One milliliter of a 0.2y0solution of Thoron was added and the solution was made up t o volume with distilled mater and mixed carefully. The pressure caused by the acetone vapor was relieved by loosening the stopper. The solution \vas allowed to stand 30 minutes, and the absorbance was then measured a t 486 mp us. a reference solution that contained all of the reagents. The amount of lithium was found by reference to a calibration curve that was obtained by plotting the ahsorbmce of lithium standards treated by this procedure. DEVELOPMENT OF METHOD
Spectral Properties. The spectral absorption of an alkaline aqueous solution of Thoron reagent (Figure 1, curve A ) , as ]vel1 as a similar solution containing lithium (Figure 1, curve B ) , was measured with water as reference. The reagent blank was prepared by diluting 0.2 ml. of a 0.2% aqueous solution of Thoron plus 0.2 ml. of a 2Oy6 potassium hydroside solution to a final volume of 10 ml. with distilled ivater. The lithium solution v a s prepared by adding to these combined reagents 80 y of lithium, the volume being maintained at 10 ml.
Lo
The measurement of the spectral absorption of the solutions containing 707, by volume of acetone showed that the absorption spectrum of the lithium-Thoron complex is shifted to longer w i r e lengths, and that the maximum absorption appears as a hroad peak from 480 to 490 mp. The acetone-water solutions (Figure 2) contained the same amount of reagents as the solutions xi-hose spectral data were shown in Figure 1. Conformity to Beer's lanin the acetone-water medium was obtained within experimental error from 0 to 6 y of lithium per 10 ml. The deviation of the ai)sorbanee from Beer's law at 10 -, of lithium per 10 ml. was :tbout 6.0%. Effect of Reagent Concentrations. The optimum amount of potassium hydroxide was found by adding various amounts of a 10% potassium hydroxide solution to 80 -1 of lithium plus 1.0 ml. of 0.2% Thoron solution and measuring the absorbance of this solution a t 458 mp us. a reference solution that contained equal concentrations of potassium hydroxide and Thoron. The amount of 10% potassium hydroxide solution that produced the most absorbance was found to be 0.4 nil. The optimum amount of Thoron reagent was found by adding various amounts of a 0.2% solution of the Thoron reagent to a solution that contained 80 -/ of lithium and 0.4 ml. of 10% potassium hydroxide solution. The absorbance of this solution was measured a t 458 mp us. a reference that contained the same iaoncentration of reagent and potassium hydroxide. The data shoiv that 1.0 ml. of a 0.27, solution of Thoron in a final volume of 10 nil. is the optimum amount. The amount of reagent also doc3 not seem to be very critical, an excess being more desirahlc than a deficiency.
0300 -
W A V E LENGTH, m p
-
7 -
Figure 1. Abborption spectra of Thoron and lithiumThoron complex in potassium hydroxide solution Curve A
B
C
Test Solution Reagent blank Reagents lithium Reagents lithium
++
Reference Water Water Reagent blank 320
The spectral data showed rather broad peaks in the region where the lithium-Thoron complex absorbed more energy than the reagent. When the absorption spectrum of a n alkaline solution containing reagent plus 80 y of lithium was measured against the alkaline reagent solution, absorption peaks were found a t 350 and 458 mp (Figure 1, curve Cj. Only 0.2 ml. of the Thoron solution was used to obtain the absorption spectrum instead of the optimum 1-ml. volume reqllired for the calibration curve, because a more dilute colored solution v a s required in order to measure the absorbances a t the shorter wave lengths. When a calibration curve of lithium concentration us. absorbance was plotted, it was found that the system conformed within experimental error to Beer's law in the concentration range from 0 to 40 y of lithium per 10 ml. The data in this region fall on a linear plot through the origin. At concentrations greater than 40 y of lithium per 10 ml. the absorbance v a s less than indicated by Beer's law; however, the data seemed to be reproducible. Several organic so1vent.s were tried without success in an effort to isolate the colored lit'hium-Thoron complex. The comples was insoluble in chloroform, butyl alcohol, and amyl acetate. However, when acetone was added to an aqueous solution of the complex, an intensification and change in hue of the color resulted. The addition of acetone was suggested by the work of Crouthamel and Johnson ( I ) , who used an acetone-water medium t o increase the sensitivity of the colorimetric thiocyanate method for the determination of uranium.
340
360
380 W4VE
400
420
440
460
480
500
LENGTH, m p
Figure 2. Absorption spectra of Thoron and lithiuniThoron complex in potassium hydroxide-acetone-water medium Curve A B C
Test Solution Reagent blank Reagents lithium Reagents lithium
+ +
Reference Water Water Reagent bIanK
I t J\ as necessary t o ascertain the optimum amount of acetone for color intensification of the lithium-Thoron complex This was accomplished by adding various amounts of acetone to 10 y of lithium plus 0.2 ml. of a 20y0 potassium hydroxide solution. Thoron reagent was added last. The order of addition of the reagents was not critical, but a more stable color was produced if the Thoron reagent was added last. The absorbances of the solutions were measured a t 486 mp vs. a reference that contained the same concentration of acetone, potassium hydroxide, and Thoron, The data show that the absorbance of the solution of the lithium-Thoron complex increases with an increase in the concentration of acetone. The limit is approximately 7070 by volume of acetone. A dark immiscible liquid is formed if the acetone concentration is as much as 80% by volume. Also, 70y0 acetone ensures sufficient allowance for the volumes of the aque011s reagents to be added.
V O L U M E 28, NO. 10, O C T O B E R 1 9 5 6 The optimum amounts of Thoron and potassium hydroxide were also determined for the acetone-water medium in the manner described for the aqueous medium, except that the absorbances oi the solutions were measured a t 486 mp. The data show t h J t 0 2 ml. of a 20% potassium hydroxide solution and 1.0 ml. of a 0.2'A aqueous Thoron solution in a final volume of 10 ml are the optimum amounts. Time Stability of Color Intensity. The color of the lithiumThoron vomplex in aqueous medium seemed to be fully developed after a 30-minute period and did not change in intensity for at least 4 hours. The absorbances of some solutions were measured 18 hours after color development and were unchanged. A solution containing 10 y of lithium and the optimum concentrations of r e a g e n t s 4 . 2 ml. of 20% potassium hydroxide, 1.0 mi of 0.2'; Thoron, 7 ml. of acetone, and sufficient water to make 10 mlwas measured periodically a t 486 mp against a reference solution containing all the reagents except lithium. The following data were obtained: Time after Color Development, llinutes 3
Id 30
45 60
Slit Width, Mm. 0 68 0 6.3 0 59 0 57 0 55
Absorbance, Lithium Test Solution 0,770 0.830 0.860 0.860 0.860
Theye data shorn- that color development of the lithium-Thoron complex in the acetone-water system is complete after 30 minutes and that the absorbance of the solution is stable for a t least 1 hour. When an aqueous reagent blank (no acetone added) was compared against the reference solution containing acetone, m t e r , and Thoron, a n increase in absorbance n-as also noted until 30 minutes had elapsed. Tiiue after Color Development, Minutes 5 15 30 45 60
Slit Width.
iMm. 0.3 0.26
0.25 0.25 0.25
Absorbance, Aqueous Reagent Blank 1 .05 1.25 1.45 1.45 1.45
The development of the colors and the measurement of the absorbances of all the soliitions v-ere made a t room temperature (approximately 25" C.). Some of the aqueous solutions were heated to 50" C. for 5 minutes, then cooled to room temperature before measurement of the absorbance. The temperature cycling from 25' to 50" and back to 25' C. apparently did not affect the filial absorbance. Interferences. Both calcium and magnesium form Thoron complexes in aqueous medium that also absorb strongly a t 458 mp. Iiuznetsov ( 5 ) recommended the addition of an alcoholic solutioii of sodium oleate t o destroy the color of the calcium complex or the separation of calcium and magnesium from lithium by fluoride precipitation. h carbonate precipitation of calcium and magnesium is also effective in separating them from lithium. To check this, a solution that contained 100 y each of calcium, magnesium, and lithium in 3 ml. was treated with 0.2 ml. each of a 2054 potassium hydroxide solution and a 10% potassium carbonate solution. The volumes were adjusted t o 5.0 ml., the solutions allowed t o stand for 2 hours, and the precipitates of calcium and magnesium carbonate centrifuged off. The supernatant liquid containing the lithium was analyzed by the procedure described previously. Lithium was recovered to within =k35:. The interferences of calcium, magnesium, cesium, rubidium, and sodium were investigated by adding known amounts of the chloride salts of these element,s t o a known amount of lithium. acetone, and reagents, and measuring the absorbances of the solutions us. an acetone-reagent reference. The results are Phon-n in Table I. These data show that interference from calcium, magnesirim, rubitii\iin, and cesium in the concentrations studied is not serious
1529
in the acetone-water medium Sodium a t 100 to 1 ratio gives :I 5% positive error. Any ion that precipitates with potassium hydroxide must be absent. Ammonium chloride in amounts of 5 mg. or more caused a positive error, but the interference could be removed by heating the sample with 0.2 ml. of the 207, potassium hydroxide solution to drive o f f the ammonia before adding the other reagents. RESULTS i N D DISCUSSION
.iqueous samples of different lithium chloride concentrations n'ere submitted as unknowns to a referee analyst for determina-
tion by the acetone-m-ater medium procedure. The stock lithium chloride solution from which the samples were prepared had heen standardized gravimetrically by the sulfate method ( 2 ) . The analyst prepared dilutions of these samples and determined lithium on suitable sized aliquots by the above procedure. The results are presented in Table 11. The data indicate that the acciiracy of the procedure with the acetone-water medium is vithin zk3mo.
The samples listed in Table I1 viere also submitted to another laboratory for analysis by means of an internal-standard flame photometer. The flame photometer used for these analyses was designed and built a t Oak Ridge Xational Laboratory. It employs a Beckman burner, Reckman Model B monochromator, a photomultiplier detector, and a Brown recorder. A solution of .odium chloride was used as an internal standard. The results obtained with the flame photometer agreed to within f 2 7 , of the amount of lithium present This shoived that the accuracy of the Ppectrophotometric method is of the same order as that of the flame photometric method.
Table I. Alkaline and Alkaline Earth Interference in Standard Color Developing Medium" Salt CaClz CaCh LiCl MgCh hlgC1,
NaCl NaCl NaCl NaCl
+ LiCl
NaCl
+ LiCl + LiCl
NaCl
+ LiCl
RbCl RhCl
+ LiCl
Concentration, y Metal/Ml.
Absorbance a t
Li Equivalent,
10 10
0.090
1 .o
0.842
9,s 0.4 10 0
486 mp
1 .o
10
0.40 0.860
1?I . "n 50 100 500 50 1 0
..
-
2.0
..
0.0
.. ..
0,540 0,840
0 900
10.5
+ 5 0
0 960
12.0 (approx.)
+20 0
0.220
1.0 10 I? n
%
1.0 2.4 6.0 9.8
0,080
100 1 0 500
Error
Y
0 000 0 860
-
..
2.0
0 0
10'0
0.0 Color developing medium and reference solution: 0.2 ml. of 20% potassium hydroxide solution; 7 ml. of acetone: 1.0 ml. of O.2y0 solrition of Thoron; water t o make a total voliiine of 10 nil. a
Table 11. Results of Spectrophotometric Determination of Lithium in Lithium Chloride Solutions Sample uo
___
Lithliim
1
Present 1 07
2
0 21 0.118
0.103 0.064 0.058
0.043 0.022
~
\\Iz/?fl-
Dlff
~
round 1 10 1 05 0 23 0 21 0 118 0 116 0 100 0 098
0 0 0 0 0 0 0 0
066 062 058 058 045 046
023 023
%
5;
:
+9 8 0 0
-1.2 -3 0
-4.0 +3.0 -3.0 0
n f4.0 f6.0 +5.0 +5.0
ANALYTICAL CHEMISTRY
1530
Use of the acetone-water medium has the advantages of greater sensitivity and reproducibility over use of the aqueous medium. ACKNOWLEDGMENT
The author is indebted to Cyrus Feldman for his translation of the Russian papers by Kuznetsov and Nikolaev and Sorokina, and to Helen P. Raaen for her careful editing of the manuscript.
Analysis,” vol. I, 5th ed., p. 888, Van Nostrand, New York, 1939.
(3) Horton, A. D., ANAL.CHEM.25, 1331 (1953). (4) Horton, A. D., Thomason, P. F., Miller, F. J., Ibid., 24, 548 (1952).
(5) Kuznetsov, V. I., Zhur. Anal. Khim. 3, 295 (1948). (6) Nikolaev, A. V., Sorokina, A. A., Doklady Alzad. Nauk S.S.S.R. 77, 427 (1951). (7) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” 2nd ed., p. 414, Interscience, Kew York, 1950.
(8) Thomason, P. F., Perry, LITERATURE CITED
(1) Crouthamel, C. E., Johnson, C. E., ANAL.CHEM. 24, 1780 (1952). (2) Furman, N. F., ed., “Scott’s Standard Methods of Chemical
M.A., Byerly, W. M., ANAL.CHEM.21,
1239 (1949). RECEIVED for review February 2, 1956. Accepted M a y 26. 1956.
Ultraviolet Determination of Phenolic Antioxidants in Rubber COE W. WADELIN Research Division, Goodyear Tire and Rubber Co., Akron 16, O h i o
The determination of phenolic antioxidants in rubber extracts by direct ultraviolet measurement is subject to interference from other extractable materials. However, in extracts of both hot and cold GR-S, the only substances which undergo a change in ultraviolet spectrum as a function of basicity are phenolic antioxidants. By choosing a wave length where the antioxidant has a low absorptivity in neutral solution and a higher absorptivity in 0.1N potassium hydroxide solution and measuring the increase in absorbance, the amount of antioxidant can be accurately determined. The method has been applied to Wingstay S (alkylated phenol), 2,6-di- tert-butyl-4-methylphenol, and methylenebis-2,2’-(6-tert-butyl-4-methylphenol).
APPARATUS AND REAGENTS
Absorbance measurements were made with a Cary Model 11 spectrophotometer and a Beckman Model DU spectrophotometer. Measurements were made in matched pairs of 1-em. quartz cells, using solvent blanks in the reference cell. Absorption intensities are plotted in terms of absorptivity, a, liter gram-’ centimeter -l(2).
1
T H E determination of phenolic antioxidants in synthetic rubber by ultraviolet absorption is subject to interferences from other materials present in the samples. This interference is more serious than that encountered in measuring amine antioxidants because the absorption maxima of the phenols occur a t shorter wave lengths than the maxima of the amines, and the absorptivities of the phenols are about one tenth as great as those of the amines. Ultraviolet measurements have been made on extracts of the rubber samples (6)or by complete solution methods (S). I n either case some average factor must be used to correct for the background absorption. The background for the complete solution method varies greatly with different samples, and accurate determinations cannot be made by using an average correction factor (4). Thr background absorbance correction for Wingstay S (alkylated phenol, Goodyear Tire and Rubber Co.) analysis by the extraction method has been found to range from 0.065 to 0.124. This is sufficient variation to cause about 15% error in the analysis. Coggeshall and Glessner have found that the ultraviolet absorption peaks of various phenols in ethyl alwhol solution are shifted about 20 mp toward longer wave lengths when the solutions are made basic and that the absorptivities a t the absorption peaks are increased (1). This shift is in the proper direction to minimize the interference shown in Figure 1. Therefore, this technique was applied to the determination of Wingstay S in Plioflex 1006 (copolymer of butadiene and styrene made at 122’ F.) and Plioflex 1502 (copolymrr of butadiene and stjrene made at 41’ F.).
220
300 WAVE LENGTH, m g
260
340
380
Figure 1. Ultraviolet absorption spectra A. B. C.
D
Wingstay S in absolute ethyl alcohol Wingstay S in 0.1N potassium hydroxide in absolute ethyl alcohol Plioflex 1502, antioxidant-free, absolute ethyl alcohol extract Plioflex 1006, antioxidant-free, absolute ethyl alcohol extract
A liM solution of potassium hydroxide was prepared by grinding 22.4 grams of the reagent with 50-ml. portions of absolute ethyl alcohol until it had dissolved and then diluting to 400 ml. with absolute ethyl alcohol. The solution was allowed to stand overnight for potassium carbonate to settle. When the solution began to turn yellow, it was discarded. The solution should be between 0.9 and 1.1N. Commercial grade antioxidants were used without purification. PROCEDURE
Sheet the sample to a thickness of 0.020 inch and cut into strips 1 x 5 cm. Weigh a &gram sample and add it to 100 ml. of absolute ethyl alcohol, one strip at a time to prevent sticking together. Reflux for 60 minutes, pour off the solvent, add 100
