E factor, in acetonitrile has indicated a general increase in the sensitivity of chronopotentiometric measurements in acetonitrile with respect to aqueous solutions. A second advantageous feature of acetonitrile as a solvent for chronopotentiometry is the small temperature For the oxidation coefficient of of dilute anthracene (10-3M) in acetonitrile it was approximately 0.8% .per degree centigrade [for the anodic oxldation of one of the sulfa drugs in liM perchloric acid it m s about 2% (S)]. Anodic Chronopotentiometry of Sulfanilamide. The anodic chronopotentiometry of sulfanilamide and several of its dcrivatives has been studied in this laboratory (9). The best precision attained in several reproducibility experiments v*3s =t1% ( u ) relative in the values of In the present investigation, the oscillographic-short transition time technique n-as applied to the ancdic oxidation of sulfanilamide in 1ilf perchloric acid. Although the transition time plateau is initially rounded, the method of measurement described in the section on procedure is satisfactory. The precision of two reproducibility experiments is not as shown in Table IV. These results are again indicative of
a significant improvement in the precision of chronopotentiometric measurements by the short transition time technique. Concentration us. The long transition time investigation of concentration ws. T1’2for sulfanilamide revealed that the chronopotentiometric constant increased with increasing transition time. Thus, the plot of P2 v s . concentration was biased upward when T112was plotted as the ordinate. I n light of the present study, this upward bias can probably be accounted for by mass transport by partial cylindrical diffusion, The results described below for the short transition time technique seem to support this conclusion. Table V shows the values of the constant, K , over a twofold range of concentration a t a constant current of 3.278 ma. The T 112 values are averaged from three to five T measurements a t the temperature quoted. When the values of the constant are corrected to an average of 21 O C. by the 2% per degree temperature coefficient previously observed for the sulfa drugs (9), the constant becomes 0.585 2~1.9%( u ) . The values of the constant shown in parentheses are those calculated from measurements of long transition times in the
same concentration range a t a total current of 494 pa., a t the same platinum wire electrode. The high values of these constants, with their significant variation over a narrow concentration range, are indicative of cylindrical diff usion in contrast to approximate linear diffusion indicated by the short transition time constants, which are essentially constant within the limits of experimental error. LITERATURE CITED
(1) Delahay, P., Berzins, T., J.Am. Chem. SOC.75,555 (1953). (2) Delahay, P., Mamantov, G., ANAL. CHEM.27, 478 (1955). (3) Laitinen, H. A., Ferguson, W. S , Ibid., 29, 4 (1957). (4) Laitinen, H. A,, Gaur, H. C., Anal. Chzm. Acta 18, l(1958). (5) Lund, H., Acta Chem. Scund. 1 1 , 1323 (1957). (6) Nicholson, M. M., J . Am. Chem. Soc 76,2539 (1954). (7) Reille , C. N., Everett, G. W., Johns, CHEM.27, 483 (1955). R. H., (8) Voorhies, J. D., Furman, N. H., Zbid , 30, 1656 (1958). (9) Voorhies, J. D., Parsons, J. S., ACS Meeting in Miniature, North Jersey Section, Jan. 27, 1958. RECEIVEDfor review April 26, 1958. Accepted October 10, 1958. Taken from a thesis b John D. Voorhies to the Department of 8hemistry, Princeton University, in artial fulfillment of the requirements of Zoctor of philosophy, June 1958.
KNAL.
Spectrophotometric Determination of Aluminum with 2-Qu iniza rinsuIfo nic Acid (Sodium Sa It) E. GUY OWENS I1 and JOHN H. YOE Pratt Trace Analysis laboratory, Department o f Chemistry, University o f Virginia, Charlottesville, Vu.
,The
reaction of 2-quinizarinsulfonic
acid (sodium salt) with aluminum ions in methanol to give a stable violet complex has been applied to spectrophotometric determination of aluminum. Tolerance levels of 46 diverse ions are reported. The method has been successfully applied to several bronze and steel samples after preliminary separation of interfering ions from aluminum with the mercury cathode. Interfering ions not thus re~ , Ti+4, moved are: Be+2, s ~ + Th+4, Y+3, Zr+4, F-’, and the rare earths. Optimum concentration of aluminum in bronze or steel is from 0.01 to 1.0%. The reaction has a spectrophotometric sensitivity of 1 part of aluminum in 50,000,000 parts of solution. The spot plate sensitivity is 0.25 y, with a concentration limit of 1 to 200,000; spot paper sensitivity is 0.5 y, the concentration limit being
384
ANALYTICAL CHEMISTRY
1 to 100,000. Effects of exposure to light, variation of temperature, and order of addition of reagents are negligible. Maximum wave length of absorbance for the violet complex is a t 560 mp and Beer’s law is obeyed up to an aluminum concentration of 1.7 p.p.rn. Development of the method included consideration of mole ratio of the complex, acidity, and rate of complex formation.
T
HE reagent, lJ4dihydroxy-2-anthra-
quinone sulfonic acid (2-quinizarinsulfonic acid), has been investigated in the colorimetric determination of beryllium in aqueous medium ( 1 ) and quinizarin, the parent compound, in the fluorometric determination of beryllium (2). 5,8-Dichloroquinizarin has been recommended for the qualitative detection of aluminum ( 5 ) . This paper describes
a new spectrophotometric method for the determination of aluminum based on a sensitive color reaction of 2-quinizarinsulfonic acid (sodium salt) with AI + 3 in methanol. A methanol solution of the reagent is bright yellow. Small amounts of aluminum in absolute methanol react t o form an intense violct complex suitable for spectrophotometric measurement. Interfering ions are separated from aluminum with the mercury cathode. An aliquot of the electrolyte is then evaporated and the residue taken up in absolute methanol. The color is developed in the methanol solution. The complex is stable, no protective colloid is needed, and no heating step is required. The use of methanol solvent without buffer presents a minor disadvantage in adjusting acidity but with a little practice no adjustment is necessary.
APPARATUS
Spectrophotometer. Beckman Model DU, with matched 1-em. Corex cells. pH Meter. Beckman Model G, with Beckman general-purpose glass electrode. Standardize with Fisher standard buffer solution, p H 4.0. Power Source for Mercury Cathode. Any regulated direct current will serve. A Fisher Electroanalyzer was used in this investigation. Electrolysis Cell for Mercury Cathode, fabricated according to the directions of Scherrer and hfogerman (8). Commercial models are available from the Fisher Scientific Co. -4watercooling jacket was uscd. REAGENTS
Reagent Solution. Dissolve 0.16 gram of 2-quinizarinsulfonic acid (sodium salt, orange-red) (LaMotte Chemical Products Co., Chestertown, 31d.) in absolute methanol with a maximum water content of 0.1%. I t may be necessary to pulverize small particles with the tip of a stirring rod t o aid solution. Make up to volume i l l a 500-nil. volumetric flask and transfer to a polyethylene bottle. This solution is stable under ordinary laboratory conditions. The reagent can be dried at 100” C. in an oven or in a rieeiccator over anhydrous magnesium perchlorate. I n either case, it should he weighed by difference, as it gains n eight rapidly on exposure to the atmosphere. If the reagent is to be iivd routinely and the equilibrium n-ater content is known, it may be weighed directly without prior drying. The small amount of water present will not interfere. Standard Aluminum Solution. Add 6.95 grams of reagent grade aluminum nitrate, A1(?J03)3 9 H p 0 , t o a 500-ml. volumetric flask. Cover the salt with 200 ml. of absolute methanol, add 10 ml. of concentrated hydrochloric acid t o dissolve, and dilute t o the inark with methanol. The solution should contain 1 mg. of aluminum per 1111.
It is important to standardize the solution a t the same time the calibration curve is made, to avoid error caused by temperature fluctuation. Pipet 25nil. aliquots of the aluminum solution to be used for standardization, and a t the same time take aliquots for further dilution and preparation of the calibration curve. Evaporate the 25-ml. portions, ignite to aluminum oxide, and neigh. Solvent. Mallinckrodt AR grade absolute methanol was used. The water content should not exceed 0.1%. Acids. C.P. hydrochloric, sulfuric, and nitric acids with specific gravities of 1.19, 1.84, and 1.42, respectively, were used. Doubly Distilled Water. The rvater nhich must be free of aluminum, n as distilled twice, then passed through a column of Amberlite I R 120 exchange resin. The residue obtained by evaporation of an aliquot
of the water in a silica dish can be conveniently tested for aluminum in a procedure similar to that for determination of aluminum in steel and bronze.
I
I .o
I ’
M
1
I
1
EXPERIMENTAL
Procedure for Steel and Bronze. After separation of interfering ions, pipet a 5- to 15-mi. aliquot of the aqueous electrolyte solution, which contains 2 to 15 ml. of concentrated sulfuric acid per 100 ml., into a silica evaporating dish. (Each aliquot should contain 30 to 70 y of aluminum.) Evaporate to near dryness on a hot plate at low temperature. When fumes of sulfur trioxide appear, increase the heat and continue to dryness. Cool, and add 10 drops of concentrated hydrochloric acid and 5 drops of concentrated nitric acid to the white, barely visible residue. A red or brown residue a t this point indicates incomplete removal of iron. Swirl the dish to ensure contact of the acid with the residue and wash down the sides with a fine stream of water from a polyethylene wash bottle. The total volume should be about 5 ml. Again evaporate over low heat to an estimated 1 drop of solution. Cool, take up the moist residue in 5 ml. of absolute methanol, and transfer with the aid of a funnel to a 50-ml. volumetric flask. Use a rubber policeman to ensure solution of all residue, The dish must be washed thoroughly with a fine stream of methanol from a wash bottle. If the dish and funnel are held in one hand with the dish in a vertical position, the entire transfer, including washing under the lip of the dish, can be made without moving the dish from its upright position. Test the dish for aluminum by adding a few drops of the reagent stock solution. If any residue remains, a violet color will result. Add 10 ml. of the reagent stock solution and dilute to 50 ml. with methanol, Upon dilution, the acidity should decrease to a value within the desired limits of “pH” 0.3 to 0.5. If the solution is on the basic side of these limits, adjust the acidity to “pH” 0.3 to 0.5 with the aid of a p H meter by touching the stopper of the flask with a dropper containing concentrated hydrochloric acid. With a little practice no adjustment is necessary. The aliquots used to fill the small sample vial of the p H meter may be discarded. Allow the solution to stand 1 hour and read the absorbance a t 560 mp against a reagent blank. Determine the amount of aluminum from a calibration curve. Absorbance Curves. I n Figure 1, typical curves of wave length us. absorbance of a solution containing 1 p.p.m. of aluminum and its reagent blank are compared. Measurements were made in 1-cm. cells against methanol as a blank. Acidity. The acidity of the methanol medium is regulated with the aid
530
550
WAVE L E N G T H ,
570
MU
Figure 1. Typical wave length absorbance curves in methanol A.
8.
1 . 8 5 X 1 0 - 4 M 2-quinirarinsulfonic acid (sodium salt) 3.7 X 10-6M aluminum ( 1 p.p.m.) in presence of A
of a p H meter standardized with aqueous buffer. Limits are from 0.3 to 0.5 on the pH scale of the meter and are obviously not true pH units. These limits nere selected for several reasons. In a more acid medium, the color develops slonly and in the extreme case not a t all; in a more basic medium, the color develops faster but decays slowly after reaching its maximum absorbance. Kith increasing basicity, interferences of foreign ions, especially ferric iron, increase, and the color of the reagent solution changes progressively from yellow to red then to blue. As the color of the aluminum complex is violet, the red and blue would interfere with its intensity measurement. Final adjustment of acidity should be made n it11 hydrochloric acid from the basic side of the limits. Color Development. Under the conditions specified in the procedure for steel and bronze, the color reaction is 96% complete after about 5 minutes. The color intensity increases slowly, reaching a constant maximum after about 40 minutes. The colored complex is stable for a t least 2 weeks. Effects of exposure t o light, order of addition of reagents, and variation of temperature over the range 15’ to 35’ C. are negligible. Beer’s law is obeyed up t o 1.7 p.p.m. of aluminum. Sensitivity. Using Sandell’s expression for sensitivity (r),2-quinizarinsulfonic acid (sodium salt) has a sensitivity of 0.002 y aluminum per sq. cm., corresponding to 1 part of aluminum in 500,000,000 parts of solution. For general use, however, the practical VOL. 31,
NO. 3,
M A R C H 1959
385
sensitivity of the reagent is 0.02 y aluminum per sq. em.-Le., 1 part of aluminum in 50,000,000 parts of solution representing an absorbance of 0.010 unit. Spot plate sensitivity is 0.25 y aluminum (0.05 ml.) with a concentration limit of 1 to 200,000. Sensitivity of drop test on filter paper is 0.5 y aluminum with a concentration limit of 1 to 100,000. The paper is warmed over a flame to develop a violet spot. Test solutions must be in methanol (acidity of sample solution adjusted to “pH” 0.3 to 0.5). For both tests, detection by fluorescence (yellow of the reagent alone changes to red in the presence of aluminum) is equally sensitive. Many ions interfere (Table I). Different lots of 2-quinjzarinsulfonic acid (sodium salt) may show differences in sensitivity for aluminum. X o difference was found between Eastman practical grade and a Du Pont experimental lot, but two lots prepared by sulfonation of quinizarin (Eastman
technical grade) by Marshall’s method (6) were 30% and lo%, respectively, less sensitive. They had a brownish cast instead of orange-red. The sensitivity of the reaction in absolute methanol is of the same order of magnitude as in diethyl ether and absolute ethanol, but in water the color does not develop a t low aluminum concentrations. Mole Ratio. The mole ratio of the complex in solution has been established as 1 t o 1 by three methods, all in good agreement: the mole ratio method of Yoe and Jones (9), the continuous variations method of and the slope ratio method Job (4, proposed by Harvey and Nanning ( 3 ) . Figure 2 shows results obtained by the mole ratio method (9). The aluminum concentration was constant a t 3 . i X lO-sM and increasing amounts of reagent solution were added. Interferences. Solutions of diverse ions were made up in absolute meth-
r Table I.
I
I
I
Interference of Diverse Ions
Ion Added Be f 2 c u +2 Fe + 3
% Error at 1 P.P.M. A1
Concentration, P.P.M.
La +3
s c+3 Th+4 Ti +4
Y+3 Zr +4 F -1
Oxalate Po4-3 (ortho) P107-4(pyro)
so4 -2
Tartrate
0 60 2 10 20 0 0 0 0 0 0 100
0 0 0 2 5 2 2 2 5 0
1 0 60 100
0 2 0 0
+ 7 0 $ 4 0 + 2 0 + 5 0 + 4 0 + l o f 5 O + 3 0 + 3 5 $ 4 0 -28 0
2
0
color developed
No
- 2 5 - 5 0 - 2.5
KO color developed
Table II. Analysis of National Bureau of Standards Samples
Type of Sample Manganese bronze
Aluminum, yo NBS Founds
NBS 62
1.13
1.12 1.13
Basic open hearth steel NBS 13 c
0.019
0.013 0.013 0.013
Basic open hearth steel S B S 14 c
0.022
0.022 0.022
0.54
0.52 0.52 0.50
Silicon bronze NBS 158
Aluminum bronze NBS 164
I /
MOLE ALlJMlNUM
ANALYTICAL CHEMISTRY
13
1 2 TO
1’4
RATIO REAGENT
Figure 2. Determination of mole ratio of complex in methanol Aluminum concentration constant (3.7 X 1 O-6M or 1 p.p.m.) 2-Quinirarinsulfonic acid (sodium salt) concentration varied
Table Ill.
Precision of Transfer and Color Development Alumi-
Aliquot num Dev. from No. (All Found, Av., ( d ) , Square of % 76 Dev., ( d I 2 5 MI.) 1 2
1.13 1.13 1.11 1.11 1.12 1.13 1.13 1.12 1.10 1.15 10 Av. 1.12
KO.of detns. 10 Av. 1.12
6.22
6.22 6.23
Each value represents average of three aliquots of separate sample.
386
I
Range 1.10 to 1 1 5 NBS value 1.13
+o . O l
0.0001 0.0001 0.0001 0.0001 0.0000 0.0001 0.0001 0.0000 0.0004 0.0009 Sum o.0019
+0.01 -0.01 -0.01 0.00 +o. 01 +0.01 0.00 -0.02 + O . 03
anol and stored in polyethylene bottles, the nitrate or chloride being used for metallic ions, if available. Most solutions of the anions were prepared from sodium or potassium salts. Results are summarized in Table I. Ferric iron causes a smoky blackpurple color. 811 other positive interferences cause a red or violet eolor. The rare earths (cerium through lutetium) have not been quantitatively investigated, but it is clear from qualitative tests that they interfere. The folloning ions cause a maximum of 5% error a t the 1-p.p.m. aluminum level when present in a concentration of 100 p.p.m.: Au+3, Ba+?, Bif3, Ca+2, Cdf2, Cof2, Cr+3, Fe+2, GaL3, Hg+2, InL3, K+l, Li+’, Alg+*, Mn+?, Si+*, Fb+2, Pd+?, Sb+3, Snf2, Snf4, Sr’?, U02+2, Zn+*, borate, bromide, citrate, iodide, and sulfide. Sitrate causes only a 2% error a t a concentration of 300 p.p.m. and silicate is not appreciably soluble in methanol. Potassium a t 100 p.p.m. caused precipitation of excess reagent but only 2% interference with the color of the aluminum complex. Kot more than 3 ml. of water can be tolerated in 50 ml. of solution; larger amounts will hinder color development. Porcelain crucibles or evaporating dishes caused high results for aluminum; silica evaporating dishes (10-cm. diameter) are recommended. Contamination from ordinary glassware was avoided by storing solutions in polyethylene bottles. The use of a mixed solvent of 10 ml. of diethyl ether and 40 ml. of absolute methanol will reduce the interference of ferric iron. In this mixed solvent, 60 p.p.m. of ferric iron interferes to the extent of only 2% a t the 1-p.p.m. aluminum level but details have not been worked out. Separation. A large number of metals can be conveniently separated from aluminum in one step by electrolysis a t the mercury cathode. Scherrer and Mogerman used this method for separation of interfering ions prior t o determination of aluminum nith hluminon (8). Their recommended separation procedure was used without modification in this work, but the electrolysis cell was fitted with a water-cooling jacket and the electrolysis times used, with a current of 4 amperes, were 4 hours for a 0.5-gram steel sample and 1 hour for a 0.5-gram bronze sample. These are not necessarily minimum times, as a considerable safety margin was allowed. Interfering ions not removed by the mercury cathode are: Be+*, S C + ~ , Th’4. Ti+4, Y + 3 ,Zr+3, F-l, and the rare earths. Results. Several bronze and steel samples issued by the Sational Bureau of Standards were analyzed objec-
tively-Le., they were issued t o E.G.O. as “unknowns.” The precision of the procedure, not including the weighing step or preliminary separation of interfering ions from aluminum, was determined by analyzing ten aliquots from the same sample solution after separation of interfering ions. Results for sample 62 (Table 11) are shown in Table 111. These values show that the change from aqueous medium to absolute methanol, with subsequent development of the color,
can be accomplished nith good precision. LITERATURE CITED
W.,Neuman, W. F., hlulvran, B. J., ANAL. CHEM.21. 1358 (1949). . ( 2 ) Fletcher, hl. H., White, C. E., Pheftel. hl. S.. IND. ENQ. CHEM.. AKAL.ED. 18. 179 119461. (3) Harvey, A: E.,’ Manning, D. L., J. Am. Chem. SOC.72, 4488 (1950). (4)Jcb, P., Ann. chim.9, 113 (1928). ( a ) hul’berg, L. M., Mustafin, I. S.,
(1) Cucci, hl.
Dokladg Akad. h-auk S.S.S.R. 77, 285 (1951). (6) Marshall, P. G., J . Chem. SOC.1931, 3206. ( 7 ) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” 2nd ed., Interscience, New York, 1950. (8) Schemer, J. A., Mogerman, W. D., J. Research Natl. Bur. Standards 21. 103 (1938). (9) Yoe, J. H., Jones, A. L., IKD.ENG. CHEM.,AZAL. ED.16, 111 (1944).
RECEIVEDfor review August 5, 1958. Accepted October 29, 1958.
Spectrographic Determination of Aluminum, Iron, Potassium, and Silicon in Tungstic Oxide THOMAS
J.
VELEKER and RUDOLPH DYCK
Chemical & Mefallurgical Division, Sylvania Elecfric Producfs Inc., Towanda, Pa.
b A method for the quantitative spectrographic determination of aluminum, iron, potassium, and silicon in tungstic oxide uses a powder-pellet spark technique for the aluminum, iron, and silicon determinations, and a high voltage alternating current arc method for potassium, The spark excitation of tungsten in the presence of graphite powder causes an erratic formation o f tungsten carbide, making it very difficult to obtain reproducible intensity ratios using secondary tungsten lines for internal standardization. In contrast, the addition of anhydrous sodium tungstate as a buffer in the preparation of the pellets gave a very reproducible spark which stopped the formation of tungsten carbide during the sparking cycle. This procedure i s used as a routine control for aluminum, iron, potassium, and silicon, in one step of the manufacture of tungsten wire and rod from the ore. It covers the following elements and ranges, aluminum 0.005 to 0.1 5%, iron 0.008 to O.l%, potassium 0.025 to 0.35%, and silicon 0.02 to 0.50%.
T
HE manufacture of tungsten wire and rod for the electrical industry requires stringent control of the physical and chemical properties throughout the stages of manufacture from the ore to the finished product. Tungsten cannot be smelted like many of the common nonferrous metals because of its high melting point. Consequently, the process involves a combination of hydro- and powder metallurgy. The control of impurities is very important in the chemical processing steps from the digehon of the ore in sodium
rial to tungstic oxide. After firing, the material is blended on a Wig-L-Bug t o ensure homogeneity. One part of sample, 1 part of SP-1 Rational graphite powder, and 1/4 part of pure anhydrous sodium tungstate by volume are mixed thoroughly for 30 seconds in a Wig-LBug using a 1-inch polystyrene vial and 3/8-in~hPlexiglas ball. Enough sample is mixed to conveniently prepare two pellets ( 3 / l e x inch) on an Applied Research Laboratories’ briquetting press. The pellets are prepared by applying 1000 pounds’ pressure (28,000 p.s.i.) for 5 seconds, the pressure is released, and then applied for 10 more seconds. A firm pellet weighing about 180 mg. is formed. Samples are sparked in duplicate. DETERMINATION OF POTASSIUM. The blue oxide is oxidized as above. The resulting tungstic oxide is mixed with dry lithium sulfate. Six hundred milligrams of tungstic oxide are mixed with 100 mg. of lithium sulfate in the KigL-Bug for 30 seconds. The sample is then loaded in triplicate into brass electrodes by tamping. ELECTRODES. High purity graphite electrodes are used for the aluminum, iron, and silicon determinations. The sample-bearing electrode is inch in diameter by 3/4 inch in length, with crater 0.194 inch in diameter and inch in depth The counter is a 15” cone-shaped electrode inch in diameter and 3/4 inch long. For the potassium determination, the electrodes are made from free machining yellow brass rod inch in diameter. The sample-bearing electrode is ‘/4 inch in diameter by inch long, with a PROCEDURE crater inch in diameter by l / s inch in Sample Preparation. DETERMI- depth. The sides have a 15” slope, so that a t the crater top the wall thicknesb NATION OF ALUMINUM, IRON, AND SILIis 0.010 f 0.002 inch. The brass counter CON. The blue oxide (WaOll) is oxidized electrode is the same design as the in fused silica combustion boats a t graphite counter electrode above. 750” C. for 1 hour to convert the matehydroxide to the final gray metal powder. The metallurgical and chemical makeup of the tungsten metal powder greatly influences the quality of the wire and rod made from it ( 7 ) . This spectrographic method offers a rapid, precise control of aluminum, iron, potassium, and silicon in one of the processing steps. Pure tungstic oxide is produced and then partially reduced to the tungsten blue oxide (W4011). At this stage several of the impurities which have been removedalumina, potassium, and silica-are carefully added in trace amounts to the blue oxide. These additives act as cleansing agents during the sintering of the ingots pressed from the powder and also as grain growth inhibitors. The iron is a residual impurity and must be controlled, because it lowers the recrystallization temperatures in tungsten Tire (6, ‘7). The blue oxide is then reduced in hydrogen to the gray metal powder. Little has been published on the spectrographic determination of trace impurities in tungsten. Pheline (6) lists a method for determining silicon in tungstic oxide using a 220-volt, 2ampere arc. Gentry and Mitchell (1) and Lounamaa (3) describe the analysis of tungstic oxide using direct current arc excitation with graphite as a buffer, and formation of tungsten carbideduring the arcing cycle.
YQl. 31, NO, 3, MARCH 1959
387
