idue from a 2-assay-ton sample indicated the presence of 0.01 troy oz./ton of osmium. I t is realized that more reproducible results probably could have been obtained for ruthenium and osmium in osmiridium, and for ruthenium in the precious metals concentrate, by gravimetric analysis of the final fractions rather than by the more convenient spectrophotometric methods which involved relatively large dilution factors. However, the results given show clearly the suitability of the proposed methods when applied to those sample materials. INTEGRATION O F PROPOSED M E T H O D S I N T O ANALYTICAL SCHEME FOR SIX PLATINUM METALS
Csually, when ruthenium and/or osmium are to be determined in materials such as ore concentrates, mineral specimens, meteorites, or metallurgical products, it is also desirable to determine the other four metals of the
platinum group. Consequently, it is necessary to have a flexible scheme of analysis that will permit the determination of all six metals on a single sample, but a t the same time permit unnecessary operations to be readily bypassed when only partial analysis is required. Such a scheme is presented in Figure 2 in the form of a flow-sheet in which it is shown how the proposed methods for ruthenium and osmium can be combined with those previously reported for platinum and palladium ( 5 ) , gold ( 6 ) ,rhodium ( 7 ), and iridium (9). ACKNOWLEDGMENT
The author acknowledges the assistance of P. E. Moloughney in certain aspects of the work described in this paper. Thanks are due to Falconbridge Nickel Mines, Ltd., for supplying the copper-nickel matte, flotation concentrate, and the precious metals concentrate, and also to Johnson, Matthey
and Co., Ltd. (Toronto), for supplying the osmiridium used in the present work. LITERATURE CITED
(1) Coburn, H. G., Beamish, F. E., ANAL.CHEM.28, 1297 (1956). (2) Currah, J. E., Fischel, A,, McBryde, W.A . E., Beamish, F. E., Ibid., 24, 1980 (1952). (3) Faye, G . H., Ibid., 37, 259 (1965). (4) Faye, G. H., Inman, W. R., Ibid., 31, 1072 (1959). (5) Ibid., 33, 278 (1961). (6) Ibid.. D. 1914. ( 7 j Ibid.; '34,972 (1962). (8)Ibid., 35, 985 (1963). (9) Faye, G. H., Inman, W. R., Lloloiighney, P. E., Ibid., 36, 366 (1964). (10) Kavanayh, J. AI., Beamish, F. E., Zbid , 32, 490 (1960). (11) Marks, A. G., Beamish, F. E., Ibid., 30, 1464 (1958). (12) Van Loon, J. C., Beamish, F. E., Ibid., 36, 872 (1964). (13) Westland, A . D., Beamish, F. E., Microchim. Acta 5 , 625 (1'357). (14) Zachariasen. H.. Beamish. F. E.. ASAI,. CHEM.34, 965 (1962). ' RECEIVED for review January 18, 1965. Accepted LIarch 1, 1965.
Determination of SmaII Amounts of Nitrite by Solvent Extraction and Spectrophotometry ANTHONY FORIS and THOMAS R. SWEET Department of Chemistry, McPherson Chemical laboratory, The Ohio State University, Columbus, Ohio The reactions between 8-aminoquinoline and nitrous acid and quinoline diazonium ion and 8-aminoquinoline have been investigated. Reaction parameters for both the diazotization and the coupling processes have been determined. A method for the determination of small amounts of nitrite is proposed which is based on the above reaction sequence and employs solvent extraction and spectrophotometry. The sensitivity of the method, expressed as the weight of nitrite resulting in an absorbance of 0.010 a t 465 mp in a 1-cm. cell, is 0.151 pg. This corresponds to a sensitivity of 0.0038 p.p.m. if a 40-ml. sample of unknown is used.
available for the detection and determination of nitrite and nitrite precursors have recently been reviewed by Kolthoff and Elving (3) and Sawicki, I'faff, and Stanley ( 4 ) . I n other publications Sawicki compared the merits of 36 new methods ( 5 ) and discussed a series of autocatalytic methods for the determination of nitrite through free radical chromogens (6). The reaction sequence employed in the present method consists of two steps. The first step (I) involves the
condensation of the amino group with nitrous acid.
+
HNOz
+ H@ -+
"2
+ 2Hz0
(I)
111
N
I n the second step (11), the diazonium ion couples with excess 8-aminoquinoline to yield a highly colored azo dye.
ETHODS
Elemental analysis of the isolated reaction product yielded results that agreed well with the theoretical values (found: C, 72.03%; H , 4.43%; K, 23.29% ; calculated : C, 72.22%; H, 4.38y0; 5 , 23.40y0). The compound decomposed at 183" C.
432 I 0
EXPERIMENTAL
Apparatus. T h e absorption curves shown in Figure 1 were made on a Cary Model 14 recording spectrophotometer. All other absorbance measurements were made with a D U spect ro p ho t o m e t e r Be c kman equipped with a photomultiplier a t tachment. The absorbance of all solutions was measured in Beckman 1-cm. quartzwindow absorption cells fitted Kith ground glass stoppers. h l l pH measurements were made with a Beckman Model G pH meter with general purpose glass electrodes and fiber junction calomel reference electrodes, or with Beckman combination electrodes, S o . 39183. Reagents. 8 - AMINOQUINOLINE REAGEKT SOLUTIONS.The reagent solution for procedure I mas prepared by dissolving 2.00 grams of 8-aminoquinoline (Eastman Organic Chemicals No. 4033) in 5 ml. of concentrated hydrochloric acid and a minimum of water. The solution was made ul) to 100 ml. with demineralized double-distilled water. For procedure 11, a 2% solution of 8-aminoquinoline was prcpared which also contained 50 ml. of glacial acetic acid per 100 ml. of solution. STANDARD
SITRITE
SOLUTIO
stock solution was prepared by dissolving 1.500 grams of reagent grade S a S O , VOL. 37, NO. 6, M A Y 1965
701
16 p g . of KO2-, add 8 Vnl. of the 2% 8-aminoquinoline acetic acid reagent solution. The pH of the solution should be betaeen 0.4 and 1.9. After about 10 minutes, adjust the pH of the mixture to 2.2 to 2.3 with 411 sodium acetate and place the bottle and contents in a constant temperature bath a t 60" C. .ifter 20 minutes, remove from the bath, cool to room temperature or below and adjust the pH of the solution to 5.25 to 6.00 with 1OJf S a O H . This should take about 6 ml. of base. Shake well and cool again. Add 5 ml. of n-heptanol, shake the mixture for about 2 minutes, centrifuge to speed up the separation of layer-, and fill a 1-cm. absorption cell with the organic phase. Measure the absorbance at 465 nip against a reagent blank. If, because of temperature changes, the heptanol layer becomes turbid, recentrifuge. Properly treated organic phases show constant absorbance for at least 24 hrs.; however, it is suggested that absorbance readings be taken as soon as possible after extraction to avoid possible turbidity.
Figure 1 . Absorption spectra of azo dye ( A ) and reagent blank ( B ) in aqueous solution Weight of NO%- = 95.5 pg.
in demineralized double-didtilled water and diluting to 1 liter. Chloroform, 1 nil., was added to prevent bacterial growth and the solution was kept Confree. One milliliter of this stock solution is equivalent to 1000 pg. of nitrite ion. The sdution was standardized by the U.S.P. method with permanganate ( 7 ) . More dilute solutions were prepared by dilution of the stock solution. DISCUSSION A N D RESULTS .ICETIC ~CIDS -ICETATE O DSOLUICM TIONS. These solutions were prepared Absorption Spectra. The absorpby adjusting the p H of a 2 M acetic tion spectra of aqueous solutions of acid solution with 2M sodium acetate the azo dye and the reagent are to the desired value. shown in Figure 1. T h e dye has a DIVERSEIONS. A11 solutions of dimaximum absorbance a t 505 mp. Beverse ions were prepared by dissolving cause the reagent also absorbs at this analytical grade salts in distilled water. Procedure I. Determination of 10 wavelength, a blank was used in all to 200 pg. of NO2 -. T o a 10-ml. determinations. The absorption maxineutral sample solution containing 10 to mum of the neutral azo dye in n-hep200 pg. of S O 2 - add 2 ml. of reagent tanol occurs a t 465 mp. AIcorrespondsolution and mix well. The pH of the ing shift, in the absorption spectrum of reaction mixture should be 0.4 to 1.9. the extracted reagent is also observed. Allow about 10 minutes for diazotizaDiazotization Reaction. EFFECTO F tion and then adjust the p H to 2.20 p H . The effect of p H on the dito 2.30 with 1M sodium acetate for azotization reaction was studied by coupling. Quantitatively transfer the mixture t o a 50-nil. volumetric flask adding 2 ml. of 27& reagent solution and make up to the mark with an containing various amounts of hydroacetic acid-sodium acetate solution, the chloric acid to 10-ml. neutra' sample pH of which has been previously adsolutions. Each sample solution conjusted to 2.25. Mix well and place tained 100 pg. of NO,-. The mixtures the flask and contents in a constant were treat,ed according to procedure I. temperature bath a t 60" C. After 20 The results of this investigation are minutes, remove the flask from the given in Figure 2, which shows that the bath and cool to room temperature. absorbance a t 505 mp is essentially Read the absorbance a t 505 mp against an identically treated reagent constant when the pH of the solution is blank. Readings should be made after between 0.4 and 1.9. Thus, the con30 minutes and not later than 90 densation of 8-aminoquinoline with minutes from the time of p H adjustnitrous acid is most effective in fairly ment for coupling. st,rong acid media, as predicted from a Procedure 11. Determination of consideration of the principle of mass 0.5 to 16 pg. of NO,:. To a 40-ml. act,ion applied to Reaction I in which neutral sample solution in a 2-02. the hydrogen ion is a reactant. screw-cap bottle containing 0.5 to
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RATE O F DIAZOTIZATION. The time necessary for complete diazotization of 8-aminoquinoline was determined by reacting 10-id. samples, each containing 100 pg. of nitrite, with 2 ml. of 2% reagent solution a t pH 1.12 for various lengths of time. The samples were further treated according to procedure I. There was essentially no change in the absorbance if the diazotization reaction was allowed to run 2 to 60 minutes. Thus it appears that the diazotization reaction is quite fast a t room temperature, and that the quinoline diazonium ion is stable under the esprrimcntal conditions. Coupling Reaction. EFFECTOF COUPLIXGTIME,pH, AXD TEMPERATURE. The effect of pH on the rate of the coupling reaction was studied by reacting a series of samples, each containing 100 pg, of NO2- in 10 nil.! with 2 ml. of 27, reagent ,solution a t pH 1.1 for 10 minutes. The pH of each solution was adjusted for coupling to a different value with 1X sodium acetate; mixtures were transferred to 50-ml. volumetric flaaks and made up to volume with 2M acetic acid-sodium acetate solutions of corresponding pH values. The coupling reaction was allowed to proceed a t room temperature and the absorbances were recorded at 505 nip for several hours. From plots of absorbance z's. coupling time for each of these solutions it was concluded that the coupling reaction is slow a t room temperature a t all the pH values investigated. Constant absorbance readings were obtained only after 180 to 300 minutes for most solutions, and in some cases incomplete reaction was observed even after 300 minutes. The effect of higher temperatures on the coupling rate was investigated a t pH 2.25. The samples for this study were treated as above for the diazotization step. The coupling reaction, however, was allowed to proceed at different temperatures and for different periods of time. From considerations of highest absorbance, shortest coupling time, and color stability, a period of 20 minutes a t 60' C. was chosen as the optimum condition for the coupling reaction, although the temperature does not have to be rigidly controlled. Figure 3 shows a plot of absorbance 1's. time for a sample which was allowed to undergo the coupling step a t 60" C. for 20
Table 1.
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1
1
2.2
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2.4
3
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Figure 4. reaction
Effect of pH on coupling
W e i g h t of NOz- = 100 pg. 0 After 2 0 min. a t 60' C. A Maximum readings obtained a t room temperoture
minutes, was cooled to room t'emperature, and mas allowed to stand for various periods of time before the absorbance readings u.ere recorded. &is t,his figure shows, samples which were treated under these conditions may be read after about 30 minutes, and preferably before 90 minutes after the adjustment of p H for coupling. Figure 4 summarizes the results of an extensive study of the effect of p H on the coupling rate for samples that were allowed to react at room temperature until constant absor'bance readings were obtained and for samples that reacted at 60' C. for 20 minutes. Essentially constant absorbance values were obtained for samples that were reacted in the pH range from 2.20 to 2.30. Coupling was retarded in strongly acid solutions. The effect of the acid is to decrease the reactivity of the amine, presumably by salt formation. h n ammonium group with a positive charge on the nitrogen atom diminishes the tendency of the molecule to couple with a positive diazonium ion. Because diazonium salts can react with water to release hydrogen ions, slo~vlyin the cold and more rapidly when heated to 60" C., the coupling reaction will also be sensitive to higher pH values. I n such a reaction, nitrogen is lost and a phenol is formed. This results in loss of diazonium ion and, consequently, in lower yields of azo dye. Effect of Reagent Concentration. To study t,he overall effect of reagent concentration on the absorbance, 2 ml. of 0.5, 1.0, 1.5, 2.0, and 47, %aminoquinoline solutions containing 5 ml.
8 - A m i noqui n o I i ne Figure 5. Effect of reagent concentration on absorbance W e i g h t of NO*.- = 100 pg.
Effect of Diverse Ions on Absorbance
100 pg. of NOz- added Moles of interference Wt., mg. per mole of NOz2500 192 742 100 479 100 1150 100 1900 100 2 01 0 244 1 75 0 212 1710 100 0 181 0 025 461 95 51 2 2 963 48 2500 212
Ion added c o-
NO3 SO4C2
Ca(I1) Rlg(I1) Fe(II1)
Fe(II) Al( 111)
Cu(I1) PO4 -,"
"4
Na(1) WI)
Table II.
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30
50
40
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Effect of pH on extraction
Error, 70 + I 88 -37 19 -4 0 0 00
+ 1 10 +9 10 + 1 41 0 00
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Masking of Fe(lll) with Phosphate
100 pg. of S O z - added Fe(II1) HJ'Od added, Absorbance added, pmoles at 50d mp Hmoles None 0 632 None None 0 690 4 36 100 0 658 4 36 200 0 636 4 36 400 0 624 4 36
8 pg. of NOz- in 40-ml. sample solution
of concentrated hydrochloric acid per 100 ml. were reacted with 100 pg. of KOz- according to procedure I. Absorbance measurements were made after 45 minutes of coupling time. Figure 5 shows the results of this investigation. For practical purposes, a 2y0 reagent solution was chosen. Effect of pH on Extraction of Azo Dye. The effect of p H on the estraction was studied by reacting 8 pg. of NOz- in 40-ml. samples according to procedure 11. After coupling was complete, the pH of each solution was adjusted to the desired value with 1, 3 , and IOM sodium hydroxide; the mixtures were cooled to room temperature and the azo dye was extracted into 5 ml. of n-heptanol. The mixtures were centrifuged and the absorbances of the heptanol layers were measured against identically treated reagent blanks. As Figure 6 shows, essentially constant absorbance values were obtained between pH 5.0 and 6.2. Further addition of base had little effect and the loss in absorbance from pH 6.2 to pH 13 was only 2.57,. Because of the presence of large amounts of acetic acid, the solutions are well buffered in the optimum pH range and it takes 2 ml. of 1 O M S a O H to raise the p H from 5.0 to 6.2. This allows considerable latitude in the measurement of the base during the p H adjustment. Effect of Nitrite Concentration on Absorbance. PROCEDURE I. T h e
effect of nitrite concentration on the absorbance a t 505 mp was determined by reacting 10-ml. samples containing 10 to 200 gg. of NOz- according to the recommended procedure. Beer's law is obeyed in the concentration range investigated (0.25 to 4.0 p.p.m.). The sensitivity of the method expressed as the weight of SO,- corresponding to an absorbance of 0.010 at 505 mg is 1.61 pg. The molar absorptivity is 1.43 X 104 mole-' cm.-' liter. PROCEDURE 11. Samples for this study were reacted according to the suggested procedure. Beer's law is obeyed in the concentration range of 0.0125 to 0.4000 1i.p.m. The method has a practical sensitivity of 0.151 pg. of nitrite which corresponds to an absorbance of 0.010 at 465 mp in a 1cm. cell. This is a sensitivity of 0.0038 p.p.m. if a 40-ml. unknown sample is taken for analysis, as suggested in the procedure. Interferences. Because this method was primarily designed for the analysis of water samples, a specific interference study was conducted on ions t h a t commonly occur in such samples. T h e solutions for this investigation were prepared using reagent grade materials and contained 100 pg. of nitrite and various amounts of chloride, nitrate, sulfate, C a ( I I ) , l I g ( I I ) , Fe(III), Fe(II), Al(III), Cu(II), X a + , K + , NH4+,and phosphate. As the results in Table I indicate, large amounts of Ca+*, l I g + 2 , . i l + 3 j VOL. 37, NO. 6, M A Y 1965
703
Table 111. Analysis of Synthetic Samples b y Procedure I
Weight
Ion
added
of nitrite
Weight, my.
found,
30 30 30 040 av. 30 32 ci, error +0 0 120 122
NO*-
0 1 0 0 5
Ca(I1) Mg(I1) Al( I I I )
c1-
NO.,-
030 6 8
pg.
0 0 2
1 33 9
Table IV. Analysis of Synthetic Samples b y Procedure II
Weight
Ion added
of nitrite
Weight, mg. 0 0 0 0 0 0 0 0 0 0
SO?
Ca(I1) JIg(I1) illlII1)
c1sop-
KO-
SO,-* (‘I K’
found, pg.
00132
16
os
004 107
av
pc error + O
00840
8
8
2h
036 16
177
1 1 1 1
8
av.
5 error
8
37 28 35 33 76 49 48 47 48
+ 0 95
CI-, ? ; 0 3 - , S04-2, K + , and S a + do not interfere in the determination. Cupric ion constitutes a serious interference. I t not only catalyzes the decomposition of diazonium salts, but also forms an insoluble complex with S-aminoquinoline ( I ) . Aimmoniareacts with nitrite to liberate nitrogen and will cause low results. Ferric ion oxidizes 8-aminoquinoline, especially at higher temperatures, to a highly colored product which absorbs a t 517 and 550 mp (1, 2 ) . This results in a serious interference in the present method. E L I M I N A T I O S O F INTERFERENCE B Y FERRIC 10s. If Fe(II1) is present in the sample, it must be removed or masked with a proper complexing agent. Trace amount? of phoslihate inhibit the oxidation of 8-aminoquinoline by ferric ions ( I , 2). This effect can be used to reduce the interference of Fe(II1) in the determination of nitrite by the present method. The data in Table I1 shox the effect of various amounts of phoslihate on the absorbance of a 10-nil. sample containing 100 pg. of SO2- and 243.5 pg. of Fe(II1). -1s the results indicate, the addition of 0.2 mmole of H 3 P 0 4 essentially eliminated all interference due to ferric ions. Care must be exercised, however, in adding
phosphate ions to the reaction misture, as the addition of too much phosphate will result in low absorbance. Analysis of Synthetic Samples. The method for the determination of nitrite was checked by the analysis of synthetic samples. Table 111 shows the results of these analyses by procedure I , while Table IT’ summarizes the results of this study by procedure 11. LITERATURE CITED
(1) Gustin, Y. K., Ph.1). dissertation, The Ohio State University, 1963. (2) Gustin, V. K., Sweet, T. R., ANAL. CHEM.35, 1395 (1963). ( 3 ) Kolthoff, I. AI., Elving, P. J., “Treatise on Analytical Chemistry,” Part 11, 1‘01. 5, p. 275, Interscience, Nex York, 1961. (4) Sawicki, E., Pfaff, J., Stanley, T. W., Rev. C-niv.Znd.Santander 5 , 337 (1963); C . A . 59, 3299 (1963). ( 5 ) Sawirki, E., Stanley, T. W., Pfaff, J., D’Amico, A., Talanta 10,641 (1963). (6) Sawicki, E., Stanley, T. W., Pfaff, J., Johnson, H., ANAL. CHEM.35, 3183 (1963). ( 7 ) “U. S. Pharmacopoeia,” p. 4 i , 1942. RECEIVEDfor review December 28, 1964. Arcepted February 15, 1965. Taken in part from the Ph.D. dissertation of Anthony Foris, The Ohio State University, Columbus, Ohio, 1965. Presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1965.
Spectrophotometric Determination of Aluminum in the Presence of BerylIium with 5-SuIfo-4’-Diet hyl a mino - 2’, 2-Dihydroxyazobenzene M. FLORENCE’ Australian Atomic Energy Commission Research Establishment, Lucas Heights, N.S. W., Australia
T.
A new reagent, 5-sulfo-4’-diethylamino 2’,2 - dihydroxyazobenzene (DDB), i s proposed for the spectrophotometric determination of aluminum in the presence of beryllium. At pH 4.7 the reagent forms a bright pink 1 to 1 complex with aluminum, which has a molar absorptivity of 41,000 at 540 mk. The beryllium complex has a molar absorptivity of less than 3 a t this wavelength, but large amounts of beryllium depress the aluminum color. Most interfering metals can be masked with (ethylenedinitri1o)tetraacetic acid (EDTA) or ferrocyanide, while gross amounts of impurities may b e removed b y a rapid triiso-octylamine extraction from 8M hydrochloric acid. The method has been successfully applied to the direct determination of aluminum in beryl and in NBL beryllium oxide spectrographic standards.
-
704
ANALYTICAL CHEMISTRY
T
most important water-soluble chromogenic reagents for aluminum are aluminon ( 6 ) , Eriochrome Cyanine R , Alizarin Red S, and Chrome Azurol S ( 4 ) . These reagents also produce strong colors with beryllium, and in most cases the reaction is actually more sensitive toward beryllium than aluminium. Since beryllium cannot be selectively masked, it must be separated comliletely before the aluminum determination is carried out. At pH 5, 8-quinolinol does not react with beryllium (6,6 ) , and aluminum quinolinate may be extracted into chloroform and the absorbance of the organic phase measured. However, a large number of other common metals also extract a t this pH, and interfere seriously with the aluminum determination. Certain di-o-hydroxyazo conipounds-e.g., Superchrome Garnet Y HE
(5-sulfo-2’,4’,2-tri hydroxyazobenzene)are known to form strong complexes with aluminum, but not with beryllium ( I ) , and have been used for the polarographic determination of aluminum in beryllium (3) and thorium (2) compounds. The weak orange color of the aluminum-Superchrome Garnet Y comples, h o w v e r , does not differ sufficiently from that of the free dye to enable aliectrol)hotonietric methods to. be used. By introducing a strongly auxochromic groui), such as the diethylamino group, into thc dye molecule, this obstacle may he overconie. l h e compound 5-sulfo-4’-diethylamino2’,2-dihydro>yaznbeiizent (DDl3) is light orange-yellow a t pH 4.7, while the aluminum com1,lex is bright 1)ink. nith a molar ahsorpti~ityof 41,000 at 540 1 Present address, Oak Ridge Sational Laboratories, Oak Ridge, Tenn.
