The rmogravimetric Pyrolysis of 8-Quinolinol and 5,'I-Dihalo8-quinolinol Chelates of Scandium, Thorium, and Uranium(YI) WESLEY W. WENDLANDT Department of Chemistry and Chemical Engineering, Texas Technclogical College, Lubbock, Tex.
solvate molecule could be thermally removed in the 5,7-dihalo8-quinolinol chelates and how these chelates compare in stability with the unhalogenated quinolinol chelates. The lack of an oside level in the uranium(V1)- and thorium 8-quinolinol chelates (4,5 ) a t elevated temperatures also warranted further expeiimental study.
The theriiiogravinietric pyrolysis of the 8-quinolinol, 5,7-dichloro-8-quinolinol, and 5,7-dibroni0-8-quinolin01 chelates of scandium, thorium, and uranium(V1) was determined. It was found that only two of the chelates can lose the extra molecule of solvation by thermal deconiposition-namely, the 8-quinolinol chelates of thorium and uraniuni(\'I). Oxide levels of the ignited chelates were found in the temperature range from 4,50" to 600" C. The sublimation curves for the three chelating agents were also investigated.
EXPERIMENTAL
Reagents. 8-Quinolinol (melting point, 74-6" C). and 5,Tdibromo-8-quinolinol (melting point, 195-6' C.) were obtained from the Matheson, Coleman and Bell Co., East Rutherford, iY.J. 5,i-Dichloro-8-quinolinol (melting point, 177-9' C.) was obtained from the Eastman Organic Chemicals Co., Rochester 3, N. P. They were used without further purification. Scandium oside of 99.8% purity was obtained from A. D. Mackay, Inc., New York, N. Y . ; thorium nitrate tetrahydrate was obtained from the Lindsay Chemical Co., West Chicago, Ill.; and uranyl nitrate hexahydrate was obtained from Alerck S: Co., Inc., Rahway, PIT. J. All other reagents were of C.P. quality.
S
CAXDIUhl, thorium, and uranium( VI) form chelates with 8-quinolinol and the 5,i-dihalo-8-quinolinols having the general formula, M(Ox),HOx, where &Iis the metal ion of oxidation number n and HOx represents the chelating molecule. Contrary to the normal chelates of 8-quinolinol with metal ions ( I ) , these particular compounds contain an extra molecule of solvation. For the scandium chelate, this extra molecule is thought to be held by weak lattice forces in the solid state and is said to be of the order of magnitude of about 1 kcal. per mole (18). Because of the favorable gravimetric factors of the 8-quinolinol chelates, they have been employed for the determination of scandium (16), thorium (Q), and uranium(V1) (3, 6, 8, 10). However, there is some question as to the temperature limits for drying the precipitated chelates. Frere ( 7 ) reported that the thorium 8-quinolinol chelate was stable up to 100" to 110' C., but lost the extra molecule of solvation between 160' and 170" C. Above this temperature the chelate decomposed rapidly. Using the Chevenard thermobalance, Dupuis and Duval(4) found that there was no loss in weight to a constant composition in the temperature range from 20" to 945' C. Using the same experimental method, Borrell and P h i s ( 2 ) found that the anhydrous chelate containing the extra molecule of solvation was stable up to about 80" C., decomposed by an intermolecular dehydration above 137" and then lost the extra molecule above 245' C. On further heating, the thorium oxide level was obtained a t a higher undesignated temperature. The uranium 8-quinolinol chelate loses the extra solvate molecule easily a t 210" to 215" C. (1.5). Duval ( 5 ) found that the chelate began to lose 8-quinolinol a t 157" and formed the normal chelate a t 252" C. Above 346" the normal chelate began to decompose, but the oxide, UaOa, was not obtained even a t 947' C. The scandium 8-quinolinol chelate has not been studied on the thermobalance, but, from thermal decomposition a t temperatures up to 165' C., it mas found that the extra molecule of solvation could not be removed without total decomposition of the chelate
1 100
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200
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300
.
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Figure 1. Pyrolysis curves of clielating reagents and scandium chelates
(18).
The 5,7-dichloro-8-quinolinol and 5,7-dibromo-8-quinolinol chelates of scandium (14), thorium ( 1 2 ) , and uranium(V1) (13) have recently been investigated. I t was reported that the extra molecule of solvation could not be removed by thermal decomposition. However, under carefully controlled conditions, chelates could be prepared which did not contain the solvate molecule. ' Because of the gross inconsistencies in the literature regarding the thermal decomposition of the 8-quinolinol chelates of thorium and uranium(VI), and the new series of the 5,7-dihalo-8-quinolinol chelates, it was decided to study the chelates on a new type of thermobalance (19). It was of interest to determine if the extra
1. 2. 3. 4. 5. 6.
8-Quinolinol 5,7-Dichloro-8-quinolinol 5,7-Dibromo-8-quinolinol
Scandium-8-quinolinol ohelate Scandium 5,7-dichloro-8-quinolinol chelate Scandium 5,7-dibromo-8-c~uinolinolchelate
Thermobalance. The thermobalance used has previously been described (19). Preparation of Chelates. The 8-quinolinol chelates of scandium (17), thorium (11), and uranium(V1) (16) were prepred. by methods previously described. The 5,7-dihalo-8-qu1nol1nol chelates of scandium ( 1 4 ) , thorium (12), and uranium(V1) ( I S ) were also prepared I.7 methods previously described.
499
ANALYTICAL CHEMISTRY
500 All of the precipitated chelates r e r e air dried a t room teniperature (25' to 27" C.) for 24 hours before thermogravimetric pyrolysis on the thermobalance. DISCUS SIOK
8-Quholhol and 5,7-Dihalo-8-quinolinols. Altho.igh priniarj interest was in the metal chelates of the three compounds, the volatility of the pure materials was first determined. The sublimation curves for 8-quinolinol, 5,i-dichloro 8-quinolino1, and 5,7-dibromo-8-quinolinol are shown in Figure 1. 8 Quinolinol is the most volatile (atmospheric pressure about 680 mm.). The first weight loss occurred a t 85' C.; the rate of weight loss became rapid between 100" and 200' C. Thus it is possible to remove an excess of this reagent by heating the precipitate5 above 85'.
I
2 0 MG.
I 100
I
9
200
300
400
TEMP.,
Figure 3.
I 500
I 600
I
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I ROO
C.
Pyrolysis curves of uranium chelates
1. Uranium 8-quinolinol chelate Uranium 5,7-dichloro-8-quinolinol chelate Uranium 5,7-dibromo-8-quinolinol chelate
2.
3.
0
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e
600
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Figure 2. Pyrolysis curves of thorium clielates 1. 2. 3.
Thorium 8-quinolinol chelate Thorium 5 7-diohloro-8-quinolinol chelate Thorium 5:7-dibromo-8-quinolinol chelate
The 5,7-dihalo-8-quinolinols show both sublimation and decomposition. The dichloro- derivative began to sublime a t 142' C., increasing to a very rapid rate of sublimation between 200" and 250' C. There was no decomposition of the compound. The dibromo- derivative began to sublime a t liso C., but decomposition followed after two thirds of the material mas removed by sublimation. At 290' a black tar remained in the thermobalance pan; the tar was gradually oxidized as the temperature increased to 600' C. Obviously, the dibromo- derivative is the least stable. Scandium Chelates. The pyrolysis of the 8-quinolinol, 5,7dichloro-8-quinolinol, and 5,i-dibromo-8-quinolinol chelates of ecandium is shown in Figure 1. From an over-all view, the decomposition curves are of the same general pattern. An interesting aspect of the three curves is that an oxide level corresponding to SclOa was found in all three cases. The 8-quinolinol chelate was the most stable, with the first weight loss occurring a t 125' C. The least stable was the 5,7-dibromo-8-quinolinol chelate, which began to lose weight a t 80' C. In no case did a constant weight level occur which had the stoichiometry for the normal chelate, Sc(C&NO)a. Rather, a mixture of the solvated chelate, Sc(CBH~NO)a. CgHsNOH, and the normal chelate was formed until the oxidation of the organic material began a t 350' to 400' C. The oxide levels were obtained a t about 600" C. for
all three of the chelates. Of course, in the analytical determination using the 8-quinolinol chelate, it is impractical to ignite to the oxide because of the favorable gravimetric factor for the chelate, i . 2 4 % scandium. However, the maximum temperature limit for drying the chelate is 125' C. The pyrolysis of the pure 8-quinolinol (Figure l), shows that the reagent began to sublime a t 85' C., so that any temperature between 85' and 125" would be satisfactory for drying. The 5,i-dihalo-8-quinolinol chelates are not recommended for the determination of scandium from the standpoint of thermal stability and also because of the difficulty in preparing the pure chelate in the desired stoichiometry. Thorium Chelates. The pyrolysis of the three chelates of thorium is shown in Figure 2. The results obtained with the 8-quinolinol chelate agree with those of Borrell and PAris ( 2 ) . The chelate was stable up to 80" C., where it began to lose €4quinolinol. A constant weight level resulted at 250' C., which corresponded to the normal chelate, Th(CgH6NO)d. The normal chelate began to decompose a t 310" C., resulting in an oxide level of ThOt beginning a t GOOo C. The results differ considerably from those of Dupuis and Duval (I), in which an oxide level was not found even a t 945' C. The reason for the difiagreement between the two previous workers using the same type of thermobalance is not known. The 5,i-dihalo-8-quinolinol chelates are about as stable a8 the 8-quinolinol compound. I n no case mas a region of constant weight obtained which would designate the normal chelates; therefore, it appears to be impossible to prepare them in this manner. There seems to be no advantage in using the dihalo chelates for the determination of thorium, except possibly a better gravimetric factor. The difficulties in precipitation would, however, more than compensate for the difference. Uranium Chelates. The pyrolysis of the three chelates of uranium(V1) is shown in Figure 3. In the case of the 8-quinolinol chelate, UOs(C9H&")z. C9H6KOH, the decomposition proceeded smoothly to the normal chelate, UO( CpH,NO)z, beginning a t about 230' C. This higher temperature for the sublimation of 8-quinolinol from the chelate indicates some degree of interaction
V O L U M E 28, N O . 4, A P R I L 1 9 5 6
501
in the solid state. As stated previously, 8-quinolinol begins to eublime a t 85’ C. However, when held as a molecule of solvation, it sublimes a t 230” C. The results seem to indicate that the lattice forces holding this extra molecule of 8-quinolinol must be of a greater order of magnitude than 1 kcal. per mole. The normal chelate was stable up to 380” C., where it decomposed to the oxide level, U30a, beginning a t 450” C. This is certainly in disagreement with Duvnl (6) who did not obtain an oxide level even at 9 4 i ” C. Although it is possible to prepare the normal chelate of uranium and 8-quinolinol by thermal decomposition, it is not possible to remove the extra solvate molecule of the 5,7-dihalo-&quinolinol chelates by heating. There was no evidence from the pyrolysis curves for the existence of the normal chelates in this series of compounds. The 5,7-dihalo chelates were even less stable thermally than the 8-quinolinol chelate; thus, there is no advantage in using them for the determination of uranium. It is difficult to prevent coprecipitation in the chelates, as is shown by the decomposition of the uranium 5,7-dibromo-8-quinolinol chelate in Figure 3. The excess reagent began to sublime a t about 200’ C. going directly to the chelate, U01(CPH1Br2K0)2.C oH4Br2NOH.
LITERATURE CITED
(1) Berg, R., “Die Analytische Verwenclung von o-Oxychinolin (‘Oxin’) und seiner Derivate,” 2nd ed., F. Enke, Stuttgart, 1938. (2) Borrell, II.,Pbris, R., And. Chim. Acta 4, 267 (1950). (3) Classen, A , , Visser, J., Rec. trau. chim. 6 5 , 211 (1946). (4) Dupuis, T., D u d , C., Anal. Chiin. Acta 3, 589 (1940). (5) Duval, C., Ibid., 3, 335 (1949). (6) Fleck, H. R., Analyst 62, 378 (1937). (7) Frere, J. F., J. Am. Chetn. SOC.55, 43653 (1933). (S) Got& H., Sci. Repts. TBhoku Imp. Univ., First Ser. 26, 391 (1937). (9) Hecht, F.. Ehrmann, FV., 2. anal. Chem. 100, 98 (1935): (10) Hecht, F., Reich-Rohrwig, TI7., ilIonatsh. 53, 596 (1929). (11) Moeller, T., Ramaniah, JI. IT., J . Am. Chem. SOC.75, 3946 (1953). (12) Zbid., 76, 2022 (1954). (13) Ibid., p. 5251. (14) Ibid., p. 6030. (15) Moeller, T., II‘ilkins, D. H., Inorg. Suntheses 4, 2G7 (1953). 23, 757 (1951). (16) Pokras, L., Bernays, P. bl., ANAL.CHEM. (17) Pokras, L., Bernays, P. AI., J . Am. Chein. SOC.73, 7 (1951). (18) Pokras, L., Kilpatrick, hl., Bernays, P. M., Ibid., 75, 1254 (1953). (19) Wendlandt, W. FV., ANAL.CHEM.27, 1277 (1955).
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RECEIVED for review November 15, 1955.
Accepted December 30, 1955
Color Reaction of Amino Acids with Alloxan, Isatin, and Ninhydrin in Circular Paper Chromatograpby ABRAHAM SAIFER
and
IRWIN ORESKES
Isaac Albert Research Institute and Department
of Physical Chemistry, Jewish Chronic Disease Hospital, Brooklyn 3, N. Y.
Color reactions of 51 amino acids and their sensitivities were determined as spot tests on filter paper with alloxan, isatin, and ninhydrin. R f values for these amino acids were obtained with the circular chromatographic method using phenol- and butanol-acetic acid as developing solvents. Alloxan gave stable and uniform colors for most amino acids and was a more sensitive reagent than ninhydrin for some of them. For aniino acids with similar R , values, simultaneous use of these reagents applied to sectors from one chroniatograni often permits the ‘‘differential” determination of a single amino acid in a mixture. Isatin is the most useful for this purpose because of the wide variety of its color reactions and lack of reactivity with many amino acids. The nature of these color reactions for amino acids and their possible application to amino acid niixtures are discussed.
T
WO general approaches have been tried for the complete qualitative identification of amino acids in complex mixtures using paper chromatography. The first approach aims a t complete resolution of amino acids into bands or spots containing but one component. A single, generally applicable coloring reagent would then suffice for the identification of all the amino acids present. However, complete resolution is difficult to achieve in practice. An alternative approach is the use of specific or selective coloring reagents to identify a single amino acid in a band or spot containing several components. Attempts to obtain more complete separation of the individual amino acids in a mixture have led to such developments as: ( a ) increased path of travel of components by using larger paper sheets (48) or ascending-descending chromatography (6, 48); ( b ) the use of buffered filter paper (26, 30); (c) the use of multiple solvent sys-
tems (14, 19, 21, 30, 37-39); (d) multiple development techniques (11, 23); and ( e ) elution from one chromatogram followed by rerunning the mixture on a second chromatogram employing a different solvent system (IS). It is the opinion of Block, Durrum, and Zweig (6) that many of these inovations represent no major improvement over the two-dimensional, two-solvent technique originally described in the classical paper by Consden, Gordon, and Martin (9). Indeed, many of these modifications have made the procedure more costly, complex, and laborious. One exception to this general trend appears to be the increasing popularity of the circular chromatographic method proposed by Brown (8) and Rutter (41, 42). In this method the substances to be analyzed are resolved into circular bands instead of spots. Recent work by LeStrange (25), as well as previous studies by hluller and Clcgg (SS), have confirmed the simplicity of the method, as well as the facts that separation occurs more r a p idly, and the definition between components is superior to that obtained with the descending technique. Careful experimental studies of some of the physical factors which influence circular paper chromatography were made by Saifer and Oreskes (43). These studies have led Block and others (6) to conclude that circular paper chromatography is a more rapid and a simpler method than the more conventional techniques. In addition to the advantages stated above, circular paper chromatography lends itself most readily t o the use of several different color reagents, because a large number of sectors can be cut from a single chromatogram (41, 4 2 ) . Such an approach permits the identification of the separate amino acids even where complrte resolution has not occurred (17). The use of such diffeIentia1 reagents which give colors with one or several amino acids has often been cited in the literature (4, 6, 17, 18, 48). Although ninhydrin comes closest to the ideal coloring reagent because of its reactivity and sensitivity for most amino acids, even it has certain drawbacks, such as the faint yellow colors obtained with proline and hydroxyproline (9, BS). For
