Thiophene Derivatives as Analytical Reagents 2-Thiophene-trans-aIdoxime, a New Reagent for Palladium S. G. TANDON and S. C. BHATTACHARYA Nafional Chemical Iaborafory, Poona, India
b 2-Thiophene-frans- a Id o x i m e has
The palladium complex is very stable.
been found to b e a highly specific reagent for palladium(ll), forming with it a weighable, light yellow, coordination complex in acidic solutions. It has been employed for the gravimet ric estimation of palladium and for its separation from diverse ions.
It can be digested in the acidic medium
M
heterocyclic organic compounds derived from furan, pyrrole, pyridine, quinoline, etc., have been employed as analytical reagents. Thiophene derivatives, which should possess strong coordinating ability, have somrwhat surprisingly escaped the attention of the analysts. Except for thenoyltrifluoroacetone (9), which has been recently used for the separation of hafnium from zirconium, no other thiophene derivatives seem to have been studied for application in analysis. Even in the complex of this compound the hetero atom sulfur does not participate in coordination (2). The principal purpose of this investigation was t o study the coordinating ability of thiophene derivatives and to utilize them as analytical reagents both for qualitative and quantitative purposes. A number of thiophene derivatives, mainly aldehydes, ketones, and their respective oximes and carboxylic acids, were examined. 2-Thiophene-trans-aldoxinie(anti) was ANY
3-j-H HO--S
found to be a highly selective reagent for palladium, forming with it in acidic solutions a light yellow, insoluble, coordination complex of definite composition, suitable for qualitative detection, gravimetric estimation, and separation of this metal from most other metals. 2-Thiophene-trans-aldoxime is stable toward heat, light, and air, and can be preserved indefinitely either as such or in solution. Its solubility in water, decinormal, and normal hydrochloric acid is 2.26 0.02, 4.29 & 0.02, and 10.94 =k 0.02 grams per liter, respectively, a t 30" C.
*
194
ANALYTICAL CHEMJSTRY
on a boiling lvater bath and is easy to filter. The precipitation of palladium is not conditioned by close control of pH because it takes place quantitatively from fairly strong acid solutions; this constitutes a distinct advantage, for palladium can be estimated in the presence of many diverse ions without recourse to complexing agents for keeping them in solution. The complex has a fairly large molecular weight with a palladium factor of 0.2470 as against 0.3167 for dimethylglyoxime ( 7 ) ,0.2743 for nioxime (12), 0.2669 for p-furfuraland 0.2009 for 3-hydroxy-1, doxime (67, 3-diphenyltriazine (11 ) . 2-Thiophene-trans-aldoxime can be compared a i t h p-furfuraldoxime ( 6 ) , which also forms a weighable, light yellow, coordination complex with palladium, and has found use as an analytical reagent for the gravimetric estimation and separation of palladium from other metals. Both reagents possess high specificity for palladium but Zthiophene-trans-aldoxime has a distinct advantage over p-furfuraldoxime in that the reagent, as well as its complex, is much more stable. Palladium-p-furfuraldoxime complex cannot be digested on a steam bath and is, therefore, not granular and is comparatively difficult to filter. According to Hayes and Chandlee (6), the complex must not be dried above 110' C. %Thiophene-trans-aldoxime offers some advantages over the widely used dimethylglyoxime reagent. The thiophene derivative is fairly soluble in water and there is no danger of the precipitation of the excess reagent; its palladium precipitate is much easier to filter and has a larger molecular weight; its selectivity for palladium is also better, as it effectively separates palladium from iron(II1) and nitrate which are known to interfere when dimethylglyoxime is used as a precipitant (1, 4 ) . 2-Thiophene-trans-aldoxime precipitates palladium from fairly strong acid solutions and in this respect it is also superior to 3-hydroxy-1,3-diphenyltriazine, whose palladium complex cannot be precipitated quantitatively below p H 2.
REAGENTS A N D APPARATUS
Several methods for the preparation of 2-thiophenealdehyde have been mentioned in the literature. I n the present investigation the method described by Schreiber (IO) was basically followed. Thus, thiophene (50 grams), phosphorus oxychloride (113 grams), and N-methylformanilide (107 grams) (8) gave 2thiophenealdehyde (44 grams) which was used without further purification for the preparation of the oxime. Hydroxylamine hydrochloride (62.5 grams) and hydrous sodium acetate (125 grams) were dissolved separately in a minimum quantity of water and were mixed together with 50 grams of 24hiophenealdehyde in 200 ml. of aldehyde-free ethyl alcohol in a 1-liter round-bottomed flask. The combined mixture was refluxed for about 2 hours. Shining white crystals of the oxime which appeared on cooling were filtered under suction, washed thoroughly with cold water, and crystallized twice from 3Oy0 ethyl alcohol as white needleshaped crystals. Yield, 45 grams; melting point, 132.0 to 132.5' C. [The other isomer (5) of this oxime is reported to be a liquid.] Analysis showed: C, 47.21; 3.86; N, 11.50; S, 25.40%; C5H@?jS requires c, 47.24; H, 3.96; N, 11.02; S, 25.22%. The oxime is readily soluble in common organic solvents and dilute alkalies. Standard Palladium Solution. A 2-gram sample of palladium(I1) chloride (Johnson, Matthey, & Co., Limited, London) was dissolved in 25 ml. of analytical grade warm concentrated hydrochloric acid and diluted to 1 liter with glass-distilled water. This solution was standardized gravimetrically by precipitation with dimethylglyoxime following the standard procedure. Reagent Solution. A 2% w./v solution of the reagent in 95yo ethanol was used. This solution is stable indefinitely. Other Solutions. The solutions of inorganic ions required for testing the specificity of the reagent against different ions and also for quantitative separations were prepared from reagent grade salts, using basically the procedure of West (13). Alkali salts were used for the solutions of the anions, and nitrates and chlorides for the solutions of the cations. I n a few cases sulfates and acetates were used. Platinic chloride, cerous chloride, ceric ammonium sulfate, and ferric alum were used as sources of Pt(IV), Ce(III), Ce(IV), and
Fe(III), respectively. The concentrations of solutions were adjusted to give about 20 mg. per ml. of the ion in question. The following procedure was employed for testing the ions: One milliliter of the solution to be tested Fas acidified with about 0.5 ml. of 6N hydrochloric acid and diluted to 5 ml. with distilled water. Excess reagent solution was added drop bv drop with constant stirring. Any development of color or formation of precipitate was carefully observed as such in the cold, as well as after warming on the water bath for 15 to 30 minutes. The pH of the solution t o be tested ranged from 0.2 to 0.8. No studies were carried out a t other p H ranges. Buffering solutions were neither used nor required. The following ions were testrd: al-
~~~
Table 1.
(Values given in grams) Pd Taken 0.009'75 0,01208 0.01625 0.02395 0.02395 0.03250 0.04875 0.06500 0.01625 0.02395 0.01625 0.01625 0.02416 0.01625 0.02395 0.02395 0.02395 0.02395 0.02395 0,01208 0.01208 0.01208 0.02395 0.02406 0.02395 0.02395 0.02395 0.02395 0.01653 0.02395 0.02395 0.02395 0.02395 0.02395 0.02416 0.02395 0.02395 0.02416 0.01625 0.02395 0.02395 0.01625 0.02395
Apparatus. A Beckman p H meter, Model H-2, was used for all precise measurements of pH. Wide-range and narrow-range p H indicator papers wrre employed for rapid pH determinations. A Beckman Model DU spectrophotometer with 10-mm. matched silica cells was used for determining the absorption spectra of the reagent and the complex. Sintered-glass crucibles, 1 0 s . 3 and 4,were employed for collecting precipitates. Silica and Rose crucibles were used for ignition. Graduated apparatus of standard calibration were u ~ e dfor measurements. PROPERTIES OF PALLADIUM COMPLEX
The palladium complex of 2-thiophene-trans-aldoxime is precipitated as a granular yellow precipitate from acidic solutions. Quantitative precipitation takes place up to p H 6.0 but is not assured above this pH. Completeness of precipitation can be tested by the simple, sensitive, and specific protective layer action test (3) using nickeldimethylglyoximate papers. For gravimetric estimations, lower p H values are preferred because of the better specificity of the reagent for palladium and also for getting larger granules. Digestion on a water bath helps coagulation of the precipitate and simultaneously converts the excess reagent into watersoluble products without affecting the complex in any way. The precipitate cannot be filtered from hot solutions because of its appreciable solubility a t higher temperatures. The palladium complex is insoluble a t room temperature in water, dilute acids and alkalies, and alcohol; it does not dissolve in dilute or concentrated ammonia but its color changes from yellow to white. It is freely soluble in
Determination of Palladium and Its Separation from Other Elements
Foreign Ion
0 . 1 UO2\(11) 0 . 2 nitrate 0 . 1 arsenate 0 lvanadate 0 . 2 sulfate 1 . 0 tartaric acid
potassium cyanide, pyridine, and dioxane, but is less soluble in acetone, chloroform, carbon tetrachloride, benzene, ether, and petroleum ether. Once formed, the complex is resistant to even cold concentrated hydrochloric acid. The complex is stable up t o 150' C. but starts gradually blackening above this temperature and melts with decomposition a t about 190" C. It can be dried a t 110" to 115' C. to a constant weight within a n hour; the time of heating can be safely extended without any fear of its decomposition. Microanalysis of the complex showed: C, 28.90; H, 2.47; N, 6.20; S, 15.70; C1, 15.54; Pd, 24.90%; Pd(CsHEOh'S)2C12 requires C, 27.82; H , 2.33; N, 6.49; S,14.85; C1, 16.42; Pd, 24.70%. The absorption spectra of the reagent and the complex were determined in 95% ethyl alcohol and chloroform, respectively. ,A,
for reagent 268 mp; log
e =
4.0977 ,A, for complex 242, 249, 306 mp; log E = 4.2914, 4.2773, 4.5369, respectively
Pd Com p1ex 0.0395 0.0488 0.0658 0.0970 0.0969 0.1314 0.1966 0,2622 0.0662 0.0969 0.0659 0.0660 0,0977 0.0662 0.0968 0.0967 0.0968 0.0969 0.0969 0.0490 0.0493 0.0489 0.0968 0,0975
Pd Found 0.00976 0.01205 0.01625 0.02396 0.02394 0.03246 0.04856 0.06477 0.01635 0.02394 0.01628 0.01630 0.02413 0,01635 0.02391 0.02389 0.02391 0.02394 0.02394 0.01211 0.01217 0.01208 0.02391 0.02408
O.OO6i 0.0972 0,0970 0.0972 0.0673 0.0969 0.0969 0,0972 0.0971 0.0969 0,0974 0.0970 0,0970 0.0976 0,0657 0,0970 0.0970 0.0659 0.0969
0.02389 0.02401 0.02396 0.02401 0.01662 0.02394 0.02394 0.02401 0,02399 0,02394 0.02406 0.02396 0.02396 0.02411 0.01623 0.02396 0.02396 0.01628 0.02394
Error
+0.00001 - 0.00003 0.00000
+o. 00001
- 0.00001 - 0.00004 -0.00019 - 0.00023
+0.00010
- 0,00001
+o. 00003
+0.00005 - 0.00003 +o .00010 - 0.00004 - 0.00006 - 0.00004 - 0.00001
- 0.00001 +0.00003 0,00009
+
0,00000
- 0.00004
+0.00002
- 0,00006 i 0.00006 -!- 0.00001
0.00006 +0.00009 - 0.00001 - 0.00001
+
t0.00006 +0.00004 - 0.00001 - 0.00010
+
0.00001 +0.00001 - 0.00005 - 0.00002 +o. 00001 +0. 00001 +0.00003 - 0.00001
EXPERIMENTAL PROCEDURE
Palladium could be estimated by 2thiophene-trans-aldoxime over a wide range of metal concentration. Its separation from all ions listed under Metal Ion Solutions, excepting Ag(I), Au(III), Sn(II), Ce(IV), Ru(III), Os(IV), and Pt(II), was readily feasible. iig(I), Au(III), Ru(III), Os(IV), and Pt(1V) do not form complexes with the reagent in cold but on heating are either reduced to the metallic state or produce nonhomogeneous precipitates. Sn(I1) interferes because in its presence the weight of the palladium complex was always a little less than the theoretical amount. Interference from platinum, if present in moderate amounts, can be avoided by the addition of ammonium oxalate to the solution before the introduction of 2-thiophene-trans-aldoxime or by precipitating and filtering the palladium complex in cold without digestion on a steam bath. This modification in procedure minimizes the reduction of Pt(1V) to lower valency states and VOL. 32, NO. 2, FEBRUARY 1960
195
thus checks its coprecipitation. Though separation from Ce(IS’), which forms a light yellow precipitate vith the reagent, was not possible, separation could be effected by reducing Ce(IV) to Ce(II1) with sufficient quantity of hydrogen peroxide and removing the excess of latter by boiling. Procedure for Palladium(I1). A solution of palladium chloride containing 10 t o 50 mg. of metal was diluted to 100 t o 150 ml. in a borosilicate glass beaker, and the p H was adjusted betn-een 0.2 to 0.8 by the addition of 3 to 7 ml. of analytical grade concentrated hydrochloric acid (sp. gr. 118) To this solution, IThich may contain varying quantities of diverse ions, an excess of 2% n-./v. alcoholic solution of the reagent was added from a buret with constant stirring. Fifty t o 60 mg. of the reagent, corresponding t o about 125% excess, was employed for every 10 mg. of the metal. Larger excess of the reagent was tolerated. The precipitated coniplex was digested with occasional stirring on a steam bath for 15 minutes to facilitate the coagulation of the precipitate and its subsequent filtration. After cooling t o room temperature (about 2 t o 3 hours) the precipitate was filtered on a weighed G-4 sintered-glass crucible, washed first with about 25 ml. of cold 1% v./v. hydrochloric acid, subsequently washed with 7 5 to 100 ml. of cold mater, dried a t 110’ C. for an hour, and weighed. The weight of the precipitate multiplied by 0.2470 gave the weight of palladium. Cu(I), Pb(II), Hg(I), and Tl(I), which form partially soluble chlorides, interfered with the analysis and were therefore oxidized to higher valency states before the addition of reagent. Direct separations from Pb(11) and TI(1) n-ere possible if the reaction mixture was diluted sufficiently to keep their less soluble chlorides in solu-
tion. In the separation of palladium from antimony, about 1 gram of tartaric acid and a n additional 5 ml. of concentrated hydrochloric acid were added to the reaction mixture to keep antimony in solution. The precipitate was washed initially nith a solution containing 2% v./v. hydrochloric acid and a little tartaric acid. When iron(111) was present, a faint brownish yellow turbidity n as produced in cold which, however, rapidly disappeared during the course of digestion.
estimations have been made so far. Some of the results are given in Table I. ACKNOWLEDGMENT
As the composition of palladium-2thiophene-trans-aldoxime coniplex corresponds to the formula [Pd(C5H50NS)2C12]. nhich is a coordination comple.; analogous to sevrral other complwes of palladium exemplified by dichlorodiamino palladium(I1) [Pd(SH3)2C12],it was anticipated that the estimation of palladium by this method might not he feasible in presence of anions other than chloride. Hom-ever, contrary to this belief, satisfactory result. n-cre obtained in presence of fairly large excesses of other anions like fluoride, acetate, nitrate, sulfate, and tartrate. The other anion complexes are not formed from the strong hydrochloric acid medium because the chloride complex is the more stable and less soluble. From palladium(I1) nitrate and nitric acid solution, and from palladium(I1) sulfate and sulfuric acid solution, the corresponding yellow insoluble complexes could be obtained n ith 2-thiophene-trans-aldoxime but both complexes were found unsuitcd for the determination of palladium by direct neighing on ing to their spontaneous decomposition a t about 100” C. Concerning estimation of palladium as such and also its separation from various divcrse ions, about 260 concordant
The authors are indebted to the Director, National Chemical Laboratory, for permitting S. G. Tandon to work as guest norker and to the Government of hfadhya Pradesh for granting study leave to him. They also thank 1‘. S. Pansare and colleagues for the microanalyses of compounds. LITERATURE CITED
(1) Charlot, G., BBzier, D., “Quantitative Inorganic Analysis,” p. 524, Wiley, Xew York, 1957. (2) Connick, R. E., JIcVey, 11’. H., J . .4m. Chem. Soc. 71, 3182 (1949). (3) Feigl, F., J . Chem. Educ. 20, 298 (1943) \ - - - - I
(4) Gilchrist, R., Bur. Standards J . Research 12, 291 (1931). (5) Hartough, H. W.,“Thiophene and Its Derivatives.” n. 313, Interscience, Ken. Tork, 1052. 1\ 6* ) Haves. J. R.. Chandlee. G. C.. .$SAL. C H E ~ I14. . 49i 11942). ‘ . (7y Hillebrahd, W: F., Lundkll, G,., E. F., Bright, H. A , Hoffman, J. I , hpplied Inorganic Analysis,” 2nd ed., p. 379, Kilev, S e i T York, 1953. (8) Horning, E. C., “Organic Syntheses,“ Collective 1-01,3. 11. 590. Wile\.. ?;ex Tork, 1955. (9) Huffman, E. H., Beaufait, L. J., J . A I ~ Chem. L Soc. 71, 3179 (1949). (10) Sfhreiber, R. S., “Organic Syntheses. Vo1. 31, D. 108, Wiley, Sew York 1951. (11) Sogani, X.C., Bhattacharya, S. C., AKAL.CHEM.28, 81 (1956). (12) T-oter, R. C., Banks, C. V., Diehl, Harvev. Ibid., 20, 652 (1948). (13) Kezt, P. IT., J . Chem. Educ. 18, 528 (1941). /
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RECEIVED for review January 26, 1959. Sccepted Sovember 12, 1959.
Rapid Test for Identification of the Isomers of Phenylenediamine R. G. FRIESER’ and P. A. SCARDAVILLE’ Central Research laboratories, lnterchemical Corp., 4 3 2 West 45th St., New York 36, ,A new specific one-reagent method for the qualitative identification of 0 - , m-, and p-phenylenediamine is described. The test has been modified for quantitative determination.
A
of the literature did not reveal a simple one-reagent method of differentiating the three isomeric phenylenediamines. Available methods are based on coupling reactions and the production of organic pigments (8) or SURVEY
196
0
ANALYTICAL CHEMISTRY
azo dyes (6). The drawbacks of these methods are that the reagent solutions must be freshly prepared and several steps are required. Furthermore, each of these methods only gives positive identification of a single i,qomer. Some methods (4, 9) permit differentiation from other amines; a titrimetric method (Z), recently published, is timeconsuming and must be carried out in an inert atmosphere. Still other methods ( 3 , 7 , 10) have been used but are not sufficiently specific. Procedures exist
N. Y. by which the three isomeric phenylenediamines can be differentiated (I, 5 , 1 2 ) but are cumbersome because they use a complicated scheme of a variety of organic and inorganic reagents. These methods do not readily lend 1 Present address, Radio Corp. of America, Semiconductor and Vaterials Division, Somerville, N. J. 2 Present address, Radiation Application Inc., 42-30 24th St., Long Island City, 3 . P.
