A positive responE$e %-as obtained likewise with very sinall amounts of solid cellulose, nitrocellulose, and cellulose ester and ether. DETECTION
OF
N-ARYLICENE COMPOUNDS
If Schiff bases, oximes, and hydrazones of aromatic aldehydes-Le., N-arylidene compounds containing the ArCH=NR-atom grciup are subjected to the procedure described above, hydrolysis occurs:
+
+
ArCH=NR HZO 3 ArCHO NHiR (R = H, OH, NH2, etc.) and the aromatic aldehyde split off in this manner will condense with thiobarbituric acid. Pertinent compounds therefore behave in the same manner as aromatic aldehydes.
aone, C6HaCHNNH2,which is isomeric with benzamidine, behaves in the same manner as other N-benzylidene compounds. Therefore it is possible t.0 distinguish between the two isomers by the test described above. DIFFERENTIATION OF HYDROBENZAMIDE AND AMARINE
Hydrobenzamide (I) and the isomeric heterocyclic amarine (11) formed from (I) by heating to 120°C. are condensation products of 3 moles of benzaldehyde and 2 moles of ammonia
I (m.p. 101’ C.) ACKNOWLEDGMENT
FOU~D /Lg. , Benzalazine Benzaniline Resorcylaldoxime Salicylaldszine PHydroxybenzaldoxime m-Xitrobenznldazine
5 10 2.5 2.5
2.5
10
The test for N-arylidene compounds just given assumes the absence of aromatic aldehydes. The presence of the latter can be detected by the spot test procedures commonly employed (6). Benzamidine, Cd&C(NH)NHt, proved to be completely inactive. This was to be expected, since this compound contains no benzylidene group and when saponified yields no benzaldehyde but instead the nonreacting benzamide. Howevw, benzalhydra-
Another way to distinguish between these two water-insoluble isomers is based on the finding that amarine, in line with its constitution, is a stronger base than hydrobenzamide. Both isomers respond to the test characteristic of basic inorganic and organic basic compounds-namely, the formation of a red precipitate on contact with nickel dimethylglyoxime equilibrium solution (6). However, this reaction proceeds much more rapidly with amarine than with its isomer. If 0.5 mg. of amarine is used, the reaction with the equilibrium solution is immediate, whereas the same weight of hydrobenzamide yields either no precipitate or a barely visible amount of nickel dimethylglyoxime.
semihgdrate, 106’ C.)
II(m.p. 130” C.; as
Inspection of structural formulas I and I1 reveals that only hydrobenzamide contains a -N=CHC&group and accordingly reacts in the manner discussed above. Therefore hydrobenzamide can be detected through the positive outcome of this test and thus be differentiated from amarine. The procedure used for detection of n-arylidene compounds was followed. Identification limit is 5 bg. of hydrobenzamide. The results were positive in the presence of any reasonable amount of amarine.
The aut’hors are grateful for the support of this work by the Conselha Nacional de Pesquisas. LITERATURE CITED
e, G. P., J .
(1916). (3) Ibid., p. 2164. ( 4 ) Feinl. F.. “ h o t Tests in organic Anal;s!s.” 6th ed.. DD. 426.591. Elsgvi \
I
~,
38,3824’( 1905).
RECEIVEDfor review January 7, 1963. Accepted July 15, 1963.
S pect rophioto met ric Dete rmina ti on of Platinum with 2,3-CJuinoxalinedithiol GILBERT H. AYRES arid RAYMOND W. McCRORYl Department of Chemisfry, The University of Texas, Austin, Texas
b Platinum(lV), after reaction with tin(1l) chloride in hydrochloric acid, gives a blue color with 2,3-quinoxalinedithiol in N,N-dimethylformamide solution. The color develops rapidly and is stable for many hours. Absorption peaks occur at 624 and 585 mp. The system con‘orms to Beer’s law, and spectrophotometric results are reliable to a relative error of less than 1.5% over the ciptimum concentration range of about 1.4 to 5 p.p.m. of platinum for measurement at 1.00cm. optical path. The? absorbance is not highly sensitive to the concentration of reagent, of tin(1l) chloride, or of hydrochloric acid. Copper, cobalt, nickel, and rhodium interfere and require separation from platinum.
Spectrophotometric solution studies and precipitate analyses show that platinum and 2,3-quinoxalinedithioI react in a 1 to 2 ratio to form a blue, uncharged complex in acid solution. This complex is a weak diprotic acid that ionizes in alkaline solution to give a red divalent anion having maximum absorption at 519 mp.
R
EVIEWS O F THE SPECTROPHOTOMETRIC methods for the platinum
metals were published by Beamish and McBryde (4) in 1953 and in 1958. Of the several methods then available for platinum, the tin(I1) chloride method (3, 6, l a ) and the p-nitrosodimethylaniline method (12) were favored.
More recently, platinum has been determined spectrophotometrically with tin(I1) bromide ( I @ , with o-phenylenediamine (l?), and with N,N‘-bis(3dimethylaminopropyl) dithiooxamide (9). The preparation of 2,3-quinoxalinedithiol by Morrison and Furst (14), and their observation that it formed colored products with a number of metal ions, has led to the use of this reagent for the spectrophotometric determination of some of the transition elements. Nickel in ammoniacal solution was determined by Skoog, Lai, and Furst (18). Ayres and Janota (2) Present address, Jackson Laboratory,
E..I. du Pont de Nemours & Co., Wilmngton, Del.
VOL. 36, NO. 1, JANUARY 1964
133
500
550 600 650 WAVELENGTH, rnp
Figure 1. A. B.
Platinum-QDT platinum QDT reagent
700
Spectral curves complex;
3.8
p.p.m.
of
determined palladium. and deduced the composition of the complexes formed in the reaction. The reactions of nickel, cobalt, and palladium with 2,3-quinoxalinedithiol in alkaline solution were Qtudiedby Stei anrevit and Draiii. (19). The h u l t a n e o u - determination of cobalt and nickel in acidic aqueousethanol .olutiona ha. been reported by Burke and Voe (6) and by hyres and .-innand ( I ). The marked interference of platinum in the determination of palladium ( 2 ) wggested that 2,3quiiioxalinedithiol might be a sensitive reagent for platinum; the analytical method and a qtudy of the reaction are pre-ented herewith. The simultaneous determination of palladium and platinum with 2,3-quinoxalinedithi01 albo ha5 been accomplished in this laboratory ( I O ) . EXPERIMENTAL
Apparatus. -1bsorbances a t varying wavelengths were recorded with a Beckman Model DK-1 recording spectrophotometer. A Beckman Model D U spectrophotometer, operated a t high, constant sensitivity, was used for precise measurements a t fixed wavelength. Matched silica cells of 1.00-em. optical path were used in both instruments. A Beckman Zeromatic pH meter and a Leeds & S o r t h r u p conductivity bridge were used for the potentiometric and conductometric titrations, respectively. Reagents. STANDARD P L a T I N U M SOLUTIOP;. A known weight (869 mg.) of Grade Xo. 1 thermocouple wire, specified 99.99yo pure, 1%-asdissolved in hot aqua regia and the resulting solution was evaporated almost t o drynew. The residue was taken up 134
0
ANALYTICAL CHEMISTRY
in hydrochloric acid, then evaporated to small volume; the treatment with Table I. Calibration and Sensitivity hydrochloric acid was repeated sevData eral times to remove nitric acid and Platinum t o destroy any nitrosyl complexes of concn., A4bsorbance Absorptivity, platinum t h a t may have formed. ppm. a t 624mp p.p.m.-lcm.-l After the final evaporation, the residue 0 70 0 096 waa taken up with 10 ml. of concentrated hydrochloric acid and diluted to exactly 1 liter to give a concentration of 869 i1.p.m. of platinum. Aliquots of the solution were analyzed by reduc3 4s 0 189 0 141 tion with magnesium; the precipitated 0 1-42 5 22 0 740 0 991 0 112 6 96 platinum wai: ignited and weighed as 5 70 1 230 0 111 metal (8). Closely agreeing replicates Av. 0 141 calculated t o a concentration of 868 p.p.m. of platinum. Working solutions of loner concentration were prepared by iolumetric dilution of the qtock solution. For preparation and isolation niiuture stand for 10 minutes. then of solid products in the study of the dilute t o yolume n i t h D N F . Measure color reaction, potassium tetrachlorothe ahqorbance against a blank conplatinate(I1) was used; assay of the taining all the components except salt for platinum content checked n ith platinum. the calculated value. 2,3 - QTZIKOXALINEDITHIOL (QDT). RESULTS The reagent (Eastman KO.7 3 1 i ) was ;1 0.1% (11. 'v.) used as received. The spectral curve of the product, solution in AY,S-dimethylformamide Figure 1. curve A, is characterized by ( D N F ) wa- prepared for w e in the two absorption peaks of nearly equal recommended procedure. inteniity a t 624 and 585 mp separated TIK(II) CHLORIDE.A 10%. (1% ./v,) by a trough a t 599 nip. These three solution of tin(I1) chloride dihydrate 17-avelengths are equally suitable for in 65f hydrochloric acid n as prepared. S . S - D I ~ ~ E T H I - L F ~ R ~ IThis . L ~solI I U E . platinum determination. The conforms to Beer's lan oier the convent (Ea>tman S o . 5870) was used as received. centration range studied (up to 8 . i OTHER RELGEXTS. Reagent grade p.p.m. of platinum, absorbance at chemical. were used in the interference 624 nip = 1.230). The optimum conthe common cations were used centration range for measurements at as chlorides, and the anions were used as 1.00-cni. optical path is about 1.4 to 5 sodium or potassium salts. Standard p.p.m. of platinum. solutions of the other platinum elements Reproducibility and Sensitivity. I n were available from previous investigaa test of reproducibility (preckion) tions in this laboratory. of the method, 17 sample., each conPreliminary Tests. Initial teqt; on taining 2.00 1i.p.m. of platinum (about application to platinuni(1V) of t h e standard procedure reported for palthe middle of the optimum range) ladium(I1) by Ayres and Janota were prepared and measured. The ( 2 ) gave little indication as t o n-hy standard deviation of the absorbances, platinum ChouId interfere PO seriously a t the three diff erent wavelengths, with the palladium determination. Mixmas 0.006 absorbance unit, or about tures of platinum(IV), hydrochloric 1.4%. D a t a for the calibration curve, acid, and QDT gave no immediate test for conformity to Beer's lair, and ex idence of reaction. Cpon standing qensitivity a t 624 mp are shown in overnight, the samples developed a brown turbidity, or, a t higher acidities, Table I ; each entry in the table is the a green color. I t appeared that the average of several closely agreeing QDT reagent might he reducing the replicates. h t 62-1 mp the *pecific platinum(IT'), followed by complexaabsorptivity of 0.141 p . p m P 1 em.-' tion with the lower oxidation state of corre.ponds t o a molar absorptiLity of platinum. T'ariou. reducing agents X 104 liter mole-' em.-' ;It 2.75 added to the platinum(1V) solution 599 and 385 mp,the qpecific absorptivibefore addition of the QDT n-ere n-ithtie%of 0.131 and 0.135 p.p.m.-' cm.-l out effect, except in the case of tin(II), correqpond to molar absorptivities of which resulted in the rapid formation of a %tableblue color highly sensitive 2.36 X lo4 and 2.64 X IO4 liter mole-' for platinum. Xfter a detailed study cm.-l, respectively. of the variables, the follon-ing standardized procedure was adopted. STUDY OF VARIABLES Recommended Procedure. Into a 23-1111. volumrtric fla-k, transfer 8.0 Te>t> in each study were made on ml. or lea? of the platinum solution; solutions containing a fixed amount of i f nwcqsary, add n-ater t o make a platinum which was color-developed by total volume of 8.0 ml. +Xdd 1 ml. of the recommended procedure except for tin(I1) chloride reagent, miu thorthe variable being studied. oughly, add 10 ml. of D M F , and again Stability of Product. Using freshly mix thoroughly and cool to room prepared reagent solution, full color temperature. Add 2 ml. of Q D T devrlopment was attained within 13 reagmt wlution, mix well, let the
minutes after addit-on of the QDT reagent, and the absorbance remained const'ant for about 50 hours. The absorbance then decreased slowly, and after about 60 hours a turbidity appeared in both t h e sample and the blank. Aged solutions of eit,her t'in(I1) chloride or QDT decreased the time stabilit'y of color-developed solutions. &Iged tin(I1) chloride solution resulted in the appearance of ii white gelatinous precil)itate, while aged solutions of QDT caused forniation of a light yellonturbidity; in both tasep, precipitate formation occurred in a shorter time as the age of the reagent solution increased. Stability of Reagent Solutions. T h e 10% solution of tin:II) chloride dihydrate‘ in 6M hydrcchloric acid was chosen to provide a reasonable shelf life without precautions against air osidation, and as a convenient means of adjusting the acidity of samples in the desired range. The tin(I1) solutions were satisfxtory for about 2 weeks for developnient of the platinum color; after tkiis interval, low alisorbances were obtained. Observations on the stability of the QDT solution in D l I F have been published previously. Solutions of QDT (0.1%) in D l I F stored in transparent bottles were s,itisfactory for 4 days for development' of the platinum color. .\fter this t i n e the reagent solution, originally dark red-brown, turned bright gellols, and slowly d e v e l o l d a yellon- deliosit on the walls of the container: use of this solution resulted in l o y nonrepr oducible absorhances for 1)latinum. *After about 1 month, the yellow deposit had dissolved, and the solution was completely ineffective for develcping the characteristic platinum colcr. QDT reagent solutions stored in brcwn bottles n-ere satisfactory for color development of ~)latinunifor a t least 13 days. Effect of QDT Concentration. Solutions wire preparcd by the reconimilnded procedure, except t h a t the volumc. of O.lyo QDT reagent was v:iricd from 0.3 t o 12 mil. niiiiimuni of 0.5 nil. was requiwd for full color tlew1ol)ment. Solutions containing 0.5 ml. or less reag'int were blue, nhrreas larger amounts of reagent gave green solutions, 3 mixture of the blue color of the platinum complex and the, yellow color of excess reagent,. Spectral curves of all t,he solutions had the same general char,tcteristics, indicating the presence of only one reaction product. Excesses of i'eagent solution up to 12 nil. were without effect on the absorbance, although the higher concentrations of reagent resulted in turbidity in the blank solutions. For the upper limit of the optimum range ( 5 1i.p.m. of platinum) the amount of Q D T reagent ( 2 ml.) used in the recommended procedure provides a mole
ratio of QDT to platinum of about I6 to 1. Effect of Acid Concentration. Full color development wquired a niinimum of 5 mmoles of hydrochloric acid per 25 ml. final volunie. .%mounts of acid up t o 19 mnioles had no significant, effect on the absorlxmce, although amounts above IO ninioles increased tlie fading rat? sonitxIJ-hat. The optimum amount of hydrochloric acid was incorporated into the tin(I1) chloride solution. Effect of Tin(I1) Concentration. The acidity of a serics of platinum solutions was adjusted so t h a t after addition of the tin(I1) reagent (which for this test was 1.11 in HC1) tlic total acid contcnt was 6 mniolw of hydrochloric acid per final volumc of 25 ml. The color was then dcveloped and measured in the usual way. Even 1 ml. of tin(I1) reagent was adequate for full color development; u p to 4 ml. of tin(I1) rcagent coultl be used, but larger amounts gave low, nonreproducible absorbanccs. Effect of Water-DMF Ratio. I n t h e determination of palladium with QDT ( 2 ) it was found that D M F 11-as the most suitable water-miscible organic solvent for preventing precipitation of the product in acidic reaction mixtures. The same was found to he the case in the present investigation. In addition. the platinum reaction product, soluhle in DNF, was insoluble in the usual waterimmiscible organic solvents, which precluded the application of an estraction technique. -2serie? of 3olutions was prepared by the recominended procedure, except that the amount of water and, or aqueous solutions wa-: varied. -1s the water content increased froni 12% to 367, of the final volume, absorliances a t 624 mp decreaqed gradually. whereas absorbances a t 599 and 585 nifi w r e constant. ;\t volumes of water above 367,, absorhances at a11 thrw wavelengths decreased quitc rapidly. I'recipitation of QDT owurrcd when the volume of water esceeded a h u t 60%. The stability of the colorcd qolutions decreased as the water content increa
