V O L U M E 22, NO. 12, D E C E M B E R 1 9 5 0
1501
DIFFERESCES RELATEDTO C=C STRETCHIXC T r ~ ~ The two terminally unsaturated cornpounds mentioned above (Ilgures 1 and 2) show a well resolved C'=C stretching band near 6 microns, but no resolved band appears at this wave length in the spectra of the various internally unsaturated compounds slioivri in Figures 1 to 4. Close examination of the spectra of the cis monounsaturated compounds of Figures 1 to 4 reveals that a slight broadening or inflection on the long wave-length side of the strong C=O band is t,he only evidence of C=C stretching absorption under the resolution employed. The prediction from theoretical considerations, that C=C stretching should be infrared active near 6 microns in a cis but not in a trans compound, was confirmed in the case of the ethyl esters of oleic and elaidic acids by McCutcheon, Crawford, and Welsh (20), but to bring out this difference they found it necessary to plot the ratio of the trarismittance of each of the esters to that of ethyl stearat,? in the 6micron region. G E N E R A L C O M M E N T S AND C O S C L U S I O N S
The foregoing discussion shows that the four classes of lonychain compounds studied are readily differentiated by their characteristic infrared absorption. Within each class, the trans monounsaturated compounds are readily distinguished from the cis monounsaturated and/or saturated compounds, and internal arid external unsaturation can easily be differentiated. Becausc, of the close similarity of their spectra, however, distinction bet w e n cis and saturated, or between various individual cis, trans, or saturated compounds within a ~I:LSS requires careful examination of the curves. In general, the spectra presented should prove useful in kwessing the potentialities of the infrared method as applied to studies of fat systems; they are primarily intended t o serve as a guidc in the development of spectroscopic or combined chemicalspwti,oscopic methods for the analysis of mixtures of fatty matcri:tls. For example, as a result of this study, the authors have devrloped an infrared spect,rophotometric method for determination of trans components in mixtures of long-chain compounds, utilizing the strong 10.36-micron band (26). .4lthough the structural correlations given are not all firmly established, when applied with caution they should be useful in the detection and possibly the estimation of functional groups in fats and other long-chain svstenis. ACKSOW LEDGhI ENT
The authors are grateful to Waldo C. Ault for the samples of petroselinic and petroselaidic acids, and to R. E. Iioos for assistance in the preparation of some of the reference materials.
~
.
~ ~ ~ ~ ~L I T E~R A .T U R E C I T E D d m . Petroleum Inst., A.P.I. Project 44. ,Spec,tra. Sational Bureau of Standards. CHEM., ~. 20, 998 Anderson, J. A , , and Seyfried, IV. I)., A i x . ~ (1948). Barnes. R. B., Gore, R. C., Liddel, C.. and l\-illianis. l-. Z., "Infrared Spectroscopy," Ken- T o r k , Reinhold Publishing Corp., 1944. Barnes, R. B., Gore, R. C., Stafford. R. IV.,niid it-illiams, T. Z.. AMAL.CHEM.,20, 402 (1948). Bauer, S. T., Oil & Soap, 23, 1 (1946). Bertram, S. H., Chem. Weekblad, 33, 3 (1936). Bertram, S. H., Oil Cololcr Trades J . . 94, 1227 (Oct. 8, 1938). Brown, J. B., and Shinowara, G., J . d n r . Cliem. Soc., 59, 6 (1937). Buswell, A. M., Rodebush, IT. H., and Roy, RI. F., Ibid., 60, 2239 (1938). Ilavies, h4. M.,.I. Chpm. Phys., 8, 577 (1940). Davies, AI. M., and Sutherland, G. B., Ibid.,6, i 5 5 (1938). Gillette, R.H., J . Am. Chem. Soc., 58, 1143 (1936). Gillette, R.H., and Daniels, F., IOki., 58, 1139 (1936). Herman, R. C., and Hofstadter. R., J . Chem. P h y s . , 6, 534 (1938). Ibid., 7, 460 (1939). Jordan, E. F., and Swern, D., J . A m . Citetn. Soc., 71, 23i7 (1949). Kitson, R.E., Experimental Station, E. I. du Pont de Nemours & Co., Kilmington, Del., private communications. Klota, I. M., and Grueri. D. M . , J . PhUs. & Colloid Chern., 52, 961 (1948). Knight, H. B.. and Swern, D.. -1.A m . Oil C'herrLists' Soc., 26, 366 (1949). JIcCutcheon, J. IT., Crawford, AI. F., and IVelsh, H. L.. Oil & Soap, 18, 9 (1941). Rao, P. C.. and Dauhwt, B. F., J . -4m.C h e m Soc., 70, 1102 (1948). Rasmussen, 11. S.,Brattain, R . R..and Zucco, P. S.,J . Chenr. Phys.. 15, 135 (1947). Sheppard, N., and Sutherland, G . B., .Vatcue, 159, 739 (194i). Sheppard, T., and Sutherland. G. R., P r o c . Roy. Soc., A196, 195 (1949). Yhrere. 0. D.. and Heether, 11.R.. .&S.AL. CHtor., 22, 836 (1950). Shreve, 0. D., Heethet,, b l . K., Knight, H. 13., and Swern, D., Ibid., in press. Smith, D. C., and lliller, E. C.. ,J, Optical Soc. A m . , 34, 130 (1944). Swern, D., and Jordan, E. F., J . Am. Chetn. Soc., 70, 2334 (1948). Swern, D., Jordan, E. F., and Knight. H. B., Ibid., 68, 1673 (1946). Swern. D.. Knight. - H. 13., and Findley. T. IV., Oil & ,Yoax), 21, 133 (1944). Thompson. H. IT.,J . Chern. Soc., 1948, 328. Thompson, H. W., and Torkington. P.. Trnna. Furctday S'oc., 41, 246 (1945).
RECEIVED March 4 , 1930. Presented in part before the Division of Organio Chemistry at the 116th Meeting of the A M E R I C A NCHEMICALSOCIETY, Atlantic City, S . J. Paper V in series "Reactions of Fatty Materials with Oxygen." Paper I V is ( 1 9 ) .
Polarographic Determination of Platinum F. L. ENGLISH Chumbers Works, E . I . d u Pont d e Nemours C C o m p a n y , Znc., Deepwater,
T
HE literature of the polarography of platinum is meage],
consisting of but one article by Willis ( b ) ,who states that this metal is not reduced polarographically. He worked apparently only with divalent platinum, but preliminary experiments showed t h a t in the tetravalent form a definite and reproducible wave is produced in neutral solution. This wave rises instantly from zero applied potential, and attains a well defined plateau in the neighborhood of -0.6 volt, from which the curve drops rather sharply a t about -0.8 volt forming a secondary plateau, approximately two thirds the height of the first one, a t - 1.2 volts. The polarogram then rises again a t the discharge potential of the supporting electrolyte. The Iieight of the first plateau above
N. J .
the starting line is linearly proportional to the concentration of platinum and serves as the basis of a quantitative determination. No starting plateau is formed, even if the electrolysis is started at a positive voltage of the dropping mercury electrode. APPARATUS
A Model SS visible recording polarograph, manufactured by E. H. Sargent & Company, Chicago, Ill., was used. The polarographic cells were of the H-shaped type recommended by Lingane and Laitinen (Z), one arm serving for the sample solution and the other for a saturated calomel cell anode. These cells were mounted in an air-agitated, vibration-insulated ( 1 ) water bath, 0 05' C. The capillary used thermostaticallv regulated to 25"
ANALYTICAL CHEMISTRY
1502
A polarographic procedure is presented for the estimation of small amounts of platinum in organic materials, such as fresh and spent catalysts prepared on charcoal. Interfering common metals are removed by extraction with hydrochloric acid in the presence of a strong reducing agent. The residue is ignited to destroy organic matter, the platinum is dissolved in aqua regia, and the filtered solution is adjusted to pH 7 and polarographed. Results are accurate to *lQ/tof the amount of platinum present.
passed 1.384 mg. of mercury per second and the average drop time over the 0- to -0.6-volt interval and in the supporting electrolyte used in the analysis was 4.58 seconds, giving an mz/at1/6 constant of 1.60mg. 2/3sec.-1/2. -4mufRe furnace, operating from a 2-kw. Variac transformer, adjusted to 650" * 20" C. n-as used for the ignition. REAGENTS
Titanous chloride, approximately 20% in strong hydrochloric acid, obtainable from the Stauffer Chemical Company, Niagara Falls, N. Y. Sodium hydroxide, 20 grams diluted to 100 ml. pH 7 Buifer. Solution A , 21 g r a m of citric acid (C6Hs0,.HnO) diluted to 100 ml. Solution B, 26.8 grams of disodium phosphate (SarHP04.7H20)diluted to 100 ml. Buffer, 8.8 ml. of Solution h plus 82.4 ml. of Solution B diluted to 100 ml. This is MacIlvaine's formula (3). ~, but five times the usunl concentration. Gelatin, 0.5 gram of Knox's No. 1 gelatin dissolved in 50 ml. of warm mater. This solution should be prepared fresh daily. Standard Platinum. For the preparation of the calibration curve, a standard solution of platinum may be prepared from pure platinum Jvire dissolved in aqua regia or from ammonium or tassium chloroplatinate, purified as recommended 1,y 3Iellor
g:
PKOCEDL RE
Weigh into a 150-nil. beaker a poition of sample containing preferably 2.0 to 3.5 mg. of platinum. Add 25 ml. of water, 25 ml. of concentrated hydrochloric acid, and 5 ml. of titanous chloride solution, boil gently for 10 minutes, filter hot through a Gooch crucible provided with a KO.1 Whatman paper, and wash with 25 ml. of hot water, discarding filtrate and washings. Set the Gooch in a 50-ml. porcelain crucible and ignite in a muffle furnace a t a dull red heat (about 650' C.) until all organic matter has been burned. This will require from one to several hours, depending upon the nature of the sample, but prolonged heating even overnight will do no harm. After cooling, add 1 ml. each of concentrated hydrochloric and nitric acids, cover with a watch glass, and heat in the steam bath for 10 minutes. Add 10 nil. of water and filter through a Gooch mounted on a 1-liter suction flask containing an 8-inch test tube to catch the filtrate. Wash with water to a volume not exceeding 35 ml. Transfer to a beaker, add 2.5 ml. of the sodium hydroxide solution, then a drop of methyl red indicator, and continup the sodium hydroxide until the solution is just alkaline. Add 2 ml. of the buffer solution, and 0.5 ml. of gelatin, transfer to a 50-ml. volumetric flask, and dilute to volume. Electrolyze the solution in the usual manner a t a sensitivity setting of 2-80 (0.02 pa. per mm.) starting a t zero applied potential. The curve will rise immediately, form a slightly inclined top plateau, and start to drop a t about -0.8 volt, a t which point the machine is shut off. Because of the lack of both a starting plateau and any substantially straight portion of the steeply rising step line, a modified method of measuring the nave height is recommended. Determine the median line of the plateau pen oscillations, extend it back to intersect the -0.3-volt ordinate, :tnd take as the step height the vertical distance from the starting line to this intersection point. EXPER1.M EDTA L
Preliminary experiments t o establish a suitable supporting electrolyte revealed that a satisfactory wave is produced only in substantially neutral solution. Numerous combinations of ammonium and sodium salts (acetate, chloride, cyanide, nitrate, tartrate, thiocyanate) were tried in the hope of producing a starting plateau, but in all cases the wave rose abruptly, irrespective of the initial voltage. The sodium salts Rere found to give more regular and reproducible curves than the salts of ammonia,
pyridine, aniline, etc. A solution of approximately 0.3 N sodium nitrate adjusted t o pH 7 n-ith MacIlvaine's buffer was hall!.adopted. For calibration purposes, solutions of known platinum content were prepared from purified ammonium and potassium chloroplatinates and by dissolving platinum wire in aqua regia. riiespectedly, it was found that none of these solutions is stable, mere standing overnight at room temperature entailing an apparent loss of platinum content of as much as 6%. This is due to spontaneous reduction of some of the platinum to the divalent state, which is polarographically inactive. Reproducible results from any of these solutions of whatever age are, however, obtainable by evaporating aliquots to dryness in porcelain crucible., dissolving the residue in aqua regia (1 ml. each of hydrochloric and nitric acid), and completing the determination as detailed above for the analysis of samples, but omitting the filtration. The final solutions for electrolysis are stable for several hours. Typical results, set forth in Table I, indicate the degree of precision that may be expected. These data were obtained with an initial potential setting of zero; span potential, 3.0 volts; sensitivity, 2-80 (0.02 pa. per mm.). The step height was taken as the vertical distance from the starting line to the intersection of the median plateau line nith the -0.3-volt ordinate.
Table I. P-t
I
AIg./oO RI1.
3.500 2,625 1.750
0,875 0.000
Calibration against Potassium Chloroplatinate Step Height a t -0.3 Volt, Mm. AV., m m . / m r B C 219.0 217.0 220.5 62.5 164.0 160.0 162.5 61.8 112.0 109.0 110.0 63.0 55.5 56.0 55.0 63.4 7.0 ... 6.0 ,..
A
zd.
CrnZ/atl/' 7.62 7.55 7.70 7.72
...
In the analysis of a catalyst, two major problems are presented: complete extraction of the platinum from the material on whirh it is adsorbed, and elimination of interfering metals that may be present, particularly in spent samples. The common metals most likely to be picked up from plant equipment are iron, copper, nickel, lead, manganese, zinc, and chromium, some of which yield polarographic waves that interfere more or less with the platinum step. Extraction of the platinum by treatment with aqua regia from ithernew or spent catalyst xithout previously destroying the organic matter !vas found impracticable because the platinum salts are so tenaciously retained by the charcoal as to require a prohibitive amount of wash water for complete removal. Ignition tollon-ed by solution in aqua regia is effective, but the temperature of the furnace must be kept below about 700" C . to avoid fusing some of the platinum into the glaze of the crucible. K i t h Vycor crucil)les, probably a much higher temperature could lie used. Separation from the common metals could theoretically be effected by reducing the platinum salts to metal in an acid medium, after ignition and solution in aqua regia, and filtering, the interfering elements being lost in the filtrate. When this was attempted, using either stannous or titanous chloride as the reducting agent, unfilterable colloidal suspensions of platinum were
V O L U M E 22, NO. 12, D E C E M B E R 1 9 5 0 obtained. Zinc dust gave :I well coagulated precipitate, but the recovery of platinum was always low, for undetermined reasons. .%nother obvious line of attack is to extract the unignited sample with hot acid, thus dissolving the metallic impurities, filter, and extract tht, platinum from the ignited residue. This expedient was tried, using 1 to 1 by volume hydrochloric acid, but again the recovery of platinum was IOT. The loss was found to be due to solution of some of the platinum because of either its extremely fiue state of subdivision or a n oxide coating. )Then titanous chloride w:ts added to the dilute hydrochloric acid, this loss was avoided. T o c - h r ~ kthe performance of the method, a standard ratalyst \vas prq):trcd from accurately wigheti quantities of powdered c4h:ucmiI : i r l t i platinum wire dismlved in aqua regia. This catalyst 1 .OOO% platinurn. Three :~nalysesof it yielded 0.992, c~)iitaiiir~tI 1.000, :ind 0.085%. T h e addition of ro1;~tivelylarge amounts (up to 10 mg. each to a 0.30- to 0.35-gram assay) of the ahovrinentioiied vonimon metals a9 various salts did not affect the rec'overy of platinum. 13ecause platinum is below nierc8ui.y in the electromotive series, it i.: possiblr that a n apprwiable propoi,tion of it precipitates on the. n i e ~ ~ c wdrops y during the in;rking of a po1:wograni. T o test this, zsuc*c~rs.sivr runs were made upon the same cell charge of platinuni .wlutiori, step heights of 166.5, 167.0, 166.0, and 167.0 nini. Ileirig o b t a i n d . The :mount of precipitation during a run is t hc,rt,fore iiegligitile. On t h e other hand, if the plutinum solution is ~ 1 1 : i k c ~ i vigorously i ivitli mercwy hefore electrolyzing, a Ionr r c o v r r ~ 'ensues, due either to precipitation or rduc,tion to the divalent state. .As a find test of the procedure imples of spent catalyst were :iii:ilyzed (Table 11). S:implw 1 and 2 contairied a high proportion of a siliceous filter : i i ( l . 1cB:tving a large residue after ignition. T o extract the platiiiuin conij)lcBtrly froin this, it IVW found necessary to use 2.5 ml.
1503 eavh of hydrochloric and nitric acids instead of the 1 ml. called for in the standard procedure. This larger amount of arid increases the concentration of the supporting electrolyte, which affects the step height. A calibration under these conditions yielded an average value of 60.6 mm. per mg. of platinum. T h e values for samples 3 and 4, which contained little silica, were calculated on the basis of the calibration figures given in Table I.
Table 11.
Analysis of Spent Catalysts
171.5 145.5 216.5 177.0 141.0 229.5
3.00 2.50 3.00 2.50 2.00 1.75 1.50 2 2.5 2.00 1.75
1,D.O
"''3.5 201.6 172 0
2.83 2.41 3.55 2.92 2.33 3.36 1.89 3.57 3.24 2.78
0,094 0.096 ( I . 118 0.117 0 11; 0.192 0 . 193 0.159 0 . 162 0 159
LITER.41'LIRE CITED
( I ) English, F. L., A s . 4 ~ C'HEM., . 20, 489 (1948). (2) Lingane and Laitinen. ISij. 1,:si:. ('H'EM., .\N.~L. h., 11, 504
(1939). ( 3 ) NacIlvaine, J . Biol. Chem., 49, 1S3 (1921). (4) Mellor, J. W., "Comprehensive Treatise on Inorganic and Theoretical Chemistry," Vol. 16. p . 316, Xew York. Longmans, Green and Co., 1938. (91 ITillis, ,J. B., J . .4m. Chcnt. .Sot.. 67, 547 (1945). RECEIVED h l a y 12, 1950.
Determination of Copper and Iron in Oils by Amperometric Titration T€IO>I.iS D. PARKS' A N D LOUIS LYKKEK Shell Dereloprnent Company, Emeryiille, Calif, .i method is described for the determination of copper and iron in inorganic residues, such as that obtained from oils, by reduction in a silver reductor and amperometric titration with dichromate ion at a rotating-platinum electrode. When copper is determined i n the absence of iron, the reduced solution is receiyed under ferric alum solution and the equivalent amount of ferrous iron produced is titrated. When both iron and copper are being determined, the reducing value of the mixture is determined on
T
HE importance of metal arialysis in new arid used lubricating oils in the evaluation of engine tests for corrosion and wear has been pointed out by Lgkken, Fitxsimnions, Tibbets, and Wyld ( 7 ) . They gave detailed procedures for the separation and determination of the metals normally encountered in these materials, including colorimetric methods for iron and copper. T h e latter methods ( 1 ) have proved to be reliable and useful but somewhat exacting in use and somewhat lacking in precision and accuracy. T h e advent of new amperomet>ric techniques suggested titration procedures for copper and iron ivhich are more rapid and precise than the common colorimetric methods. 1
Present address, Stanford Research Institute, Stanford, Calif.
an aliquot of the reduced solution received under ferric alum, the iron is determined by titrating a separate reduced aliquot after selectike oxidation of the copper by aeration, and the copper is calculated by difference. The titration procedure is rapid, precise, and accurate; it is sensitite to 0.02 mg. of iron or copper. The method is particularly suitable for the analysis of copper and iron in lubricating oils because other elements that are normally present in these oils do not interfere.
Walden, Hammett, and 12dmonds (8) introduced the silver reductor for preparation of ferrous ion solutions for titration with ceric sulfate, and Birnbauiii and Edmonds ( 2 ) extended the use of this reductor to the preparation of cuprous ion solutions. They showed that solutions of those metals could be quickly prepared for analysis even in the presence of various other coinnion ions. Kolthoff and May (4)titrated very dilute solutions of potassium dichromate with ferrous iron at a rotating-platinum electrode and suggested the reverse titration of ferrous iron with dichromate solution. In the procedure given bcloiv for the determination of copper iii lubricating oils, the metals are separated from the oil by air
