Efficiency of Chemical Desiccants FRED TRUSELL and HARVEY DIEHL Department of Chemistry, Iowa State University, Ames, Iowa
b The efficiency of 21 chemicals as drying agents for gases has been determined b y collecting the residual water in a liquid nitrogen trap and weighing. The best of the desiccants tested and the residual water in micrograms per liter of gas are: anhydrous magnesium perchlorate (0.2), barium oxide (2.8), alumina (2.9), phosphorus pentoxide (3.6), and Molecular Sieve 5 A (3.9).
T
of their removal of water from gases varies widely among the chemical drying agents was early recognized, by Dumas (8) for esample, by Pettenkofer (l7), by Favre (IO),and by Morley who measured the water escaping from sulfuric acid (14) and from phosphorus pentoside (15,16). A review of the earlier work, beginning with the use of calcium chloride by Berzelius and Dulong (S) and continuing with the studies of other desiccants of Erdmann and Marchand (Q), Regnault (It?), Fresenius (II), and Dibbits (6) is found in the first of Morley's papers (14), and Morley himself contributed the classic paper in the field (16),for his work on phosphorus pen& oxide has not only not been repeated in the 70 years which have elapsed but has been the basis of all subsequent work. T h a t is to say, phosphorus pentoside has been accepted as the ultimate drying standard and only once has the suggestion been made that another agent may be better (24). Xor have the subsequent studies been few in number or uncritical in character as witnessed by the studies of Baster and Warren [calcium bromide, zinc bromide, zinc chloride, calcium chloride (211, Johnson [aluminum oxide ( I S ) ] ,Baxter and Starkweather [sodium hydroxide, potassium hydroxide ( I ) ] , Dover and Marden [copper sulfate, calcium oxide (7)], Willard and Smith [magnesium perchlorate ( S 4 ) ] , Smith [magnesium perchlorate] barium perchlorate (19, 20)], Walton and Rosenblum [boric acid (SS)], Booth and hfcIntyre [barium oxide ( 4 ) ] , Hammond and Withrow [calcium sulfate ( l a ) ] and , Bower [various agents already mentioned (6)1. The number of drying agents which has been seriously proposed is about 20 or so; there are excellent ones in the list, some with high capacity, some with extraordinary efficiency, and one remarkable one, anhydrous magHAT THE EFFICACY
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ANALYTICAL CHEMISTRY
nesium perchlorate, in which the highest known capacity is combined with the greatest effectiveness. An ingenious device was employed by Morley for determining the moisture remaining in a gas dried by passage over phosphorus pentoside. It worked in the following manner. Air under pressure, two to 13 atmospheres, was dried by passage through phosphorus pentoside. The gas then passed into a weighed vessel in which three operations were carried out: a little water was introduced into the gas by passage over hydrated calcium chloride; the gas was expanded to atmospheric pressure by passage through a capillary; the gas was dried by passage over phosphorus pentoside. Any loss in weight of this vessel represented water carried away by the air and specifically by that volume of air by which the exit air is greater than the entering air. The loss was about one twentieth of a milligram in 4300 liters of air and ". . .therefore so little, that in 4300 liters i t cannot be detected with much probability by even the most delicate weighing." Morley himself pointed out that the difficulty here arises from using the very small difference between two large values: this is a difficulty inherent to all such gravimetric methods. The more serious problem, namely the volatility of phosphorus pentoside] Morely answered in his 1904 paper. H e did indeed find phosphorus pentoside carried along by a gas stream; the amount was about equal to the total loss in the earlier esperiment and he concluded with the statement: ". . . that no gravimetric experiments which the scientific world has in hand at present would need to take account of the moisture which phosphorus pentoside leaves in a gas. " As already mentioned, in all of the subsequent measurements of the efficiency of drying agents phosphorus pentoxide was used as the reference standard. Bower (6),who did the most recent and extensive study of desiccants, employed phosphorus pentoxide as the ultimate standard although he made most of his determinations against a secondary standard thus introducing a possible compounding of errors. The Bower value for anhydrous magnesium perchlorate is particularly suspect for phosphorus pentoxide will not dry magnesium perchlorate below the tri-
hydrate, even over a period of four months [Willard and Smith ( 2 4 ) ] . I n the present re-examination of this problem the chosen mode of attack was to pass humidified nitrogen over the desiccant and to trap the water whicli escaped in a cold trap immersed in liquid nitrogen. Using a sufficiently large volume of gas, the weight of water escaping the desiccant can be found by direct weighing. By extrapolating the vapor pressure curve of ice, calculation shows that at equilibrium a t the temperature of liquid nitrogen there is but one molecule of water vapor for each 1800 liters of gas (23). Even if equilibrium was missed by several orders of magnitude, a weighable amount of water would not escape this trap. EXPERIMENTAL
Chemicals. With t h e exception of t h e following, desiccants were obtained from t h e usual sources a n d used as received. ALUMINA. Prepared by heating reagent grade alumina powder to 400' C. in an electric furnace. AKHYDROCEL. Anhydrous Calcium sulfate prepared by dehydrating gypsum (G. Frederick Smith Chemical Co., Columbus, Ohio). ANHYDROXE. Magnesium perchlorate, almost anhydrous (J. T. Baker Chemical Co., Phillipsburg, K. J.). ASCARITE.Sodium hydroxide on an asbestos base (-1.H. Thomas Co., Philadelphia, Pa.). BARIUM OXIDE. Porous barium oxide, carbide free (Barium & Chemicals, Inc., Willoughby, Ohio). BARIUMPERCHLORATE. Anhydrous barium perchlorate obtained from G. Frederick Smith Chemical Co. was dried in a vacuum for 16 hours at 1 2To C. CaLcIulf CHLORIDE, AXHYDROUS. Prepared by heating the commercial, granular, eight-mesh material in a vacuum a t 127' C. for 12 hours. CALCIUM CHLORIDE, ANHYDROUS. Dried a t a high temperature. Prepared by drying for 16 hours at 245' C. in a vacuum. CALCIUM OXIDE. Prepared by heating calcium carbonate 6 hours in a furnace at 900' C. DRIERITE. Anhydrous calcium sulfate (W. A. Hammond Drierite C o , Xenia, Ohio). LITHIUMPERCHLORATE, .hHYDROUS. Prepared by neutralizing reagent grade perchloric acid with reagent grade lithium hydroxide, isolation of the trihydrate, and drying at 70' C. for 12
r= A
8 Figure 1,
c
O
E
F
G
H
I
Absorption train used in gravimetric determinations of residual water in nitrogen dried over desiccants A. 6. C. D. E.
F. G. H.
I,
hours in a vacuum :md for another 12 hours a t 110' C. hfAGNESIUM OXIDE. Prepared by igniting magnesium carbonate in an electric muffle at 800" C. for six hours. MAGNESIUXPERXLORATE, ANHYDROUS. Anhydrous magnesium perchlorate, (G. Frederick Smith Chemical Co.) was further drisd by heating in a vacuum a t 245" C. for 48 hours. ~IAGNESIUM PERCHLORATE, A"YDROUS, INDICATING. Magnesium perchlorate plus potassium permanganate ('2.Frederick Smith Chemical Co.). MIKOHBITE. Sodium hydroxide on expanded mica. (G Frederick Smith Chemical Co.) hfOLECULAR SIEvP, 5A. I n the form of l/ie-inch pellets (Linde Co.). SILICAGEL. (Eagle Chemical Company). This material was sold as a drying agent and, in the absence of manufacturer's instructions to the contrary, was used withcut any preliminary treatment. Analytical Methods. When pract,ical the desiccants were analyzed before use. After a run a, portion of the desiccant from the inlet and outlet sides of tube F (set! later description of apparatus and Figure 1) was also analyzed. With one exception, t,he desiccants which are salts of alkaline earth metals were analyzed by titrating the alkaline earth metal ion with :ethylenedinitrilo)tetraacetate nt pII 10, employing Eriochrome Black 1' a.; indicator. The single esce1)tion to thi: was the analysis of calcium sulf:ite (bnhydrocel and Drierite). The weighed sample of this material was stirred overnight in a beaker with a slurry of 10 grams of wet Amberlite 11%-120caldon exchange resin in the hydrogen form. The resulting sulfuric acid was t.ien titrated with standard sodium hydroside. I n the case of anhydrous, indicating magnesium perchlorate, the potassium permanganate, the indicating agent, was reduced with h,ydroxylammonium chloride, and the total manganese and magnesium was determined by titrating a t pH 10 with (ethylenedinitrilo) tetra-
Compressed nitrogen Preliminary freeze-out tube in liquid nitrogen Three-outlet distributor Magnesium sulfate, heptahydrate in 25' C. water bath Calcium chloride (when used) in 25' C. water bath Desiccant under test in 25' C. water bath Freeze-out tube in liquid nitrogen Anhydrane safety tube W e t test meter
acetate. The end point was markedly sharper if the solution was heated to about 40' C. The manganese was determined colorimetrically as the permanganate in an aliquot of the same sample. The magnesium was found by difference. Ascarite and Mikohbite were treated with excess standard hydrochloric acid and allowed to stand for 1 hour with frequent swirling. The excess acid was then titrated with standard sodium hydroxide. Alumina was dissolved by refluxing for 20 minutes in concentrated perchloric acid. The perchloric acid was neutralized with ammonium hydroxide, excess (ethylenedinitrilo) t e t r a a c e t a t e w a s added, and the solution warmed briefly. The excess (ethylenedinitrilo) tetraacetate was then titrated with copper nitrate, employing Calcein as indicator. The end point was detected by the quenching of the fluorescence of the indicator under ultraviolet illumination. Apparatus and Procedure. The absorption train used is shown in Figure 1. Nitrogen, obtained from the cylinder A , was used as the carrier gas. Tube B was made of IO-mm. borosilicate glass and was immersed in a Dewar flask of liquid nitrogen. Contaminants in the carrier gas which will condense out a t -196" C. nere removed at this point. C was a distributor connection having one inlet and three outlets (only one showni. Each outlet was fitted with a stopcock, permitting regulation of the gas flon- of each stream. From this point on there were three separate, parallel trains. making possible three detcrminations to be run simultaneously. Tubes D , E. and F were Kinias number 4G050 Schwartz drying tubes 14-mm. i.d. and 150 mm. deep, fitted a t the inlet end n ith a 12/5 groundglass socket and a t the outlet end n-ith a 12/5 ground-glass ball. They were immersed in a bath a t 25" C. Tube D was filled with magnesium sulfate, heptahydrate, to feed a small amount of water back into the carrier gas. In the runs of Anhydrocel, .%searite, anhydrous barium perchlorate, calcium chloride, anhydrous calcium chloride,
anhydrous calcium chloride dried a t high temperature, calcium oxide, Drierite, magnesium oxide, Mikohbite, potassium hydroxide, and sodium hydroxide, tube E was empty. During all other runs it was filled with calcium chloride as a preliminary desiccant. Tube F contained the drying agent under t c 5. In cases where the desiccant n a s pretreated by ignition or vacuum drying, it was loaded into tube F while still warm. A borosilicate glass-wool plug was inserted on the outlet side of the. ci tubes to prevent mechanical carry over. Tube G was immersed in liquid nitrogen during a determination to freeze out all of the water in the carrier gas which was not removed by the drying agent in tube F . -410-mm. borosilicate tube, H , containing Anhydrone, prevented diffusion of water back into the system. The volume of gas used was measured by the n-et test meter, I . All ground-glass joints, n i t h the exception of the ball and socket joints on tube G, were lubricated with Apiezon N grease, and were securely taped into place with Scotch Brand electrical tape to prevent the pressure from inside the absorption train from dislodging them. Before starting a determination the tubes IT-ere loaded as specified above, and six to 10 liters of nitrogen were passed through the system to swcep it out. During this time the flow rate was regulated to approximately 225 ml. per minute. Tube G was then taken to a room maintained a t 25' C. for weighing. The relative humidity of this room wa? between 50 and 60y0. Before removal from the train the inlet stopcock was closed, but the outlet stopcock nay left open and allowed to remain open for 3 minutes. This permitted any oxygen which had condensedout to boil off harmlessly andescape to the atmosphere. The outlet stopcock was then closed. After 1hour tube G was weighed to the nearest 0.1 mg., using an identical tube as a counterpoise. Before weighing, the outlet stopcock was briefly opened to permit the pressure within the tube to become equal to the atmospheric pressure. Weighings were made a t 30VOL. 35, NO, 6, MAY 1963
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minute intervals until two successive meighings agreed mithin 0.1 nig. Gen erally, two weighings sufficed. This gave the initial weight of tube G. Tube G was then placed back into the train, and the determination was begun. Once a determination was started i t was allowed to proceed without interruption. -it the completion of a determination tube G IT as n eighed in a manner identical to that described above. The gain in weight represented the amount of water not reinoved by the desiccant in tube F . After each run a portion of the desiccant from the inlet side and a portion from the outlet side of Tube F )vas analyzed t o certify that the incoming gas
Table 1. Determination of Water Vapor Remaining in Nitrogen Dried over Anhydrous Magnesium Perchlorate
Runs 1
2
Tube G Initial wt., g. 9,0972 9.3572 Final wt., g. 9.0974 9.3574 Gain in wt., 0.0002 0.0002 g. Vol. of nitro1200 1136 gen, liter Time, hour: min. 87:42 87:42 Flow rate, -998 216 ml./min. Residual water, pg./liter 0.17 0.17 Av. residual water, pg./liter 0. 17
3 9.4250 9.4252 0.0002 1199 57:42 228 0.17
mas wet and that unexhausted desiccant was still present in the outlet side. Discussion of Design and Operation of Apparatus. The flow rate, 225 nil. per minute, was chosen as loiv as possible and yet permit t h e passage of a total volume of gas which would give a significant result in a reasonable period. This flow rate is less than that employed by Hammond and M7ithro~\ (19) and by Smith (19) but greater than that used by others quoted; the rate used by BoIver ( 5 ) cannot be calculated from the data given. Most workers have ubed rates of 1 to 5 liters per hour and the volumes of gas used were so small as to open questions as to the significance of the results. As customary in gravimetric, gas absorption work, the ground-glass joints of the absorption vessels were left dry and the problems of removal of lubricant n-ere thus obviated. It was the original intention to use air as the carrier gas, pulling it through the absorption train by applying a slight vacuum a t the outlet end. Honever, it was found that oxygen, boiling some 13’ C. higher than nitrogen. n-a‘ condensing out in excessive quantities. This indicated that the carrier gas \\as cooled to at least -183” C. as it passed through tube G. At this temperature the equilibrium vapor pre9wre of n ater over ice was calculated to be 2 3 X mm. Hg ( 2 3 ) . Thu., in a determination uring a total volume of 1000 liters of carrier gram of gas, not more than 4 X water escaped detection. It is, of course, conceivable t h a t ice crystals could be carried out of the liquid nitrogen trap mechanically; i t was felt, however, that the condensation of water 1va.s occurring on the cold m i l s of the trap and remaining there, KO proof is offered of
this other than that the tray was functioning well as evidenced by the condensation of oxygen. Supplemental Experiments w i t h +4n effort Electric Moisture Meter.
was made to learn if the slow gravimetric method just described could be replaced by continuous measurement with the Moisture Monitor (Consolidated Electrodynamics Corp., Type 26-302). This instrument is essentially the one described by Taylor (91). The sensing element of the instrument is tn o interwound but separate helices of platinum wire connected by a thin coating of phosphorus pentoxide. The phosphorus pentoxide removes the mater vapor from the gas stream, and it is electrolyzed by impressing a potential between the platinum electrodes which exceeds the decomposition potential of water. The current from this electrolysis is proportional to the water in the gas stream. The instrument, as received from the manufacturer, is calibrated in parts per million of mater vapor by volume at. a flow rate of 20 ml. of gas per minute. The instrument was used under exactly the conditions prescribed by the manufacturer, the flow rate of 20 ml. per minute being measured by the soap film flow meter recommended by the manufacturer. Covar glass-to-metal joint m-as used to attach the glass U-tube to the stainless steel connection into the instrument. The water content of a tank of nitrogen was determined gravimetrically as 325 pg. per liter, This tank was coiinected directly to the U-tube containing the drying agent through a lubricated 12/5 ball and socket joint. Kitrogen was permitted to flow for 24 hours before a final reading n a s taken. I n some later
Summary of Water Vapor Remaining in Nitrogen Dried over Various Desiccants Flow rate, Total vol., Desiccant (initial composition) ml. /min. liter Residual mater, pg./liter Table II.
Magnesium perchlorate. anhydrous (Mg( C104)z .0.12H20) 2 16-228 1136-1200 Anhydrone (Mg(ClO1)2.1.48H20) 195-233 1056-1258 Barium oxide (96.293 BaO) 210-229 233- 254 A41umina(A1203.0.00H20) 213-233 251- 275 Phosphorus pentoxide 220-224 ,555- 576 Moleciilar Sieve 5-4 203-231 200- 230 Magnesium perchlorate, anhydrous, indicating (58% l\lg(C104)n, 0.867, KMnOa) 211-232 411- 45; Lithium perchlorate, anhydrous 223-228 260- 274 Calcium chloride, anhydrous (.CaC12.0.18H20) 227-231 30- 36 Drierite (CaSOL.0.02H20) 230-235 228- 236 304- 330 222-241 Silica gel 43- 4 1 223-228 Ascarite (91.0% SaOH) 57 222-226 Calcium chloride (CaCL 0.28H2O) Calcium chloride, anhydrous, dried at high 227-229 30- 31 temperature (CaCL~O.OOH20) Anhydrocel (Ca30r’0.21H~0) 206-230 646- 720 Sodium hydroxide (SaOH. 0.03H2O) 221-230 17.4-15. 2 Barium perchlorate, anhydrous 27- 28 (Ba(C104)g *O.OOH,O) 220-221 Calcium oxide (CaO.O.OOH20) 222-23 1 50- 5’7 220-222 22-22.1 Magnesium oxide (RIg0.0.00H20) 222-229 18 2-18.6 Potassium hydroxide (KOH.0.52Hz0) 63- 72 Mikohbite (68.77, KaOH) 200-228 Washing from Tube G in Run 3 contained 0.14 pg. P per liter of gas passed; a better value over phosphorus pentoxide is therefore 3.5 pg./liter. * No unexhausted desiccant remained in the outlet side of tube F .
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ANALYTICAL CHEMISTRY
0.2,0.2,0.2, 1,3,1,6,1.5, 3 . 1 , 3 . 0 ,2 . 4 ,
2.6,2.8,3.3, 3.8, 3.5, 3 . 6 . 4.1, 4 . 0 >3 5 ,
-4v.: 0 . 2 Av.: 1 . 5 Av.: 2 . 5 Av.: 2.9
Av.: 3.6” Av.: 3 . 9
4.3, 4.6, 4.2, 13, 13, 12,
Av.: 4 . 4
67, 67, 6:, 69! 65, 66, 60, 70, 72.
Av.: 67
93, 93, 93. 98, 100, 98.
Av.: 70 Av.: 93 Av.: 99
136, 138, 137, 208, 210, 204, 534, 511, 495;
Av.: 137 Av.: 207 Av.: 513
604, 59i, 596;
Av.: Av.:
650, 653, 664; 775, 752, 731; 928, 955, 935; 1570, 1260, 1310;
.Iv. : 13
Av.: 67
599 656
Av.: 753*
Av.: 939 .4v.: 1378
for the residual water in the gas passed
experiments a .daides:; >tee1U-tube \\-as employed in the hope that t'he adxorption of water on the ~ a l l sof the t,nbe would be less of a problem on this material than on glass. Holvever, in no case was a stable reading obtained any faster, nor n-as the firm1 reading significantly different froin t h a t obtained ming tho glass F-t'ubci. RESULTS AND CONCLUSIONS
Typiczl data obtained in a deterniination of residual moisture are given in Table I, for t,liree runs on anhydrous magnc,siuin pcrchlorate. -1sumniary of the results on variow drying agents is given in Table 11, the drying agents being lidcd in t,he rrrtivr of their efficienc !-. 1.: a clr.-iccant, anh;,-drousmagnesium perchlorate is iri a el I5 times: as much Tater being left in a gas stream by the ncxt best desiccant, barium oxide. This .fficirnc:y, coupled with eahe of handling and capncity unmatched by any of the otlier desiccants, makes anhydrous magnesium perchlorate the desiccant of rhoice. The coninion practice of following Ascarite and AIikohhite vr.ith a desiccant in U-tubes in which these materials are used as absorbents for carbon dioxide is confirmed by the fi:idings that these material.? arc poor dr? ing agents. I n accord 11-ith the findings of Morley ( I @ , a nc~gligihleweight of phosphorus pentoxide was vapor. zed when used as :t desiccant a t 25' C. to dry a gas stream. H o w w r . contrary i n Norley, a dis-
tinctly measurable amount of wate'r remained in the gas s t r m n . Readings obtained with the 1Ioi.ture Illonitor departed widely from the values for the residual water obtained by the liquid nitrogen-freezing-graT imetric method just described; tliu., for anhydrous magnesium perchlorate. reading 3.3 fig. of water per liter (0.17 pg. of water per liter by the grai imetric method); Anhydrone 4.6 (1.5); barium o d e : 2.0 (2.8); phosphorus pentoxidc. 2.4 (3.5); I\lolecular Sieve 5-4: 2.4 (3.9); lithium perchlorate: 21 (13): calcium chloride, anhydrous: 9 (67) ; Drierite: 113 (67); silica gel: 133 (70'1; calcium chloride (CnCI2.0.28 H20): 111 (99) ; Anhydrocel : 503 (207) ; barium perchlorate: 98 (599) ; calcium oxide: 3.7 (656); liquid nitrogen trap (helium used a5 carrier ga.): 1.7 (reference). Readings of zero were not obtained on the Aloisture Monitor rvhen phosphorus pentoxide iva4 tested nor when liquid nitrogen trap TI as u-ed to d r y the entering gas. Thus, in our hands the Moisture Monitor has proved unreliable for the measurement of the small amounts of water remaining in a gas stream dried with a chemical desiccant. LITERATURE CITED
(1) Baxter, G. P., Starkweather, H. IT., J . Bm. Chem. SOC.38, 2038 (1916). (2) Baxter, G. P., LJ-arren, R. D., Ibid., 33,340 (1911). (3) Berzelius, J. J., Dulong, P. L., A n n . Chim. Phys. Sei-. 15, 385 (1820;.
(4) Booth, H. ESL. CHEM
S , NcIntyre, L , I A D
, AAAL
ED.
2 , 12 (1930).
( 5 ) Bower, J. H , J . Iiea Vatl. Bur. Std.
12.241 119S4). (6) Dibbits, H. C., Z. A n d . C'heni. 15, 150 (18i6). (7) Dover, M. I-., llarden, J. IV., J . A m . Chem. Sac. 39, Id09 (1917). 18) Dumas. A. B.. Ann. Chim. Phvs. 9er. ~~, 3, 8, 193,'210 (1842). (9) Erdmann, 0 . L., Marchan?, R. F., J . I'rak. Chem., 26, 464 (1842,. (10) Favre, h f . P.-*4., A n n . C'him. I'hys. Fer. 3, 12, 223 (1844). (11) Fresenius, R., 2. Anal. ('hem. 4, 177 (1865). 112) Hammond, IT'. 1., Kithrow, J. It., Ind. Eng. Chem. 25, 653 (1933'8. (13) Johnson, F. 11, C., J . Am. ( ' h e m . SOC.34, 911 (1912). (14) llorley, E. IT.,d m . J . Sei. Per. 3, 30, 1-10 (1885). (15) hlorley, E. W., Zbid., Per. 3, 34, 199 (1887). (16) Morley, E. W,, J . Am. Cheifl. Soc. 26, 1171 (1904). (17) Pett,enkofer, >I.,Ann. Chew., Suppl. 2, 29 (1862); Sitzungsber. kiin. hnyerischen Akad. Il'iss. Jfiinchen. 11, 59 (1862). (18) Regnault, M. V., A n n . Chitn. Phys. Ser. 3, 15, 152 (1845). (19) Smith, G. F., Ind. Eng. Chem 19, 41 1 (1927). (20) Smith, G. F., Brown, lI., Ross, J. F., Ibzd., 16, 20 (1924). (21) Taylor, E. C., Rejrzg. Eng. 64, SO. 7,41 (1956). (22) Kalton, J. H., Rosenblum, C. IY., J . Am. Chem. SOC.50, 1648 (19% 1. (23) Washburn, E. IT.,Nonthly T e a t h e r Rev. 52, 488 (1924). (24) Willard, H. H., Smith, G. F., J . Am. Chem. Sac. 44, 2255 (1922'. RECEITED for reviex October 12, 1962. Accepted February 1.3, 19ti8. This work was done under a grant from the Xational Science Fonndation, XSF-GI 0012.
Anodic CIxidation of Triethylamine ROLAND F. DAPO (2nd CHARLES K. MANN Department of Chemistry, Florida State University, Tallahassee, Flu.
b The oxidation of triethylamine at a platinum anode has been investigated in dimethylsulfoxide with a Pb-Pb(ll) reference electrode. Coulometric analysis indicates that 1.02 0.04 electrons per molec:ule are involved in the reaction. Ail examination of chronopotentiometric data indicates that the electrode rc?action i s diffusion controlled and involves a reversible charge transfer followed b y an irreversible chemical reciction. The major product of the reaciion i s the triethylammonium ion, which was identified by preparing its picrate!. The postulated reaction mechanism involves loss o f an electron to form the radical ion, followed b y reaction with the solvent to form the triethylammonium ion. Quantitative analysis can b e performed under optimum conditions with a relative standard deviation of 2y0.
S
of the electrocliernical reactions of amines that have been reported to the present generally involve aqueous systems. The amine group is sufficiently difficult to oxidize that i t is generally necessary for a favorable form of the reaction product to be available if the reaction is t o be observed in aqueous solution. For example, the ortho and para isomers of phenylenediamine are readily oxidized to the corresponding imines, n hile the meta isomer, having no quinoid ring structure available, is relatively difficult to oxidize ( 2 ) . Anodic oxidations of amines apparently involve abstraction of one of the unshared pair of electrons from the nitrogen atom, followed by some chemical rearrangement. On this baqis, i t seems reasonable to expect that oxidations of amines should generally be possible, but in the case of a compound TUDIES
that lacks a favorable structure, higher potentials than are available in aqueous systems might be necessary. I n addition, i t has been pointed out that the compound must be available as the free amine if the reaction is to be carried out smoothly ( 7 ) . Loveland and Dimeler (51 have given peak potentials for voltammetric oxidation of some ahphatic amines in acetonitrile, but have not reported any investigation of the electrode reactions. \Ye have examined the oxidation of several aliphatic amines in various nonaqueous solvents and nish to report on a detailed investigation of the reaction of triethylamine in dimethylsulfoxide (DMSO). This has involved an examination of the chronopotentiometric behavior of the system, the performance of preparative and coulonletric controlled VOL. 35, NO. 6 , MAY 1963
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