Oxygen in Organic Compounds Direct Microdetermination bv the Unterzaucher Method J
VETO A. ALUISE, ROBERT T. HALL, FRANKLYN C. STAATS, AND WALTER W. BECKER Hercules Experiment Station, Hercules Powder C o m p a n y , W i l m i n g t o n 99, Del. The promising micromethod of Unterzaucher for the direct determination of oxygen in organic compounds has been investigated. The procedure involves pyrolysis of the compound in a stream of nitrogen and conversion of all the oxygen in the pyrolysis products to carbon monoxide over carbon a t 1120" C. The carbon monoxide is oxidized to carbon dioxide by iodine pentoxide, and the equivalent amount of iodine liberated is determined titrimetrically. A furnace meeting the high-temperature requirements is described. Criteria are presented for the selection of carbon which gives quantitative conversion of the
oxygen to carbon monoxide, and iodine pentoxide for quantitative oxidation of the carbon monoxide. The results obtained to date in this laboratory show over-all precision and accuracy somew-hat less than those now obtained in carbon and hydrogen analyses. The method, however, possesses several outstanding advantages over other methods for oxygen determination, in that the apparatus is somew-hat less elaborate, the presence of other compounds, such as sulfur, nitrogen, and halogens, has no effect on its applicability, and catalyst poisoning does not present a problem.
A
LTHOUGH oxygen is one of the most commonly occurring
it was decided to present results obtained with this method thus far, although certain improvements, especially in the apparatus, are still under investigation. The procedure involves pyrolysis of the compound in a stream of nitrogen, conversion of all the oxygen in the pyrolysis products t o carbon monoxide over carbon a t 1120" C., and oxidation of t h e carbon monoxide t o carbon dioxide by iodine pentoxide. The equivalent amount of iodine liberated from the iodine pentoxide is determined titrimetrically, by oxidation to iodate, subsequent reduction to iodine, and titration n-ith sodium thiosulfate solution. The method requires less elaborate apparatus than either the complet'e combustion or thc catalytic hydrogenation method. Catalyst poisoning is not a problem as in the latter method, and the presence of elements other than carbon, hydrogen, and oxygen has no effect on the applicability of the method. Finally, no complicated calculations are necessary and the titration of 12 atoms of iodine for 5 atoms of oxygen is a favorahle one. Since the water gas reactions involved
constituents in organic compounds, t'here is, a t present, no entirely satisfactory method for its direct determination. The lack of such a method is clearly emphasized in the recent excellent review by Elving and Ligett ( 2 ) , who present a critical examination of the methods that have been investigated for the determination of thi5 element. They suggest that probably the best method is that based on thermal decomposition of the compound over carbon, first proposed by Schutze ( 5 , 6 ) and later improved by Zimmermann (8) and Unterzaucher (7). Iiorshun (3) examined Schutze's procedure and found it satisfactory. Since the need for a satisfactory direct osygen determination arises frequently in this laboratory in the course of problems involving identification of unknown compounds and measurement of extent of polymerization and condensation reactions, investigation of the thermal decomposition method was undertaken. Essentially, the apparatus and procedure described by Unterzaucher ( 7 ) were used. After overcoming a number of difficulties, reasonably accurate and reproducible results have been obtained, and the method is now being used routinely. I n view of the general interest in the problem of direct determination of oxygen,
A-OXIDATION FURNACE 8-REACTION FURNACE C-MOVABLE BURNER D-PURIFICATION UNIT E-BUBBLE COUNTER
H20
+C
Hz
+ CO
co, + c = 2co
F-QUARTZ TUBE CAP 6-QUARTZ TUBE H,H' REVERSE STOPCOCK5 K-VICREUX ABSORPTION TUBE 0 - M O T O R I Z E D REDUCER SPHERICAL J O I N T S 12/5
7
Figure 1. Diagram of Apparatus for Direct Determination of Oxygen in Organic Compounds
347
(1)
(2) require a high temperature to proceed quantitatively, a n essential requirement is a furnace capable of continued operation above 1000" C. Both Schutze (6,S ) , who first proposed this method and used it on a semimicro scale, who adapted it to a micro scale, and Zimmermann (8), used a minimum temperature of 1000 C. Lnterzaucher (7) subsequently found that a minimum temperature of 1120 ' C. !vas necessary to obtain quintitative results. Because of this high-temperature requirement, the first problem was to design a furnace in which the temperature of the portion of the quartz reaction tube containing the carbon could be maintained a t about 1120" C. A special furnace fulfilling these requirements and equipped with automatic temperature control x a s designed in cooperation with the Lindberg Engineering Company, Chicago, I11 I n the earlier phases of this study, a suitable carbon was not available which gave both (1) quantitative conversion of all the oxygen to carbon monoxide and (2) a I o n blank. Eventually, two samples of carbon xyere prepared and tn.o commercial samples were secured n.hich fulfilled requirement 1. The authors were able to establish conditions for obtaining consistent and reasonably low blank values,
348
ANALYTICAL CHEMISTRY
oxygen in the detcrmination of carbon and
I.
to be nec&sary. The bubble counter, E, contains concentrated sulfuric acid; the adjacent arm of the U-tube, Ascarite; the other, phosphorus pentoxide. The quartz reaction tube, G, should be tested for leaks by Using vacuum and a Tesla spark coil. The detailed dimensions are shown in Figure 3. I t s design and connections permit reversal of thc flow of nitrogcn through the tube, in ordar to prevent the entrance of air
Figure 2.
Apparatus for Dir
hut have not been able to reduce reported by l'ntersiiucher (7).
:en
"..,....~ "....
yy
".._
vrrrL~e
AITARArUS
.1 diagram of the apparatus is shown in Figure 1, and the dual unitusedin thelabolatoryispict,urcdinEigure2. Thefurnace, B ,
used t,o hcat t.he carbon is a Lindherg typc CF-ZS, dual combust.ion tubc, equipped with four Globar heatin: elements, variable transformer, ammetcr, and fine and coarsc tap switches. The furnace has an over-all length of 32.5 em. (13 inches). It has a power consumption of 1500 watts and is maintained a t a constant temperat,urc by means of a Whcclco Model No. 244 (:apacitrol. I n ordri 1,hst tho furnace may be movcd latcrally to permit hcsting of bhat part, of thc rczdion tube passing through the insulating qldls, it is equippcd with a special gear and track mounting. The iodine pentoxide tubc (SCC also Figure 4) for convcrting t.he carbon monoxide to carbon dioxide is installed in a thermostatically-controlled aluminum block electric furnace, A (Arthur €1. Thomas Co.). T o make sure that the evolved iodine is swept coniplctcly into the adjoining Vigreux absorption tube, K , the ground-glass joint is likeivisc kcpt a t an elcvated temperature inside the furnace. A Fishcr high-temperature burner, C, is adequate for pyrolysis of the samplc. T o confine the flame, a small P s h a p e d hood constructed of Nichrome gauac, with asbest.os ends, is permanently fastencd to the top of tho burncr. A roll of Nichrome wire gauze is also coiled around the reaction tube and maintained in position directly above the burner. I n addition, a section of Transite is placcd betwecn t,hc burner and t,he cap, P, as 8. heat shield. By this means, a tcmpcrature of 1000" C. is easily maintained in the ---..',,:" -..--l..--_l end scetion of tho tubc where t bL -r 1"LLLpL" 1 1 py'U'y'L". Tho burner is moved by aut omatic propulsion, at a rate of 2.5 cni. in 8 minutes, by a i M d:1 SG-25 motorized reducer, 0 (2 r.p.m. output, Xerkla-Korff Gear Company, Chicago, Ill.), r h i c h turns CL threaded rod. J'. A coil spring, S, on the burner carriage forces the threads bf snlit nut aeainst the thrcads of tho rod, P, thereby propellinr
oxygen,.whether oresent as suc h or in tho form of water or carbbn dioxide, a. purification systern was used consisting of: (1) a suitable leneth of a Vvcor or Pvrcx ela.ss microcomhustion tube caittsinine a"l0-em. 5xection of copper grit (40- to 60-mesh) or ribbon mounted vertically to avoid channeling of the filling and maintained a t about 500" C. by an electric preheater furnace, D (Arthur 13. Thomas Co., Philadelphia, Pit.); and (2) an absorption tube, R, vhich contains a %cm. section of phosphorus
in Organic
sample. Once assembled, precautions should be taken t o prevent air from entering the apparatus. On overnight standing, a slow stream of nitrogen. one or t v o bubbles ner second. is nassed through the apparatus di;cctly from'thecylinder, by adjusting the gas pressure regulator and reducing valve. For long periods of idlenoss it is conveniont to U E a gasometer, which is designed to maintain the aooaratus under oressure of nitrogen, the end dfihe tube containing the iodine pentoxide being closod. The gasometer is filled with nitrogon through the three-
way T-stopcock, L. A Mariotte bottle is used to aid in maintaining tho proper flow of nitrogen and reaction products through the sppmatus. The platinum combustion boat is 4 X 12 X 2.5 mm. in size. Where rubbor tubing connections are made, heavy-aalled paraffin-irnprpgnated rubber t.ubing is used.
satisfactory for convorsion of tho carbon monoxide to oarbon dioxide. In order to pepmit the free flow of gases, i t is prepared for use either by screening to exclude particles finer than 100mesh, or by miring as received, r i t h approximstcly one fifth of its weight of aa'ter-washed and dricd pumice tone, 20- to 30-mesh. The prepared iodine pcntoxide is introduced into thc axidstion tube (Figure 4) and conditioned in a strcam of dry nitrogen for 24 hours or more a t 230" to 240" C. followed by 40 to 50 hours at 150" t,o 160" C. Aftercoolinginastreamofnitragcn theeontents of the tubc should be packed firmly by repeatcd tapping in order to avoid formation of channcls during service. Whon this oxidant is used, the temperature of ihe oxida1,iontube furnace, A, is set at 120-121" C. Iodine Pentoxide-Vanadium Pentoxide. This oxidation tube filling is prcparcd as dcscribcd by Brown and Felger (l),eroept t h a t 20- to 30-mesh Armstrong A-25 insulating brick is usod instead of 9- to 14-mesh. With this filling, s t,cmperature of 1 1 s 111" C. was found satisfactory. Furthcrmorc, bccsusc of the uniform part.icle size, a frecr flow of the gases was obtainod. Howcvcr, this filling has becn found unrclizblc far some compounds containing nitrogen. Sodium Hydroxide, A 20% solution of Bakcr's C.P. pellets in A:"&:,,-J
Bromine. Baker's C.P. Potassium Acctate-Glacial Aectic Acid. . A !OYo solution of Bakcr's C.P. potassium acetate in glacial acetic acid Sodium Acetate. A ZOO/, solution of Baker's C.F. sodium m e -
CAPILLAR" TUBlNG 6.7 MM.0.D. 1-2 MM. 1.0.
Figure 3.
7 5 - 8 M M . ,.D. 10 - I I M M . O . D .
6-7MM.O.D 2 - 4 M M . I.D. STOPCOCK To END CAP 2 MU.
Quartz Reaction Tube and Filling
349
V O L U M E 1 9 , NO. 5, M A Y 1 9 4 7 Potassium Hydroxide. Merck's reagent-grade pellets are crushed to 8- to 10-mesh and tightly packed into the carbon monoxide scrubber tube for removal of interfering gaseous product,s formed when elements, such as halogens and sulfur, are present in the subst,ance being analyzed. Nitrogen. Airco dry nitrogen, Air Reduction Co. Carbon. The benzene soot was prepared in the laboratory by burning a wick immersed in benzene (thiophene-free), and collecting the soot on a glass surface. The acetylene soot was prepared by burning acet,ylene from a cylinder, using a welder's torch with no auxiliary air or oxygen, and collecting the soot on a glass surface. These carbons and a commercial carbon, herfloted Arrow carbon black, obtained from J. AI. Huber, h e . , S e w Tork, S.T., were pelleted a t a pressure of 250 kg. per sq. em. (3500 pounds per square inch). The pellets were cut into small pieces, and the fraction passing through a 30-mesh but ret'ained on an 80-mesh sieve was dried a t 150" C. .1 second commercial carbon, IT-yes Compact, Black, J. AI. Hubcr, Inc., is available (hrthur H. Thomas Co.) in a particle size which is suitable for use in thc reaction tube n-ithout pellcting or screening; it, need only be dried at 150" C. and conditioned as described below for the othrr carbons. The sieved carbon is placed in a tube and heated iii a slow stream of nit,rogen, the temperature being gradually incrcased to 550" C. and maintained for a period of several hours. This treatment removes volatile constituents and sinters the carbon, thereby preventing channeling when the carbon is used later in the reaction tube.
is moved back and the furnace ininiediatcly returned to its original position. This ensures complete pyrolysis of any decomposition products which may have condensed on the cool portion of the tube. The burner is turned off and the sweeping continued for about 15 minutes. The absorption tube, K , is then removed, and the contents are immediately rinsed with about 125 ml. of dist'illed water into a beaker containing 10 drops of bromine and 10 ml. of a 10% solution of potassium acetate in glacial acetic acid. After stirring, the contents of the beaker are transferred to a 250-ml. Erlenmeyer flask containing 10 ml. of a 20y0 solution of sodium acetate in water, and the excess bromine is destroyed by slorvly adding, with shaking, 15 to 20 drops of 90To formic acid. AftCii. 4 or 5 minutes, about 0.3 gram of potassium iodide and 5 ml. of lOC7, sulfuric acid are added and the flask is gently swirled. Thtt liberatod iodine is t,itrated immediately n i t h 0.02 S sodium thiosulfate.using starch indicator. (A 0 . 2 5 aqueous solution of amylosr, G. F. Smith Chemical Co., \vas found to he an escellent indicator.) CAPILLARY TUBING 7-8 M M 0 D GLASS 15-2 MM I O WOOL
PROCEDURE
The rtlaction tube is packed n i t h quartz chips, 6- to 10-mcsh, quartz wool, carbon, and a s:cmd s:ction of quartz wool, as shown in Figure 3, n i t h repeated tapping in order to avoid channeling in service. The apparatus is assembled as shown in Figure 1. The copper in the preheater furnace, D , is reduccd in situ by a don. stream of hydrogen introduced through the thrw-way T-stopcock, AI; the gas stream escapes through the three-way T-stopcock, S . The tempc,rature of the preheater furnace is maintained a t 500" C. K i t h the reaction furnace, B , ' a t room tcniperature, a s l o stream ~ of nitfogrn is passed through t,he apparatus for several houi and the temperature gradually has been passed through the perature, it is ready for use. Before btarting a n analysis, the Nariotte flask is connectcd to the apparatus and the valve on the nit'rogen cylinder is a t o providc a flow of 8 t o 10 nil. per minute through the with furn:ice B at the operating temperature. h gage pre 4 to 6 pounds is sufficient to maintain this rate, which can be checlted as required by observing the bubble counter. The platinum boat is flamed just before USE and cooled in a desiccator, and a sample containing 1.0 to 1.3 nig. 6f o. the boat. Liquid samplcs arc ivcighcd in p1:iccd in the boat.
\
/W00~~14,3s
4-
10-C M .
I5 CM.
Figure 1. Oxidation Tube arid Filling
A blank run is made by introducing into the reaction tube 817 empty platinum boat and proceeding exact'ly as above using 500 to 600,nil. of nitrogen. The per cent oxygen is calculated as follolvs: (1' - b ) X normality of Sa&O:< X 0.1333 X 100 -
-~
cc os!.gcn
0.0200 X milligram.: of sample
~ v h e r c1~ = ml. of S a ? S Qrtquircd for sample
b = ml. of S a ? S Or e q u i r d for blank 0.1333 = milligrams of oxygen equivalent t'o 1 0.0200 S Sa$&
id.
of
DISCUSSIOS
In the e:irl>- stages of tlir inre-tigation, the fir-t lot of cnrhoii wed was :I commercinl 1:impbl:ick: it gave di-couraging!y high blanks and l o ~ vrecoveries for oy-grii. 0 1 1 :in:i to contnin 11.7$ :ish, consirtiny miiinly of s ni:iynesiiini, c:ilciiim, and iron, piwlxibly present :ss the oxide-. This sample of cnrl)oii IV:I*, tlierc,iorc, rejkcteti as unmit:ilile. Sub-equently, the follo\ving five tylies of cnrt)oii ryere tested: Designation Spectroscopic electrode graphite Aerfloted Arrow carbon black Benzene soot hcetylene soot \Yyex Compact Black
burncr contacts I j (40 to 50 minutes), t h e furnace is movcd a?)out 5 cm. (2 inchrs) to thr left. Thc burner is then adv:mcid manually and thar portion of t h r tube pwviouuly protected tly the, insuhting \vall of thr. furnarc. is hr>atcclfor 5 minute-different character. I n like manner other alkali sulfides can be activated. It is not impossible that this effect can be used in organic chemical preparations where alkali sulfides are employed as reduct,ors in alkaline media. THEORETICAL CON SIDERATION S
There is no doubt that in all the reactions mentioned above, sulfide ions act as reductors and selenium accelerates the reaction velocities. The general nature of the "selenium effect" can be elucidated by consideration of the mechanism of the methylene blue reduction. The decoloration of methylene blue by excess alkali sulfides can be formulated by the following equation:
2 11B
+ S-- + 2 H?O
----)
2 HMB
+ 2 OH- + So
(1)
where 1 I B represents methylene blue and HhIB represents the leuco form of methylene blue. As an excess of sulfide ion is present, the free sulfur formed (Equation 1) is dissolved by formation of polysulfide:
so + s-- + is. . SO]-*
(2)
I n the case where selenium-containing sulfide solutions react \vit,h methylene blue, it is necessary t o consider the st'ate of the
