Semimicrodetermination of Carbon and Hydrogen E. C. HORNING' AND 81. G. HORNING' Cnioersity of Michigan, A n n Arbor, Mich. The semimicroproceduredescribed for the determination of carbon and hydrogen is applicable to samples of 12 to 13 mg., has a high degree of accuracy and reliability in routine work, is as rapid as the micromethod, and may be used under ordinary laboratorj conditions.
A
T THE present, time the Pregl combustion microprocedure,
,
\vith various refinements, is in general use for rout,ine analystis in organic work of all kinds, although in most instances there is no necessity lyhatever for the use of a microsample. The g~cliieraladvantage of rapidity of operation with satisfactory avcwracy and precision has made up for the recognized disadvantages inherent in the combustion micromethod. I n the Iiands of an experienced analyst, seniimicroprocedures are as twwrate and reliable, usually more so, as micromethods, and for less-experienced analysts the results in terms of effort and practice needed to obtain accurate analyses are far better. Unfortunately, the good features of combustion semimicroproccdures are generally established a t the expense of a considerable incrrase in the sample size and in the time required for a single itiialysis. The sample size may be increased t o as much as 20 to 30 nig., and the time needed for the conibi~stionmay be twice that rt,quircd for a microsamplt,. Thc semimicroprocedure descritlcd hcrv \vas evolved to meet qualifications: I t was nccessary to analyze samples as s 12 to 13 mg., arid a general level of 15 nig. for the sample WNS desired; it was necessary to have a high degree of accuracy and reliability in routine work: and it vias desired t o work as rttpidly as in the micromethod. Furthermore, circumstances made it necessary to work under ordinary laboratory conditions with forced or windo17 ventilation, under all conditions of tempwature and humidity throughout the year. The chief guide was pixwiticd by the work of Belcher (1, 2 ) who demonstrated that it \vas possible to employ a flow rate of 50 nil. per minute of oxygen i i i both micro and macro combustion procedures, with a consequent shortening of the timc requircd for the combustion. The ~ oxygen employed in the procedure described here is mtc of f l o of about 30 mi. per minute (27 to 28 ml. per minute for nitrogenrontaiiiing compounds). Sinw the rate usually recommended 'o or semimicro work is 5 or 7 to 10 i d . per minute, an t o a rate of 30 t o 50 nil. per minute might well be erpclctcci to result in incompleto absorption of the combustion products a t thc propcr*point, incomplete combustion, or both. 111this work, no tvidence of incomplete absorption of xater or c:irboii dioxide in the ahsorption tubes, or of halogen and sulfur oxiticr o n tlirb ni1vc.r wool plugs, \vas found. The absorption tubes \ v < ~ wtiit' usual xcmimiero type, and the combustion tube w-as of st antlard micro diinension.. h 4-em. silver wool plug TT-as eniploycxi at each cnti of the combustion tube filling for the absorption of halogens and sulfur oxides. Thr complete combustion of the sample, in both micro- and seiiiiriiicroprocedur(,s, is generally not a difficult problem except for wrtain types of cwmpounds. Condensed ring compounds, particularly those xvith angular methyl groups, and those with long hydrocarbon chitins, are very resistant to complete comh s t i o n (4,7 ) . K i t h an increase in the rate of flow, however, complete combustion brcomes a niore serious problem, and it is ncwssary to increase the effectiveness of the burning operation. T h e best ways of doing this arc to increase the furnace temperature, to increase the length of the heated zone of t,he combustion 1
I'resent address, University of Pennsylvania, Pliiladelphia, Pa.
688
tube, and possibly to include a cat,alytic zone of platinized asbestos or platinum gauze in t,he combustion tube. For their rapid microprocedure, Belcher and Spooner (1) employed an unfilled quartz tube, heated to 800" C. I n this work, Vycor combustion tubes were used; the furnace length was 30 cm. (12 inches7 and the furnace temperature n-as 730" to 750" C. The tube filling was of a simple type, consisting only of wire copper oxide with a silver ~voolplug a t each end of the filling. Platinum gauze or platinized asbestos n-as not included. Under these conditions no evidence of incomplete combustion was ever found. This point was interest,ing, since in some laboratories incomplete combustion has caused difficulty, but this could not be investigated as completely as \vas desired. ' A series of samples, each of which was a polycyclic compound containing an angular methyl group, was analyzed; the results indicated complete combustion in each case. I t has been reported (9) that both quartz and Vycor combustion tubes are unsatisfactory in the usual microprocedures, and that failure of S'ycor tubes (by cracking) appears to be due to fusion of silver halides and sulfate'into the tube. KO such failures were encountered during this work; the limit of service was determined by fogging which occurred slowly over the area heated by the free flame used under the sample boat. Tubes removed for inspection after about 100 analyses were in excellent condition, and one tube, in service for about 200 analyses showed fog areas wit,h stains but was otherwise in satisfactory condition. I t may be that a more rapid rate of gas f l o ~is responsible for this difference in behavior from that noted elsewhere (9); Vycor or quartz tubes are essential here because of the higher temperahre used. With satisfactory combustion and absorption assured, a rapid oxygen flow has several advantages. It, permits the burning of the sample in a short time, and it, makes it unnecessary for the operator to devote very much care or attention to the burning operation. There is a minimum of back-diffusion of sample vapor, and it is unnecessary to reburn or flame the tube back of the boat. il large sweeping volume is used, yet the time required for sweeping is as short, as that needed for a microsample. Thr: sweeping operation is also simplified by the fact t,hat a rapid oxygen flom- in combination with a modified Mariotte bottlc, carries water droplets through the capillaries of the waterabsorption tube without difficulty. Although water always condenses in these capillaries, no flow stoppages occur, and it is unnecessary to warm the capillaries a t any time. Faulty wiping of the absorption tubes before weighing is thcx probable source of most of the errors made in practicing the usual micromethod. The omission of t,his step has been recommended previously for micro work (6, 8) and it should be omitted in semimicroprocedures. The tubes should not be touched by hand at any time, but the tips should be wiped with a square of rayon or other dry, lint-free cloth. It is necessary, however, that the conditions of temperature and humidity near the balance be approximately the same as those near the combustion train. The.: conditions are achieved satisfactorily whenever the balance and train are in t,he same or connecting rooms.
SEPTEMBER 1947
689
Results obtained in routine operation with cornpounds used as lahoratory standards &re shown in Table I. The results on many research Samples not containing nitrogen indicated that hydrogen values were likely to be 0 to 0.10% high. Tho ertrhan values obtained on these standards and on many research samples indicated that the precision of the carbon determination was apparently determined by the precisian of the balance. It is estimated that the precision of the weighing8 on the balance used in this work was about * 0.03 mg.
Table I.
Analyses of Laboratory Standards Deviation
Compound
Sample
Methylene-bis-dimethyldihydroresoroinol 2,6-Diben.aloyolohexsnone Acetanilide P-Toluenesulfonamide
MO. 14.65 14.34 15.81 14.15 13.92 11.34 12.53 10.82 14.95
13.00 12.62
3,3.6,6-Tetramethyl.9.(~. nitrophenyl)-1.8-diketo1,2.3,4.5.6,7.8-ootah~d~~14.58 15.37 xanthene
Analysis. Found
C H % % 69 85 8.36 69.80 8.30 87.51 6 . 8 2 6.83 6.85 6.74 6.69 6.80 5.32 4’3.18 5.40 49.15 5.43 87.48 87.67 71.14 71.11 71.21 49.08
70.02 6.35 69.75 6.25
from
Theoretiral
C % +0.01
-0.04 - 0 05 -0.08 +0.11 +KO5 f0.02 +0.12
-0.02
+0.08
+0.05
H %
+0.09 +Q.O3 +0.21 +0.22 +0.24 +0.04 -0.01 +0.10 +0.02 +0.07
occurring in the water-ahsorption tuho, hut the resulting h,ydrogen error was so small that it was difficult to doeidc whethcr or not it was within the experimental precision of the method. Tho results of about 60 analyses, on a variety of compounds cont,aining widely different nitragcn structures, wcrc compared with ICsUk8 obtained under the same general conditims using t,hr samc kind of furnace in the same room and with the same hslsnca, with compounds not containing nitrogcn. The hydrogen values WOK almost without exccption 0 to 0.30% high, with almost, a11 t,he values falling in the rango of 0.10 to 0.20% ithovo tho thoarctical; for compounds not containing nitrogon, the results wcrc in bho range 0 to 0.10% high. The error introduced here by absorption of nitrogcn asidcs is therefore not of major importance, but the authors int,roduced a correction factor of -0.15y0 for all hydrogen vrtluos, snd in subsequent work this correction was applied without rcgard l,o tllc amount or structural type of nilrogen present in tho sxmplo. Analyses of lahoratory standards wore carried out in routine fashion over a considerable period of time in order to providc information on the accuracy and precision of tho method. From the results i t was possible to estimate the precision of thc hydrogen determination. About 85% of the valuos were withiu +O.lO’%, and about 15% of the valucs worc in the mngc hctwcn
* o , ~ - ..
..
.
.~
+0.13
+0.17 -0.02 -0.10
-0.12
nitrogen-containing compounds presented a specm promem, in that i t seemed likely that a load dioxide
packing of tho usual kind would not absorb nitrogen oxides in satisfactory fashion a t high rates of oxygen flow. Belcher and Spooner (8) reported that the metbod of Elving and hIcElray (6) for tho absorption of nitrogen oxides was satisfactory for micro samples, a t a flow rate of 50 ml. per minuto of osygcn, and this method was followed here. According to Elving and .XcElroy, an absorption chamher containing a solution of potassium permanganate or potassium dichromate in concentrated sulfuric acid, placed between the water and carbon dioxide tubes, can he used in a semimicromethod instead of a lead dioxide packing. To be sucoessful such a procedure requires the nonabsorption of nitrogen oxides by the dehydrating agent in the wat.or-absorption tube, quantitative retention of nitrogen oxides in the absorption chamber, and satisfactory sweeping of carbon dioxide from the absorption chamber. The original artiole (5) reported satisfactory results on a somimicro scale, but for micra work conflicting rcports have appearod. Some workers (8, S) hiwe obtained atisfactory results in the micromethod, hut Clark and Stillson (4) found that there was some absorption of nitrogen oxides by the Dohydrite packing of their water absorption tube. It was found in practice that the amount of absorption was sufficient to lead to highly inaccurate hydrogen values and in separate experiments with nitric oxide it was demonstrated that there is a small hut definite absorption of this gas. The combustion of a nitrogen-containing compound under the usual conditions of analysis may he expected to give nitrogen, nitrogen dioxide, or nitric oxide, or mixtures of these gases, in the combustion gases. In the case of nitro compounds, and particularly polynitro compounds, the amount of nitrogen dioxide may be large enough t o give a characteristic red-brawn color to the gas stream. There is no way of predicting the relative amounts of nitrogen and.nitrogcn oxides to he expected from the combustion of a given compound, however, and it is likely that no t v o combustions would give exactly the same quantities. The authors preferred to use a practical and empirical approeoh in testing the suitability of this method in their procedure. Preliminary experiments indicated that a Slight absorption of nitrogen oxides I V ~ S
C I I I L ~ O ~ana I . ~ aasoromg soLuuons provrueu inrormauon an tnc functioning of the absorbing sgcnt,. Some of the valucs in Tabla I wcrc obtained with thc use of a potash bulb containing a solution of potassium permanganstc in conccntrated sulfuric acid. Unfortunately, aft,er this method had boon in use for some t,imc, it was found that tho potassium permanganate solutions WCK ~h:tcriorsting on standing. Tho solutions wcrc a t first quitc stablc, and could he kept in a potash bulb for a rvcck or mare. Aftor some time, a rapid dotoriomtion, similar to that caused by shsorption of nitrogen oxidcs, occurrod within a day or two after tho solution was prepared. A simpler type of absorption chambcr (Figure I), was used in latcr work with a saturated solut,ion of potassium dichromate, containing undissolvod dichromabc, in concentrated sulfuric acid. The dt:grce of retcntion of nilrogen
690
V O L U M E 19, NO. 9
oxides, and the completeness with which the chamber could be swept, were satisfactory. This procedure is now preferred. When it is necessary to include an absorption chamber in the train, the time of sweeping must be increased slightly. The time required for a single combustion of a nitrogen-containing compound is 35 to 37 minutes, about 5 minutes longer than is needed for compounds not containing nitrogen. Incomplete sweeping will result, of course, in low carbon values. The accuracy and precision of the carbon determination, with the sweeping period recommended, are quite satisfactory.
Absorption Chambers. Two types of absorption chambers have been used. One of these was originally designed for use as a potash bulb, and was of unidentified glass (German import). The second, Figure 1, was of Pyrex and was constructed from the bulb of a micro-Kjeldahl flask (30-ml. capacity). The absorbing solution used in the first type of chamber consisted of a solution of potassium permanganate in concentrated sulfuric acid. The second type was filled with a potassium dichromate solution, prepared by adding 0.50 gram of potassium dichromate to 20 ml. of concentrated sulfuric acid in the chamber. (The potassium dichromate does not dissolve completely.) The side-arm drying tubes were filled with Anhydrone, held in place with Pyrex glass wool plugs, and the joints were sealed with beeswax. PROCEDURE
APPARATUS
Balance. The balance used in this work was a Henry Troemner analytical balance. The milligram sensitivity for loads of 20 to 25 grams was 2.5 scale divisions. It is essential that the balance be in the same room as the combustion train, or in a connecting room where the temperature and humidity conditions near the balance approximate those near the train. Pressure Regulator. A standard Pregl pressure regulator is used. The height of the regulator is adjusted to provide a head of 4 to 5 cm. of 1to 1 sulfuric acid during operation. If the nitrogen oxide absorber is in the train, a head of 11 to 12 cm. is needed. Preheater. A standard micropreheater with electrical heating cap is used. The cooling coil need not be immersed in lyater; air cooling is sufficient. Bubble Counter and Absorption Tube. The standard micro pear-shaped bubble counter is not satisfactory; the rapid oxygen flow will carry sulfuric acid from the bubble chamber into the Ascarite packing. A larger semimicro bubble counter and tube are satisfactory. Combustion Tubes. Vycor combustion tubes, of 96% silica glass No. 790 (Corning Glass Works), are used. These tubes are of standard micro dimensions (10.5 mm. in diameter, 520 mm. in length) with side arm. Combustion Boats. Micro platinum boats are used. Combustion Tube Filling. A silver wire plug 4 em. in length is placed in the end of the tube, followed by a 22-cm. zone of wire copper oxide. A second silver wire plug 4 cm. in length completes the filling. The total length of the filling is approximately 30 cm. X short length of platinum wire is inserted in the tube tip in the usual fashion. The authors have found it convenient to construct the silver wire plug in the front part of the furnace in the following fashion. Silver wire (No. 34 gage) is wound on a short piece of metal rod 1 mm. in diameter. The coil thus obtained is wound on a hookshaped wire form about 4 cm. long, bent from a length of Nichrome wire (KO. 18 gage). This form carrying the plug can be withdrawn readily and a new plug can be inserted whenever necessary. The Sichrome wire disintegrates slowly, and a new form should be bent each time a plug is inserted. The copper oxide filling and the silver plug a t the exit end of the tube are kept in place until the tube is discarded. Combustion Furnace. The furnace is 30 em. (12 inches) long, and operates a t 730” to 750” C. Sample Burner. A Meker burner is used for burning the sample. The distance from the top of the burner to the tube should be about 5 cm. Absorption Tubes. Semimicro absorption tubes are used. The authors have found it best to fill these tubes in the following fashion. A thin flat disk of Pyrex glass wool, about 5 cm. in diameter, is worked into shape with the aid of scissors. One quarter of this is cut out, and the remainder is rolled into a cone and inserted in the tube with the aid of a spatula. With a little practice, the glass wool can be inserted so that a cone-shaped space is left for the front portion of the filling. The tubes are then filled as usual with Anhydrone or Ascarite, and Pyrex glass wool plugs. The shape of the filling permits swelling of the absorbent (particularly Ascarite) in the front of the tube without impeding the necessary rapid flow of gas. Mariotte Bottle. A modified type of Mariottc bottle is employed to control and measure the rate of oxygen flow during combustion. The usual side arm is replaced by a short drain tube which includes a section of rubber tubing. A screw clamp on the section of rubber tubing serves to control the rate of flow, Rubber Connections. Paraffin-impregnated or other special tubing is not necessary. A good grade of tubing, cleaned in hot aqueous sodium hydroxide, is used for all connections. The connection on the tip of the combustion tube is of thick-walled (0.64-cm. 0.25-inch) tubing, and is wired into place, The absorption tube connections and caps are lubricated lightly with glycerol. X neoprene stopper is used on the combustion tube.
I t is convenient to maintain separate combustion trains for nitrogen-containing compounds, but the same apparatus, including the combustion tube and its packing, can be used for all types of compounds. If nitrogenous compounds are being analyzed, an absorption chamber, filled as described, is inserted in the train between the water and carbon dioxide absorption tubes. The rate of flow of oxygen is slightly lower and the sweeping time slightly longer, when this chamber is used. Weighing of Sample and Absorption Tubes. All weighings are calculated to 0.01 mg. The procedure followed in weighing the tubes and sample does not differ greatly from microchemical procedure, except that the tubes are not touched by hand a t any time, but are handled always by the caps or by forceps. Before weighing, the caps are removed and the tips polished with a small square of rayon. I t is essential that there be no contact of the hands with the tubes and that the wiping of the tubes be limited to the tips over the area covered by the rubber tubing connections and caps. It is not necessary to time the weighings. Oxygen Flow. A rate of flow of 30 nil. per minute of oyygen is maintained throughout the determination. The total volume of oxygen needed for both burning and sweeping is 900 ml.; the time required for passage of this volume is 30 minutes. The oxygen flow to the pressure regulator should be adjusted so that it is only slightly greater than the rate needed for the combustion. If nitrogen-containing compounds are being analyzed, the rate is adjusted to 27 to 28 nil. per minute, The total volume needed is 980 ml., and the total time 35 to 37 minutes. Burning the Sample. The platinum boat is placed in the combustion tube 2 to 3 cm. from the furnace. The Sichrome gauze over the tube is adjusted so that one half of the boat is within the gauze area, and a lleker burner is placed so that the flame area covers the end of the gauze an-ay from the boat. The combustion is started with a moderate flame. The intensity of the flame is increased slomly, and the gauze and burner are advanced until the gauze rests against the furnace; the burner is then adjusted to full flame. This operation should be carried out fairly rapidly; the time which should be taken for complete burning of the sample (or for distillation into the furnace zone) is about 7 minutes (a maximum of 9 to 10 minutes may be allowed). More rapid burning has not led to detectable error; some samples have been burned in less than 5 minutes. To ensure complete combustion the burning is concluded by allowing the full flame of the Jfeker burner to remain on the gauze for an additional 5 minutes. Reburning or flaming anv part of the combustion tube back of the gauze i s unnecessary. I t is unnecessary to burn an unweighed sample to condition the combustion tube. ACKYOW LEDGM ENT
The furnaces used in this work were designed by Arthur C. Cope and were constructed by R. Law of Bryn hlawr College. LITERATURE CITED
(1) Belcher, R., Fuel, 20, 130 (1941). (2) Belcher, R.,and Spooner, C. E., J. Chem. SOC.,1943,313. (3) ~, Bureer. K..Die Chemie. . 55., 260 (1942). (4) Clark, R.O.,and Stillson, G. H., IKD.ENQ.CHEX.,ANAL.ED., 17. - - ,520 - ~ 11945). - --,(5) Elving, P. J., and McElroy, n’. R., Ibid., 13,660 (1941). (6) MacNevin, W.M.,and T‘arner, J. E., Ibid., 15,224 (1943). (7) Niederl, J. B., and Niederl, V., “,Micromethods of Quantitative Organic Analysis,” 2nd ed., New York, John Wiley & Sons, 1942. (8) Royer, G. L., Norton, A. R., and Sundberg, 0. E., IND. ENQ. CHEM.,ANAL.ED.,12, 689 (1940). (9) Steyermark, A , , Ibid., 17, 523 (1945). I
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