V O L U M E 23, NO. 7, J U L Y 1 9 5 1 (11) Hybbinette, A. G., and Sandell, E. B., IXD. ESG. CHEU., An.4~.ED., 14,715-16 (1942). 112) Job, P., Ann. chim., (10) 9, 113 (1928).
(13) Johnson, E. B., and Dennis, L. M., J . Am. Chem. Soc., 47, 790-3 11925).
Kitson, R. E.; and Mellon, M. G., IBD.ENG.CHEM.,ASAL. ED., 16, 128-30 (1944). (15) "Lange's Handbook of Chemistry," 7th ed., pp. 1127-8, Sandusky, Ohio, Handbook Publishers, 1949. (16) Orliac, AT., Bull. soc.franc. mineral., 72,9-240 (1949). (17) Orliac, AI., Coinpt rend., 221, 500-1 (1945). (14)
1027 (18)
Poluetkoi, N. S..Zacodskaya Lab., 5, 27-8 (1935); Z.anal. Chem., 105, 23-6 (1936).
Prideaux, E. B. R., "Theory and Use of Indicators," p. 358, London, Constable and Co., 1917. (20) Vassallo, E., Gazz. chiin. ital., 41,11, 204-12 (1912). (21) Vosburgh, W. C., and Cooper, G. R., J . Am. Chem. SOC.,63,
(19)
437 (1941).
RECEIVED October 21, 1950. Work supported by a grant from the National Research Council (Canada).
Dumas Microdetermination of Nitrogen in Refractory Organic Compounds Modijfication of Zimmermann's Method PAUL D. STERNGLANZ, RCTH C. THO-MPSON, AND WALTER L. SAVELL Chemical Research Division, Remington Rand, Inc., South Norwalk, Conn.
A""-
\ IEW of procedures for the Dumas microdetermination of nitrogen in refractory substances has shown the need for a simple method applicable to both tractable and refractory compounds. Some of the existing methods which claim excellent results (9,18)are not of general applicability-for instance, the potassium chlorate method of Spies and Harris (18) and the copper acetate method of Hayman and Adler (9) were applied to pteridines without success ( 1 ) . Brancone and Fulnior ( 1 ) were able to analyze pteridines merely by raising the temperature of the movable burner to 900" C . ; oxidation aids were omitted. The results, however, show a somewhat greater spread than might be desired. Other Dumas modifications for refractory compounds have been published. One (11)involves the use of nickel oxide as combustion tube filling, a temperature of 1000" C., and a rather coniplicated setup. In another (ZZ), oxygen generated by the decomposition of 30y0 hydrogen peroxide is introduced. -411 these methods have in common an inherent drawback of the conven3
A review of procedures for Dumas microdetermination of nitrogen in refractory substances has shown the need for a simple method applicable to both tractable and refractory compounds. Some existing methods are not of general applicability, and all require a carefully controlled combustion with painstaking observation of bubble speed. Zimmermann's method, designed to facilitate the analysis of tractable compounds, eliminates concern about bubble speed. An attempt was made to simplify this method and to extend it to the analysis of such refractory substances as certain pyrimidines, purines, and pteridines. The procedure, simplified by adapting the tube filling to a boat combustion, was successfully applied to tractable compounds. Refractory compounds required the addition of an oxidation catalyst. Cobaltic oxide was found to be suitable. The movable burner was kept at a temperature of 800" to 830" C. The mixing chamber was redesigned. The automatic setup was replaced by inexpensive standard American equipment. The proposed method is quick and simple, and requires no special care on the part of the operator.
tional Dumas method : A carefully controlled combustion with painstaking observation of bubble speed is required, if accuracy and reproducibility are to be achieved ( 1 4 ) . If the Dumas method is adapted to the analysis of refractory compounds by adding oxidation aids ( 7 , 9, 18), the changes involved may cause inconsistent results, as some investigators have reported (1, 8, 10, 17). Without certain modifications, however, refractory compounds, such as certain pyrimidines, purines, and pteridines, give extremely erratic results (1, 8-10, 12, IS, 17, 18). Concern about bubble speed was eliminated in a method published by Zimmermann in 1943 ( 2 3 ) . His Dumas modification was designed to facilitate the analysis of compounds of normal combustion behavior. In his setup, the precautions which would be required during the combustion period in the conventional Dumas method became superfluous. During the combustion period the gases are blocked a t the three wag stopcock of the nitrometer, and are conducted into a special mixing chamber located at the opposite end of the tube. Here the gases are mixed and diluted with carbon dioxide. Gases to be reduced, such as oxides of nitrogen, and gases to be oxidized, such as carbon monoxide, are now present in a lower concentration. The dilution in the chamber allows the mixed gases to be driven through the combustion tube much faster (25) than is feasible in the conventional Dumas method. Equally important is the fact that a uniform rate of gas flow through the combustion tube is readily maintained by this procedure (3). Although Zimmermann's method is a considerable improvement over other Dumas modifications, adherence to Pregl's way of mixing the sample with copper oxide and transferring it from a mixing tube to the combustion tube is objectionable, as several investigators have stated (13, 18). .41so, Zimmermann's change of the temporary filling from the 40- to 100-mesh copper oxide (as specified by Kiederl, 1 4 ) to a fine powder seemed undesirable. Such fine powder in a 15-em. layer retards the passage of the combustion gases (16). It seemed preferable, therefore, to distribute the sample in a combustion boat and to use coarser copper oxide. With these changes, the method was successfully applied in this laboratory t o tractable compounds. 4 n attempt was then made to extend the Zimmermann method to the analysis of refractory compounds. The results which were obtained with 4-amino-6-hydroxy-2-thiol pyrimidine monohydrate were found to be low. It was, therefore, necessary to develop a modificat,ion of the method. Introduction of an oxidation aid was considered. Because the gaseous combustion products enter the mixing chamber rather than the nitrometer during the combustion, t,he increased rate of evolution of gases could
ANALYTICAL CHEMISTRY
1028 not affect the rate of gas flow into the nitrometer, Potassium chlorate or chromates were inconvenient to use because they are likely to fuse the combustion boat to the combustion tube. Cobaltic oxide, however, was found to be suitable. Its catalytic oxidizing efficiency has been reported by Campbell and Gray ( 4 ) . A temperature of 800' to 830 O C. for the movable burner was found adequate for even difficultly combustible compounds. A manual adaptation of Zimmermann's setup was devised, so that available, inexpensive, American standard equipment (14, 20) could be used. The mixing chamber was redesigned, as discussed under Apparatus. The modification described gives accurate and precise results for tractable and refractory compounds, REAGENTS
Coarse and fine copper oxide, as described by Siederl ( 1 4 ) ; cobaltic oxide, black, c.P., Eimer & Amend. APPARATUS
The apparatus is arranged as shown in Figure 1. Carbon dioxide is supplied by a double Kipp generator, according to Xiederl (14, bo).. The gasometer used is of standard make ( 1 4 , 20). A modification of Zimmermann's mixing chamber is inserted between the Siederl gasometer and the combustion tube (Figure 2 ) . One of the capillary tubes joins the mixing chamber with the Xederl gasometer by means of pressure tubing. The other capillary tube, drawn out to a tip and fitted with a micro rubber stopper, joins the chamber with the combustion tube. The mixing chamber was redesigned to guard against dead space, the mercury can be raised almost to the stopcocks, displacing the greater part of the combustion gases. The chance of forcing mercury into the combustion tube is minimized by having the original horizontal capillary exit changed to an almost vertical one. Stopcock E (Figure 1j. not present in the Zimmermann chamber, was added because of its usefulness during the sweeping-out process. A stationary electric furnace (Eimer & Amend, Item 20-286), a 520-mm. standard length microcombustion tube of Vycor No. 790, and a movable burner of the Meker type with a 12-mm. grid (Eimer & Amend, Item 3-902) are utilized. The nitrometer used is of the Stehr type (19), as supplied by Arthur H. Thomas Co., Philadelphia. Instead of an iron wire to loosen nitrogen bubbles from the mercury surface ( l j j , an iron bar encased in borosilicate glass (about 15 mm. long and 5 mm. in diameter) is used, as well as several steel chips. The chips and bar are held by a horseshoe magnet a t the mercurypotassium hydroxide interface just above the place where the gases enter the nitrometer. Forced by this device to rise directly, the bubbles cannot combine a t the mercury-potassium hydroxide interface to form oversize bubbles which stick in the neck of the nitrometer. METHOD
mixing is completed. Filled to the brim with fine copper oxide, the boat is pushed into the tube against the 3-cm. layer. Then a 2-cm. layer of fine cop er oxide is added, followed by enough coarse copper oxide to filrthe rest of the tube. The system of clamping the tube tip down in an almost horizontal position before installing the boat and the temporary filling was found to be highly satisfactory. The copper oxide can be charged into the tube with little trouble by n?eans of a chute (any conveniently bent metal sheet) and a wire poker. The tube should be tapped lightly during the charging to prevent the formation of air gaps. The temporary filling may be re-used until the copper oxide shows signs of being reduced. Procedure. The tube is attached to the train; care is taken that a 4- to 5-cm. layer of copper oxide extends outside the stationary furnace to serve as cold zone. The air in the system @ displaced by carbon dIoxide while the furnace heats up to (00" C. At about 550' C., the usual test for microbubbles should be made. If the result is satisfactory, stopcock D is closed, E is opened, and F is opened to the air. The carbon dioxide in the mixing chamber is displaced by mercury up t o the capillary tubes by raising leveling container C-2. Then the two mercury surfaces should be on the same level. The threeway stopcock, F , is then moved so that it is partially open to the nitrometer. To prevent the gases from entering the nitrometer, C-3 is raised as high as the top socket joint ( 3 ) . Khen the stationary furnace is hrated to 700" to 750' C., the combustion is started. The LIeker burner is set under the tube a t a point about 13 cm. from the sample and is sloivly moved toward the furnace. The first combustion lasts 7 minutes; the second, 5 minutes. A 10-cm. square of Sichrome gauze, conveniently bent, is used to reflect the heat toward the top of the tube; a Sichrome gauze roll 7 em. long surrounds the tube. Care must be taken, especially during the first combustion, to have 4 cm. of the tube red hot before the hleker burner is moved ahead ( 2 3 ) . During the first combustion, C-2 is lowered as needed to avoid having excess pressure in the system ( 3 ) . At the end of the second combustion, C-2 is lowered so that its mercury level is about 5 mm. below the mercury level of the miving chamber. Then C-3 is set on a level with the bottom of the nitrometer. Stopcocks C and D are opened in this order to allow 50 ml. of carbon dioxide to pass within 7 minutes from the Niederl gasometer into the mixing chamber under a pressure head of 50 mm. of mercury. Stopcock F was left partially open to the nitrometer a t the beginning of the combustion. If the gases enter the nitrometer too rapidly now, a fine adjustment of this stopcock should be made ( 3 ) . After the 50 ml. of carbon dioxide are displaced by gradually raising C-1, D is closed and the gases in the mixing chamber are displaced (in about 4 minutes) by gradually raising (2-2. Sweeping is now begun. E is closed and C-2 is lowered until its mercury level is about 60 mm. lower than that of the mixing chamber. It is important to drive the carbon dioxide into the chamber with a considerable Dressure head (about 100 mm. of mercury), so that the entering carbon dioxide stirs and mixes the remaining traoes of nitrogen. When 30 ml. of carbon dioxide are driven in, D is closed and E is opened. Again the gases in the chamber are displaced by mercury. This sweeping procedure should be carried out two or three times, depending on the diminution of the gas bubbles to microbubbles. The
Tube Filling. The proposed modification requires changes in Zimmermann's tube filling as well as in his procedure. The permanent filling consists of a 27-cm. layer of coaise copper oxide i n t e r r u p t e d twice bv reduced comer JL gauze rdls (each a b o u t 2 . 5 cm. long) and the appropriate a s b e s t o s plugs. The first copper roll is located 7 cm. from the tapered end of the tube; the second, 16 em. (Figure 1). At the 27-cm. B C D E mark the temporary filling begins. This consists, first, of a 3-cm. layer of fine copper oxide to which about 20 mg. of copper acetate may be added. The sample, weighed in a charging tube, is transferred to a 5-cm. porcelain boat (size 00). Then 50 mg. of cobaltic oxide are mixed ine-l c-2 c-3 timately a i t h the sample by Over-411 Yiew of .4pparatus and Enlargement of Filled Combustion Tube Figure 1. means of a bit of platinum wire. The wire is allowed to Capital letters designate stopcocks C-1, C-2, C-3 designate leveling bulbs remain in the boat after the
A
V O L U M E 23, NO. 7, J U L Y 1 9 5 1
1029
v o l u w is read after 30 minutes, and a 1.1% volume correction applied. The temperature of the air near the nitrometer is taken as the temperature of the collected nitrogen. DISCUSSION AND RESULTS
A temperature of 800 to 830" C. for the part of the tube heated by the movable burner has been found adequate for the combustion of all the compounds analyzed. The temperature n-as measured inside the combustion tube using a platinum-platinum 10% rhodium thermocouple. Zimmermann had simply stated that a high temperature is needed for the movable burner ( 2 3 ) . It seemed inadvisable t o use a temperature lower than 800" C. During the pyrolysis of the sample, methane might be formed. Because methane is not completely oxidized when passing over copper oxide heated lower than 800" C. ( a l ) , high results would be obtained. Temperatures much higher than 830" C. are undesirable because they shorten the life of the combustion tube.
+
T I
h
\.\H
i 0
Table I.
Nitrogen Results Obtained by Modified Zimmermann Method Nitrogen, 70
Proposed modification Proposed without Compound Calcd. modification cobaltic oxide Tractable Compounds Glycine 18.66 18.67.18.76 18.80, 18.72 18.76 18.62 Benzyl isothiourea hydro13.82 13.84,14.01 13,73, 13,96 chloride 13,76,13.76 Refractory Compounds 4-Amino-6-hydroxy-2-thiol 26.07 26.22,25.95 25.59,25.91 pyrimidine monohydratea 25.80,25.94 25.48.25.58 25.88,26.09 25.62 Vitamin BI (thiamine hy16.61 16.72, 16.70 16.73 drochloride) 16.54, 16.72 16.89 16.65 2-.lmino-4-hydroxyl-639.53 39.56,39.74 ... methylpteridine 39,31,39.47 39.62,39.55 2,4-Diamino-6,7-diphenyl26.74 26.77, 2 6 , 9 1 26.87 pteridine 26.74,26.79 26.74 6-Amino-2-h droxy purine 24.73 2 4 . 7 6 , 2 + .65 24.38 D-riboside$ 2 4 . 6 2 ,2 4 , 7 0 24.40 24.48 Supplied through t h e courtesy of L. IT. Hartzel. Remington Rand, Inc., South Norwalk Conn. h Supplied t i r o u g h the courteay of L. AI. Bra Division, Ainerican Cyanamid Co., Pearl River, Supplied throngh t h e courtesy of C . IC. Cain Philadelphia, Pa. d Supplied through the,courteey of .J. I t . Spies, Allergen Research Division. S. Department of Agricriltiire, \Va.hington. D. C.
r.
on tn-o compounds of normal combustion behavior. The results are shown in Table I. After these preliminary tests, the method was tried on refractory compounds; satisfactory results were obtained (Table I). Experiments were then repeated, omitting cobaltic oxide. The results obtained with 4-amino-6-hydroxy-2-thiol pyrimidine were low and erratic (Table I). Although the other refractory compounds could be analyzed without cobaltic oxide, it is considered safer t o use it in all cases. The compounds chosen in Table I have been reported in the literature as refractory (1, 9, l d , 18) and, therefore, were considered suitable for testing the proposed modification. All substances were checked for purity by carbon, hydrogen, and moisture analyses, and for chlorine and sulfur, when those elements n-ere present. I n particular, 2,4-diamino-6,7-diphenylpteridine was chosen because it was reported that nitrogen analyses could be obtained only by the Kjeldahl method ( I d ) . To compare the precision of the proposed modification with that obtained by Brancone and Fulmor ( I ), 2-amino-4-hydroxyl6-methylpteridine was analyzed. The average deviation of the arithmetical mean of 39.54% is =+=0.041%for the proposed modification, compared to their *0.082%. The accuracy of the arithmetical mean of 39.5470 is satisfactory in both cases, the theoretical value being 39.53y0.
ivcM*YFigure 2.
Mixing Chamber
Leveling container Mixing chamber, 50-ml. capacity, made of wide borosilicate glass tubing C. Precision stopcocks with 2-mm. bore capillary tubing (Eck and Krebs item 5004) 50-om. length of rubber tubing joins A and B
A. B.
Cobaltic oxide was chosen because of its catalytic oxidizing efficimcy ( 4 ) . Other attributes favorable to its use are nonvolatility before and after reduction, and lack of affinity for nitrogen at the temperature applied ( 5 ) . Cobaltic oxide is converted to cobalto-cobaltic oxide a t 372" C. ( 2 ) . Its dissociation pressure acl / 2 0 2 is about 20 mm. cording to the reaction CoiO, + 3 c o o of mercury a t 825" C. (6). Therefore, it functions to some ext'ent as a generator of oxygen, which is especially needed for the combustion of refractory compounds. In t,his respect it is superior to copper oxide, which has a m,uch lower dissociation pressure a t this trmperature (about 2 mm. of mercury). Copper acetate may be added to the temporary filling of the comhustion tube to minimize concern about bubbles sticking in the constricted part of the nitrometer. The carbon dioxide produced when the movable burner heats the copper salt helps t o drive the combustion gases out of the tube, into the mixing chamber. I n this way, there is an effective mixing and dilution of the gases with carbon dioxide. Later, \Then the diluted gases enter the nitrometer, the bubbles which rise are smaller than they would be, were the copper salt omitted. It was established that amounts of cobaltic oxide up to 100 mg. do not increase the blank. To make sure that cobaltic oxide has no other adverse effects, the proposed modification was tested
+
ACKh-OWLEDGMENT
The authors wish to thank L. AI. Brancone, C. K. Cain, L. TT'. Hartzel, and J. R. Spies for their kindness in submitting the compounds required for this investigation. Appreciation is extended to V. Jane Wilson for determining the purity of the compounds by carbon, hydrogen, and moisture analyses. LITERATURE CITED
(1)
Brancone, L. A I . , and Fulmor, IT., XSIL. CHEM.,21, 1147
(1949). (2) Burgstaller.
S.. Abliarrdl. dcut. n a t z u w - m c d . T'er. Bohmen "Lotos," 3, 83 (1912). (3) Bussmann, G . , H e h . Chim. A d a , 32, 995 (1949). (4) Campbell, J. R.. and Gray, T. H., J . SOC.Chem. Ind., 49, 44i50
T (1930).
ANALYTICAL CHEMISTRY
1030 Ephraini, F., “Inorganic Chemistry,” 2nd ed., p . 563, London, Gurney and Jackson, 1934. (6) Foote, H. IT.,and Smith, E. K., J. Am. Chem. Soc., 30, 1346 (5)
(1908).
(7) (8) (9) (10) (11) (12)
Gysel, H., Hela. Chim. Acta, 22, 1088 (1939). Hallett, L. T., IND. EKG.CHEM.,ANAL.ED., 14, 975 (1942). Hayman, D. F., and .4dler, 8.. Ibid., 9, 197 (1937). Kirner, \T. R., I b i d . , 7 , 297 (1935). Kirsten, W.,A x . 4 ~CHEM., . 19, 925 (1947). hlalette, M. F., Taylor, E. C., Jr., and Cain, C. K., J . -4m.
Pomatti, R., IKD.EKG.CHEY.,.~N.AL. ED..18, 63 (1946). Pregl, F., and Roth, H., “Die quantitative organische hfikroanalyse,” 4th ed., pp. 84-105, Berlin, Julius Springer, 1935. ENG.CHEM..A N ~ LED.. . 12. 304 11940). . . (17) Ronaio. A. R.. IND. (18) Spies, J. R., and Harris, T. H., Ibid., 9, 304 (193:). (19) Stehr, E., Ibid., 18, 513 (1946). (20) Steyermark, 81, Alber, H. K., Aluise, V. A., Huffman, E. W.D., Kuck, J. A., hforan, J. J., and Willits, C. O., AKAL.CHEM.. (15) (16)
21, 1555 (1949). (21)
Ubbelohde, L., and Castro, C. de, J . Gasbeleucht., 54, 810
(22) (23)
Unteraaucier, J., Chem. I n g . Technik, 22, 128 (1950). Zimmermann, W.,Mikrochemie m. Microchim. Acta, 31, 42
Chem. Soc., 69, 1815 (1947).
(1911).
Miher, R. T., and Sherman, M. S., IND.ENG. CHEM.,As.4~. ED., 8, 331 (1936). (14) Niederl, J. B., and Siederl, V., “Micromethods of Quantitative Organic Analysis,” 2nd ed., pp. 79-94, S e w York, John ITiley & Sons, 1946. (13)
(1943).
RECEIVED December 12, 1950
Constant-Current Supply for Coulometric Analysis CH.4RLES N. REILLEY, .W. DONALD COOKE,
AND
N. HOWELL FURMAN
Princeton University, Princeton, N . J .
”ERE has been much recent interest in coulometric methods in which the time required for a constant current to complete a chemical change is measured. The constant current has been obtained by the use of a high-voltage source, such as a rack of storage batteries ( d ) , or a regulated power supply (1, Q), in conjunction with a resistor in series to regulate the current. These systems work well for coulometric micro “titrations,” where currents of 20 ma. or less are employed. If only small constant currents are available the determination of macro quantities of substances extends the time requirement unduly. With these systems for constant current, macro work requires either an averaging of the current (IO)or a continuous varying of a series resistor ( 8 ) to compensate for changes in the resistance of the electrolytic cell. There are certain circuits known that maintain a constant current automatically. Trishin (11) utilized a balanced circuit in which a galvanometer-phototube combination regulated the operation of a motordriven potentiometer to adjust the voltage applied to the cell. Such circuits have the inherent slow response of mechanical feedback loops. Lingane ( 7 ) described another consta6t-current- circuit that has a rather wide dead-zone regulation and a slow response caused by mechanical feedback. Elmore and Sands ( 3 ) have published details of a constant-current circuit for a mass spectrometer magnet that has a rapid response and excellent stability. This circuit is of a heavy-duty, high-current type that is more bulky and complicated than the circuit described here. Gittings (6)reported a general five-tube circuit of quick response, and regulation within 0.015%, according to theory. Details of circuit values and actual test runs were not reported. The simple three-tube constant-current source described in this paper has been in use in this laboratory for 6 months. The current that it supplies to the electrolytic cell is almost independent of changes in cell resistance. In fact, a change of only 0.01% in the current through a standard resistor occurred when the cell was shorted a t 150 ma. A constant voltage source of 200,000
volts would be required for a similar result with a conventional power supply. Regulation is so rapid that no transient deflection is seen on the galvanometer of the potentiometer that is used in testing the constancy of the current. By holding the current within very narrow limits, this supply allows the operator’s attention to be directed to the course of the titration, particularly near the end point. Several cells may be used with the same power supply; the number depends on the currents drawn, the resistive values of the cells, and the regulation desired. Thus, with several cells, the operator can be preparing for additional titrations while one or more are in process. A trigger circuit similar to that of Lingane and Muller (8) has been used in this laboratory to open the time and current circuits a t the end points of titrations. DESCRIPTlON OF CIRCUIT
The circuit in Figure 1 has two outputs, a regulated voltage output intended for use in microtitrations and a regulated current output for use in macrotitrations. Switch & is open when constant current is employed and closed when constant voltage is
-
I
- *
a
+
CONSTANT-CURRENT
CON STANT- VOLT 4Q.E
Figure 1. Schematic Diagram of Constant-Current a n d ConstantVoltage Supply G. 2-mfd., 1000-volt capacitor Cz. 8-mfd., 1000-volt capacitor Cs. 0.1-mfd., 600-volt capacitor CX. 10-henry, 200-ma. filter reactor
F.
2-ampere fuse L. 3- or 6-watt, 110-volt tungsten lamps M . 0-200-ma. meter Ri, Rz. 100-ohm, 10-watt resistors
Rs, Rs. 12,000-ohm, 25-watt resistors R I , Rs. 470,000-ohm, 2-watt resistors
R,. 15,000-ohm, 25-watt resistor Re. 10,000-ohm, 25-watt variable resistor SI, Sz, 4 . S.P.S.T. toggle switches Ss. 3-circuit, 5-position switch T. 425-0-425-volt, 165-ma. power trans-. former
