DecimiIIigram Determination of Nitrogen with a Simple Sealed Tube Combustion Method KEllCHlRO HOZUMP Institute of Medical Chemistry, university of Uppsala, Uppsala, Sweden
b An improved method for the decimilligram determination of nitrogen is described which involves sealed tube combustion for 2 hours at 750" C. and weighing volumes of mercury equivalent to the volumes of collected nitrogen which the mercury displaces. The use of mixed barium and magnesium oxide as an absorbent for the carbon dioxide and water evolved in the decimilligram combustion constitutes an improvement over the sodium hydroxide previously recomAn empirical correction mended. amounting to +1% of the nitrogen value found is indicated; it represents nitrogen occluded by the reagents in the decimilligram combustion tube. The method reported is simple, rapid, and practical. It calls for no special equipment, although use of an ultramicro balance instead of an ordinary microbalance may be desirable for accurate decimilligram-sample weighing.
A
NEW METHOD for ultramicro gas volumetric determination of nitrogen has recently been described by Hozumi and Kirsten (1). A subsequent paper (3) describes an ultramicro and micro method which includes some technical improvements for the preparation of sealed combustion tubes and the measurement of the gas volume. I n the original method, a sealed combustion tube-which, before sealing, had been drawn out to a capillary a t one end and to a fine tip a t the other end-containing sample, copper, sodium hydroxide, and pure oxygen, was heated in a furnace. After combustion, the tube was cooled with dry ice to freeze out the water completely. The fine tip was then broken off in mercury, collecting the gaseous nitrogen a t the top of the capillary. The volume of nitrogen was determined by weighing the mercury which replaced the nitrcgen. Although the method has worked quite well hitherto in the author's laboratory, it appeared desirable to eliminate the freezing step before
18 wires per cm. is heated with glacial acetic acid for a few minutes and boiled several times with distilled water. It is dried in a quartz tube and heated to 500" C. for 5 minutes in a current of air. The air is replaced with nitrogen and the temperature is raised to 800" C. The heating is continued for 20 minutes and finally the copper is cooled under nitrogen . Reagent Mixture. Reagent grade barium oxide is crushed in a mortar and ground to a fine powder preventing the access of moisture as well as possible. At the same time reagent grade magnesium oxide is placed into a porcelain crucible and heated with a strong gas flame to red heat for 1 hour. After cooling in a desiccator, about equal volumes of the two reagents (the weight ratio is approximately 7 to 1) are placed in the mortar and ground together. The mixture should be kept free from moisture in a small glass bottle with a polyethylene cap. Procedure. About 500 pg. of sample is weighed on a microbalance and placed a t the bottom of the combustion tube. Sample of aqueous solutions may be pipetted out and evaporated as described in the preceding paper (3). Copper gauze (100 mg.) is cut out as a square and wound on a 3-mm. metal rod. It is inserted into the tube as shown in Figure 1A. About 100 mg. of the reagent mixture is also placed into the tube a t a distance of 20 mm. from the copper gauze by means of a long-handled microspoon. The tube is then brought into the wide flame of a bat-wing burner (flame spreader) fed with hydrogen as shown in B. It may be better to heat the
breaking the combustion tube. The freezing step is somewhat troublesome and it requires some practice to break off the tip of the combustion tube quickly enough after taking out the tube from the freezing vessel. The step can be eliminated by replacing sodium hydroxide with other reagents which would absorb completely not only acidic gases but also the last traces of water. Several reagents were therefore tested under different conditions and among these, a mixture of barium oxide and magnesium oxide was found to be suitable for this purpose. The investigation was carried out on the decimilligram scale to be able to run the analyses with an ordinary microbalance, and because there appeared to be considerable need for a fast and reliable method a t this level. EXPERIMENTAL
Combustion Tube and Reagents. Combustion Tube. Supermax or Pyrex 1720 glass tubing with 5.0- t o 5.2-mm. i.d. and a wall thickness of 0.8 t o 1.0 mm. is first heated with glacial acetic acid on a water bath for 1 hour and then with distilled water. After cooling, i t is washed several times with distilled water and dried. The tube is drawn out in a thin, hot gas flame a t the point 300 mm. from the end t o a fine and bent tip as illustrated in Figure 1 A . Copper Wire Gauze. Copper gauze with a wire diameter of 0.15 mm. and A
-1 L\
8 0 rnm.-j-20rnrni
,Supremax
or P y r e x 1720 i
Ea
I
\
Sample
1
Cu Q a u z e
L70
rnrn.-
B
-c c
I I D 50-52rnrn. W a l l t h 0 8 - 1 Ornm,
BaOtMgO I
E a -
i
1i
1
-
/I
Present address, Faculty of Pharmacy, Kyoto University, Kyoto, Japan. 1
666
ANALYTICAL CHEMISTRY
Figure 1.
Sealing of combustion tube
Scotch tape to a small rule resting on
a desk (Figure 2). Although any type
I
/
Rule
of rule may be used, a finely graduated glass rule which is taken out from a broken thermometer is recommended for this purpose. The position of the meniscus against the rule is noted, taking for a reading the mean height bet,ween the highest and lowest points of the arc. The mercury is then flipped out and the weight of the rule with the empty capillary still attached is determined. Mercury is then injected into the capillary from a microhypodermic syringe until the reading of meniscus exactly agrees to the previous value as illustrated in Figure 2. This arrangement is to minimize the error caused by a difference between the actual volume of the nitrogen and the mercury. By reweighing the rule and capillary the weight of mercury which has replaced the nitrogen can be obtained.
Capillary
C,om bus t i on t u be
\\\\\\\\b
Figure 2.
I
1
I
Measurement of gaseous nitrogen
CALCULATION
reagent mixture briefly with the flame first to sinter and fuse it slightly to the inside wall of combustion tube so that the band of material does not widen out when the tube is subsequently 10tated in the flame. A length of 50 mm. is heated, rotating the tube slowly and drawing out a capillary of 100- to 150-mm. length. A capillary i.d. of 1.5 mm. may be most convenient for decimilligram analysis, but the accurate drawing of this diameter of course requires some practice. The tube is moved along over a stainless steel capillary as shown in C, and 5 ml. per minute of pure oxygen, generated electrolyticdly, is used t o sweep out all the air from the tube (2-4). After sweeping; for 5 minutes, a split type furnace with a temperature of 500" C. is drawn forward over the reagents and kept theime for 5 minutes. The furnace is then pushed back and the tube is partially withdrawn from
Table 1.
Test compound Acetanilide
3,5-Dinitro-
benzoic acid
Thiourea
Phenacetin Trifluoroacetanilide Sulfathiazole
about the steel capillary and its narrow portion is sealed off with B small flame as shown in D. Several combustion tubes thus prepared are maintained in a horizontal position pending their introduction into a furnace a t 750" C. for 2 hours. The furnace is then turned off and allowed to cool to room temperature. After the tubes are taken out, each is held vertically and vibration is applied wit.h a massage apparatus. The copper gauze and reagent mixture should fall down to the bottom of the tube. The end of the combustion tube with the fine tip is then pushed down into the glass thimble filled with mercury with minimum force and the tip is broken off. As a result of previous vacuum, the level of mercury in the tube slowly rises until it has collected all nitrogen a t the top of the capillary. The capillary is cut off at a point about 20 mm. below the meniscus and attached with
T:mz.,
694 605 576 639
465 E116 486 E62 3 28 347 3 68 3 67 E 15 790
732 811 770 857 1.442 1.503 1.595 1.601 748 707
54.1 59.9 56.9 63.3 106.6 111.1 117.9 118.3 55.3 52.2
23 23 25 25
5869 5 64 556 367
752 48 1 1.091 723
55.6 35.6 80.6 53.4
24 24
wt., pg. 553 4:95 471 b10
Hg wt., mg.
The calculation may be carried out with the following equation: %N
=
x 100
Sample The factor F may be found from the nomograph of Koch, Simonson, and Tmhinian (6) after reading the room temperature and the barometric pressure. As below, it was better to apply a correction of +1% to the volume of the nitrogen. Results of a series of analy..qes are listed in Table I. The blank value obtained with 500 pg. of lauric acid was 0.3 pl. which could be neglected in the calculation. 13.53
Mg.
DISCUSSION
Selection of Absorbent.
Although sodium hydroxide was originally used as the absorbent, two disadvantages
Decimilligram Analyses of Several Organic Compounds
Nz vol., pl. 51.3 44.7 42.6 47.2
Sample
OF RESULTS
23 25 25
23 22 25 25 24 24
25
Barom. press., mm. Hg. 747 747 751 751
Nl wt., pg. 58.2 50.7 48.3 53.5
747 747 751 751 747 747 75 1 751 754 754 754 754 751 751
N, % Found Uncorr. Corr.
Theory
10.52 10.24 10.26 10.49
10.62 10.34 10.36 10.59
10.37 10.37 10.37 10.37
61.4 68.0 64.5 71.8 121.0 126.5 133.7 134.2 63.2 59.6
13.20 13.17 13.27 13.00 36.89 36.45 36.33 36.56 7.75 7.54
13.33 13.30 13.40 13.13 37.26 36.81 36.69 36.93 7.83 7.62
13.21 13.21 13.21 13.21 36.83 36.83 36.83 36.83 7.81 7.81
63.5 40.7 91.4 60.6
7.31 7.22 16.44 16.51
7.38 7.29 16.60 16.67
7.41 7.41 16.46 16.46
VOL. 35, NO. 6, MAY 1963
667
i
+O
t 10;
Figure 3.
:
;
2,o
3,O
Table II.
Sample
Test compound Acetanilide 3,5-Dinitrobenzoic acid
wt.,
pg.
448 417
770
lo^. The cause was found to be incomplete absorption of carbon dioxide. It ti-as therefore concluded that sodium aluminate does not react quantitatively with carbon dioxide unless a considerable amount of water is present. The favorable properties of potassium permanganate mere also considered: it is commercially available in a very high purity, it is not hygroscopic, and I t decomposes to K20.2A1n02and oxygen a t 500" C. The resulting ?\In02 will also serve as a combustion catalyst and a slow source of oxygen at 700' C. This latter assures complete combustion of relatively large samples. Low results-around - 0.57&--were, however, observed with 3,5-dinitrobenzoic acid even when the combustion was carried out a t 750" C. for 2 hours, instead of 700' C. and 1 hour, which had been sufficient with sodium hydroxide. The reason has not been clarified yet, but probably potassium nitrate and/or nitrite which might be formed during the combustion is rather stable especially in the absence of water. Dehydrated sodium permanganate was likewise rejected because it gave low values for the same test material.
393
Acetanilide 3,5-Dinitrobenzoic acid
605 584 517 299
574 407 778 1.249
398 440 629 557
466
Thiourea Acetanilide 3.5-Dinitrobemoic acid
0
ANALYTICAL CHEMISTRY
Dimensions of Combustion Tube.
Figure 1 gives suitable combust'ion tube dimensions for deciniilligrani analysis with the recommended quantity of catalyst. Since the inside volume of a sealed tube prepared according t o instructions in this paper is about 4.5 nil., there is enough space t o accommodate the 2 ml. of oxygen necessary to consume a 1-mg. organic sample containing up t o BOYo carbon and 10% hydrogen. Wider and short'er combustion tubes of about the same inside volume were also t'ried with an eye to a shorter furnace. However, t,hese ivere inconven-
Effect of Temperature and Time on Decimilligram Combustion 700' C., 1 hour Barom. s , yo Hg wt., X2vol., Temp., press., s2 wt., Found, uncorr. Theory r g. mm. Hg P1 . ' C. mg. 6.80 10.37 759 30.5 26.6 25 360 6.18 13.21 759 25.8 22.5 25 305 13,21 94,O 12.21 25 759 1,110 82.0 35.39 36.83 25 759 139.1 121.4 I.643
Thiourea
668
4,096 N i t r o g e n
Distribution of analytical errors with several standard substances
subsequentlj becanicl evihe>e were incomplete absorption of n-ater geiieratcd in the coinbustion. and a tendencj t o attach the glass nall of the combustion tube. The lattcr effect was revealed through the appearance of cracks in the glass ahich were often associated with unsatisfactory analyses. Several other reagents Tvere tested and the follon ing information was obtained. Sodium peroxide, which is coinniercially available in a high grade of purity, and which also reacts with water to give oxygen, was unsuitable because it rvas not completely decomposed in the furnace at 500' C. Also, the amount of residual oxygen liberated by the 50 mg. of reagent in the combustion was too large for the 100 mg. of copper to absorb. Sodium aluminate, which was presumed to break down under high heat to sodium and aluminium oxides, was next tested. It mas used only after previous dehydration for 1 hour a t 700' C. and under reduced pressure. Very high analytical results were frequently obtained, especially when the hydrogen content of the samples was
Thiourea
:
The finely po\r-dered barium iiionoxide previously referred t o ( 2 ) was next tried, but uiidcr different, conditions. H ~ K ever, this reagpnt also had t,o be rejected in turn because. like sodium hj-droxide, the barium hydroxide formed in the presence of n-ater at,tacked the glass. -1mixture of bariuni osidp and magnesium oxide vas finally found to be suit'able and free from the disadvantages mentioned above. lIagnesiuni oxide appears not oiily to mitigate the contact betmen the barium oxide and the glass wall, but also to abiorb water as a strong drying agent,. Of several possible mixtures of the tT1-o reagents, a 50,'50 mixture by volume of each in finely powdered form gave good results and was most convenient. Teniperature of combustion is iniportant in connection with the complete decomposit'ion of any barium nitrate or nitrite t'hat may form in the decomposition of certain nitrogen coiitaining structures. Use of a furnace long enough to heat' the n-hole tube uniformly and precise autoiliatic temperature control are recommended. Table I1 shows that 1 hour a t 750' C. is better than 2 hours a t 700' C.
685 957 2.374
700' C., 2 42.4 30.1 57.5 92.3
hours 24 24 23 24
756 756 756 756
48.5 34.5 66.1 105.7
8.01 5.91 12.78 35.35
10.37 13,21 13.21 36.85
750" C., 1 hour 34.4 23 50.6 23 70.7 24 175.5 23
756 756 756 756
39.5 58. 1 61.0 201.6
9.92 13.20 12.87 36.19
10,37 13.21 13.21 36.83
ient becau3e of the difficulty of drawing out capillaries of the right dimension. Proper Amounts O F Reagents Used in Combustion Tube. If cuprous oxide is taken a‘ the probable ieaction product in the coinbustion instead of the usually considwed cupric oxide. only 50 mg. of coplx’r is theoretically nece’sary t o combine nith 4.5 nil. of oxygen. Furthermorf,, a favorable decrease by l to 1.5 nil. in the oxygen volume proceeds froiii the oxygen uptake by the buriiing sample. Consequently the recommended amount of 100 mg. of copper p-okidcs an ample niargin of safety. In connection n i t h the reagent mixture, a siniilar iliargin of safety is provided for in the amount recommended. For TT ork a t the dei~imilligram level, 100 nig. of the niilture is suggested, although as little as 50 mg. has been found sufficient t o ab-orb all carbon dnoide generated in the combustion of lauric acid control miples up to 1 5
mg. On the other hand, incomplete water absorption was observed when sample weights of the same test material exceeded 1 mg. Here small spots of condensed water were discovered between the inside wall of the capillary and the mercury. Consequently, for samples under 1 mg. the recommended amount of reagent mixture mas increased to 100 mg. for security. Empirical Correction of Analytical Results. A number of analyses were cairied out with >creral standard substances t o determine the distribution of error (Fig~ire3). hlthougli the de\ iatiori usunlly icll m-it!iin Pregl’s limits for nitrogen prrcentages up to 2075, it \vas more serious in the case of thiourea (36.837,) where the analytical values were frequently outside the lower limit of acceptability. X possible explanation for low results in the case of liigher nitrogen percentage may lie in significant occlusion of nitrogen gas by the porous reagent mixture and the
copper gauze. The absolute error caused by this effect should be proportional t o the nitrogen content of the sample. From the rough estimation in Figure 3, a relative correction of +1%, shown by the dotted line, is indicated for the samples analyzed. ACKNOWLEDGMENT
The author thaiiks IT’. J. IGrsteii for valuable suggestions. LITERATURE CITED
(1) Hozumi, K., Kirsten, W. J., ASAL. &EM. 34,-134 (IgB‘S). ( 2 ) Kirsten, IT. J., 2. Anal. Chrin. 181, 1 (1961).
(3) Kirsten, W. J Hozumi, I
