n hen titrated in acetone 11ith triethyl-nbutylammonium hydroxide. Because of the electron-withdratving effect of the two phenyl groups, dibenzoylniethaiie would be expected to be the stronger acid. K h e n titrated in acetone with sodium or potassium methoxide, dibenzoylmethane actually is a Qtronger acid than 2.4-pentanedione. An even more striking effect is in the titration of succinic acid. I n water the pKl is 4.2 and pKz is 5.6. From this, the curve would be expected to have little if any inflection when one hydrogen is neutralized, and a sharp end point 11hen the second hydrogen is
neutralized. The curve actually obtained (Figure 9) has a n excellent break when one equivalent of base has been added, and only a relatively small break a t the second inflection point. There is no visible precipitation a t any point in the titration. KOexplanation is offered a t this time, but further study of these effects is planned. LITERATURE CITED
(1) Bruss, D. B., Wyld, G. E. A,. ASAL. CHEAT. 29, 232 (1957). ( 2 ) Cundiff, R. H., Markunas, P. C., Ibid., 28, 792 (1956). ( 3 ) Deal, 5'. Z., Wvld, G. E. A , Ibid., 27, 4 i (1955).
( 4 ) Fritz, J. S., Ibzd., 24, 306 (1952).
( 5 I Ibid., p. 674. ( 6 ) Fritz. J. S.. Keen. R. T.. Ibzd.., 24., 308 (19521. ' ( 7 ) Ibzd., 2 5 , 179 (1953): (8) Fritz, J. S.,Lisicki, S. LI., Ibid.,
23, 589 (1951). (9) Harlow, G. A , Sohle, C. XI., Wyld, G. E. -4., I b i d . , 28, 787 (1956). (10) Moss, 11. L., Elliott, J. H., Hall, R. T., Ibid., 20, i 8 4 (1948).
RECEIVED for review October 18, 1956. Accepted February 27, 1957. Division of Analytical Chemistry, 130th Meeting, &\CS,iltlantic City, N. J., September 1985. Contribution No. 520. Work performed in Ames Laboratory of the t-. S Atomic Energy Commission.
Dumas Microdetermination of Nitrogen WOLFGANG J. KIRSTEN Institute o f Medical Chemistry, University o f Uppsala, Uppsala, Sweden
b A very fast and highly accurate method for the microdetermination of nitrogen is based upon pyrolysis in carbon dioxide and subsequent combustion over nickel oxide. In continuous work a complete analysis including weighing out, running the analysis, reading, and calculating the result takes 15 minutes. The standard deviation of a series of 21 analyses of pure solid samples was 0.037% of nitrogen. A procedure for the analysis of aqueous solutions and biological fluids is given.
A
accurate method for the semimicrodetermination of nitrogen, based upon pyrolysis of the sample in carbon dioxide a t 115OOC. and combustion of the formed gases over nickel oxide, described by Iiirqten ( d ) , was applied to the deeimilligram scale by Kirsten and Grunhauni ( 5 ) . I n the semimicroprocedure backway sneeping was used (3) in connection n i t h a temporary filling. nhich n a s changed for every anal\ 4s. In the decimilligram procedure back-n a y sneeping was avoided, and the temporary filling was found unnecessary with the samples used. Simultaneously n i t h ( 5 ) Clark and Dando ( 2 ) published a method for the microdetermination of nitrogen based upon the same principle. also without back-n a y sn eeping and without temporary filling. The decimilligram apparatus ( 5 ) 17 as provided 17 ith a leak arrangement TT hich allorred fast aiid clean introduction of the sample and had been found ~ e r y useful for several purposes. The possibility of wing all these VERY
1084
ANALYTICAL CHEMISTRY
innovations in the design of a fast, simple, and accurate method for the microdetermination of nitrogen (milligram scale) was investigated. The method finally adopted works with a back-way sweeping system and with a temporary filling contained in a capsule of platinum gauze. The leak arrangement, combined with a quartz rod for introduction of the sample which remains in the tuhe during the combustion, reduces backway sit-eeping time to a very short period. -4 faster rate of combustion can be used as a consequence of the use of smaller samples. Use of an overpressure (above atmospheric) in the nitrometer during the combustion, in combination with a nitrometer head which is not greased, allows the reading of the nitrogen volume without a waiting period immediately after the combustion. Opaque quartz is better able to withstand the stresses of the high temperature combustion than clear quartz. APPARATUS
The apparatus s1ion.n in Figures 1 to 6 is available from Sicroma, Klara T'astra Kyrkogata 7 , Stockholm, Sweden. Thc carbon dioxide tank is provided n-ith a reducing valve. welded to the flexible copper tube, -4. The gas passes into the branch tube, B , connected with the pressure regulator, C, which is filled with mercury. Tube D leads to the exhaust. Tube E leads to the rotameter, F , which is connected with the combustion tube through tube G , joint H , and side tube K . A capillary, L , is placed in joint H as shoan in Figure 2. From K part of the gas pasqes through -If, through
capillaries S and P (Figure 5 ) out to free air. Another part passes along rod Q through R into the long furnace, 8. Capsule T , made of platinum gauze and filled with nickel oxide, is situated in the wide part, C,of the combustion tube, which is situated in S. The nickel oxide filling, T', is held in the tube with quartz wool, W . The gas passes then through the thick-v alled capillary, A B , through joint AC into tube AD, in which the Hopcalite filling, AE, is held \Vith borosilicate glass wool, AF. The Hopcalite i s heated by furnace AG. From threeway capillary stopcock, AH, the gas passes into the nitrometer through connection AK (Figure 3). The connection is filled with mercury, which makes it diffusion-tight. AL is a short bit of rubber tubing. Through tubes AM and A S the gas passes through mercury into the nide part of the nitrometer, AP, and into the graduated part, A&, which ends n i t h funnel AR, closed with joint
iis
During the back-n ay sweeping, rod Q i4 taken out of the tube and stopcock AH is turned so that carbon dioxide passes from branch tube B through copper tube AT and joint AC with capillary .4V (Figure 2), through stopcock AH, tube AD, and the combustion tube out the back way. The drying furnace, AW, is used in the analysis of aqueous solutions. I t s temperature is dependent on the nature of the solution to be analyzed. For serum 90" C. is suitable. B A is a motor-driven splittype furnace, held a t 1050" to 1100" C., S is a tube furnace, held a t 1000" to 1050O C., and AG is a small tube furnace, held a t about 150 O C. Figure 4 shons the head of a nitrometer. nhich nas found very suitable for this type of analysis. BC is the graduated, very thick-walled nitrometer tube, the upper End of which ic ground conical
and provided with the groove, BD. BE is a funnel of clearly transparent plastic, cemented upon the joint of tube BC with plastic cement. The purpose of BD is to give a firm hold for the cement. The end surface of BC is ground flat, and the cementing is done so t h a t the surface lies about 0.5 to 1 mm. above the bottom surface of t h e funnel. B F is a flat ground glass which fits to the flat
end of BC. It is connected to glass rod BG with rubber tubing, BH. The ends of BF and BG which meet each other are rounded, and a soft rubber is used for BH to provide for flexibility. Screws BK hold rubber band BL, which holds the glass rod, BG. BC is graduated u p to the flat end. The conical part is clearly transparent, so t h a t the calibration lines can easily be seen.
Figure 1.
REAGENTS
The nickel oxide, Hopcalite, and potassium hydroxide have been described (4). Tank carbon dioxide (9) is treated as described ( 5 ) . Quartz wool is used to hold the tube fillings. ADJUSTING THE APPARATUS
The apparatus is assembled as shown
Apparatus for determination of nitrogen
Connection to gas tank Branch tube of brass. Copper-tubing branches are soldered to branch tube, which is held by clamp on carriage of apparatus T-tube pressure regulator filled with mercury C. D. Connection to exhaust. There should be no strong suction Connection of flexible copper tubing with metal joints soldered on E. Rotameter F Flexible copper tubing with metal joints Eoldered on G. Ground joint connection metal to quartz, compare Figure 2. Widest diameter of ground H. part 10 mm. Side tube of quartz. Total length 170 mm., inner diameter of tubing 1 mm., outer 9 mm. K. Back branch of combustion tube. Length 150 mm., inner diameter 10 mm., outer diampter M. 14 mm. Clearly transparent quartz .v . Quartz capillary sealed upon quartz rod &. Inner diameter 1 mm., outer diameter 7-8 nim Fine glass capillary fixed upon capillary N with rubber tubing P. Quartz rod. Diameter 7-8 mm. Compare Figure 5 Q. Branch of combustion tube between side tube K and furnace S. Length 180 mm., other R dimensions like M Furnace, length 230 mm. S. Capsule of platinum gauze. Tots1 length 50 mm., outer diameter 9 mm., gauze 36-mcilh, T. wire diameter 0.23 mm. c. Part of combustion tube in furnace S. Length 340 mm., inner diameter 11 m m , outer diameter 16-17 mm., opaque quartz v. Tube filling of granulated nickel oxide Quartz wo6l W. A B . Quartz capillary. Inner diameter 2 mm., outer diameter 8 mm. Total length with joint 100 mm. -4 c. Ground joint glass-quartz, widest diameter of ground part 10 mm. A D . Hopcalite tube. Length of vide part, total Tvith joint 70 mm., inner diameter 7 mm. Length of capillary 20 mm., inner diameter 1-2 mm. A E . Filling of granulated hopcalite, length 15 mm. A P. Borodicate glass wool AG. Small heating furnace, length 30 mm. -4H. Three-way, T-type, capillary stopcock A K . Special connection to nitrometer. Compare Figure 2 .4 M , Thick-walled capillary. Inner diameter 1 mm., outer diameter 7-8 mm. A N . Gas inlet tube to nitrometer. Inner diameter 2 mm. Tip ground oblique A P. Wide part of nitrometer. Length 200 mm., inner diameter 200-250 mm. AQ. Graduated part of nitrometer. Length 200 mm., volume 1.0 ml. AR. Funnel AS Tapered ground joint. Diameter of narrowest ground part equal to that of tube A& A T . Flexible copper tubing with metal joint soldered on AC. Ground joint metal-glass. Compare Figure 2 AW. Split-type furnace, length 120 mm. BA. Movable s lit-type furnace, length 120 mm. B M , B N , BP. &round joints metal-glass BQ. Glass capillary. Dimensions as A X A. R.
I
VOL. 29, NO. 7, JULY 1957
1085
in Figures 1 to 6. The fillings of the combustion tube and the Hopcalite tube should not restrict the gas flow appreciably. Capillaries L, P, and AV are prepared by drawing out a wider capillary to a fine tip and cutting off the tip with a pair of tongs until a suitable orifice is obtained. The gas pressure and the capillaries are adjusted t o give a gas flow of 4 to 5 ml. per minute through the combustion tube during combustion and a flow through the leak of about 2 ml. per minute. The total gas flow is read on the rotameter. The flow through the leak is measured by connecting thin rubber tubing to P , leading the gas into a fermentation saccharometer for a suitable time, and then reading the volume. The flow through the combustion tube is the difference between total flow and the flow through the leak. The backflow through tube A T , , joint AU, and capillary AV is adjusted in the same manner to a rate of about 20 ml. per minute. Joints H , AU, B M , B N , AC, and BP are lubricated with silicone grease (Dow Corning D C high vacuum grease) ; stopcock AH is lubricated with Leybold grease, R. The length of rod Q is adjusted so t h a t the end of the platinum boat is about 4 cm. from the end of furnace S.
Figure 2. Connection metal to glass or quartz
BE.
Horns on quartz tube to hold springs which keep together ground joints BS. Horns on metal joints H, A U . Metal joint, compare Figure 1 L, AV. Capillary, compare Figure 1 G, A T . Copper tubing, compare Figure 1 Connection AK is prepared by cementing the rather thick rubber tubing, A L , upon the side tube, A X , of the nitrometer with Kronig's glass cement. The empty part is filled with mercury. The apparatus is then adjusted so that carbon dioxide passes out through tube BQ. The end of BQ is cautiously warmed and lubricated with Kronig's glass cement. The nitrometer with its rubber tubing is then cautiously connected with it. The mercury fills the space between AM and BQ, giving a diffusion-tight connection, and the excess flows down through AM into the nitrometer. The speed of the motor is adjusted so t h a t furnace B A moves about 7 cm. in 4 minutes. T o obtain fast combustion it is important that capsule T be situated completely inside furnace S, and that furnace B A stands tightly together with S at the end of the combustion. When all is assembled, the carbon dioxide flow is turned on. Rod Q is taken out and stopcock AH is turned,so t h a t the gas passes through the back way. The furnaces are heated to temperature. If it is desirable to have the apparatus in operation very quickly, 1086
ANALYTICAL CHEMISTRY
AL
I
Figure 3. Connection between three-way stopcock and nitrometer
A L . Rubber tubing the temperature of furnace AG can be raised to 250" C. and that of furnace S to 1100" C. The temperature of AG is lowered to 150" C. after 15 minutes and that of S to about 1050" C. after 30 minutes. Rod Q is slowly introduced again and the air is expelled from tubes BQ, AM, and AN by passing carbon dioxide into the nitrometer for a few minutes. The apparatus is now ready. If AG and S are not heated above working temperature, the apparatus must stand for about 3 hours with gas passing through i t before i t is ready for use. Two analyses are now carried out with different kinds of substances, to determine combustion and sweeping times and to condition the apparatus. One or two blanks are run; 0.002 ml. of nitrogen is the blank value usually obtained. The apparatus is now ready for routine work. The apparatusmay be kept hot overnight with a very small flow of carbon dioxide through the back way. The quartz rod with leak is replaced with a rubber stopper with a narrow capillary during this period, to avoid clogging of the leak capillary with contamination products from the tube. The apparatus is thus immediately ready for use in the morning. If the heat and the carbon dioxide have been turned off overnight, it takes about 1 hour before the apparatus is swept clean enough for accurate analysis. After about 100 analyses the nickel filling should be reoxidized. High nitrogen results may otherwise be obtained with compounds which form large amounts of methane upon heating. The Hopcalite filling should be renewed at the same time. The Hopcalite tube with three-way stopcock is removed from the apparatus, and rod Q is taken out. Oxygen is blown through the hot tube at A B , or suction is applied a t the other end of the tube and air drawn through it for about 15 minutes. The Hopcalite filling is renewed in the nieantime. The apparatus is then rearranged as described above by raising the temperatures of the furnaces or by allowing it to stand for 3 hours with carbon dioxide passing through. PROCEDURE
The sample is weighed out in a platinum boat. Stopcock AH is turned so that the carbon dioxide passes through the back way, and rod Q is taken out of the tube. Capsule T is filled with nickel oxide and introduced into the furnace. The boat with sample is introduced into the combustion tube with rod Q, so that it stands at BU, where side tube K branches off. Q remains in the tube
behind the boat. After about 40 seconds AH is turned to connect the combustion tube with the nitrometer only, thereby causing carbon dioxide to enter the combustion tube through F and G. Rod Q is pushed in and stopper BT is immediately fixed in the end of the combustion tube. Furnace B A is drawn over the tube, so that its end is situated about 0.5 cm. from the platinum boat, and the motor is switched on. The next sample is now weighed out and the second platinum capsule is filled with nickel oxide. With most substances microbubbles are obtained after 7 to 8 minutes, but to be sure in all cases, a combustion and sweeping time of 10 minutes is used in the author's laboratory.
Figure 4. Head of nitrometer for small volumes
BC. BD.
BE. BF. BG. BH.
BK. BL.
Graduated part of nitrometer. Dimensions like A&, Figure 1 Groove in tapered joint ground on BC. 1 mm. broad, 0.5 mm. deep Funnel of clear transparent plastic Flat glass joint ground to end of tube BC Glass rod connected to BF nith rubber tubing BH Soft rubber tubing Screws Rubber band
Furnace B A is now moved back froni the tube, stopcock AH is turned so that the carbon dioxide passes through the back m-ay, rod Q is taken out, capsule T is drawn out with a steel wire, the next capsule is introduced, and the next sample is introduced to BU. The gas volume in the nitrometer and the teniperature are now read, and the gas hubble is slipped out. Stopcock AH is then turned, the sample is pushed in, the furnace is drawn over the tube, and the next combustion is run, etc. If a fast working one-pan balance (hlettler, Zurich, Switzerland) is used for the weighing, 10 minutes of time, during which combustion and sweeping go on, are sufficient for both weighing the next sample and calculating the result of the most recently completed analysis. Rubber stopper BT with rod Q must be fixed immediately in the combustion tube in one operation with the pushing in of the sample. Sometimes the area in which the boat is placed in the tube has not cooled enough from the preceding analysis and this can cause decomposition of the substance. If the combustion tube is not closed immediately, decomposition products can pass out the back way. A series of analyses carried out according to this procedure is given in Table I.
Electrostatically charged compounds are covered with a layer of nickel oxide before introduction into the tube, in order to prevent particles of the sample from flying aivay. I n this case the back-way sweeping time should be increased to about 1.5 minutes. Volatile substances are weighed out in quartz capillaries, which are bent in about right angles a t both ends and are about 70 to 80 mm. long. The capillaries are introduced and crushed in one operation with the pushing in. The capillary is thus inevitably crushed, because the space betryeen rod Q and capsule T is too short for it. If the sample has a very high vapor pressure, the area in \\-hich the capsule is crushed may be cooled n i t h dry ice before the introduction. N
Q
P
BT
Figure 5. Detail of combustion tube
Glass capillary (2. Quartz rod ,V. Quartz capillary B T . Rubber stopper P.
Aqueous solutions are pipetted into platinum boats with silicone-treated ultramicropipets and introduced with rod Q into the middle of part iM of the comblistion tube, which is always kept hot by furnace AW. Rod Q is drawn out. A sample of 50 pl. of serum is dry after 5 minutes and can be introduced and burned in the ordinary manner. Some analyses carried out according to this procedure are given in Table 11. Though somewhat low results are obtained with most pure organic compounds, results with serum appear to be considerably better than those with the conventional micro-Kjeldnhl methods. DISCUSSION
Sweeping System. The first apparatus and procedure tried were essentially t h e same as t h e decimilligram apparatus ( 5 ) ,with no back-way sweeping, except that a temporary filling in a n Inconel capsule (4) and an ordinary micronitrometer were used. The method gave good results, and generally a tube filling could be used for a great many analyses. Sometimes, however, the Hopcalite filling was inactivated after very few determinations and no more microbubbles were obtained. This generally occurred when halogencontaining compounds had been burned. It appeared t h a t some halogen compound passed into the Hopcalite and poisoned i t , A number of halogencontaining compounds were now analyzed. Chlorine, bromine, and iodine poisoned the Hopcalite. When iodine had been used, the Hopcalite could be reactivated by heating it to 300’ C. nnd drawing a current of air through it.
A black mixture of solid and liquid distilled out of the Hopcalite, apparently mostly iodine, and the filling could then be used again. When chlorineand bromine-containing compounds had been analyzed, the filling could not be reactivated in this manner. Different means were now tried to retain the halogen before it entered the Hopcalite. Introduction of silver woo1 into the end of the combustion tube between the nickel oxide and the Hop-
Table
I.
calite had no effect. A4pparently it was the nickel halide which was volatilized, and was not decomposed by the silver. Scrubbing out the nickel halide with different liquid scrubbers and adsorption agents was tried, but without much success. Moist silica gel with a granule size of about 30 mesh gave the best effect, but delayed the inactivation for only about five analyses. This inactivation had never been observed with back-may sweeping
Analysis of Pure, Solid Organic Compounds
Weight of Sample,
Substance p-‘iitiobenzoic acid
hIg. 3 880 3 709
ritrogen, % Found Calcd. 8 38 8.38 8 43 8 43
Nitrogen, Ml. 0 286 0 275 0 255 0 270 0 251 0 218 0 321 0.262 0.305 0.290 0.20‘3 0.217 0.211 0.217 0.122 0.129 0.122 0.120 0.523 0,614
3 425 3 615 3 368 d.l-5erine 2.123 2 742 2,235 2 580 2 460 Glucosamine hydrochloride 3.645 3.792 3.669 3.744 Behenamide 3.405 3.546 3.372 3.305 1.610 Thiourea 1.888 1.487 0.481 Standard deviation, O.O37a/, of nitrogen. Table
Substance .4nirnonium oxalate
PI.
50 50
Glucosamine hydrochloride
50
d,L-Serine
50 10 30
Serum B
8 44 8 42
13 29 13 31 13 27 13.36 13.32 6.53 6.51 6.50 6.55
13 33
4.08
4.12
4.13 4.11 4.10 36.77 36 75 36.78
6.50
0.00
$0.05
+o. 05 +O.OG +o. 04 -0.04
-0.02 -0.06 + O . 03 -0.01 fO.03 to.01 0.00
f0.05
36.81
-0.04
f0.01 -0.01 -0.02 -0.04
-0.06
-0,03
II. Analysis of Aqueous Solutions Volume of Sample,
Thiourea
Seruni -1
Deviation,“ %
50 30 50
Weight of Substance, Mg. 1.027 1.027 0.987
0.987 1.853 1.853 1.355 1.355
Xitrogen, 111. 0 164 0 164 0 319 0 320 0 105 0 105 0 153 0.154 0 075 0.219 0 220 0 361 0 190 0 190 0 362
DevinNitrogen, % tion, Found Calcd. % 18.11 19.72 -1.61 18.11 -1.61 36.65 36.81 -0.17 36.77 -0.04 6 43 6.50 -0.07 6.45 -0.05 12 87 13.33 -0.4G 12.96 -0 37 0.834 0.815 0.818 0.809 0.71 0.71 0.72
Serum A Kjeldahl 0.75 First laboratory 0.78 Second laboratory Serum B Kjeldahl First laboratory 0.66 Second laboratory 0.67 Sera were obtained from clinical laboratory which re orted the nitrogen values “first laboratorr.” Dumas determinations mere then run. 8era were then given to another laboratory, where the analyses “second laboratory” were run. In this laboratory the samples were pipetted out with silicone-treated ultramicropipets in the same manner as in the author’s laboratory. In the first four analvses shown for Serum A, the results are lower where larger volumes of water have been pipetted out, which probably is caused by the longer time of contact of the sample n-ith hot water.
VOL. 2 9 , NO. 7,JULY 1957
1087
and it therefore appears probable that the halide passes slowly through the nickel oxide tube with the carbon dioxide, and is swept back, by the hackstream between the analyses, and every morning while the apparatus is heated. When no backstream is used, the carbon dioxide flow, which always goes in the same direction, acts as a gas chromatographic eluting agent and slowly carries the halide into the Hopcalite. The back-way sweeping system was therefore used again. Leak Arrangement. It was previously found ( 5 ) that silicone grease (Dow Corning DC high vacuum grease) absorbs nitrogen reversibly. Therefore, in the decimilligram apparatus mentioned (6) the ground joint, which closes the combustion tube a t the end where the sample is introduced, is provided with a leak. A continuous stream of carbon dioxide passes out through this leak and prevents the nitrogen absorbed by the stopcock grease during introduction of the sample from entering the main tube. This leak obviates the necessity for using very tight joints a t this place. Theground joint could therefore be replaced with a rubber stopper. For simple and fast introduction of the sample, a thick-walled long quartz capillary closed a t one ,end was inserted into a bore of this stopper. A hole was blown in the wall of the capillary near the thinner end of the stopper. The capillary acts as a leak, and its long end can be used to push the sample into the tube (Figure 1). It obviates the need for allowing the hack-way sweeping t o go on some time after introdnetion of the boat, and it can be used as ampoule breaker when volatile liquids are an* lyzed. The same arrangement is now used in the author's laboratory in all dry combustion methods. Temporary Filling. Use of t h e apparatus without a temporary filling (B,5)was then tried. When a new tube filling was used, a low result was generally obtained in the first analysis. The next six or seven analyses were correct, but sweeping time, which had been 8 t o 10 minutes in the first analysis, increased with every burned sample, and a t last 25 to 30 minutps was no longer enough to sweep out all nitrogen; lower and lower results were obtained, though the microbubbles seemed t o indicate that all nitrogen had been swept out. This agrees with the experiments reported by Belcher and Macdonald (1). Then an attempt was made t o reoxidize the nickel oxide filling after ahout seven analyses. The temperature of the nickel oxide furnace was increased to 1100" and that of the Hopcalite furnace to 250' C. The inner part of the three-way stopcock of the back-way sweeping was removed and air was drawn through the tube by applying slight suction a t the other end of 1088
ANALYTICAL CHEMISTRY
Figure 6.
Mounted Dumas nitrogen determination apparatus
Nitrometer provided with metal block with thermometer and plastic window (not shown)
the combustion tube for about 10 minutes. The suction pump was then removed and the stopcock was rearranged. When carbon dioxide had passed through the apparatus for 15 minutes, the temperatures were lowered to their ordinary values, and the apparatus was ready for use again. This regeneration was generally done just before lunch and a t the end of the day, The results obtained were not unsatisfactory, hut were not so good as those obtained with a temporary filling, and as troubles with regeneration were greater than those caused by the temporaryfilling, the author returned to the old method. I n the first semimicromethod (4) capsules of nickel gauze were used t o hold the temporary filling. Like Belcher and Maedonald i f ) , the author found that these quickly became brittle. An Inconel capsule could be used for up t o 100 analyses, hut it is difficult t o make, and must he glowed in carbon gas first for some hours in order to free it from nitride. The capsules are very hard, and when grains of nickel oxide fall out into the tube the capsules can be jammed between the tube walls and the grains, and are sometimes very difficult to, take out. The use of platinum gauze had not been tried, because it appeared that the a t mosphere of the tube was too deleterious for this metal. A trial showed, however, that capsules of platinum gauze are very satisfactory. They are not badly attacked, they do not contain nitrides, and they are so soft that operation inside the tubes causes no trouble. Belcher and Macdonald i f ) introduced simple, easily prepared boats of nickel gauze for the temporary filling, which also appears to be a satisfactory solution of the problem. When small
samples are analyzed, d capsule filling can conveniently be used for two combustions. Combustion Tube. Frequent breakage of the quartz tube has been reported (1, 8, 6 ) . I n the author1 laboratory opaque quartz with a wal diameter of 1.5 to 2 mm., used for the part of the combustion tube situated inside the long furnace, was good for several months of daily use. Recently, clear quartz was used in an experiment; even very thick-walled tubes cracked after a short period of use. Opaque quartz appears therefore t o be much superior. Probably the slight elasticity of the quartz caused by the great number of small bubbles inside the material makes it more able to withstand the thermal strains. Nitrometer. Recently Schoniger (IO) reported that the gar volume in a micronitrometer can be conveniently read 5 minutes after the completed combustion. This was found correct. I n the Dumas nitrogen determination the leveling bulb of the nitrometer is usually placed in a low position during the comhnstion. This causes an underpressure (below atmospheric) in the graduated part of the nitrometer. If the stopcock or joint a t the top of the nitrometer is not quite tight, this underpressure causes potassium hydroxide from the funnel t o pass slowly down, and form a layer of liquid above the gas bubhle. The position of the bubble has, therefore, t o be adjusted after the combustion. As the film of potassium hydroxide on the walls of the nitrometer is disturbed by this adjustment, a waiting time is necessary to obtain a reproducible reading. When the stopcock or joint of the nitrometer is kept tight with grease, the upper meniscus of the gm bubble ia not. autc-
matically obtained in a reproducible iituation, but generally has to be :idjusted, which also makes necessary a period of waiting. Wlien the joint between the combustion tube and the nitrometer is quite pas-tight, a n overpressure, (above atmospheric) in the tube does no harm, and the leieling bulh can be placed above the graduated part of the nitrometei (luring the combuction. An overpresqure i i now obtained in this part, and if the joint or dopcock used is not ahsolutelv tight, potassium hydroxide will d o n ly p s i upward through i t into the funnel. If the overpreqsure is not very high, all potassium hydroxide above the gas bubble passes out, but the surface tenqion prevents the gay from folloiiing it. The gas bubble is thus autoniatically adjucted to an exactly reproducible situation during the conibustion. I n the author’s Iaboratory the leveling bulb is general$ placed so that its liquid level is 3 to 7 m. above the joint of the nitrometer. The position i., however, not critical. No subsequent adjustment is necessary, and the gas volume can be read immetliately after the combustion, without w e n waiting 5 niinutec. The nitrometer deicrihetl bv 1Iilner and Sherman (8) is very suitable for thiq purpose, while nitrometers with $topcocks are difficult to use without grease. For the measurement of small volumes of nitrogen it is necessary t o use narrow nitrometers; very small tapered joints must be used, nhich increases the risk of breakage. A new nitrometer head provided n i t h flat ground joints was therefore constructed. The flat joint is obtained hy grinding a thick-walled capillary with precision-ground bore. The head of the nitrometer is shown in Figure 4. Spherical joints (12) are probably :is good or better, but are difficult to ohtain with a precisionground hore The nitrometer is at-
tached to the apparatus with a mercurytightened rubber connection, similar to a connection used earlier b y Lindner (7’). It is diffusion-tight, but flexible enough to avoid breakage. Analysis of Aqueous Solutions. An a t t e m p t was made t o analyze aqueous solutions by pipetting them into t h e platinum boat a n d introducing a n d burning them like other samples. T h e combustion had t o b e carried out very slo~vlyand cautiously, because the large amount of water, nhich was volatilized a t once, caused a very fast gas flow through the tube and this caused incomplete combustion. The Hopcalite filling was inactivated after a number of such analyses. Probably the m-ater vapor carried wtalyst poisons into the Hopcalite. The author therefore tried t o evaporate the aqueous solutions before the combustion. Furnace i i W was mounted as s h o r n in the figures. The solution was pipetted into the boat, and the boat was introduced into the back end, M , of the tube. The quartz rod was drawn out again. I n the rapid carbon dioxide flow 50 pl. of serum were dried in 5 minutes. The sample was then introduced in the ordinary manner. The results obtained with pure compounds Tere somewhat low. Correct results were obtained when the solution in the boat was dried in a vacuum instead of being evaporated in the tube. It was first suspected that ammonia was lost during the evaporation, but addition of oxalic, formic, sulfuric, and hydrochloric acids did not improve the results. The speed of evaporation was, however, important. Lower results were obtained when a long evaporation time \\as used, even though the temperature was lower. A temperature of 90 O C. was suitable for serum. The other solutions were evaporated a t 110’ C. Siliconetreated Kirk ultramicropipets (11) were used for pipetting.
All weighings reported in the present paper were carried out with a Mettler one-pan microbalance, Model M-5. The nitrogen volumes read in the nitrometer were corrected by first subtracting a blank of 0.002 ml. and then subtracting 0.8% of the obtained volume as a correction for the liquid film and the vapor tension of the potassium hydroxide. The result was then calculated in the usual manner. ACKNOWLEDGMENT
The author is indebted to Einar Stenhagen for his interest in the work, to Leif Klinga for carrying out the analyses, and to Endel Sepp for drawing the figures. The work mas made possible by a grant from the Swedish Medical Research Council LITERATURE ClTED (1) Belcher, R., Macdonald, A. M. G., Mikrochim. Acta 1956, 1111. (2) Clark, S. J., Dando, B., I b i d . , 1955,
1012. (3) Gysel, H., Helv.Chihim. Acta 22, 1088 (1939). (4) Kirsten, K. J., Mikrochemie ver. Mikrochim. A d a 40, 121 (1952). (5) Kirsten, W. J., Grunbaum, B. W., AXAL.CHEM.27, 1806 (1955). (6) Lacourt, A,, Departement de Microchimie de l’Universit6 de Bruxelles, Belgium, private communication. (7) Lindner, Josef, “Mikromassanalytische Bestimmung des Kohlenstoffes und Wasserstoffes mit grundlegender Behandlung der Fehlerquellen in der Elementaranalyse,” p. 315, Verlag Chemie, Berlin, 1935. (8) Milner, R. T., Sherman, M. S., IN?. ENG. CHEBI.,ANAL. ED. 8, 331 (1936). (9) Royer, G. L., Xorton, A. R., Foster, F. J., Ibid., 14, 79 (1942). (10) Schoniger, W., Mikrochemie ver. Mikrochinz. Acta 39, 229 (1952). (11) Sisco, R. C., Cunninghamn, B., Kirk, P. L., J . Bid. Chem. 139, l(1941). (12) Stehr, E., IXD.ENG.CHEM.,ANAL. ED. 18, 513 (1946). for review July 21, 1956. AcRECEIVED cepted hlarch 11, 1957.
Microdetermination of Heavy Elements Such as Mercury and Iodine, in Solution, by X-Ray Absorption JEAN LEROUX, PATRICIA A. MAFFETT, and J. L. MONKMAN laboratory Services, Occupational Health Division, Department of National Health and Welfare, Ottawa, Canada ,The use of x-ray absorption to measure micro amounts of heavy elements in solution has been investigated, By careful selection of the absorption cell and radiation, a fraction of a microgram can be determined in a sample volume of less than 1 pi, The sensitivity of the technique for a
particular element or compound can be predicted from a simple empirical equation.
D
the past 10 years many authors (7, 8) have discussed the principles of quantitative analysis of liquids by x-ray absorption. ApplicaRING
tions included determination of lead (1) and sulfur (6) in hydrocarbons. The lowest concentrations measured were about 0.02y0 and the minimum volume of sample required W&S of the order of 10ml. I n microchemistry it is often necessary to measure elements at concentrations VOL. 29, NO. 7, JULY 1957
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