High Temperature Furnaces for Organic Elementary Analysis WOLFGANG KIRSTEN Znstitute of Medical Chemistry, University of Uppsala, Sweden
several purposes in organic elementary analysis ( 2 , 4 ) it is During the war hightemperature analytical furnaces were not available in this country, and suitable furnaces had to be designed and constructed in the laboratory. Platinum-wound furnaces were tried first. The low electrical resistance of the material made it necessary to use very thin nire, which made the windings rather fragile. And the volatility of the platinum at higher temperatures decreased the durability of such furnaces. This and the high price of the platinum led to trying Kanthal .1-1 wire. Kanthal A-1 is an alloy, supplied by Kanthal AB, Hallstahammar, Sn eden, containing aluminum, chromium, and cobalt, TI ith a specific resistance, a t 20" C., of 14.5 ohm-em. X 10-5, and a normal working temperature of 1325" c. ( 6 ) . I t was desirable to have a uniform temperature in the whole length of the furnace. In order to obtain this, a sparse winding in the middle of the furnace with the winding increasing in tightness gradually toward the ends of the furnace has often been recommended. Experiments showed that this was not a suitable design, a t least in the case of these small furnaces, because the heat losses are the same in the n hole length of the furnace except at the very ends, where the heat losses in the axial direction are added to the losses in the radial directions. In order to obtain a uniform temperature the winding, therefore, had to be made uniform over the T\ hole length of the furnace except a t the very ends, where a few much tighter windings had to be added. Experiments with different methods of insulation showed that a uniform temperature could not be obtained with a furnace too n-ell-insulated, because the differences between the heat losses at the ends and the middle of the furnace were so large that compensation could not be achieved by a few tighter-wound twins. Less well-insulated furnaces gave, therefore, better uniformity of temperature. Less well-insulated furnaces also have an advantage; it takes much less time to heat them to working temperature using the worlting voltage because the ratio
voltage furnaces have a disadvantage; they require heavy transformers and special switches. As the high-voltage furnaces worked as well, except possibly in the temperature ranges around 1250" C.-Le., near the temperature limit of the resistance material-the high-voltage furnaces are generally preferred. The layout of a tube-furnace of the high-voltage type is shown in Figure 2.
TOR
E desirable to use high temperatures.
R =
I
L
A
K r i i
Ld Figure 2.
Layout of High-Temperature Furnace
A , alundum cement; B, windings of Kanthal A-1 spiral coil; C, Kanthal wire insulated with porcelain pearls; E , kieselguhr; F , vulcan asbestos: G, top of thermocouple; H , wires of thermocouple: K , screw for attachment of measuring instrument; L, metal housing of furnace; N, front view of furnace
In the case of the furnaces used for temperatures higher than 1100" C., it was considered desirable to make arrangements for temperature measurement and automatic temperature control. This is desirable, for instance, in the oxygen determination method according to Schutze-Unterzaucher ( 4 , 5 ) and the sulfur determination method described by Kirsten (1, 2 ) , where constant temperature is necessary. Unterzaucher's recommendation (6)of mounting the thermocouple outside the heating element was found inconvenient, because the thermocouple responds too slowly to temperature changes inside the tube, which, in the case of automatic control, causes the temperature curve to become a wavy line instead of a nearly straight line. And it was considered desirable to have the thermocouple indicate the right temperature without corrections. The correctness of the indication of the thermocouple vias determined as follows: A thermocouple was mounted inside a combustion tube, and the latter was inserted into the furnace. The indications of the thermocouple mounted outside and of that mounted inside were then compared. The construction shown in Figure 2 was found to give correct indication and rapid response of the thermocouple.
is much lower in such a furnace. This was considered important, because of the practice of heating the furnaces to working temperature using a high voltage, and then decreasing the voltage, Such a practice is undesirable as the furnace may be destroyed if the operator forgets to cut down the voltage \Then the working temperature has been reached.
84---
E
H
Energy needed for heating of furnace material Energy needed for maintaining of working temperature
\
B
,N
I' _-___----__,
Figure 1. Kanthal A-1 Ribbon Baked into Alundum Cement
Table I.
Dimensions and Capacities of Furnaces Shown in Figure 3
Voltage Amperage Highest Used a t Used a t Furnace Working Temperature Temperature Hole Outer (See Temperature Listed Listed Total DiDiFigure in Regular in Column in Column Length, ameter, ameter, 3) Use, O C. 2 , V. 2, Amp. Zlm. Mm, Mm.
Two different types of furnaces were tried-one with thick Kanthal ribbon wound as sh0Lv-n in Figure I, using low voltage and high amperage; and one with higher voltage and lower amperage. In the second type the heating element used was in the shape of a spiral coil of round wire which was wound around the furnace tube. Both types worked very well. The low-
A B
C
D E F G
805
1150 1100 1150 1200 1150 1150 1150
120 100 140 100 140 95 200
3.5 3.2 5.0 3.5 6.0 1.7 2.0
230 75 120 125 180 75 120
;: 23 18 25 18 33
60 75 75 60
75
60 75
806
ANALYTICAL CHEMISTRY
Figure
3. Automatic Combustion Carriage with Mounted Furnaces
The furnaces of this type were found to have excellent uniformity of temperature in the whole length of the tube. They have working temperatures up to 1250" C. Several furnaces have been heated up to 1350" C. for short times without failing, and some of them have survived such shocks for several years of intermittent use a t 1100' to 1200" C. The cost of the furnaces is only a fraction of that of most earlier described furnaces. Splibtype furnaces based on the principles outlined above hiEve also been developed. They are constructed for working temperatures up to 1150" C. The furnaces can be oontrolled using automatic temperature controllers, input controllers, variable transformers, or resistors. Stepped transformers (Figure 3, H)are especially Convenient, bersuse one transformer e m feed several furnaces and other apparatus. The furnaces and accessories have been commercially available for some time (Nieroma, Klara. Viistra Kyrkogatan 7, Stockholm), and hundreds of them ara in use in Swedish laboratories. Dimensions and capacities of the furnaces shown in Figure 3 are given in Table I. ACKNOWLEDGMENT
I n the first furnaces made, the heating element uras wound around a refractory tube. This was found less convenient because heating the furnaces to working temperature takes a long time, and the arrangement, with the windings rather far away from the center of the furnace, increases the heat losses a t the ends and upscts the uniformity of the furnace tmperature. The method of baking the wire into a refractory cement was, therefore, preferred. Alundum cement (Norton Behr-Manning, Overseas, In?., Worcester, Mass.) wa8 found to be B very con venient cement far Xanthal wire.
The author is much indebted to E. Stenhagen for his interest in the work, to A. Edenstrom, Uppsala, to E. Svanheck, Nicroma, Stockholm, for assistance with the technical designs, and to E. Sepp for drawing the figures. LITERATURE CITED
(1) Kirsten, Wolfgang,ANAL.Cnm., 25,74 (1953). (2) Kirsten, Wolfgang, Mikrochemie DE?. Mikrochim. A d a , 35, 174 (1950). (31 IbkZ., 8.217. (4) Untersctucher. J.. Be.deul. chem. Ges., 73,391 (1910). (5) Unteraeuoher, J., C h a . Iny. Tech.. 22, 39 (1950). ( 6 ) Walden. L., J . Sci.Inst7.. 16,l (1939). Recmvro for review August 18. 1962. Aoaepted December 15, 1852.
Determination of Niitrate in Plant Materials R. E. UNDEKUVWN nwealth Division of Biochemistry and General Nutrition, Commo University of Ade [aide. So
G . B. JONES
AND
mocedures far the estimation of nitratenitrogen r h i c C dependon the nitration of phenol disulfonicacid in the presenc URREXT
of strong sulfuric acid have proved satisfactory for inorganic s m des. but as chloridesand organic matter interfere, the estimation c
silver sulfate; various measures have been advocated in order to remove the orgenie matter which is extracted from tho plant tissues, and successful procedures have been evolved for the estimation of nitrate in certain types of plant materials. I n this laboratory the problem centered round the nitratenitrogen content of grasses, particularly the young shoots of a perennial grass, Phalaris luberosa. The phenol disulfonic acid methods of Gilbert, Eppson, Bradley, and Beath (5),and Eastowe and Pollard (9) were found to give little consistency between duplicate analyses, and the recovery of added potassium nitrate was incomplete. With mast grasses and with mature P. tubema, the method of Johnson and mrich (7) was much more satisfactory, hut with young shoots of P. luberosa recoveries wem quite low. Modifications of the original xylenol method suggested by Blom and Treschow (8)proved to be more successful. Barnes (I) bas advocated the use of 2.4xvlenol. the nitration product of which is
tion. They found the latter tho more consistent when the nitrs, tian product is distilled in s t e m , and applied the method to the determination of nitrate-nitrogen in soil extracts made d h copper sulfate solution and from which organic matter and excess copper were removed by precipitation with cdcium hydroxide and magnesium carbonate. The method of Piper and Lewis (If) was adapted by us to deal with the greater quantities of organic matter in the extracts from plant material. Activated carbon was added during the clarification stage to produce a colorless extract, and the solutions were chilled in ice water in order to minimize the reduction of nitrate by any orgmio mattor during the addition of the 83% sulfuric acid. This method gave consistent results with 95 to 100% recoveries of added potassium nitrate in the case of rye grass (Loliurn spp.), wild oats (duma saliua), and mature P. luberom. However, with both the phenol disulfanic acid method of Johnson and Ulrich (7) and the modified method of Piper and Lewis (ff), recoveries from young shoots of P. luberosa were consistently in the region of 50%, hut as the growing season progressed, recoveries from more mature plant material improved up to 90%. The young shoots clearly contained substances which interfered seriously with the determination of nitrate. The method described below was evolved to overcome the interference by such substancea. Complete recoveries of nitrate added to all of the grasses tested and consistency of malytical results were achieved by the retention of the nitrate ions ou au anionic exchange resin from which the interfering organic matter was separated by washing with water.
