A Simplified Combustion Pipet G. H. NELSON, H. D. WEIHE,
AND
D. F. J. LYNCH, Agricultural By-products Laboratory, Ames, Iowa
S
EVERAL improvements in the design of combustion pipets have been made since 1876 when Coquillon (2) proposed the use of an electrically heated platinum spiral for slow-combustion analyses of hydrocarbon mixtures. Weaver and Ledig ( 7 ) suggested the use of a fine platinum spiral in a Pyrex or quartz capillary tube for the anal sis of small quantities of combustible gases. Bayley (1) descriged a pipet into which the gas was admitted through a small platinum jet in the side of the pipet directly onto a heated platinum spiral. He claimed that the danger of explosion was less with this apparatus, since mixing was much better. Matuszak (5) employed a pipet with the upper portion terminating in a cone containing an inverted V-type of platinum coil. Porter and Cryder (6) described a combustion pipet in which the mixture of combustible gas and oxygen was passed through a vertical platinum tube 0.9 mm. in diameter, located within the pipet. The glowing platinum spiral was located just above the opening of the platinum tube.
ably allow more intimate contact with the gas a t the combustion temperature. Carbon monoxide and hydrogen are completely removed before the residual sample of gas, which contains only methane and nitrogen, is passed into the explosion pipet for analysis. The use of a heating element which can be removed and replaced quickly and conveniently without the necessity of removing the mercury should interest the commercial gas analyst, for whom delays due to repairs may seriously interfere with plant control. The use of an interchangeable groundglass joint, in addition to facilitating the removal of the platinum spiral, precludes the necessity of replacing or repairing anything more than the broken part, in case of an accident.
Description of Combustion Pipet The modified apparatus is shown in Figure 1. The combustion pipet i s constructed of a Pyrex glass tube, A , to which are sealed two glass side tubes through which pass 20-gage wires, CB and DE, each consisting of a piece of platinum wire spot-welded to a piece of tungsten wire. The p1at)inum portion is used within th$ tube and is bent to form a hook a t B and E in order to keep the platinum spiral from slipping off. The tungsten portion of the wire provides a satisfactory means for making a gas-tight seal with the glass. The use of the tungsten wire-glass seal has proved very satisfactory and the authors have experienced no difficulty with the glass cracking a t this point. The wire is of sufficient length (over 2 em.) outside the tube so that it can be soldered to copper leads without danger of the solder’s being melted by the heat transmitted from the glowing spiral. The spiral heating element is made from a 12.5-cm. (5-inch) piece of 26-gage platinum wire and is hooked on at B and E. The source of current for heating this ie 110-volt alternating current which, by means of a transformer, is reduced to about 6 volts and controlled with a small rheostat. Should the platinum coil burn out or break, it can be removed and replaced readily with a pair of narrow tweezers. An interchangeable ground-glass joint, G, T 19/38, is located at the lower end of tube A, providing a means for attaching the 1Pbcc. flask, H . The ground joint is lubricated with a small amount of vaseline and kept tightly closed by means of two springs. A practice, common with all types of combustion pi ets, is to provide a shield for protection of the operator in case o?an explosion. For this purpose a wire gauze (16 meshes per 2.5 om., 1 inch) is placed in a position between the operator and the pipet. This gauze does not materially obstruct the operator’s vision and will serve as a protection from flying glass if an explosion should take place.
The commercial type of apparatus usually employs a comparatively wide combustion pipet, the heating element being near the top where the tube may be somewhat constricted. In the proposed pipet the smaller cross section and the arrangement of the heating spiral parallel t o the gas flow prob-
Procedure
235CM.
Air or oxygen may be used for the combustion, though oxygen is preferable as it allows the use of a larger sample., A sample containing 10 to 15 cc. of methane thoroughly mixed with about 85 cc. of oxygen was found to work satisfactorily. The wire gauze shield was adjusted in place between the operator and the pi et. Then the spiral was heated to a bright yellow color and %e combustible gas mixture passed over it at the rate of about 10 to 20 cc. per minute, while controlling the temperature of the spiral (to prevent overheating as the gas burns) by means of a rheostat. The mixture was again led over the spiral at a somewhat higher rate and this procedure was continued until four passages had been made. The gas mixture was then coole to room temperature by a small air blast and its volume measure before and after removing the carbon &oxide. .,
!
FIGURE 1. DIAGRAM OF
The gas analyzed consisted of methane with a small amount , of nitrogen present. In the modified Orsat type of apparatus, such as was employed here, hydrogen :and carbon monoxide are determined before the methane combti$tion is made. This proposed pipet was designed primarily to replace the corna mercial type of methane pipet formerly used and the authors do not recommend its use for combustion of hydrogen or car-.!: bon monoxide.
PIPET
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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TABLE I. ANALYSIS OF TYPICAL RESIDUAL FERMENTATION GASES Sample a
b
Gas Analyzed
Oxygen
cc. ...
cc.
Methane %
14.6 14.85 13.8 13.8 9.8
84.3 82.15 84.4 84.0 84.1
79: 6 80.3 83.1 83.1 83.4
Air
a d
5.0 5.0 9.0 9.0
95.0 95.0 91.0 91.0
95.9 98.1 91.2 91.2
Using either air or oxygen for the combustion, the data given in Table I show the analysis of typical residual fermentation gases, after the oxygen, carbon dioxide, and hydrogen had been removed. Generally, oxygen yielded closer duplicate results than air.
Discussion and Conclusions I n the use of the slow-combustion pipet the recommended procedure is to make several passes of the gas over the hot coil (S, 4). To determine the efficiency of this combustion pipet, after removing the carbon dioxide produced during the first four passes, two additional passes of the gas were made. Results of these additional passes showed 0.91 to 2.67 per cent
VOL. 11, NO. 3
(average, 1.61 per cent) residual methane left after the first four passes. These percentages are based on the original sample and conform to plant control practice. The apparatus described in this paper was constructed with the idea of incorporating several desirable features in one combustion pipet and a t the same time keeping the construction simple and inexpensive. These features are: (1) the location of the heating spiral in a tube of comparatively small cross-sectional area; (2) the attachment of the spiral in such a manner that it can be quickly and easily removed and replaced; and (3) the use of a standard ground joint just below the heating coil. This combination of features is a simpIification of the apparatus hitherto used in routine gas analysis.
Literature Cited Bayley, C. H., Can. J . Research, 7,680 (1932). Coquillon, J., Compt. rend., 83, 394 (1876); 84,458, 1503 (1877). Fieldner, A. C., et al., U. S. Bureau of Mines, Tech. Paper 320 (1935).
Matuszak, M. P., Gas-Analysis Manual, Pittsburgh, Pa., Fisher Scientific Co., 1934. Matuszak, M. P., IND.ENG.CHBM.,Anal. Ed., 6, 77 (1934). Porter, D.J., and Cryder, D. S., IND.ENG.CHEM., Anal. Ed., 7, 191 (1935).
Weaver, E. R., and Ledig, P. G., J. IND.ENG.CHEM.,12, 368 (1920). RECEIYED May 21, 1938.
A Flask for Efficient Stirring AVERY A. MORTON, Massachusetts Institute of Technology, Cambridge, Mass.
M
UCH attention has been paid to different kinds of stirrers, but very little to the type of container which is by far the more important. After a considerable variety of stirrers had been tried, all with little benefit because swirling in round flasks kept the heavy particles in an outside belt, the author observed a marked improvement upon changing the shape of the container. I n particular, vertical creases in the side of the flask had a pronounced effect. As many as four creases, each 7.5 to 10 cm. (3 to 4 inches) long by 0.6 to 2.5 om. (0.25 to 1 inch) in depth, tapering in width from 0.3 cm. (0.125 inch) at the innermost part to 3.75 cm. (1.5 inches) a t the circumference, have been made in flasks whose capacity varied from 0.5 to 2 liters. The depth of the crease was not proportioned to the size of the flask but depended partly on the speed with which the stirrer was to be operated. For instance, an 0.5-liter flask in which deep wedges had been sunk in the circumference appeared as if sectioned into four parts about a central portion in which the stirrer rotated a t high speed in order to agitate the mixture. Moderately deep creases break up the average swirling but may fail to prevent the funnel-shaped depression about the stirrer when it is operated a t high velocity. In operation a stirrer of the propeller type with blades pitched a t about a 45" angle has proved satisfactory. The mixture is usually pushed downward against the bottom of the flask and up between the sections, from whence it falls again onto the stirrer, With thick slurries the stirrer is driven faster and faster until the mixture moves readily. The agitation in such cases is remarkable. Observations on the efficiency of stirring were usually made with a mixture of sea sand and water. With the ordinary flask the sand would collect in belts and layers, depending on the speed of rotation and the position and shape of the stirrer, These results were contrasted with the even dis-
persion obtained in the new flask. I n making sodium sand for use in reactions with sodium in progress in this laboratory, the particles were more uniform and were a third to a fourth the diameter of those obtained in round flasks. Reaction mixtures so thick that the stirrer rotated without appreciable agitation in the ordinary flask were readily mixed, particularly if the speed were increased. A great advantage in all cases was a continued improvement in the results as the rotation velocity was increased. Previously, comment has been made (I) on an improvement in the dispersion of sea sand in water when a square bottle was used. Such containers, however, are apt to crack when heated, have corners in which solid particles lodge, and will allow a funnel to form around the stirrer a t high speeds. Rectangular containers overcome the swirling but not all of the pocketing. The author has, however, had very satisfactory r&sults making sodium sand in rectangular apparatus. Baffles placed in beakers also have pockets in which heavy particles can collect. Most of these containers cannot be used readily with experiments in which the mixture must be refluxed or kept in an inert atmosphere. Fortunately the ordinary flask with its rounded bottom, so helpful in avoiding pockets, and its partially closed top, can be converted into an ideal container for stirring with the changes noted above. Construction is relatively easy for any competent glass blower, the principal precaution being good annealing.
Acknowledgment The author is indebted to Mr. Wayringer for his cooperation in constructing different shaped stirrers and these flasks.
Literature Cited (1) Morton and Palmer, J . Am. Chem. Soc., 60, 1428 (1938). RECEIYED November 19, 1938.
