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T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
voi. 9 , N ~8 .
f r o n t support in Figs. I and 2. The swinging yoke For convenience in packing for transportation t h e is kept from vertical movement by pieces of tubing handle of t h e lever may be unscrewed a t a point near slipped over t h e rear standard, one below and one t h e bend. Where much variety of analytical work above t h e yoke. These are clamped with just suffi- is t o be done, i t will also be found convenient t o have cient tightness b y t h e lock nut immediately under- two handles, one about 30 in. long for tough, fibrous neath t h e upper yoke. The tube over t h e front roots, and one not over I j t o 2 0 in. long for use with standard is in one continuous piece from t h e base softer materials. t o t h e upper yoke, a portion, however, being cut The press now being used by t h e writer was made away in such a manner as t o form a support for t h e a t t h e Government Navy Yard in Washington a t a free end of t h e swinging yoke when t h e latter is in cost of about % j j . O O . However, i t would not be position for t h e operation of t h e plunger. The two possible for other t h a n Government laboratories t o standards are screwed into t h e base and when t h e nuts have work done a t t h e Navy Yard. As yet no ara t their tops are drawn tight, t h e framework of t h e rangements have been completed for t h e commercial press is locked solidly together. production of t h e press, b u t pending such arrangeT h e pulp may be received either in open vessels, as ments blue prints of t h e complete working drawings beakers, or in wide-mouth bottles or flasks. I n any will be furnished upon request from interested parties. case, t h e t o p of t h e receiving vessel should be raised The materials required in building t h e press are such high enough around t h e lower part of t h e cylinder t o as may be found in stock in almost a n y machine insure against loss from spurting sidewise. I n t h e shop. writer’s practice, potato samples are received directly U. S. DEPARTMENT OF AGRICULTURE BUREAUO F PLANT INDUSTRY into weighing bottles about 45 mm. in diameter by 6 5 \VASHIYGTON. D . C. mm. high. The bottles are immediately stoppered and t h e samples thus held without possibility of loss by evaporation until after weighing. I n some analyses t h e pulp is delivered directly into a DETERMINATION OF GAS DENSITY’ tared vessel containing t h e solvent t o be used for exB y JUNIUS DAVIDEDWARDS traction or digestion. A second weighing then gives Received April 11, 1917 t h e amount of t h e sample. This procedure minimizes Because of its importance in orifice meter measuret h e possibility of chemical changes due t o contact of t h e finely divided sample with t h e air. It also en- ments, as well as t o t h e gas industry in general, t h e problem of determining t h e density of gases has been ables t h e instantaneous inhibition of enzymatic action. For certain work in this laboratory there have been investigated at t h e Bureau of Standards. T h e comdesired raw cultures of finely divided substance, plete results of this investigation are t o be published sterile, except for such inoculations as may already as Technologic Papers N o s . 89 a n d 94, entitled “A exist in t h e interior of t h e material. T h e press herein Specific Gravity Balance for Gases” and “Effusion described has been found t o be well adapted t o t h e Method of Determining Gas Density.’’ The present making of such culture preparations. The cylinder paper presents a summary of t h e results and conclumay be removed, wrapped and placed in t h e sterilizer, sions of t h e work. T h e direct method of determining gas density and while t h e plunger may either be treated similarly or be t h e one capable of yielding t h e most accurate results sterilized without removal b y immersing its entire working portion in a suitable sterilizing fluid. T h e is t o weigh in a closed glass globe a known volume balance of t h e press may also be readily sprayed with of t h e gas a t a definite temperature a n d pressure. any suitable noncorrosive liquid. Plugs of t h e culture When conducted with every refinement of technique material are then prepared with flamed tools a n d a n d apparatus, this method may be made t o give slipped into sterile test tubes, which are closed with results accurate t o I part in 1 0 , 0 0 0 or 20,000. T o cotton plugs until t h e moment for insertion of t h e secure a n accuracy of even a part in 1000,is, however, plugs into t h e press cylinder. The pulp will, of course, a tedious and delicate operation. I n order t o secure precise results with convenience be received directly into t h e sterilized tubes or flasks in which t h e cultures are t o be grown. This work a n d rapidity, a specific gravity balance employing a n has not been fully developed as yet, b u t preliminary indirect weighing method was developed. T h e printrials with only ordinary precautions have resulted ciple of t h e method is based upon t h e laws of t h e compressibility and t h e buoyant effect of gas. According very satisfactorily. From t h e writer’s experience with a number of t o Boyle’s law t h e density of a gas is proportional t o sampling devices it is believed t h a t t h e press which its pressure, and t h e buoyant force exerted upon a has been described possesses distinct points of superi- body suspended in a gas is proportional t o t h e density ority which would make i t especially useful t o Experi- of t h e gas a n d therefore t o its pressure. Therefore, ment Station chemists as well as t o others having much if t h e buoyant force exerted upon a body is made t h e work with t h e class of materials t o which i t is ap- same when suspended successively in two gases, then plicable. The finely divided pulp produced b y this t h e densities of t h e two gases must be t h e same a t process can be extracted in a small fraction of t h e these pressures; or t h e densities of t h e two gases a t time required where t h e pieces are merely chopped 1 Published with the permission of the Director of the Bureau of Standards. or minced b y hand or in a vegetable grinder.
Aug., 1917
T H E J O C R N A L OF I N D C S T R I A L A N D ENGINEERING C H E M I S T R Y
normal pressure are in inverse ratio t o t h e pressures when of equal buoyant force. The construction of t h e specific gravity balance is shown in Fig. I . The central water-jacketed gas
FIG. D DETAILS
OF C O N S T R U C T I O K OF
EDWARDS GASDENSITYBAL.4iYCE
chamber is closed gas-tight by screw caps, one of which is shown a t t h e left, a n d is provided with a gas inlet, a connection for attaching a mercury U-gauge, and a needle valve. The balance beam carries on one
R
791
glass tubing. I n operation t h e gas chamber is filled with air and t h e pressure accurately adjusted by means of t h e needle valve until t h e beam just balances a t a predetermined point. This is judged by observing t h e position of a pair of cross lines, on t h e end of t h e beam, with respect t o a fixed mark on a small glass window (shown in end view). A small magnifying glass mounted on a movable a r m aids in reading the position of t h e beam. The operation is then repeated with t h e gas whose density is t o be determined. The specific gravity of t h e gas is t h e ratio of t h e total pressure (gauge plus atmospheric pressure) a t which t h e beam balances in t h e air t o t h e total pressure a t which it balances in t h e gas. The success obtained in t h e use of this apparatus is mainly due t o t h e high sensibility afforded by t h e needles which support t h e balance beam. The needles are easily adjustable and, in contrast with a metal or quartz knife edge, can be obtained readily a t low cost; they also can be replaced as often as necessary. T h e beam is left on its support when not in use, although i t may be removed for transportation. The balance is capable of giving results accurate t o about 0 . z per cent and requires no calibration. The effusion method, which has been widely used for technical work, is based upon t h e fact t h a t t h e times of escape of equal volumes of two gases under t h e same pressure and through t h e same small orifice in a thin plate are approximately proportional t o t h e square roots of t h e densities of t h e gases. Experiments made at t h e Bureau in cooperation with experienced observers using effusion apparatus, showed t h a t results in error by I O per cent or more were not unusual. I n order t o discover t h e sources of error in t h e method, t h e theory of t h e effusion process was investigated. However, t h e number of factors involved, such as t h e ratio of t h e specific heats. t h e vislee 101
IO0
s9 90
R
97 96
95 94 93
r
r
FIG. 2 - c U R V E S S H O W I N G T H E V A R I A T I O N I N T H E A P P A ~ N T SpEc~p~c GRAVITIES (REFERREDTO AIR) OF DIFFERENT G A S 8 9 A S DETERMINED WITH THE SAME ORIFICEAND AT DIFFERENT PRESSURES
FIG. 3-cCRVES S H O W I N G THE APPARENT S P E C I F I C GRAVITIES OF A S A M P L B OF METHANE (SPECIFIC GRAVITY= 0.583) AS DETERMINED
end a closed globe and on t h e other a pair of adjustable counterweights. At t h e center of t h e beam is mounted a small cross a r m bearing a t its ends two very sharp needles which form t h e beam support or “knife edge;” these needles rest in a half cylinder of
cosity, t h e thermal conductivity, etc., made the problem very complex; i t will be discussed in full in another paper. The apparent specific gravity of a gas as determined by t h e effusion method is affected by t h e character
WITH
DIFFERENT ORIFICES
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T H E J O U R N A L OF I N D U S T H A L A N D EIVGINEERTNG CHEIWISTRY
of t h e orifice used, t h e effusion pressure a n d t h e confining medium. T h e effect of different effusion pressures with t h e same orifice a n d different gases is shown in Fig. 2 , where R is t h e ratio of t h e apparent specific gravity t o t h e t r u e specific gravity a n d t h e effusion pressure r is expressed as t h e ratio of t h e atmospheric pressure t o t h e total effusion pressure. Different orifices give curves for t h e same gas which are similar in form b u t have different values for t h e apparent specific gravity. This is shown b y t h e curves in Fig. 3 for six different orifices when using methane. The results may be summarized as follows: I n general, t h e deviations are greatest a t t h e lowest effusion pressures (highest values of I ) a n d a low effusion pressure should be avoided; but it is not true that t h e higher t h e effusion Pressure t h e more nearly correct will be t h e there may be a pressure a t which a certain orifice will give correct results (i. e . , correct ratio t o air) with a certain gas, there is no pressure a t which i t will give correct results with all gases. The combination of factors which produces such a low result in t h e case of carbon dioxide or methane produces just t h e opposite effect with hydrogen. Decreasing t h e diameter of t h e orifice or increasing t h e thickness of t h e orifice plate tends t o give a lower apparent specific gravity with methane or carbon dioxide; t h e same changes give a higher result with hydrogen, as previously explained. Examination of t h e results obtained with 30 orifices shows t h a t orifices of t h e following characteristics give t h e most satisfactory results: t h e orifice should be round a n d smooth, with all burr a n d ragged edges carefully removed. For apparatus using a mean effusion pressure ratio of 0 . 9 0 (with mercury as a confining liquid) a n orifice 0 . 1 j mm. t o 0.30 m m . in diameter in a plate 0.05 mm. or less in thickness should prove satisfactory. For apparatus using a mean effusion pressure ratio of 0 . 9 9 (with water as a confining liquid) a n orifice 0.18 mm. t o 0.30 mm. in diameter in a plate 0.04 mm. or less in thickness may be used. These limits are merely given as a guide t o aid in constructing suitable orifices, as such orifices do not necessarily give accurate results, although they can probably be relied on t o within a few per cent. Mercury and water have bee-, t h e tn.0 confining mediums most widely used in effusion apparatus. When using water, t h e gas must be measured saturated with water vapor; a n d under these circumstances, erratic results are often produced b y condensation of water vapor in t h e orifice. This trouble is eliminated by t h e use of mercury. Lower effusion pressures are usually obtained with water; b u t t h e pressure should not be allowed t o fall below 2 in. of water in a n y case, even at t h e end of t h e test. T h e effusion method, although not adapted to securing absolute results, should prove well adapted for control work or other uses where only relative values are desired. It is suggested t h a t t h e apparatus be calibrated for use with a gas of t h e character under test. The method will give results accurate t o I
Vol. 9,
KO.8
or 2 per cent when properly carried out and a somewhat greater precision can be secured b y t h e calibration recommended. Suitable forms of apparatus, operating directions a n d precautions as well as other details will be found in t h e Bureau of Standards, Technologic Paper No. 94. . op
WASHINGTON, D. C.
A SIMPLE IMPROVISED APPARATUS FOR HYDROGEN SULFIDE PRECIPITATION UNDER PRESSURE By AUBREYVAII. FULLER Received April 20, 1 9 1 i
The ordinary method of conducting hydrogen suifide precipitations b y allowing t h e gas t o bubble through t h e solution is a t Once time-consuming, wasteful, a n d uncleanly from both chemical and hygienic standpoints. In order to obviate these objections attending the customary practice, the has constructed the apparatus pictured, joo cc. Kipp generator is provided with a two-holed rubber stopper which carries a one-way stopcock, B , and a small-bore glass t u b e about 3 feet long which terminates a t its upper end in t h e reservoir bulb C, of about I O O cc. capacity a n d whose lower end extends several inches below t h e acid level. T h e gas outlet is fitted with a rubber stopper carrying a two-way stopcock, A . The operation of t h e device is as follows: t h e flask in which t h e precipitation is t o be conducted is closed with a rubber stopper carrying two glass tubes, one of which is provided with a short length of rubber tubing a n d spring pinchcock, t h e other being connected Of rubber tubing to the gas wash-bott1e by (not shown). is Opened and the gas passed through the Cock flask in order t o sweep out t h e air, t h e pinchcock of having been removed' The gas is then shut o f f , t h e pinchcock replaced, and t h e two-way cock A is opened t o t h e air t h e acid level rises almost to the tubulature, when t h e cock is turned SO as t o connect t h e flask with t h e generator, and cock B is closed. T h e acid level is t h u s caused to sink in t h e reservoir of t h e generator proper and t o rise in t h e t u b e as shown in t h e figure. When through using t h e generator, the is Opened in O r d e r t o the cock the apparatus is and is allowed t o remain open
9'
of
It is evident t h a t with an effective pressure column I meter height a n d employing I : 3 hydrochloric
,
