October 15, 1931
INDUSTRIAL A N D ENGINEERING CHEMISTRY
383
A Simple and Easily Constructed Gas-Pressure Reg ulator 1,2
A. C. Robertson UNIVERSITY OF ILLINOIS, URBANA, ILL.
URING the course of an investigation concerning combustion, it became necessary to burn very small amounts of city gas at a constant rate. The supply
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pressure sometimes ranged as high as 16 inches water column and made the problem of governing the pressure a little more difficult than usual. The customary form of gas-pressure regulator, such as those used for the purposes of gas calorimetry, was found to be unsuitable for handling small quantities of gas, permitting enough increase in pressure to nullify its usefulness entirely. Upon considering the construction of the pressure regulator being used, it became apparent that the design was faulty, and that repairing or adjusting the device would not correct its faults. Regulators of this sort are designed to handle gas in the quantities needed for calorimetry and cannot be expected to control minute amounts.
Figure I-Large
Size Gas Regulator
A pressure regulator was accordingly extemporized from materials easily available in the laboratory in an attempt to avoid the recognized difficulties. The new form of regulator has worked so reliably and so successfully that the author wishes to describe it even though it is so simple that it has possibly been used before. A cursory search of the usual literature has revealed no description of a similar device, and hence the following details are submitted in the hope that others will find the apparatus useful. It may be constructed on a small scale when exact pressure regulation is not vital, but is better made larger, when regulation is very exact and free from rapid fluctuations. The larger container is therefore described, and illustrated in Figure 1. A 4-gallon (15-liter) crock was used for the outer container which holds a “bell” consisting of a metal can 10 inches (25.4 cm.) in diameter having soldered to its top a boss for holding in an axial position 6/le-inch (0.79-cm.) rod which serves as a guide. A small petcock soldered to the top of the can near its edge serves as an outlet for the gas. The inlet consists of an elongated S-shaped glass tube which fits loosely between the bell and the walls of the crock. This tube with IReceived May 18, 1931. *Published by permission of the Director of the Engineering Experiment Station, University of Illinois.
two other tubes, having hooked ends to fit over the edge of the crock, serve as guides to prevent the bell from rubbing against the sides of the outer container. One of these tubes could be used for an outlet tube, but this is not advisable since condensate may collect in it and obstruct the flow of gas. The guide rod on the top of the bell passes through a short sleeve of loosely fitting glass tubing with fire-polished ends. The guide may be fastened to a shelf or to a ringstand which may also serve to support the valve mechanism. The regulating mechanism is simple. It consists of a glass tube, with a small bulb to catch any overflow of mercury, fastened to the bell or the guide rod and connected by small gum tubing to the valve chamber proper. I n the simplest form the valve consists of a tube which has a bottom entrance for the leveling connection, a side arm for the exit gas, and an axial entrance tube for the gas supply. The mode of operation is as follows: The bell, being full of gas, is emptied in use and descends. I n doing SO, it lowers the level of the mercury in the valve and leveling tube to such an extent that the inlet port is uncovered and the gas is fed through the valve into the bell, which thereby rises and forces the level of mercury above the entrance port again. The cycle repeats itself as demand is made upon the reservoir. The pressure upon the exit side of the bell is governed by its weight per unit area and remains very constant. Indeed when the load is proportioned to the size of
384
ANALYTICAL EDITION
the bell, a pressure can be maintained constant to about 0.02inch (0.05-cm.) water column, although when the load is large and fluctuating the variation may reach 0.15-inch (0.38-cm.) water column, depending upon the resistance of the piping at higher velocities of flow. When much gas is being used or the difference of pressure between the entrance and exit sides is great, the mercury within the valve may be sprayed about violently by the stream of gas passing through the nearly closed entrance port. Therefore, it is well to make a float carrying a needle which will reduce the flow of gas just before the cut-off is complete. The shape of the valve is shown in Figure 2. Care must be taken that the needle makes contact at the points indicated and that the valve cannot jam in any position when the surfaces are finally ground to a fit. The needle is loaded with mercury so that its weight will pull it out of the seat promptly. The needle valve will generally stop the flow of gas as well as throttle it, but if contact is not perfect there will be some leakage past the ground surfaces. Therefore, the distance from the entrance port to the side arm must be such that the mercury column between these points will exert a pressure greater than that produced by any anticipated load. The regulator described above has several advantages. The valve will not stick or leak, allowing the pressure to build up gradually to a high value when the rate of withdrawal of gas is slight. The pressure is so very constant for small loads that it is quite practical to do away with the use of
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flowmeters and to use calibrated capillary tubes in their place. The crockery container is immune from corrosion and easily obtained in nearly all laboratories; no special fittings are needed, and a cylindrical tin can will serve as well for the bell as a special container. A large beaker or battery jar may have a guide rod cemented to its base with de Khotinsky cement. When glass is used, a buoyancy effect can be noticed, whereby the pressure of the gas within the bell shows a slight dependency upon the position of the bell instead of being virtually independent of its position, as is the case when iron is used for the material of construction. The most difficult operation involved in the construction of this device is the preparation of the ground joint and inner seal, a construction which is much simplified if Pyrex glass is used for this purpose. Soft glass is preferable for the needle valve. By the use of two such regulators, it is possible to mix gases in any proportion and to change the ratio easily. The enrichment of a fuel gas with sulfur may be taken as an example. One regulator, for the main supply, operated at 4-inch (10cm.) water column pressure. Another regulator, working a t 6-inch (15-cm.) water column, saturated the gas with carbon bisulfide by passing it over the liquid held in a constanttemperature bath. The enriched gas was led into the main stream through a fine capillary tube whose size was found by trial. After the sulfur content was found by analysis, other concentrations could be secured rapidly by substituting measured lengths of the same capillary tube.
Anodic Precipitation of Lead Peroxide' M. L. Nichols DEPARTMENT OF CHEMISTRY, CORNELL UNIVERSITY, ITHACA, N. Y.
T HAS long been known that the best method for the electrolytic determination of lead is by the anodic precipitation of lead peroxide from a solution containing fairly large amounts of nitric acid. All of the conditions governing the accuracy of this determination, including the current density, temperature, presence of other metals and acids, method and temperature of drying, etc., have been the subject of numerous investigations, but little has been done to explain the mechanism of this precipitation. In the book by Classen (29 we find the following statement: "Two explanations have been suggested to account for the formation of the lead peroxide, neither of which is entirely satisfactory. According to Liebenow, the bivalent lead ions are oxidized to negatively charged PbOz-- anions, which are discharged a t the anode. Another explanation is that lead tetranitrate is formed by anodic oxidation and from this lead peroxide is formed by hydrolysis. It is not easy to determine whether the mechanism of the reaction is correctly explained by either of these assumptions." Fischer and Schleicher (8) state that, in order to explain the inclusion of water, nitric acid, and other components of the electrolyte in the precipitated lead peroxide, the divalent lead ion is oxidized at the anode to tetravalent. The so-formed Pb(NO& is hydrolyzed to Pb(0H)r and "08, and the Pb(0H)e then splits off water and is pressed against the anode by cataphoresis. Vortmann ('7) had much the same idea. However, Topelmann (6) says that the divalent lead ion is oxidized at the anode to a tetravalent lead ion, which reacts immediately after its formation with water forming lead peroxide, and the latter, with partial dehydration, will be pressed against the anode by cataphoresis. He also states that the
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Received March 6, 1931. Resubmitted May 25, 1931.
intermediate formation of lead tetranitrate from the tetravalent lead ion and nitric acid plays a very subordinate role. Jewett (4) also believes that the lead is oxidized to negatively charged lead peroxide, and that this is carried against the anode by cataphoresis as there is no way that lead ions can be carried to the anode and no reason a t present to assume the existence of any appreciable amount of lead as part of a complex anion. Consequently, the only lead ions that can be oxidized to tetravalent lead are those which happen to be a t any moment in contact with the anode. Experimental Procedure
A solution of lead nitrate was prepared from Kahlbaum's pure crystalline salt, recrystallized several times from water, and the solution standardized by electrolysis according to the method recommended by Topelmann (6). A mixture of 20 ml. of this solution, together with 5 ml. of a solution of copper nitrate (200 grams per liter) and 100 ml. of nitric acid (1 to 4) was diluted to 200 ml. and electrolyzed at room temperature with 1 amp. for 30 minutes. A platinum gauze cathode and a cylindrical platinum gauze anode rotating a t 500 r. p. m. and having an area (1) of 50 sq. cm. was used. The lead peroxide was washed with water and alcohol, and dried for 1 hour a t 190" to 200" C. 20 ml. of the lead nitrate solution gave 0.2138 gram of lead peroxide, When the electrolyte was stirred a t 500 r. p. m. with both the anode and cathode stationary, the same result was obtained if the time of electrolysis was increased to 1hour. I n order to determine whether the lead is carried to the anode by electrolysis or mechanically, the above experiment was repeated with the anode enclosed in a parchment diffusion thimble and all of the lead nitrate in the cathode compart-
