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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
Vol. 3, No. 4
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
I
I
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-
October 15, 1931
INDUSTRIAL AN11 ENGINEERING CHEMISTRY
ment. I n this case two runs gave only 0.0046 gram and 0.0038 gram, or an average of 0.0042 gram of lead peroxide deposited in 1 hour. This small amount of lead peroxide precipitated is undoubtedly due to diffusion (5) of the catholyte into the anode chamber, as some diffusion will take place in all, but quite exceptional, circumstances. If the lead is carried to the anode mechanically, as indicated by the previous experiment, then the rate of precipitation ah o uld i n c r e a s e w i t h increased rate of stirring. All conditions were kept the same as in the standardization of the solution, except t h a t t h e electrolysis was run at 0.25 amp. for exactly 15 minutes of d i f f e r e n t rate of rotation of the anode. RPM The results are given Figure 1-Results of Electrolysis under in Table I and curve Various Conditions A, Figure 1. If the lead, after being carried to the anodemechanically, is oxidized to negatively charged lead peroxide and carried against the anode by anaphoresis, then it should be possible to increase the rate of stirring to a point where, after the lead is oxidized to lead peroxide, the rotation of the anode would throw it away before it could be pressed against the anode. This should result in a decreased rate of precipitation a t very rapid rates of stirring. The previous experiment does not
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through D at the rate of 545 ml. per minute. Under these conditions the electrolysis was carried out for exactly 5 minutes a t 0.5 amp. with the anode rotating at varying rates of speed. The results are given in Table I1 and curve B, Figure 1. This experiment did not show the desired result, which was thought to be due to the fact that the rate of flow of the electrolyte was not high enough in proportion to the rate of precipitation of the lead peroxide. The experiment was therefore repeated using the same electrolyte, a current of 1.75 amp., and electrolyzing for exactly 3 minutes with a platinum gauze anode having an area ( I ) of 95 sq. cm. The results are given in Table I11and curve C, Figure 1. Table I-Results RATEOF ROTATION R . p . m. 0 500 1000 1500 2000
of Electrolysis w i t h Ordinary Cell LEADPEROXIDE 1 2 Gram Gram 0.0659 0.0655 0.1430 0.1430 0.1545 0.1543 0.1599 0.1617 0.1620 0.1621
Av. Gram 0.0657 0.1430 0.1544 0.1608 0.1621
Table 11-Results of Electrolysis w l t h Special Cell RATEOF ROTATION LEADPEROXIDE R. p . m. Gram 0 500 1000 1500 2000 3000
of Electrolysis w i t h Special Cell and Changed Conditions LEADPEROXIDE RATEOF ROTATION 1 2 Av. R. p . m. Gram Gram Gram 0 0.0130 500 0.0426 .... 1000 0.0562 2000 0.0609 0.0605 0.0607 0.0526 0.0489 0.0509 3000
Table 111-Results
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Discussion
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Figure 2-Special
Cell for Electrolysis of Lead
show this effect, since the electrolysis was made in a cell where any negatively charged lead peroxide that was thrown away from the anode would immediately be thrown back against it. If this is the correct reason, then if the electrolysis were carried out in such a manner that the lead peroxide once oxidized could be removed from the cell before it was thrown back against the anode, the decrease should be obtained. Using a solution of the same concentration as in the previous experiments, the electrolysis was carried out in the cell shown in Figure 2. The electrolyte entered through tube A , flowed past the anode B and the cathode C, and then out
These results confirm the conclusion of Jewett (4) thatithe lead is not carried to the anode as ion but is merely brought in contract with the anode by stirring the solution. If the lead is first oxidized to Pb(NO& and then the lead peroxide formed from this by hydrolysis, it should be rather easy to decrease the rate of deposition of the lead peroxide with high rates of anode rotation, as the hydrolysis would take an appreciable time. Since this was not the case, Topelmann’s (6) conclusion that the intermediate formation of Pb(NOJ4 plays a very subordinate role is confirmed. It appears, therefore, that the anodic precipitation of lead peroxide occurs by the lead being mechanically carried to the anode, there oxidized to negatively charged lead peroxide (8),and this then pressed against the anode by anaphoresis. This explains why a roughsurfaced anode must be used, and since the negatively charged lead peroxide is probably a hydrous oxide (8),the difficulty of removing the last traces of water in the drying of this precipitate (6) is readily understood. Literature Cited (1) Classen-Boltwood, “Quantitative Chemical Analysis by Electrolysis,” p . 114, Wiley, 1903. (Formula given should be area 3 2 dlb 6.) (2) Classen-Hall, ”Quantitative Analysis by Electrolysis,” p. 193, Wiley, 1919. (3) Fischer and Schleicher, “Elektroanalytische Schnellmethoden,” p . 244, Enke, 1926. (4) Jewett, J . Phys. Chem., 33, 1024 (1929). (5) Sand, J . Chem. Soc., 93, 1589 (1908). (6) Topelmann, J. prakf. Chem., l a l , 289 (1929). (7) Vortmann, Ann., 361, 283 (1907). (8) Weiser. “Hydrous Oxides,” p. 230-231,McGraw-Hill, 1926.
