April, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
that Vu = 1.97 and V4 = 2.63 as required by values of 8s and 8: of 23.5 and 31.3 dynes per cm., respectively. The lower index line connecting w = 0.46 with V , = 1.97 is produced to the a axis where a is read as 11.7". The index line for w = 0.46 and = 2.63 is not shown but would result in a value of /3 of 15.6". The total lens angle of 11.7" 15.6" or 27.3" can be compared with the approximate value obtained by aligning w = 0.46 with 4.6 on the volume scale, resulting in 26.5". The left-hand V , scale and the left side of the CY axis are used throughout. Similarly, for carbon tetrachloride and water the total volume of a liquid lens 0.60 cm. in width is 0.00466 cc.; the
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volumes of the upper and lower segments are 0.00290 cc. and 0.00176 cc., respectively, corresponding to tension values of SZs = 43.7 and Si = .26.5 dynes per cm. The upper index line connecting w = 0.60 with V , = 2.90 is produced to the a axis where a is read as 7.8". The index line for p is not shown, but alignment of w = 0.60 with Vo = 1.76 results in /3 = 4.8". The total lens angle is '7.8' 4.8" or 12.6". The right-hand V u and a scales are used throughout. The approximate value of the total angle, found by aligning w = 0.60 with the total volume of the lens, 0.00466 cc., proves to be 12.4', in excellent agreement with the sum of CY and B since the latter are so small.
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PREPARATION OF FLUORINE GEORGE H. CADY, DOUGLAS A. ROGERS, AND CLARENCE A. CARLSON2 University of Washington, Seattle, Wash.
The preparation of fluorine by the elecinsulator of portland cement. LTHOUGH the belief has Near the bottom of the diatrolysis of a solution of potassium fluoride existed for more than phragm are several pieces of twenty years ( 1 ) that in anhydrous hydrogen fluoride is still metal with a V-shaped cross large quantities of fluorine rather difficult, in spite of the fact that section. They were formed by might be prepared a t a modermany years of research have been spent pressing copper rings and are ate cost, the element has not upon the process. Even when a cell finally held in place with copper rods become an article of commerce as indicated. This type of produces the gas at a reasonable rate, the because the demand is small. c o n s t r u c t i o n w a s used t o If the gas is ever produced on power efficiency is very low, due largely to avoid mixiin@; of hydrogen and a large scale, those who develop overvoltages at the electrodes, and the curfluorine liberated a t the electhe process will probably enrent efficiency is usually below 75 per cent. trodes. Tests made with the counter a number of chemical Under certain circumstances the anode is cell indicated that the rings problems; the preparation of were no great improvement. severely attacked and at times explosions the gas, even on a laboratory The cell operated with about scale, is a troublesome task. occur within the cell. This paper discusses the same current efficiency as The purpose of this paper is to these difficulties and, for the first time, reanother (8) in which the diadiscuss the operation of fluorine ports a high hydrogen overvoltage, amountphragm was cut away to leave cells with particular emphasis ing to about 5-10 volts, which sometimes large holes near the anode. Alupon some of the difficulties. though dimensions are not nearly doubles the potential that must be It is hoped that those who engiven on Figure 4,they may be counter the troubles for the applied to the cell to cause a particular readily estimated by considerfirst time will be helped. current to flow. ing the large portion of the The only method now availanode to have a length of 15 cm. able for the production of fluoThis cell was copied in several ways after that of-Lebeau and rine is the electrolysis of a solution of one or more fluorides Damiens ( l a ) . It was operated by a procedure already pubin liquid hydrogen fluoride. Several variations of the prolished (2). Among the features superior to those of many cedure have been described, and those who are interested should study the bibliography a t the end of this paper. fluorine generators described in the literature are: (a) a method of heating (steam passing through water in the jacket) Typical examples of cells which have been used successfully which allows the electrolyte to be rapidly melted without any are represented in Figures 1, 2, and 3. Still another cell is danger of overheating, (b) an anode with a large enough surshown in Figure 4. Since this device has been used to make some of the measurements to be presented later, a brief deface to permit the use of a current of as much as 20 amperes, (c) a large cut-away area in the diaphragm resulting in comscription is given. paratively low internal electrical resistance. A disadvantage An electrolyte having approximately the composition of molten KF.2HF fills the lower two thirds of the steel pot. is that the type of heating system makes the cell difficult to move. The anode, a piece of nickel or graphite rod 15 cm. long and 3.6 cm. in diameter, was at first fastened directly to the Production of Nearly Pure Fluorine copper plate used as the bottom of the diaphragm. After it was found that the copper was a t times rapidly attacked durNo generator has been described which produces pure ing electrolysis, the anode was suspended as shown, using an fluorine. The gas is always contaminated with hydrogen fluoride and frequently with air, oxygen, oxygen fluoride, 1 Present address, Lever Brothers Company, Cambridge, Mass. carbon fluorides, and other gases, The content of the latter 8 Present address, Johns-Manville Corporation. Lompoa, Calif.
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol. 34, No. 4
authors is that the best electrolyte for use in a laboratory generator has a composition close to that of the compound KF.2HF. As the solvent is used up, it may be replaced by adding anhydrous hydrogen fluoride from a cylinder of the liquid. Impurities in the electrolyte are responsible for many of the troubles encountered. Before fluorine can be liberated in a satisfactory manner, traces of water, hydrogen chloride, and other impurities must be decomposed by electrolysis. The low current efficiency at which such a cell frequently runs may be due to an impurity which is oxidized at the anode and reduced a t the cathode. (This has not been established.) Some of the explosions which occur at times are probably caused by the decomposition of the product resulting from the reaction of fluorine with particles of free carbon. It is known that an explosive material can be formed by this reaction (go), Explosions also take place when the flow of fluorine is blocked and causes bubbles of the halogen to mix with hydrogen. .,,., ..,, ., ,i,
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