The Preparation of Weston Standard Cells WARREN C. VOSBURGH and PAUL F. DERR Duke University, Durham, North Carolina
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T HAS been the experience of one of the authors that the Weston standard cell is one of the easiest of cells to reproduce. W i l e the preparation of cells of the highest degree of reproducibility and con' stancy requires experience and much care,l most chemists should be able to make cells suitable for standards when the highest degree of precision is not required. To test this opinion, a set of directions was written and was followed carefully by several students who kindly cooperated in the test. With one exception the students were either seniors or graduate students. None had had previous experience in making Weston cells. The chemicals used were of the best commercial grade, with no further purification, except that the mercury had been redistilled and the mercurous sulfate was specially prepared by the method given. DIRECTIONS FOR THE PREPARATION OF WESTON CELLS
Vessels.-It is assumed in the directions following that glass vessels of the H-type will be used, with sealed-in platinum (or tungsten) wires for connection to the two electrodes. The quantities of the materials specified are estimated for vessels made of 12- to 15-mm. (inside diameter) glass tubing and 15 to 18 cm. high and about 5 cm. wide. For vessels appreciably larger, the quantities would need to be increased. The tops of the vessels may be closed by rubber stoppers, or sealed in a flame if desired. The vessels should have platinum (or tungsten) wires sealed in near the bottoms of the legs of the vessel; for convenience and for protection of these wires it is well that copper wires of fairly small diameter be fastened to the sealed-in wires and tied securely to the glass vessel. The copper wires may be soldered to the platinum wires. If the vessels are of pyrex glass and the sealed-in wires of tungsten, the outer ends of the tungsten wires may be covered with brass by heating in an oxygen-gas flame with a piece of brass brazing rod, with borax as a flux. The copper wire can then be soldered to the brass. Mercurous Sulfate.-Dissolve about 10 g. of finely ground crystals of mercurous nitrate in a solution prepared by the dilution of 6 ml. of concentrated nitric acid to 100 ml. When the crystals are completely dissolved, dilute to 200 ml. Filter through asbestos or glass wool if the solution is not clear, or if any material remains undissolved. Add the mercurous nitrate solution drop by drop with vigorous stirriig to 200 ml. of 2 M sulfuric acid solution. The latter may be prepared by the addition of 35 ml. of concentrated acid to 300 ml. of water, a third of the resulting solution being
' Woma AND W , "Clark and Weston standard cells," Bur. Slanderds Bull., 4, 1-80 (Dec., 1907); ATERS
Vossunon, "Condi-
tions decting the reproducibility and constancy of Weston standard cells." I. Am. Ckzm. Soc., 47, 125567 (May. 1925).
diluted with an equal volume of water and retained for use as a wash solution. Wash the mercurous sulfate precipitate (which should be pure white in color) by decantation with five 15-ml. portions of the 1 M sulfuric acid wash solution and then add the remaining 1 M acid solution and allow the precipitate to digest a t room temperature in the dark overnight or longer.% Cadmium Sulfate Solution.-Prepare 60 ml. of either 0.015 M sulfuric acid solution or 0.1 M acetic acid solution, the concentration correct within 10 per cent. To this solution add about 70 g, of finely ground cadmium sulfate crystals (3 CdSO&HzO) and stir vigorously (by means of a mechanical stirrer) for at least 30 minutes. Filter this solution, preferably through asbestos or pyrex glass wool, if any foreign matter is present in suspension. Suspended cadmium sulfate is not objectionable. Cadmium.-Heat one end of a stick of cadmium in a burner flame until the metal melts and falls in drops. The drops should be allowed to fall on a watch glass or other clean surface where the metal solidzes to form a thin disc that can be easily cut into pieces of any desired size. Preparation of Cells.-Clean the cell vessel thoroughly and dry it. Weigh the vessel to the nearest tenth of a gram. Introduce into one leg of the vessel enough mercury to cover the sealed-in wire completely and weigh again. A quantity of cadmium equal to between 10 and 12 per cent of the mercury is to be added next to make cadmium amalgam. Weigh the required amount of cadmium to the nearest tenth of a gram and add it to the mercury in the vessel. Cover the cadmium and mercury to a depth of one to two inches with approximately 0.01 M sulfuric acid solution. Immerse the amalgam leg of the vessel in warm water, heat the water, and shake the vessel until the cadmium is completely dissolved and the amalgam is of uniform composition. Allow the amalgam to cool, and remove the acid solution by means of a pipet to which a rubber tube is attached, so that the pipet can be operated while watching the lower end. Rinse the amalgam once with a small portion of saturated cadmium sulfate solution, then cover it to a depth of two centimeters with the saturated cadmium snlfate solution. Grind some selected cadmium sulfate crystals, and drop the I finely ground crystals into the solution until the amalgam is covered to a depth of about one centimeter. Then add coarser crystals up to the level of the cross arm. If some of the solution runs over into the other arm it will do no harm.
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'For cells of the highest quality it is desirable that the mercurous sulfate be digested under the acid solution, together with a little mercurv. .. at a temoerature near the boiline - .mint for several hours.
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Add enough mercury to the empty leg of the tube to cover the sealed-in wire, and then enough cadmium sulfate solution to cover the mercury to a depth of about two centimeters. Separate the mercurous sulfate from the acid solution (under which it has been kept) by filtration in a Gooch crucible with a small disc of filter paper covering the bottom. To rinse the mercurous sulfate from a beaker or k k , a 0.01 M (or more concentrated) sulfuric acid solution may be used, but not water. Wash three times on the filter ,with 5-ml. portions of the cadmium sulfate solution. Avoid expaswe of the mercurous sulfate to air any longer than necessary after washing. Transfer the mercurous sulfate from the crucible to a small beaker, add 10 ml. of cadmium sulfate solution and about 2 g. of finely ground cadmium sulfate.P Mix thoroughly, allow to settle, and decant the solution. Wash with two more 5-ml. portions of cadmium sulfate solution, add 5 ml. more, and then by means of a glass tube narrowed a t one end transfer some of the mercurous sulfate paste to the mercury side of the cell vessel, allowing it to fall through the solution above the mercury and come to rest on top of it. Enough of the paste should be added to give a layer about one centimeter deep when it has settled. Do not allow mercurous sulfate to get into the amalgam side of the vessel. Add finely ground cadmium sulfate crystals until a layer about one centimeter deep has been formed above the mercurous sulfate, then add coarse crystals up to the cross arm of the vessel. Add cadmium sulfate solution until the level of the solution is above the cross arm. Stopper the vessel. The cell may be expected to come to equilibrium within 0.1 mv. in about a week and within 0.03 mv. in about a month.' After preparation, the cells were placed in an oil bath a t 25' and measured frequently by the authors by means of a Leeds and Northrup Type K potentiometer and a group of standards probably reliable to 0.01 mv. It is of interest to compare the cells of this investigation with similar cells constructed by experienced workers with more highly purified materials. For electrolytes 0.01 molar with respect to sulfuric acid (satwatinp a 0.015 M acid solution with cadmium sulfate dilutes the acid to 0.011 M),the usual value a t equilibrium is 1.01805 v. and under the best conditions deviations should not be more than 0.01 mv. For cells with electrolytes 0.07 molar with respect to acetic acid, the value is 1.01802 or 1.01803 v.; this is a little less certain than the other because fewer cells of this type have been made.6
*The amount of cadmium sulfate to be added to the mer-
curous sulfate should be from one-quarter to one-half of the amount of the latter, estimated roughly by volume. 4 By careful exclusion of oxygen and evacuation to insure removal of oxygen from the amalgam, cells have been made to come to equilibrium within 0.01 mv. immediately. See ref. 7 below. 6 Two cells containing this urncentration of acetic acid have been observed for s period of over nine years. These cells and others made subsequently have been as satisfactorywith respect
The largest deviation from the correct value within one day of preparation was 0.67 mv. or about 0.07 per cent, while the average deviation for the twenty cells was 10.15 mv., or less than 0.02 per cent. A week later the maximum deviation was 0.2 mv., or 0.02 per cent, and the average deviation 10.06 mv., or appreciably less than 0.01 per cent. Some time after the preparation of eight of the cells it was discovered that by mistake the mercurous sulfate used in them had been prepared by precipitationfrom a solution containing too little sulfuric acid. Some of the cells made with the inferior mercurous sulfate attained values lower than the normal, but did not vary greatly. Ten of the twelve cells made with good mercurous sulfate had an averagedeviation of less than 0.03 mv. (0.003 per cent) after two months. The average electromotive force of eight of these cells made with acetic acid was 1.01802 v. and that of two cells made with sulfuric acid was 1.01806 v. If the students cooperating in this test may be considered typical of inexperienced workers, it may be concluded that relatively inexperienced technicians should be able to construct saturated Weston cells giving an electromotive force a t equilibrium within a few hundredths of a per cent of the correct value. It should be mentioned that these cells are saturated Weston cells and differ from the unsaturated standard cells available from commercial sources in having a larger temperature coefficient. A change of temperahue from 20' to 30' C. causes a change of 0.49 mv. or 0.05 per cent in the electromotive force. An a p proximate temperature correction may be made by means of the formula Et = at - O.W0046(1
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in which t is the Centigrade temperature. This formula should hold within 0.003 per cent between 18O and 30' and within 0.01 per cent between 15' and 35'. For a more precise correction, Wolff's temperature formula6 should be used. A modified Weston cell has been described7 that has a temperature c d c i e n t between a quarter and a third that of the ordinary saturated cell. The modified cell is a little more d i c u l t to make than the ordinary cell, but with proper precautions it is possible to reproduce i t 1
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to constancy as cells containing 0.01 M sulfuric acid. In one respect they may prove to be superior. So far, no reaction of acetic acid of this concentration with the cadmium of the amalgam to give hydmgen has,be+ observed. Occasionally, however, cells contauung snlfunc acid are spolled by the 'generation of hvdroeen at the amaleam. thoueh most of them show no &an; of &h action over p&i& of many years. It seems probable that this reaction is Eatalyzed by ceinai impurities. men impurities may be pr-t and when long l i e is of more importance than hich orecision. acetic acid is orobablv the betterchoice of the two acrds: 6 WOLQQ, "The temperature formula of the Weston standard cell,'' Bur. Shndardr Bull.. 5 , 309-37 (Sept., 1908); Vosswce, "The applicability of Wolff's temperature formula for the Weston standard cell." J. 0 wl Sac. Am. and Rev. Sci. Imtrumcntr, 12, 511-517 (May. 19267' VOSBURGH, GUAGBNTY, man CLAYTON. "Saturated standard cells with small temperature w5cients. 11," J. Am. C h . Soc., 59, 12&8 (July. 1937).
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about as well. To the cadmium amalgam is added 10 to 15 parts of bismuth for every 100 parts of 10 to 12 per cent cadmium amalgam.8 Also, the electrolyte of the modified cell is saturated with the double salt of cadmium and sodium sulfates, CdSO,. NanSOr. 2HzO as well as with hydrated cadmium ~ u l f a t e . ~Wherever Vossuaon AND PARKS, J . Am. Chmr.Soc., 61, 652-4 (1939).
solid cadmium sulfate is placed in the ordinary Weston cell, a mixture of cadmium sulfate and the double salt must be substituted in the modified cell. Cells of this type haveshownexcellent constancy for over three years and there is no reason to expect any greater number of failures than with the Weston cell.
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Fire FightingIn the Wheel of National Defense - A Cog AUBISON T. BURTSELL College of the City of New York, New York City UNITED Press report from London, England, October 5, 1940, read: "Fifty men of the London F i e Brigade lost their lives fighting bomhstarted fires last month, and 501 others were injured, some seriously." An article appearing in Life entitled, "London F i e Fighter," portrayed the activities of 30,000 members of the Auxiliary Fire Service who augment London's regular fire brigade. The members of this service receive four pounds a week and their keep, and devote all of their time to fighting fires started from incendiary bombs dropped on London. Frequently these duties are performed while under fire from machine guns, bombs, etc., and the risks are made especially dangerous because the location of the men is apparent from the light of the fires that they are fighting. Therefore, they are ready targets for aerial attacks. Daily press reports reveal that fighting fires in modern warfare is one of the most important defense activities. I t is also significant that three men from New York City's F i e Department were recently sent to England to study fire-fighting activities there. During the past year, the author has developed a chemistry course in the Division of Public Service Training for students preparing for the fire department in the city of New York. Since it has been apparent that fire is a modem weapon of warfare, the Department of Chemistry a t the School of Business and Civic Administration has placed special emphasis on the fact that extinguishing a fire can be thought of as reversing or slowing down the reaction taking place in the combustion. All the methods for altering the speed of a chemical reaction apply to the reverse reaction as well as the forward reaction, hence it seems logical to teach fire fighting along these lines. Fire extinguishing involves, therefore, slowing the reaction by cooling or by decreasing the concentration of reacting materials, or both, which in turn involves nothing diierent from the same means of slowing any other chemical reaction. It is only necessary to slow the reaction down to the point where the heat liberated by the reaction is produced at a slower rate than heat is lost to the environment; then the fire dies a natural death.
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This approach has been adopted by the author in teaching fire extinguishing to the students preparing in the Division of Public Service Training for fire department service. The students have previously learned the means of altering the rate of a chemical reaction, and this approach has stimulated exceptionally lively discussions, as well as a better understanding of the methods of fire fighting. The author feels that he is not presumptuous in taking this opportunity to call the attention of all teachers of chemistry throughout the United States t o their obligation to teach to every student of chemistry the fundamentals of fire fighting, since this new weapon of defense is of prime importance in modem warfare. I t seems obvious that in modem defense warfare every citizen must be alert to the danger of fire and prepared to extinguish, often single-handed, small fires before they grow large. This is a cardinal rule in safety at all times, and becomes more so in present-day warfare. Since the writer is meeting such success in fitting this type of work into the pattern of fundamental chemistry, it is further suggested that the task may be done without necessarily changing the courses already established in the various schools. A start in the right direction would be to use the reaction of comhustion for illustrating the application of the means of altering the reaction rate. From the author's experience, it is predicted that if this approach is taken, the teacher will find that the students are anxious to continue this type of thought beyond the mere introduction of the idea; the teacher will find good reason to expand upon this topic as the individual circumstances permit. Since we are preparing for defense, we owe it to our country to teach every student passing through our hands more about fire extinguishing than is customary, so that this horrible weapon "fire" can be combated successfully. I t is inevitable that, should this country he forced to defend itself from attack, every able-bodied citizen who knows something of fire fighting will he so much more of an asset. Chemistry students are the best fitted to learn fire fighting, and the author urges all his colleagues to accept the challenge before us.
