New Applications of the Silver Reductor Determination of Uranium and Copper NATHAN BIRNBAUAl AND SYLVAN RI. ED4IOSDS City College of the College of the City of New York, New York, N. Y.
T
UOZ+l
HE silver reductor coupled n-ith oxidation by ceric sulfate, using o-phenanthroline ferrous complex indicator, has been applied in the determination of iron (5) and molybdenum (1). The advantages of this method (rapid and selective reduction, the convenience of the use of the reductor column, and the sharpness and reproducibility of the end points) have led the authors to investigate its further applications. They have found that uranium and copper, respectively, are reduced quantitatively in the silver reductor under proper conditions; the reduced solutions are titrated with ceric sulfate using o-phenanthroline ferrous complex indicator,
Furthermore, increasing the chloride-ion concentration should lower the potential of the silver-silver chloride reductor. I n agreement. with these considerations the authors have found t h a t as the concentration of hydrochloric acid is increased the reduction to quadrivalent uranium becomes more nearly complete. I n 4 M hydrochloric acid a t 60" to 90" C. the reduction to the quadrivalent state is quantitative. The following procedure was finally developed in which the uranium in hot 4 M hydrochloric acid is reduced to the quadrivalent state in the silver reductor and reoxidized with ceric sulfate in the presence of phosphoric acid, usins o-phenanthroline ferrous complex as indicator.
Determination of Uranium Uranium is usually determined volumetrically by reduction in the Jones reductor followed by titration with potassium permanganate. As is well known, uranium is partially re-
METHODOF ANALYSIS. The silver reductor is similar to that described by Walden, Hammett, and Edmonds ( 5 ) . Fifty milliliters of solution containing from 0.1 to 0.4 gram of uranium were made 4 -1.I with respect to hydrochloric acid and the solution was heated to 60°to 90" C. If much less than 0.1 gram of uranium is present the titration has to be made with 0.02 M ceric sulfate. Titrations with such dilute ceric sulfate are unsatisfactory, for the end point is difficult to discern and the blank correction variable and large. Fryling a.nd Tooley ( 2 ) have shown that in cold 1 Ai' hydrochloric acid sufficient hydrogen peroxide is formed in the silver reductor to interfere in the microdetermination of iron. In hot 4 ill hydrochloric acid the formation of peroxide is apparently greater than in cold more dilute acid. However, with amounts of uranium over 50 mg. and using 0.1 iM ceric sulfate for the titration, the end point is sharp and the results are accurate. The solution was then passed through the silver reductor, which had been preheated with hot 4 J1 hydrochloric acid, a t the rate of 20 ml. per minute. The reduced solution was caught in a 400-ml. beaker and the reductor column washed with 150 ml. of hot 4 211 hydrochloric acid. The rate of passage through the reductor of the last 100 ml. of wash liquid was increased. Willard and Young (6) have shown that quadrivalent uranium may be titrated vith ceric sulfate using o-phenanthroline ferrous complex as the indicator provided that the titration be carried out a t 50". The authors have found that titrati.ons in hot solution using this indicator are unsatisfactory. The indicator dissociates rapidly at 50°, so that the pink color fades perceptibly long before the end point is reached. In fact, in some titrations where the approximate titer was not k.nown and the ceric sulfate had to be added slowly, the color faded entirely, and it was usually necessary to add more indicator nt:ar the end of the titration. The titration at room temperature is unsatisfact>ory,apparently because of the slowness of the reaction between ceric ion and quadrivalent uranium. The authors have found, however, that in the presence of a small amount of phosphoric acid the reaction is rapid and the end point sharp. Therefore, after passage through the reductor t,he solution was cooled, 3 ml. of 85 per cent phosphoric acid and one drop of 0.025 M o-phenanthroline ferrous complex indicator solution were added, and the solution was titrated with 0.1 ,Li' ceric sulfate. When the solution cools a precipitate of silver chloride usually forms but it in no Tvay interferes ~viththe detection of the end point. At the end point a fraction of a drop of the oxidant sharply bleaches the pink color of the solut,ion. Blank determinations were made on all materials used. Corrections varying between 0.02 and 0.09 ml. of the 0.1 ill ceric sulfate solution were required.
duced below the quadrivalent state, but never quantitatively to the trivalent, in the Jones reductor. However, a few minutes' aeration prior to the titration ensures quantitative reoxidation to the quadrivalent state. Uranium is quantitatively reduced in the silver reductor to the quadrivalent state. The reduced solution may then be titrated with ceric sulfate using o-phenanthroline ferrous complex as the indioator. MATERIALS,Approximately 0.1 111 ceric sulfate solution was prepared from ceric ammonium sulfate by solution of the salt in 1 AI sulfuric acid. It was standardized against Bureau of Standards sodium oxalate by the method of Walden, Hammett, and Chapman (4). Approximately 0.02 J1 potassium permanganate prepared in the usual way was standardized against Bureau of Standards sodium oxalate and also checked against the ceric sulfate through ferrous sulfate. An aqueous uranyl sulfate solution, approximately 0.05 .M, was prepared from c. P. uranyl acetate which was free from detectable amounts of iron, vanadium, and molybdenum. The salt was treated with a large excess of concentrated sulfuric acid plus a small amount of concentrated nitric acid and evaporated to sulfur trioxide fumes. The nitric acid treatment was repeated and the mixture was finally heated until almost all the sulfuric acid had been expelled. Enough water was then added to make an approximately 0.05 111 solution. The solution was standardized gravimetrically by diluting three 25.00-ml. portions to 150 ml., precipitating the uranium as ammonium uranate, and igniting the precipitate to u308. The weights found were 0.3507, 0.3507, and 0.3511 gram of U&, corresponding to 0.01190 gram of uranium per ml. It was also standardized by reduction in the Jones reductor and titration with permanganate following the method given by Hillebrand and Lundell (3). The results of three determinations on 25.00-ml. portions were 0.2983, 0.2984, and 0.2985 gram of uranium, corresponding to 0.01194 gram of uranium per ml. All chemicals used were reagent grade. Calibrated apparatus was used throughout the investigation.
ACTIOP; OF U ~ a i s ~ v aI Ni THE SILVERREDUCTOR. Sexivalent uranium in 1 M hydrochloric acid is not reduced in the silver reductor. Consideration of the oxidation potentials E o U o Z t t / ~ t t t= + 0.358 volt;
+ 4H30+ + 2e + U+++++ 6 H 2 0
E'A,CI/A~ = 0.2245 volt
EXPERIMEKTAL RESCLTS. The uranyl sulfate solution (25.00-ml. portions) , which had been standardized as described above, was analyzed for uranium b y the silver reductor-ceric sulfate method, with the following results: found, 0.2978, 0.2982, 0.2985, and 0.2970 gram (average 0.2979) of
indicates t h a t reduction should be quantitative. At 60" to 90" C. there is slight reduct'ion, equivalent to a few drops of 0.1 ~ l fceric sulfate. Increasing the acidity should increase the uranyl potential, since the reaction involves oxonium ion: 155
INDUSTRIAL AND ENGINEERING CHEMISTRY
156
uranium; average deviation, 1.7 parts per 1000, taken (gravimetric) 0.2970 gram of uranium, (Jones reductor-permanganate) 0.2984 gram of uranium. Ten-milliliter portions were also analyzed with these results: found, 0.1189, 0.1190, and 0.1196 gram of uranium; average deviation, 2.5 parts per 1000; taken (calculated from the gravimetric) 0.1190 gram of uranium, (calculated from Jones reductor-permanganate) 0.1194 gram of uranium. EFFECTOF ACETICACID. Large amounts of acetic acid have no effect on the determination of uranium with the silver reductor. Thus with 1, 5 , and 10 ml. of glacial acetic acid present in 50 ml. of 4 M hydrochloric acid solution containing the uranium, there was found 0.2970, 0.2967, and 0.2973 gram of uranium, average deviation 0.7 part per 1000, as compared with 0.2979 gram in the absence of acetic acid. EFFECTOF SMALL AMOUSTS OF NITRIC ACID. When passed through the reductor, 0.2 gram of nitric acid in 50 ml. of 4 M hydrochloric acid introduces no substance capable of oxidation by the ceric sulfate. However, in the presence of 0.3 gram of uranium the titer was always low by several per cent, the agreement poor, and the end point difficult to discern. It is therefore essential that nitric acid be absent.
TABLEI. DETERMINATION O F COPPER Solution I C u taken, gram Cu found, gram 0.1704 0.1704 0.1704 0.1704 0.1705 0.1704 0.1706 0.1704 0.2386 0.2386 0.2735 0,2727 0.3340 0.3340
Solution I1 Cu taken, gram C u found, gram 0.1679 0.1675 0.1679 0.1679 0.1679 0.1679 0.1679 0.1680 0.2351 0.2352 0,2687 0.2691 0.3291 0.3289
Determination of Copper The aut'hors have found that in 2 JI hydrochloric acid solution a t room temperature copper is quantitatively reduced in the silver reductor to the cuprous condit'ion. The solution containing the reduced copper is caught in ferric alum solution and the iron titrated with ceric sulfate, using o-phenanthroline ferrous complex indicator. MATERIALS.A 0.5 AI ferric alum solution was prepared by dissolving the salt in 0.5 i l l sulfuric acid. The approximately 0.1 M ceric sulfate solution was prepared as described above. Two solutions of copper sulfate, approximat'ely 0.1 N , were prepared by treatment of metallic copper with nitric acid, double evaporation t o sulfur trioxide fumes after addition of sulfuric acid, and dilution of the resulting solution to 1 liter. Solution I was prepared from 6.8159 grams of Leeds & Xorthrup copper wire specified for copper-constantan thermocouple measurements. Solution I1 was prepared from 6.7160 grams of Hilger spectroscopically standardized copper rod, with an assay of at least 99.993 per cent copper. In both cases the copper was carefully cleaned to eliminate any surfare contamination. Cupric ion in 2 119 hydrochloric acid is reduced in the silver reductor to cuprous chloride complex ion. Consideration of the Oxidation potentials involved (Eocu+ +/cucls- = 0.435 volt; EoAgC1/Ag = 0.2245 volt) indicat'es t h a t the reduction should be quantitative. Preliminary determinations in which the reduced solution was titrated with ceric sulfate using o-phenanthroline ferrous complex indicator gave results which were low by several per cent. Stirring the reduced solution for several minutes before titration yielded even lower results, indicating air-oxidation of the reduced copper. I n the method of analysis finally adopted, therefore, the reduced solution is collected under ferric alum and the resulting ferrous iron titrated with the oxidant.
METHODOF AXALYSIS. Fifty milliliters of solution, containing from 0.1 t o 0.4 gram of copper and 2 M with respect t o hydro-
VOL. 12, NO. 3
chloric acid, were passed through the silver reductor a t the rate of 25 ml. per minute. The reduced solution was collected under 20 ml. of 0.5 M ferric alum solution with stirring and the reductor column was washed with 150 ml. of 2 M hydrochloric acid. One drop of 0- henanthroline ferrous complex indicator was added and the soition was titrated with ceric sulfate. The end point is extremely sharp and the color imparted by the cupric ion in no way interferes with detection of the end point. T h a t the method is precise (average deviation less than 1 part per 1000) and accurate is indicated by the results given in Table I. EFFECTOF KITRICACID. The presence of comparatively large amounts of nitric acid has no effect on the determination of copper with the silver reductor. Thus with 0.2, 0.2, 1, and 3 grams of nitric acid present in the solution containing the copper, there was found, respectively, 0.1681,0.1679,0.1679, 0.1678 gram of copper as compared with 0.1679 gram of copper taken. T o determine the applicability of the method to solutions containing other metallic ions, 25.00-ml. portions of Solution I. containing 0.1704 gram of copper, were mixed with solutions containing Zn++, Sn++++, As04--- Bi+++, and Cd++, and the mixtures were analyzed with h e silver reductor. Finally, complete mixtures containing all these elements were also analyzed. The results are summarized in Table 11. . ~ N A L Y ~ IOF S STANDARD SAXPLE.A brass sample, Bureau of Standards S o . 37b, was analyzed for copper as follows: Approximately 0.3 gram of sample was treated with 1 ml. of concentrated nitric acid and 4 ml. of concentrated hydrochloric acid, and heated until completely dissolved. The solution was diluted to 150 ml. and the iron separated by a double precipitation with ammonia. The solution was acidified and evaporated to 40 ml. Sufficient hydrochloric acid was added to make the solution 2 M with respect to hydrochloric acid, and the copper was determined by the method given above, using the silver reductor. Five determinations were made and the following results obtained: 70.36, 70.39, 70.30, 70.37, and 70.46 per cent; average, 70.38 per cent; average deviation, 0.6 part per 1000; Bureau of Standards certificate value, 70.36 per cent. TABLE11. EFFECTOF Ioss wo. of Determinations
Mixture ;inalyaed
CuSOa (Solution I) 0.5 gram ZnSOa CuSO4 CuSOa 0.6 gram SnClr CuSOa 0.4 gram SaZHXsO' CuSOa 0.45 gram BiC13 CuSOa 0.5 main C d C h Complete mixture: Cu30: 100 mg. Z n + + , Sn++++, B i + + + ,C d + - , and 50 in:.
++ ++ +
+
4 4 4
4 4 4
4
Cu Found
Averap Deviation Parfs per
Gram
1000
0.1705 0.1706 0.1705 0,1706 0.1706 0.1706 0 1705
0.5 0.8 0.6 0.3 0.8 1.0 1 4
-i +s ++++
Summary I n hot 4 1 ' 9 hydrochloric acid solution, uranium is reduced quantitatively to the quadrivalent state in the silver reductor. The uranium may be determined by titration in the cold with ceric sulfate using o-plienanthroline ferrous complex indicator, provided phosphoric acid is present. Large amounts of acetic acid do not interfere. Sitric acid, iron, molybdenum, vanadium, and copper must be absent since they are reduced in the silTer reductor In 2 A ' hydrochloric acid solution copper is reduced quantitatively to the cuprous condition in the silver reductor. The reduced copper is collected under ferric alum and the ferrous iron titrated with ceric sulfate using o-phenanthroline ferrous complex indicator. Comparatively large amounts of nitric acid do not interfere. The presence of zinc, tin, arsenic (quinquevalent), bismuth, and cadmium offers no interfer-
MARCH 15, 1940
ANALYTICAL EDITIOIV
ence. Iron, molybdenum, uranium, and vanadium must be absent.
Literature Cited (1) Birnbaum and Walden, J . Am. Chem. Soc., 60, 64 (1938)
(2) Fryling and Tooley, Ibid.,58, 826 (1936).
15;
(3) Hillebrand and Lundell, “ilpplied Inorganic knalysis”, p. 370, Ken. York, John R’iley & Sons, 1929. (4) Walden, Hammett, and Chapman, J. Am. Chem. Soc., 55, 2649 I , no“\
\rYso).
Walden, Hammett, and Edmonds, Ibid.,56, 350 (1934). (6) Willard and Young, Ibid., 55, 3260 (1933). (5)
Still for Producing Metal-Free Distilled Water J. S. 3ICHARGUE ‘ N D E. R. OFFUTT h;rntiirky Ipricwltural Experiment Station, Lexington, Ky.
A
?;I -4DEQUATE supply of distilled water, which is free
from metallic and nonmetallic compounds, is of fundamental importance for research investigations planned to show the necessity of the minor elements in the economy of plants and animals. I n this article the term “metal-free dietilled water” is used to designate water containing total metals of the order of one part per billion or leis
A
B
C
FIGURE1. FORNS OF QUARTZ CONDESSER TUBES
It is well known among chemists that distilled water that is produced with the ordinary type of water still contains small amounts of a number of metallic elements, including copper, zinc, tin, lead, manganese, nickel, and iron. These elements can be detected when several liters of the distillate are evaporated to dryness and sensitive tests are carefully applied for the respective elements. It has been shown in previous work that distilled water made with the type of still commonly used in chemical laboratories contains a sufficient amount of copper and zinc to supply the requirements of plants for these elements, when it is used as the source of water in water or sand cultures. hIetallic elements arc introduced into distilled mater from the processes of oxidation, erosion, and solution of the
condenser tube, which is usually constructed from brass t u b ing and coated on the condensing surface with tin. The principal ore from which tin is smelted is cassiterite, or tin oxide @nos), n-hich in the crude state contains arsenic, bismuth. zinc, copper, manganese, and iron. The usuai process for the purification of tin seldom frees it from traces of these metali Condenser tubes made from metals are used because the\ are cheap and conduct heat more efficiently than tubes maclc~ from earthenware materials such as porcelain ot silica. Purc platinum is undoiibtedly the most desirable metal for makirig condenser tubes. h i t the cost is prohibitive. It is a1.o like]\ that traces of this metal could be detected in a few liter,. 01 distilled water made n i t h a platinum condenxi. In recent yeat + quartz vessels of small size for making snixll quantities 01 conducti\ity water hare been sold by dealer. I I I cheniical appaiatu+ For several yc’arb the senior author ha% endeavored to produce distilled ~ ’ ater r free of metals by usiiig condensers made of quartz, as shown in A and R, Figure 1. A represents a straight, thin-walled quartz tube which wplaced the metal condenser tube, in a gas-heated n a t e r htill of the Stokes type. B shons a condenser tube made froin quaitz by the Thermal Syndicate oi S e w Tork according t o the authors’ design K h e n proper11 housed and installed tin. type of condenser will produce about 5.7 liters (1.5 gallon.) uf metal-free distilled water per houi . K i t h the recent inqtallation of a central heating plant I t this university >team became available for usi’ in the 1aboi:itories. X larger size quartz condenser of the fcrm shonn by (’ in Figure 1 nas designed by members of this department anti n-ab cuonstructed according to their plans by thl: Amersil Company of S e w ’Jiork. The boiler and housing for the condenw were constructed in the shops of the Departnifnt of Buildings and Grounds of this university. Figure 2 is a detailed drawing of the still. A and E are thP brass end plates of the condenser jacket which were cast and machined to the desired form and dimensions; B and F are packing joints a t the ends of the cordenser tube; C and I are 907-gram (32-ounce) sheet copper tinned on the inside, from which the boiler and the condenser jacket vere made; D and H arc brass rings attached t o the walls of the condenser jacket and boiler, respectively. Grooves were machined into the upper surfaces of these rings in which is placed rope packing to make tight joints when plate E and boiler lid G are bolted in place. J is a steam coil; K and L make up a device for maintaining a constant water level in the boiler; Af is the overflow from the condenser jacket; N is the steam pipe t o the condenser; and 0 iy the clamp for holding the porcelain cap in place. The heavy arrows indic:rt,p the path of steam from the boiler t o the condenser tube. The cock on the tube connected to the bottom of the boiler allows the slush and scale Ivhich accumulate in the boiler to be washed out when necessary and permits the boiler t o be kept comparatively clean from residue. Figure 3 shows the still mounted and ready fcir use. It has a capacity of about 11 liters (3 gallons) of metal-free distillate per hour a t the temperature of the tap lmter, but’ when the distillate
