Determination of Phosphate in Perchloric and Sulfuric Acid Solutions of Uranium Phosphates Ion Exchange Separation and Amperometric Determination E. G. COGBILL1, J. C. WHITE, and C. D. SUSAN0 Analytical Chemistry Division, O a k Ridge N a t i o n a l Laboratory, O a k Ridge, Tenn.
Standard Phosphate Solution, 1 mg. per ml. Dissolve 1.433 grams of potassium dihydrogen phosphate-dried a t 120" C.in water, and dilute to 1 liter. Potassium Chloride, 1~11. Dissolve 75 grams of potassium chloride in 1 liter of water. Bromocresol Green Indicator, 0.1 w./v. 70. Dissolve 50 mg. of the solid indicator. Eastman Organic Chemicals Co., S o . 1782, in 50 ml. of ethyi alcohol. Sodium Hydroxide, 231. Dissolve 80 grams of solid sodium hydroxide in 1 liter of water. Hydrochloric Acid, 0.1N. Dilute 1 ml. of concentrated hydrochloric acid to 120 ml. with n-ater. iicetic -4cid, 0.1X. Dilute 1 ml. of glacial acetic acid to 170 ml. with water. ilbsolute Alcohol. Reagent grade ethyl alcohol, 95 volume %, may be substituted. Apparatus. Iox EXCHANGE CoLcim. Leach overnight the cont,ents of a 1-pound bottle of Dowex 50 (Salcite HCR) resin, 50- to 100-mesh, with about 2 liters of 2.M sodium hydroxide. Wash Fith water by decantation and leach for 1 hour with three successive, l-lit,er portions of 6.M hydrochloric acid. Wash several times, by decantation, with water to remove fines. Shorten the outlet tubing of a Jones reductor tube of the usual dimensions (body of tube approximately 21 inches long and '/* inch in diameter) by cutting it off about 1.5 inches below the stopcock. Fit the outlet tube with a short length of tubing, carrying a Hoffman pinch clamp and a short glass delivery tip. Half-fill the tube with water, then expel the air bubbles from the outlet tube. Insert a plug of glass wool into the body of the tube and pack it tightly a t the bottom, working it with a glass rod to expel entrapped air. Mix the resin with water and with the aid of a wash bottle, wash it into the column. Continue to add resin until a bed 24cm. deep is obtained. Pass 200 nil. of 5 to 6.11 hydrochloric acid through the column, followed by several hundred milliliters of water. The column is then ready for use. I n operation, the flow rate of liquid through the column can be regulated by the pinch clamp with the Bt'opcock wide open. The maximum flow rate is determined by measuring the column of water delivered per minute when the liquid level is at the top of the tube. The preadjusted setting of the pinch clamp may then he left undisturbed while a sample is being put' through the column and the flow can be stopped or started a t will with the stopcock without altering the flow rate. This permits shutting off the flow just when the liquid level has reached the top of the resin bed, so that each portion of wash liquid can be drained completely before the next, portion is added. This aids the effective elution of the column m-ith a minimum of wash liquid, and is a convenience when several columns are operated Pimultaneously. After separation of uranium, the column should be eluted with about 200 ml. of 5 to 6M hydrochloric acid, a t a flow rate of approximately 10 ml. per minute, and finally washed with water. POLAROGRAPH. A manually operated Fisher Elecdropode was used. TITRATIOK ASSEMBLY.The titration cell consisted of a cylindrical glass vessel, 4.8 em. in diameter and 10 em. high, attachable to the buret, assembly by a 35/55 spherical ball joint. The usable volume of t,he cell was about 100 ml. The cell carried a side arm, 8 mm. in diameter, through which the gas inlet tube was inserted, and a wide side arm near its bottom in which was mounted a 20-mm. fritted-glass disk. This arm was filled with saturated potassium chloride-agar gel up to the fritted disk, the gel extending through a flexible bridge-tube of Tggon tubing into a saturated calomel electrode. SOURCE Helium (nitrogen or argon may be used) was gen by passing it through a series of t'hree gass half-filled with Oxorbent solution (Burrell Corp., Pittsburgh, Pa.), 2-11 sodium hydroxide, and an aqueous solution containing 20 volume % of alcohol, respectively.
The amperometric titration of phosphate with uranyl acetate has been applied to the determination of phosphate in sulfuric and perchloric acid solutions of uranium phosphates. Uranium is removed by cation exchange separation with Dowex 50 resin. The coefficient of variation was 0 . 6 q ~for solutions containing 65 to 150 mg. of phosphate and 180 mg. of uranium (as UO*++)and having acid concentrations as high as 8 M perchloric acid and 11M sulfuric acid.
T
HIS in-vestigation was undertaken to study the applicability of an amperometric method for the determination of phosphate in solutions of uranium phosphates which are strongly acidic with respect to sulfuric and perchloric acids. The range in concentration of phosphate considered in this report is of the order of 0.1 to 0.4M. Kolthoff and Cohn ( 2 ) have shown that phosphate in low concentrations (0.01 to 0.0002it1) can be determined by amperometric titration with uranyl acetate. The method depends upon the precipitation of a slightly soluble alkali uranyl phosphate such as: UOz++ K + PO,--- +KU02POa (1)
+
+
The alkali uranyl phosphates are very slightly soluble in weakly acidic solutions, and their solubility may be further decreased by the addition of alcohol. Kolthoff and Cohn ( 2 ) carried out the titration in acidic solutions (pH 3.5) having an ethyl alcohol concentration of 20%, and used either potassium chloride or nitrate as the supporting electrolyte. The end point was found by measuring the diffusion current of uranyl ion a t an applied voltage of about -0.7 volt (us. S.C.E.). They were able to determine phosphate in concentrations as low as 0.0002N in solutions of pure potassium dihydrogen phosphate with an accuracy of 1% or better This report describes the application of a combination of the ion exchange separation of phosphate from interfering substances and the amperometric titration of phosphate with uranyl acetate, to the determination of phosphate in sulfuric and perchloric acid solutions of uranium phosphates. REAGENTS AIVD APPARATUS
Reagents. Uranyl Acetate, Stock Solution, 0.105M. Dissolve 45 grams of uranyl acetate dihydrate and 8 ml. of glacial acetic acid in about 800 ml. of water. The salt dissolves slowly and may require several hours' standing, iyith occasional mixing. Dilute to 1 liter with water. Uranyl Acetate, Titrant Solution, 0.035U. Dilute 1 volume of the stock 0.105M solution with 2 volumes of water. Each milliliter is equivalent to 3.3 mg. of phosphate. Standardize the solution by the amperometric titration of aliquots of a solution of potassium dihydrogen phosphate as follows: Take an aliquot containing about 10 mg. of phosphate and transfer it to the titration cell. Dilute to 50 ml. with water, add 5 drops of bromocresol green solution, and adjust the pH with dilute sodium hydroxide and 0 . l M hydrochloric acid until the solution has a shade of yellow green. -4dd 0.5 ml. of 0.1S acetic acid, 7 ml. of 1M potassium chloride, and 17 ml. of ethyl alcohol, then titrate. Diffusion current readings are conveniently taken at 2.0, 2.5, 3.25, 3.50, 3.75, and 4.00 ml. of standard solution. Present address, Chemistry Charlottesville, Va. 1
Department,
University
of
PROCEDURE
Procedure Adopted. Transfer a 5-ml. aliquot which contains a t least 25 mg. of phosphate to a 50-ml. Erlenmeyer flask and
Virginia,
455
ANALYTICAL CHEMISTRY
456
KO attempt was made in this work to study t,he polarography of uranium, except in Concentration, Molar Ei/s, Volt Region of PerFirst Second Iconrrsnt, so far as it was necessary to Acetate Chloride chlorate Sulfate Sitrate wave nave Volt select the proper applied volt0.002 0.1 ... ... ... -0.25 -1.27 -0.33 to -0.80 0.003 ... 0.27 ... ... -0.26 -1.28 -0.35 to -0.95 age for the amperometric ti0.003 ... 0.27 ... -0.53 -1.3 - 0 . 8 5 to -1.15 0,003 0:067 0.067 ... 0.10 -0.28 -1.2 -0.40 to - 1 . 0 0 tration under certain specified experimental conditions. Only Table 11. Precision of ,imperometric Titration of Phosphate with Uranyl Acetate in the diffusion current plateau Various Supporting Electrolytes corresponding to the first stage Composition KH~POI Titrant Ratio Deviation from of the reduction of uranyl ion of Supporting Taken, CO~(CzH30z)2. Phosphate, UOz(CzH302)s Per Over-all Electrolyte A1 1 A1 1 h lg RI1 of K H ~ P O I Average Ratio is of interest; the voltage -0.002 0.725 4.00 2.90 9.60 0 . 1 s Cl selected must lie within the 0,730 + O . 003 5.00 3.05 12.00 -0.004 0.723 6.00 4.33 14.40 region of constant diffusion Av. 0 . 7 % current between the first and 0 . 0 7 M Clop 4.00 2.91 9.60 0.728 +0.001 second naves. As the half0 . 1 0 M NOa 2.90 0,725 -0.002 0 . 0 7 M C1 5.00 3.63 12.00 0.726 -0.001 wave potentials of these waves 3.62 0.724 -0.003 3.67 0.734 t0.007 may change with varying con3.62 0,724 -0,003 centration and kind of indifAv. 0.727 f e r e n t e l e c t r o l y t e , it was 0.07M SO1 4.00 2.92 9.60 0.730 f0.003 0.1024 KO3 2.91 0.728 f0.001 n e c e s s a r y to determine the 0 . 0 7 M C1 5.00 3.62 12.00 0.724 -0.003 3.62 0.724 -0,003 shape of the polarograms of 3.63 0.726 -0.001 uranyl ion in media similar to 3.61 0.722 -0.005 Av. 0.726 that in which the titration 0.727 Over-all average volume ratio of standard solutions was to be carried out For Standard deviation 0.0033 this purpose, current-voltage Coefficient of variation 0.5% c u r v e s were r e c o r d e d f o r uranyl acetate in solutions h a v i n e c o n c e n t r a t i o n s of neutralize with concentrated ammonium hydroxide to a methyl chloride, sulfate, perchlorate, and nitrate approximating the red end point. The contents of the flask should be cooled in an highest concentrations of these ions to be expected in an actual ice bath and swirled constantly to prevent spattering. Add 5 determination of phosphate. Similar polarograms were made ml. of concentrated nitric acid, and boil gently for 1 or 2 minutes with solutions in which the supporting electrolyte was potassium to oxidize the uranium to the hexavalent state. Dilute the solution with about 300 ml. of water. Pass the solution through a chloride, sodium sulfate, or sodium perchlorate. An ORNL 24 X 2 cm. column of Dowex 50 resin a t a maximum flow rate Model Q-1160 recording polarograph (1) was used in these exof 5 ml. per minute. Catch the effluent in a 500-ml. volumetric periments Pertinent data from the polarograms obtained are flask. Rinse the beaker with water, and pour into the column. shown in Table I. With the exception of the first solution in When the level of liquid in the column reaches the top of the resin bed, shut off the flow and wash the column with 25-m1. Table I (0 I N potassium chloride, pH 4) which contained no portions until 500 ml. have been collected. Transfer a 50-ml. alcohol, the ethyl alcohol concentration of all solutions was 20 aliquot of the effluent to the titratiqn cel,l, add 5 drops of bromovolume % and the pH was approximately 3 5 . cresol green solution, then neutralize with 2 M sodium hydroxide. The data show that, of the anions considered, sulfate causes the Add O.1N hydrochloric acid dropwise until the solution is yellowish green. Bdd 0.5 ml. of 0 . 1 5 acetic acid. Stirring may be greatest displacement of the first wave of the uran3l reduction effected by passing a slow stream of oxygen-free inert gas, nitrofrom its position in a supporting electrolyte which is predomigen, helium, or argon, through the solution. Add 7 ml. of 1111 nantly potassium chloride. The region of constant diffusion curpotassium chloride and 17 nil. of ethyl alcohol. Allow the solurent extends from 0.45 to - l 00 volt and any applied voltage betion to cool to room temperature. Attach the titration cell to the buret assembly, then pass inert gas through the solution for tween these values should be suitable for the amperometric titra10 minutes. Set the applied voltage a t -0.7 volt ( u s . S.C.E.). tion The value of -0 7 volt n-as therefore adopted, and used in Titrate with the standard solution of uranyl acetate by the usual this work. amperometric technique. Precision of Titration. The precision of the titration was Alternative Procedure. A slightly different method from that described above was tested in which the solutions were diluted determined by titrating aliquots of a standard solution of potasto a known volume of 500.0 ml. before passage through the resin sium dihydrogen phosphate both in a supporting electrolyte of column. The first 400 ml. of the effluent were then rejected, and 0.1N potassium chloride and in supporting electrolytes containthe 50-ml. aliquot to be titrated was taken from the.last 100-ml. ing perchlorate, sulfate, and nitrate ions. The results of these portion of effluent without further dilution. This technique permits maximum dilution of the influent and obviates the timedeterminations are given in Table 11. consuming elution of the column which is necessary if the soluThe average volume ratios (0.726, 0 727, 0.726) of the standtions are to be made up to exact volume after the ion exchange ard solutions of uranyl acetate and potassium dihydrogen phosseparation. It does not, however, consume appreciably less phate which were found in the experiments involving three diftime than the usual procedure. A dilution of 300 ml. of the influent appears to be adequate for efficient action of the exchange ferent supporting electrolytes are for practical purposes identical, resin. thus indicating no interference, in the concentration ranges studied, with the titration of perchlorate, sulfate, or nitrate ions. RESULTS AND DISCUSSION The experiment reveals that the coefficient of variation is of the order of 0.5%. Polarographic Reduction of Uranyl Ion. Current-voltage Separation of Uranium by Ion Exchange. Samuelson (4) curves for the reduction of hexavalent uranium a t the dropping showed that phosphate can be separated quantitatively from mercury electrode are markedly affected by the composition of cations by means of a cation exchange resin. This technique the supporting electrolyte ( 3 ) . Especial interference is exerted was applied to the separation of phosphate from uranium in by the nitrate ion (v-hich is catalytically reduced in the presence of uranyl ions) and by anions, such as acetate and sulfate, with perchloric and sulfuric acid solutions of uranium phosphates. which the uranyl ion forms complex anions. The optimum conditions that were selected for this separation Table I.
Polarographic Reduction of Uranyl Ions in Presence of Certain Anions
-
457
V O L U M E 27, NO. 3, M A R C H 1 9 5 5 Table 111. Determination of Phosphate by -4mperometric Titration with Uranyl Acetate after Cation Exchange Separation [Dowex 50, 2 5 X 2 1 cm column. Composition of original solution, uranium 180 nig ( U O z + “ ) , HClOI, 0 6 6 3 1 , H2901, 10 9.111 I’olume of Influent Flow R a t e , Phosphate, h l g . Difference, Solution, LI1. per Taken, Found, B - A B bll. Rlin. A 0 . (i 99.7 100 3.5 99,l 100.6 -0.2 100.8 3.1 300
4.7 4.4 4.6
64.8 68,5 100.0
08.8
0 .1 0.7 1 1 -0.0
09 2 101.7 99.5
4.3
100.1
300
3.7 3.5
148.1 148.1
147.8 148.0
-0.3 -0.1
500
4
100.1
99.9
-0.2
were: size of resin column, 24 X 2.1 em., and flow rate, 3 to 5 ml. per minute. The longer resin column effectively eliminated breakthrough of uranium (in strong sulfuric acid solution) as the anionic uranyl sulfate complex. Phosphate in solutions as concentrated as approximately 11M with respect to sulfuric acid was successfully determined. Perchloric acid presented no difficulty in the separation. The longer columns were also satisfactory with regard to holdup of phosphate in the column. The volume of influent was not critical. Precision of Method. Synthetic samples of uranium phosphate in volumes of 5 ml. were prepared which contained varying amounts of sulfuric and perchloric acids. Each contained 1 nil. of 0 67111 uranyl sulfate (180 mg. of UOz++) and 65 to 150 mg of phoqphate. The latter was added as a solution of potassium dihydrogen phosphate containing 92.56 mg. of phosphate per gram of solution. The portion of the standard solution of phosphate used to prepare each sample was measured by weighing it in a stoppered flask. Typical data, which were obtained under the optimum conditions, are shown in Table 111.
Khen the removal of uranium by the ion exchange resin appeared to be complete, in no case was the amount of phosphate found to differ from that taken by more than 1.1%. The average difference of 14 samples from the known values (68 to 100 mg ) was 0.5% and the coefficient of variation was 0.6% on a 95% confidence level. Seither precision nor accuracy appear to have been affected adversely with changes in flow rates ranging from 3 to 5 ml. per minute. Concentration Range of Applicability. The phosphate content varied from 65 to 150 mg.; hence, the quantities of phosphate in the aliquots finally taken for titration lay between 6.5 and 15 mg. S o difficulty should be expected in applying the method to more concentrated samples, because the phosphate content of the solution uhich is to be titrated can be brought within this range by taking a smaller aliquot either of the original sample or of the effluent from the ion exchange step. At the lower extreme, the limiting factor should be the titration itself. Kolthoff and Cohn ( 2 ) ,titrating with 0.01M uranyl acetate, were able to determine phosphate in solutions as dilute as 0.0002M with an accuracy of 1%. This dilution corresponds to 1 mg. of phosphate in a 50-ml. aliquot. At two to four times this dilution, results (using 0.00534 titrant) w r e high by 2 to 11%. Under the experimental conditions applied here, and using 0.035M uranyl acetate as the titrant, 3 mg. of phosphate can be determined within 1 %; for smaller quantitiee, the results are high. LITERATURE CITED (1) Kelley, lf. T., and Miller, H. H., A K ~ L CHEW, . 24, 1895 (1952). (2) Kolthoff, I. M., and Cohn, G., IKD.ENG.CHEX.,ANAL.ED.,14, 412 (1942). (3) C. J., “Analytical Chemistry of the Manhattan Project, p. 602, McGraw-Hi11 Book Co., New York, 1950. (4) Samuelson, O., “Ion Exchangers in Analytical Chemistry,” p. 146, Wiley, Kern York, 1953.
Redd;?,
RECEIVEDfor review M a y 8, 1964 Bccepted November 12, 1954 The Oak Ridge h’ational Laboratory is operated by the Carbide & Carbon Chemicals C o , a Division of Union Carbide & Carbon C o r p , for the -4tOmiC Energy Commission Work carried out under Contract S o K-7406-eng-2G
Weighing Pipet Method for Preparing Infrared Gas Standards for Ether and Alcohol FRANK PRISTERA and ALEXANDER CASTELLI Picatinny Arsenal, Dover,
N. J.
A method is described for the preparation of infrared gas standards of ether and alcohol using a weighing pipet (micro). The method was found satisfactory when applied to sy-nthetics containing known amounts of ether and alcohol. It should be applicable also to any reasonably vola tile substances such as the numerous organic solvents which have widespread commercial application.
I
K T H E ilianufacture of solid propellants, ether and alcohol
are widely used as solvents for the nitrocellulose. The finished propellant is then usually placed in a solvent recovery house ivhere the concentration of ether and alcohol may reach appreciable proportions. It was in connection with the infrared analysis of the air for ether and alcohol in such a solvent house that the weighing pipet method for preparing infrared gas standards was developed. The infrared analysis of a gas ( I ) , in essence, consists of obtainingthe infrared absorbance of the sample and relating such absorb-
ance (usually a t some selected absorption band) to concentration from a previously established relationship (working curve) of absorbance versus concentration. The establishment of the working curve necessitates the preparation of standards containing known and varying amounts of the gas. Such gas standards are usually prepared by the introduction of controlled amounts of gas into an infrared gas cell, making accurate measurements of their pressure, and relating such pressure measurements to concentration. This method is lengthy, very difficult to control accurately, and in the case of ether and alcohol it would also be very difficult to apply as these substances are not normally in the gaseous state. For these reasons a more suitable method for preparing standards was considered desirable. In the field of organic quantitative microanalysis ( 2 ) , volatile liquids are handled in capillaries called weighing pipets. It therefore appeared reasonable to anticipate that a suitable technique could be developed to introduce known amounts of ether and alEohol in an infrared gas cell using similar weighing pipets. This paper describes the developed technique and presents some
