Determination of Uranium(V1) in Presence of Anions Ammonium Thioglycolate as a Colorimetric Analytical Reagent W. H. DAVENPORT, JR., AND P. F. THOMASON Oak Ridge National Laboratory, Oak Ridge, Term# Hexavalent uranium forms a yellow-orange soluble complex with ammonium thioglycolate in a basic solution. The color intensity remains constant over a wide pH range and is independent of the exact reagent concentration. The complex is fairly stable and unaffected by many common anions. A number of cations interfere either by reacting with the reagent or by precipitating in ammoniacal solutions, but some of these interferences may be eliminated by the use of excess reagent or oomplexing agents. The method is best suited to the determination of uranium in the concentration range 0.100 to 1.600 mg. in 25 ml. Solutions containing uranium in this range obey Beer's law.
C
OLORIMETRIC methods for uranium are usually concerned with the estimation of the element in the presence of other cations. Among the reagents reported in the literature are ammonium thiocyanate ( I ) , which has been shonn to he fairly specific with respect to cations, potassium ferrocyanide (d), sodium salicylate (4), and others ( 4 ) . More recently a number of other colorimetric methods have been developed by workers on the Manhattan Project. A comprehensive review of these methods, edited by Rodden and Warf (a), has been prepared for the National Nuclear Energy Series. The method used to the greatest extent was based on the uranylperoxide complex formed in alkaline medium. The interference of anions such as oxalate, fluoride, tartrate, and citrate in most colorimetric determinations has been noted. When these anions are present the colorimetric procedure usually involves a preliminary separation such as the destruction or volatilization of the anions by fuming with acid. Because uranium forms strong complexes with these organic anions with a resultant effect on precipitation and separation procedures, a direct colorimetric uranium analysis in the presence of these anions 0.4
would be useful. 111the separatiori of the rare earths by ion exchange columns ( b ) ,the rare earths are eluted with citrate ion and precipitated with oxalate ion. The analysis of the eluate for uranium would require a method unaffected by citrate and oxalate ions or R procedure for the removal of these ions. Analysis of uranyl hexafluoride and uranyl sulfate solutions containing excess sulfate ion is subject to the same limitations. In an investigation of conditions affecting the determination of iron with aninioniuni nirrcaptoacetate (thioglycolate) Swank and bIellon (6) reported that uranyl ions formed a strong orange complex with the reagent. Anions had little effect on the iron complex. This suggested the possibility that the reagent might be used for uranium solutions containing anions that would interfere in other colorimetric methods. The authors have found that the uranium thioglycolate complex may be usrd as the basis for a fairly sensitive colorinietrir method for uranium, and t>hatit is unaffected by relatively large amounts of chloride, nitrate, sulfate, perchlorate, oxalate, tartrate, citrate, acetate, and fluoride ions. Unfortunately, many cations do interferr, but in many cases the interfering cations will not be present in the solutions described above. When the interfering cations, aluminum, thorium, titanium, and zirconium, are present in small amounts it may be feasible to complex them with tartaric acid. EXPERIMENTAL
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0.3
3 5
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2 0.2
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2 0.1
390
Figure 1.
400
500 WAVE LENGTH, MN.
600
Spectral Absorption of Uranium Thioglycolate and Iron Thioglycolate
4. 1.060 mg. U 8. 0.100 mg. Fe Complexes were developed by adding 2 ml. of a neutralized 10% -r,lution of thioglycolio acid, adjusting to about pH 10.1 with ammonium hydroxide (1 to l), and diluting t o 25 ml.
Solutions and Methods. AmioruIuM THIOGLYCOLATE. Teu milliliters of thioglycolic acid, obtained from the Eastman Kodak Company, were diluted with about 50 ml. of water, neutralized with 1 to 1 ammonium hydroxide, and made up to a final volume of 100 ml. This concentration of ammonium thioglycolate waa used throughout the investigation. STAXDARD URaiwuni SOLTTIOX.A solution of uranium, 10.6 mg. per ml., was prepared from c.P., uranyl nitrate hexahydrate and was standardized by evaporating ali uots to dryness and igniting to the oxide, U308,a t 850' C. An Jiquot of the standard solution was diluted to give a solution 0.106 mg. per ml. A m ~ o x i uHYDROXIDE. ~ A 1 to 1 solution was prepared from C.P. reagent. FOREIGX Iox SOLUT~OS. Reagent grade salts were taken for the preparation of all foreign ion solutions. The potassium salts of the nitrat,e, chlor'ate, chromate, and bichromate ions were used and the remaining anion solutions mere prepared from their ammonium salts. The cation solutions were prepared from chloride salts with the exception of lead nitrate. Redistilled water was used for all solutions. The complexes were developed in a 26-1111. volume by adding 2 ml. of ammonium thioglycolate and 2 ml. of 1 to 1 ammonium hydroxide in excess of neutrality. Solutions were read against a reagent blank in R Beckman spectrophot,omrtc~rusiiig 1-em. cells.
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ANALYTICAL CHEMISTRY
Uranium Thioglycolate Complex. The uranium thioglycolate complex must be developed in a basic medium, but critical adjustment of pH is not necessary. I n this investigation the concentration of ammonium hydroxide added gave a pH of about 10, but no change in color intensity resulted on varying the p H over the range 7.6 to 10.7. The concentration of ammonium thioglycolate is not critical within limits. From 1 to 4 ml. of the neutralized 10% reagent may be added without affecting the color. The color intensity increases slightly as excess reagent is added. The color is stable for a t least 30 minutes in direct light. A dei'rease of about 4% in color intensity results on standing 2 hours. The yelloworange complex does not show a sharp absorption maximum; fairly strong absorption takes place over the range 330 to 400 mp (Figure 1). -4plot of extinction against concentration for 0.100 to 1.600mg. of uranium gave a straight-line curve, indicating that the color obeys Beer's Law (Figure 2).
Table I.
Effect of Anions on Uranium Thioglycolate Complex
(0.530 mg. of uranium in 25 ml. of solution) Anion Change in Approximate Anion Concn., Color Intensity, Limiting Added Mg./25 M1. % Concn., Mg. Chloride 500 *O >500 Nitrate 500 *O >500 Sulfate 600 =+O > 500 4 Fluoride 300 -9 200 5 Tartrate 600 14 200R 6 Nitrite 600 Intense yellow ... 7 Acetate 600 +4 400 8 Citrate 200 Fades rapidly 20 9 Carbonate 10 - 30
