Colorimetric Determination of Moderate Concentrations of Uranium in Perchloric Acid Solutions LOUIS SILVERMAN
LAVADA MOUDY
and
Atomics International, North American Aviation, Inc.,
P. 0. Box 309, Downey,
amounts of uranium from fractional to milligram quantities, while the aqueous thiocyanate methods niny use 1 t o 3 mg. per ml. T h e procedure described in this piper uses onll- higher amounts of uranium, 10 to 70 mg. per nil. The minimum volume of sample which may be used with the usual Beckman spectrophotometric setup is 4 nil., showing that this perchloric acid solution method is applicable onlj- when higher concentrations of uranium perchlorate are to be determined. The technique is brief. The sample (freed of sulfate) is evaporated to fumes n-ith perchloric acid, cooled, and diluted n,ith n-ater-perchloric acid to a predetermined concentration of perchloi,ic acid. .Ifter elimination of chloline gas, the sample is cooled to 25’ C. and read on the Beckman spectrophotometer. In laboratory practice, the technique resembles Bastian’s ( 1 ) differential procedure. The reference solution is a uranium solution, the concentration of which is sliglitl). less than that of the unknon-n. -1second st,andard solution of uranium which is slightly higher than that of the unknonn is used to “bracket” the unknovn. h n attractive feature of the method is the fact that those elements (which cause the greatest difficulties in other methods and require preliminary separation8 from uranium) do not interfere in the perchloric acid method.
In perchloric acid solution (2 to 65% by volume), at selected wave lengths, the colorimetric absorption of uranium solutions is proportional to the concentration. Lranium maj be determined colorimetricallj as uranium perchlorate, under these conditions. ibove 65’3 perchloric acid (b) volume) the absorbance \alum deviate from Beer’s law, showing that a new species of uranium is present. These new species are postulated. Aluminum, cadmium, lead, iron, thorium, and zirconium perchlorate solutions do not absorb in the 413 to 420-mp range.
T
HE photomet,ric absorption of the soluble compounds of
uranium has been t,he subject of extensive research in aqueous and nonaqueous solutions. Kaplan, Hildebrandt, and Ader (8)postulated formation of [UO,(SO,),] - in nitrate solution. Further work by the same authors ( 7 ) indicates that (UO*)+’ and [VO,(SO,)] may be found in nonaqueous solutions. Scott and Dixon (11) based their method on the absorption of uranyl salts in solution: maximum absorption is obtained in uranium solutions (limited amounts of chloride, fluoride, or nitrate) if the solution contains 10% sulfuric acid or 4% phoephoric acid (iron interferes). Jones and Strong ( 5 , 6 ) and Muller (9) also reported ~ o r on k absorption spectra for uranium solutions (acetate, fluoride, nitrate, sulfate). Blake, Lowrie, and Brown (2) explained the importance of p H in their spectrophotometric investigat,ion of the fluoride complexes. Sutton ( 1 4 ) also considered the importance of acidity; he worked in the acid range from about p H 0.2 to the point of hydrolytic separation. LIost of these investigations have been confined t o determinations of small amounts of uranium, the best being t’hose involving the use of thiocyanate ( 3 , 4, I B ) , 8-quinolinol ( I S ) , and peroxide (10). The procedures for the latter two are concerned with
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SOLUTIOhS AND APPARLTUS
Uranium Perchlorate, O S M . Fume 71.52 grams of uranium trioxide with 55.3 ml. of perchloric acid for about 10 minutes. After cooling add 150 ml. of water, a n d boil the solution t o expel free chlorine; filter off the white precipitate (silica). Dilute the filtrate to 500 ml. in a volumetric flask with water containing 1 t o 100 perchloric acid, to prevent hydrolysis, and adjust to volume. One milliliter contains 118.8 mg. of uranium (gravimetric) ( 1 3 ) . Perchloric Acid, c . P . , specific gravity 1.67, normality approximately 11.6. Spectrophotometer. A Beckman Model DU spectrophotometer with 1-cm. silica cells is used for the colorimetric work. PROCEDURE
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To an unknown solution add 5 ml. of nitric acid and 15 ml. of perchloric acid. Evaporate down to heavy fumes of perchloric acid. Cool. Add water and heat on a steam bath for about 15 minutes t o expel free chlorine. Transfer t o a 25-ml. volumetric flask and dilute t o volume with distilled water. Adjust the solution t o 25‘ zk 0.3” C., and read the absorbance a t selected wave lengths (415 to 420 mp) with water as reference. Optional Procedure. Determine the “approximate” concentration of the unknown uranium solution. Prepare tv, o standard uranium solutions, one of
