1200
ANALYTICAL CHEMISTRY
The method is applicable only to preparations containing pure thiamine or thiamine in mixtures with pure synthetic vitamins. Certain heterocyclic amines and quaternary ammonium salts, which produce insoluble reineckates, interfere in the method and where such compounds are present, the fluorometric method must be used. The reineckate method has an advantage over the fluorometric method in the determination of thiamine in the presence of iron salts, as iron apparently does not interfere. Analysis of thiamine reineckate which had been recrystallized several times from acetone-water mixtures gave the following results calculated on an equimolar basis: Nitrogen, % Calcd. Found 23.33 23.38
-
Sulfur, 9% Calcd. Found 26.69 26.64
It would seem, from these results, that thiamine reineckate is a mole for mole combination formed by metathesis of the thiamine salt with ammonium reineckate. Precipitation of thiamine as the reineckate is apparently dependent upon the fact that thiamine is a quaternary ammonium salt. On alkaline decomposition of thiamine, conversion to thiochrome, or sulfite fission, the end products did not yield insoluble reineckates under the conditions of the procedure. It is significant that apparently none of these latter end products are
quaternary salts. Perhaps only the intact thiamin? molecule might be precipitated as the reineckate in the method. Precipitation of thiamine reineckate in the presence of relatively large quantities of nicotinic acid, nicotinamide, and pyridoxine gave reliable and reproducible results, and although there wa9 no evidence of coprecipitation of these compounds with thiamine, this possibility was nevertheless investigated. Thiamine reineckate was found to decompose, upon heating, in a range of 157" to 160' C. Thiamine reineckate which had been precipitated in the presence of ten times its weight of nicotinic acid, nicotinamide, or pyridoxine decomposed a t 157" to 160" C. Thiamine reineckate and thiamine of mixed melting point precipitated in the presence of the B-complex vitamins, decomposed a t 157" to 160" C., but thiamine reineckate with reineckates of the other three vitamins under consideration had a lo&er decomposition temperature. LITERATURE CITED
(1) Bandelin, F. J., J . Bm. Phnrm. Assoc., Sci. E d . , 37, 10 (1948). (2) Ibid., 39,493 (1950). (3) Bandelin, F. J., Slifer, E. D., and Pankratz, R. E.. Ihid.. 39, 277 (1950). (4) Pankratz, R. E., and Bandelin, F. J., Ibid., 39, 238 (1950). ( 5 ) Pharmacopoeia of the United States, XIV, Mack Printing :CTo., Easton, Pa., 1950. RECEIVED for review lfitrch 17, 1952. Accepted April 23, 1953.
Colorimetric Determination of Uranium with Dibenzoylmethane JOHN H. YOE, FHITZ WILL, 1111, AND ROBERT A . BLACK2 Pratt Trace Analysis Laboratory, Department of Chemistry, University of Virginia, Charlottesville, Vu.
The purpose of this investigation was to make a critical study of the reaction of dibenzoylmethane w-ith uranium(V1) and to develop a sensitive colorimetric method for its determination. A yellow complex forms instantaneously and has a maximum absorbance at 395 mp. The optimum pH is 6.5 to 8.5 and the mole ratio of reagent to UO:' is 2 to 1. The color reaction conforms to Beer's law and has a practical sensitivity of 0.05 p.p.m. of uranium when absorbance measurements are made in 1.00-cm. cells. The tolerance of the colored complex to many diverse ions has been established. A separation of uranium €rom interfering ions is accomplished by ether extraction of uranyl nitrate. The procedure shouldbe useful where there is need for the determination of small amounts of uranium, especially in trace quantities.
M
ANYorganic compounds react with uranium, usually as the uranyl ion (UOz++),to give colored substances. Compilations of these reagents are given by Ware (IS) and Rodden (9, pp. 108-9). Rodden's table includes some of the more common methods for the colorimetric determination of uranium. According to Sandell (IO), the colorimetric methods for uranium are insensitive and not well suited for its determination in trace amounts. A survey of the literature revealed that many of the uranium reagents which give colored complexes are aliphatic a-hydroxy and keto acids, as well as aromatic hydroxycarboxylic acids and especially higher phenols (8). Certain sulfur-containing compounds also give characteristic colors with uranium. Most of the reagents that form uranyl complexes, however, are not specific; the chief interference is usually iron. Among the 1 Present
address, Aluminum Co. of America, New Kensington, Pa.
* Present address, Joseph E. Seagram & Sons, Inc., Louisville, Ky.
uranium reagents for which Sandell (IO) gives procedures are thiocyanate, hydrogen peroxide, ferrocyanide, and diethyldithiocarbamate. A sensitive reagent for the colorimetric determination of uranium is needed. During a systematic study of the reactivity of a large number of organic compounds with inorganic ions, it was observed that the uranyl ion gives a bright yellow stable color with dibenzoylmethane (lj3-diphenyl-1,3-propanedione,according to the Geneva system of nomenclature), C~HSCOCHZCOCeHS. Of the many ions tested, only vanadium, iron, copper, and molybdenum showed evidence of slight color or precipitate formation. Feigl and Backer (3) reported dibenzoylmethane for the detection of thallium, but made no mention of its reaction with uranium. This paper deals with the study and development of dibenzoylmethane as a reagent for the colorimetric determination of uranium( VI ).
V O L U M E 25, NO. 8, A U G U S T 1 9 5 3
1201 I
APPARATUS AND REAGENTS
Instruments. Absorbance measurements were made with a Beckman spectrophotometer, Model DU, using 1.00-cm. cells. -411 pH measurements were made with a Photovolt line-operated glass electrode pH meter, Model 100. B s the color is formed in a 5770 ethyl alcohol solution, the pH meter obviously registers only an apparent pH. Visual color comparisons were made in 50-ml. Nessler cylinders (tall-form). Standard Uranium Solution. A stock solution of 1000 p.p.m. of uranium (as UOz++) was prepared by dissolving either 1.7818 grams of uranyl acetate, V02(CzH302)z.2Hz0, or 2.1094 grams of uranyl nitrate, UOz( N03)~.6H20, in distilled water, diluting to 1 liter, and mixing. The uranium stock solution was standardized by reducing the uranium(V1) to uranium(1V) in a Jones reductor and then titrating with a standard potassium dichromate solution (7). Solutions of greater dilution were made from the stock solution as required. Reagent Solution. A solution containing 1 gram of dibenzoylmethane in 100 ml. of 957, ethyl alcohol was used. The compound was obtained from the Eastman Kodak Co. I t s alcoholic solution is colorless and stable. Solutions of Diverse Ions. Reagent-grade salts, usually the chloride or nitrate, were employed in the preparation of solutions of the inorganic ions. Except in a few cases, these solutions contained 1 mg. of the element per milliliter of solution.
uranium. Figure 1 shows the absorption peak for uranium dibenzoylmethane t o be a t 395 mp, as measured against a reagent blank solution, also a t p H 7. EFFECT OF pH AND ALCOHOL CONCENTRATION
pH. Solutions used for the study of the effect of p H on the color reaction were prepared as directed above, except that the final pH values ranged from 3.5 to 8.5. The pH should be between 5.0 and 5.5 before addition of the reagent, if the colored solution is to give its highest absorbance for a particular pH. The colored complex develops its full color intensity over a p H range of 6.5 to 8.5, the absorbance being constant over this range. K i t h increasing p H values above 8.5, the color intensity decreases. All absorbance measurements were made a t 395 mM.
ABSORPTION CURVES
Dibenzoylmethane Solution. A “blank” reagent solution was prepared by adding 28 ml. of 95% ethyl alcohol to 10 ml. of water in a 100-ml. beaker. The volume was brought to about 45 ml. and the pH adjusted to between 5.0 and 5.5. One milliliter of the 1% reagent solution was added, the pH adjusted to 7, and the solution diluted to 50 ml. in a volumetric flask and mixed. Because of the 577, ethyl alcohol solution, pH paper was found to be unsatisfactory.
3 9 5 mu I
2
3
4
5
6
7
8
9
1
-I 0
MOLES REAGENT PER MOLE UOz
Figure 2
\
D i benzoy I me hane
1 1
Ethyl Alcohol Concentration. The reagent is only slightly soluble in water, but is soluble in ethyl alcohol to the extent of 4.43 grams per 100 ml. at 20” C. Hence, the uranium and reagent solutions are mixed in an ethyl alcohol medium. Equal color intensities were obtained in 47, 57, and 95y0 (by volume) ethyl alcohol. However, the colored complex is more stable in the higher alcohol concentration; consequently a concentration of about 57% (by volume) was selected for most experiments.
‘ . . I Blank
380
400
420
440
460
480
WAVE LENGTH, Mr
Figure 1
The pH readings are thus merely reference numbers and not an arcurate measure of the hydrogen ion concentration, because measurements are made in alcoholic solutions. Figure 1 shows that the reagent solution begins to absorb a t about 400 mp, with appreciable absorption below 390 mp. Hence, the amount of reagent used must lie accurately measured. The absorbance of the reagent solution was measured over the range 320 to 430 mp, the spectrophotometer being set on distilled water as a blank. Uranium(V1) Dibenzoylmethane. The yelloff complex of uranium(V1) dibenzoylmethane was formed by adding 28 ml. of 95% ethyl alcohol to 5 ml. of a 50 p.p.m. uranium (as UOz++) solution in a 100-ml. beaker. The volume was brought to about 45 ml. with water and the pH adjusted to between 5.0 and 5.5. One milliliter of a 1% reagent solution was pipetted into the solution, the p H adjusted to 7 , and the solution diluted t o 50 m]. in a volumetric flask. The resulting solution contained 5 p.p.m. of
REAGEIVT CONCENTRATIOV AWD MOLE RATIO
Spectrophotometric measurements were made according to the method of Yoe and Jones (14) to determine the effect of reagent concentration on the color intensity, in an effort to ascertain the mole ratio of reagent to uranium in the colored complex. A series of solutions was prepared in which the mole ratio of reagent to the uranyl ion varied from 0.5: 1 to 10: 1. Best results were obtained when the components were mixed a t a pH of 5.0 to 5.5, with a final adjustment to 7 after mixing. The absorbance was measured for each solution a t 395 mp. The fact that no sharp peak occurred (Figure 2) indicates that the complex is appreciably dissociated in solution. Full color development of the complex is ensured, however, a t a 10 to 1 ratio of reagent to uranyl ion. As a mole ratio value could not be obtained from the preceding data, the method of continuous variations, proposed by Job (6) and extended by Vosburgh and Cooper ( l a ) , was tried. The uranyl and reagent solutions were mixed in proportions to give the x to (1 - x) ratio of components in 50-ml. total volume, where z is equivalent to milliliters of 3.7 x 1 0 - 6 M reagent and ( 1 - 2) is equivalent to milliliters of 3.7 X 10-6 M uranyl ion. The final concentrations of uranyl ion and reagent were equivalent to adding (1 - 2) milliliters of 3.7 X 10-5 M uranyl ion to x milliliters of 3.7 X 10-6 M reagent. The correct amounts of uranyl ion and reagent were mixed in 15 ml. of 95% ethyl alcohol. The solution after mixing waa slightly acid. IF-ater and ethyl alcohol were then added in
ANALYTICAL CHEMISTRY
1202 amounts required to give a 50-ml. solution of 47% (by volume) ethyl alcohol. The p H of the resulting solution was adjusted to 7 . The absorbance measurements were taken within 10 minutes of the time of mixing. I n Figure 3, the absorbance values a t 395 and 420 mp, respectively, are plotted against the volume of reagent solution (expressed as z milliliters added to (1 - z) milliliters of uranyl solution). Job ( 6 ) developed an equation relating a Y function, which is defined as the observed absorbance minus the calculated absorbance assuming no reactions between the colored complex components.
Y
= Eobsd.
- d[ei z v l ( 1
-
Z)
-/-
I
0.151
e2 ; l f X ]
For a given wave length, el and e2 represent the molar extinction coefficients of uranyl ion and the reagent, respectively; L l f the molarity of the solutions; d the thickness of the transmittance cell: (1 - z) and z the volumes in liters of the solutions miued. .Uthough the reagent begins to absorb a t 395 mp, a measurement of the 3.7 X 10-5 &" solution showed that the absorption could be neglected a t this low concentration. Hence, e2 is considered to be zero. As the uranyl ions showed no appreciable absorption a t 395 mp, the term el M (1-2) was also neglected. Thus, the Y term is equal to the observed absorbance only. Yosburgh and Cooper (12) later showed that if the Y function is plotted against 2, the point where Y passes through a maximum or minimum represents a value n in the formula U02R,, where and R is the reagent 1- x Thus, from the graph in Figure 3, it is seen that the peak occurs 0 675 a t 2: = 0.675. Therefore, n = - 2.07 and the ratio 1 - 0.675 of reagent to uranyl ion is 2 to 1-Le., U02R2. It is known that dibenxoylmethane exists in the enolic form, C~H&OCH=COHC~HS(2); hence, the following structure of the colored compleu is suggested:
a=-
H To ensure full color intensity, the reagent concentration used for analyses was 0.02%-i.e., 1 ml. of 1% reagent solution in 50 ml. of solution. Rate of Reaction and Stability of Complex. The color formation of uranyl dibenzoylmethane was found to be instantaneous, there being no further increase in intensity after mixing. Upon standing 3 weeks, the colored complex showed no decrease in its original intensity. Beer's Law. The uranyl dibenzoylmethane complex obeys Beer's law over the concentration range of 1 to 10 p.p.m. of uranium, with a practical range of 2.5 to 9 p.p.m., where the absorhancw occur between 0.2 and 0.7.
Oo5I
I
I
I
, I
I
I
UO
+ +
I
11
0.7 08 09 - X ) ML. OF
x ML. OF REAGENT SOLUTION ADDED TO (1
SOLUTION
Figure 3
The most sensitive colorimetric method for uranium reported by Sandell (10) is the thiocyanate. For an absorbance reading of 0.001 in a 1.00-em. cell, Sandell reports a sensitivity of 0.07 p.p.m. of uranium; this was confirmed by the authors. Recently, Crouthamel and Johnson ( I ) used an acetone medium in the thiocyanate method with a slight increase in sensitivity-Le., 0.05 p.p.m. of uranium. Adopting Sandell's way of reporting sensitivity a t an absorbance reading of 0.001, the uranium dibenzoylmethane reaction has a sensitivity of 0.013 p.p.m. of uranium. Hence, the dibenzoylmethane reagent has a sensitivity that is five to six times that of the thiocyanate. EFFECT OF DIVERSE IONS
Tests were made on spot plates betn-een dibenzoylmethane and the following 78 inorganic ions, in neutral, acid, and ammoniacal solutions wherever possible: -4gL, AI+++, AuCll-, As+++, .4-0 a 1---,Boa---, Ba++,Be++,Bi+++,Br- COI--,Ca+', CdT+, C e + + Ce++++. ~ CI-, Co++ C r - + - Cs+ C u + +t Dy + + + , E r + + + E u + + + , F-, Fe++, Fe'+-, G a r + + G d + + + G e + + + - H f + + + + Hg+. Hg+T, I - I n + + + ,IrC16--, K + , La+'+. Li+, Mg++.l I n + + , Mo04--, S a + ,Xb+""'. Y d A + ,Si'+.S O J - , Os++--HPO,--,
Table I. Ion
Tolerances to Diverse Ions Added as
Limiting Concn., P.P.I\I
