Determination of Microgram Quantities of Thorium in Water Extension of Colorimetric Method ALBERT E. TAYLOR
AND
RICHARD T. DILLON, Idaho State College, Pocatello, Idaho
ET’ERALmethods are available for determining thorium colorimetrically ( 1 , d ) . Themost sensitive of the methods surveyedis that originally reported as a visual method by Kuznetsov ( 2 ) and later developed into a spectrophotometric procedure by Thomason, Perry, and Byerly (4). This method utilizes the disodium salt of l-(o-arsonophenylazo)-2-naphthol-3,6-disulfonicacid to form a colored complex with thorium and is reported to be sensitive to 1 microgram of thorium per milliliter by visual comparison or 5 micrograms of thorium when used with the Beckman DU spectrophotometer. In this paper a method is described for concentrating the thorium present in water, so that i t can be determined spectrophotometrically by a modification of the procedure given by Thomason et al. The thorium was concentrated by precipitation of thorium oxalate with calcium, as a carrier, under controlled conditions of pH; conversion of the oxalate to carbonate by heating; dissolving of the resulting carbonates in hydrochloric acid; and determination of the thorium concentration in the resulting solution according to the method previously mentioned. The method is sensitive to less than 1 X 10-9 gram of thorium in 1 gram of water TTith an average error of approximately 1 X 10-10 gram of thorium per gram of water.
Table I. CaCOs
Concn., hIg.
Calcium Effect Absorbancy
I
I1
I11
thorium. These solutions were compared against a standard as above. The results presented below show that Beer’s law holds over the range covered. Thorium, y Absorbancy
0
0.017
5 0.045
10 0.073
15 0.101
20 0,130
It is evident that calcium added to the unknown solutions as the carbonate has a small linear effect upon the absorbancy of the solution with respect to the concentration of the calcium and no effect upon the linearity of the relation between the thorium concentration and the absorbancy of the solution. To determine the precision of this general method, a series of samples was prepared containing 150 mg. of calcium carbonate plus varying amounts of thorium (as the nitrate). The added calcium present in these samples was precipitated as the oxalate, converted to the carbonate by heating, weighed, and dissolved in hydrochloric acid. The resulting solutions were prepared by adjusting the p H and adding the dye solution, after which their absorbancies were compared with a standard containing an amount of calcium carbonate equivalent to the average found to be recovered. The amount of thorium present was calculated by noting the increase of absorbancy due to addition of a known amount of thorium to two of the four samples. The results are tabulated in Table 11. Interferences. The substances commonly present in water in any appreciable amount were tested to determine whether they exhibited any adverse effect upon this method. Of the substances checked-magnesium, sodium, potassium, silicon, and iron-iron was the only one adversely affecting the determination. Its effect was made negligible by boiling the solution with 1 ml. of 10% hydroxylamine hydrochloride solution to reduce the iron to the ferrous state as previously suggested (4). ANALYTICAL PROCEDURE
EXPERIMERTAL
Materials. Reagent grade chemicals were used throughout unless otherwise specified. Redistilled water was used to make all solutions. The dye was prepared according to the method of Reed, Byrd, and Banks ( 3 ) with modifications. Part of the work of Thomason, Perry, and Byerly was repeated in order to establish the similarity in behavior of the dye. Apparatus. A Beckman Model DU quartz spectrophotometer with a set of matched 1-cm. cells was used for all transmittancy measurements. It n-as found that a wave length of 545 mp was the best, while a spectral band width of 2.0 mp was selected, requiring a slit width of 0.0355 mm. These settings were used during all measurements made with the instrument. General Technique. I n preparing solutions for transmittancy measurements the optimum relation between dye and solution was maintained, re uiring 1 ml. of a O.lY0 aqueous solution of the dye per 10 ml. of solution, as obtained by other workers. The p H of the final solution was adjusted to slightly less than 1.0.
The following procedure was adopted for the determination of the thorium content of water on the basis of the above data. Heat four 1.5-liter samples of water almost to boiling, and add 1 ml. of 3 7 7 , hydrochloric acid to each. To two of these samples add 10 micrograms of thorium. Carry out the precipitation of
Table 11. Precision of Method
%rial
Calcium Effect. A series of solutions was prepared containing varying amounts of calcium over the range that was expected to be encountered in the samples to be analyzed. These solutions were compared against a standard containing no calcium Three cells of each solution were compared three times, and the average of the cells are reported in Table I. If these values are plotted, it can be seen that over the range under consideration the effect is linear and very slight. This effect can be corrected by adding to the atandard, against which the samples are measured, a known amount of calcium carbonate equal t o that in the sample. To determine whether the calcium present had any effect upon the application of Beer’s law, a series of solutions was prepared containing 50 mg. of calcium carbonate and varying amounts of
Thorium Added, y
Thorium Found, y
10 10 5 5 2 2 0
10.14 9,57 4.98 5.13 2.22 1.95 0.18
Error, y t0.14 - 0 43 -0.02 f O . 13 +0.22 -0.05 + O . 18 Av. error =to, 17
Table 111. Analyses of Synthetic Samples Thorium Present, y 10
Composition, .\I&
CaCOs 100
?K: Si
8.1 6 5 1.2
Thorium Found, y (10 9)
(9.3)
12.0 Av. error
10.8
A r . error
-0.6
10
624
V O L U M E 2 4 , N 0.
OCTOBER 1952
calcium oxalate as described by Willard and Furman ( 5 ) . Thoium oxalate appears to be effectively precipitated along with the rcalciuni under these conditions. Use a filtering crucible (Selas No. 3010). After heating in the electric muffle furnace for conversion of the oxalate to carbonate, weigh the crucibles and contents. Place the crucibles in Gooch filtering funnels with the outlets leading into 15 X 125 mm. test tubes on which graduation marks a t 6 and 10 ml. have been scratched. Add from a pipet 1 ml. of 3700 hydrochloric acid solution to each, allowing the acid to run down the sides of the crucibles. Turn on gentle suction and wash each crucible with a very fine stream of redistilled water until the volume of solution in the test tubes is nearly 6 ml. Dry and weigh the empty crucibles. Remove the test tubes from the filtering funnels, add 2 ml. of a lO7’ solution of hydroxylamine hydrochloride to each, and boil gently for about 1 minute. Add 5 drops of 3i7’ hydrochloric acid and 1 ml. of a 0.1% solution of the sodium salt of l-(o-arsonophenylazo)-2-naphthol-3,6-disulfonic acid. Dilute all solutions to 10 ml. Prepare a reference solution for use in the spectrophotometer by suspending in 10 ml. of redistilled water an amount of calcium carbonate equivalent to twice the average weight of calcium carbonate found in the above four samples. Add 1to 1hydrochloric acid dropwise until 10 drops in excess of the amount needed to dissolve the carbonate are present. Add 4 ml. of 10% hydroxylamine hydrochloride solution and 2 ml. of the dye solution, and dilute the resulting solution to 20 ml. Compare the absorbancy of each of the four samples, two of u hich have 10 micrograms of added thorium, against the reference solution by use of a Beckman ?\lode1DU spectrophotometer.
1625 Set the slit opening a t 0.0355 mm. and the wave length a t 545 mp. Determine cell corrections by comparing the absorbancy of the reference solution in different cells. Determine the amount of thorium present by the relative absorbancy of the “spiked” with the plain samples when compared to the reference solution. Analysis of Synthetic Samples. A series of synthetic samples was prepared to test the determination of thorium in the presence of other materials. The results of these determinations are presented in Table 111. LITERATURE CITED
(1) Hall, R . H., U. S. Atomic Energy Commission, AECD-2437 (Piov. 12, 1948). (2) Kuznetsov, V. I., J . Gen. Cheni. (C.S.S.R.), 14, 914-19 (1944). (3) Reed, S. A.. B y r d , C. Pi.,and Banks, C. V., U. S. .Itoniic Energy Commission, AECD-2565 (.-lpril 18, 1949). (4) Thomason, P. F., Perry, SI. h.,and Ryerly, TT, SI,,ANAL. CHEM.,21,1239 (1949). ( 5 ) TTillard, H. H., and F u r m a n , Pi. H., “Elementary Quantitative Analysis,” 3rd ed., pp. X39-4:3, Sew York. D. Van Kostrand Co.. 1940. RECEIVED for review March 30, 1962. Accepted June 16, 1962. Based on work done for the Atomic Energy Commission under contract AT(10-1)-310 xith Idaho State College.
Qualitative Detection of Urea in Commercial livestock Feeds GR4EME L. B4KER AND LEON 13. JOHNSON D e p a r t m e n t of C h e m i s t r y Research, Montana S t a t e College Agricultural Experiment Station, Boaeman, M o n t .
.XSY state l a ~ require s that commercial livestock feed containing nonprotein nitrogen be so labeled and its equivalent crude protein value be given. Urea is the most common source of nonprotein nitrogen employed and is usually determined b~ means of a urease digestion followed by a Kjeldahl-type distillation of the ammonia released as recommended by the Association ot Official Agricultural Chemists ( I ) . This method is too timeconsuming to be employed for the sole purpose of qualitativel\ checking for unreported nonprotein nltrogen, especially when a large number of samples is involved. Many methods have been proposed for the detection of urea. but none can be conveniently applied to the problem encountered in commercial feed analj The method proposed by Sanchez (11-13) is too tedious for routine application. The method of Fearon ( 3 ) is not advantageous since guanadine, creatine, and gelatin all give results similar to urea. A second method of Fearon ( 4 ) results in a golden-yello~vcolor as a positive test This is difficult to read because of coloration of many feed e\tracts. Schemes using urease and detecting the released ammonia by means of acid-base indicators have been proposed ( 2 , I O ) , but are rather difficult to control in laboratory operation. involving many samples. A method which detects the released ammonia with a silver-salt test paper (6) has the same objections. The problem of detecting urea in animal and vegetable nitrogen o ~ 3materials was investigated by Moore and White (8). Their method involved the uqe of bromine water and caustic soda to release nitrogen from the urea. A positive test resulted in thc effervescence of nitrogen. The method required close observation and would be difficult to conduct when many samples are to be tested. -1rapid, qualitative method has, therefore, been devised which may be used as a preliminary check for urea in a feed sample The method utilizes the normal urease digestion followed by a teqt for ammonia b) means of a modified Sessler reagent. The use of a modified Sessler reagent as a qualitative test was suggested by the use of Nessler reagent for the quantitative determination of urea in hlood and urine. The reagent as used in
IM
the pi esent piocedure is considerably more concentrated than that applied by Folin and Wu ( 5 )and others ( 7 , 9 ) . REAGENTS
Urease (Jack Bean), Arlington Brand. Sodium hydroxide, 6 S. Modified Nessler reagent. Dissolve 28 grams (0.5 mole) of potassium hydroxide, 166 grams ( 1 mole) of potassium iodide, and 227.5 grams (0.5 mole) of mercuric iodide in 500 ml. of distilled Tvater. PROCEDURE
A 0.3- to 0.5-gram sample of the feed to be checked is added to 10 ml. of distilled water in a 15-ml. centrifuge tube, and to this is added approximately 0.01 gram of urease. The materials are shaken vigorously in order to dissolve any urea that may be present in the feed and then centrifuged a t 2500 r.p.m. for about 20 minutes. Approximately 1 ml. of the supernatant liquid is decanted into a test tube and diluted to 10 ml. with distilled water. Three to five drops of the modified Nessler reagent are added to the diluted sample, followed by the addition of 1 nil. of 6 N sodium hydroxide. The presence of urea will be indicated by the production of a red-brown precipitate of dimercuric ammonium iodide (XHgzI), or by an orange to red-brown coloration of the solution, in cases of low urea concentrations. DISCUSSION
This method has been used to check different commercial feeds for urea. I n the preliminary study with 120 different commercial feeds, the feeds were first examined by the approved official method ( 1 ) for urea determination, to ensure the validity of the tests made by the method. Fourteen of 120 samples contained urea, and all were readily detected by the suggested method. The urea content in the feeds examined ranged from 1.5 to 13.37’ equivalent crude protein. To establish the sensitivity of the test, trials were made on feed samples adulterated with varying amounts of urea. Urea vias detected positively in all samples n hich contained a t least 0.5% urea (1.570 equivalent crude protein) in the original feed sample. The procedure is designed specifically for feed analy If it is t o be utilized for other purinpoqeG n heregreater sensitivity i q deqired, the sarrple neight and
