1173
V O L U M E 2 4 , NO. 7, J U L Y 1 9 5 2 Small quantities of tantalum do not interfere with the tartaric acid modified method, but large amounts of titanium and iron must be avoided. Tantalum does not interfere with the unmodified method, but large amounts of nickel as well as titanium and iron are harmful. Ions forming strong complexes n-ith niobiumfor instance, fluoride, oxalate, and phosphate-quench the color and must be absent. Tartrate and small amounts of sulfate do not interfere. Many cations inhibit the color formation if present in very large quantities, but on the whole the method is very selective. So far, no direct application has been made t o the determination of niobium in minerals, alloys, or similar complex samples. S o clear picture is available of the nature of the colored complc~s. Sandell (6) assumed the product to be a complex of trivslent niobium produced b y the reducing action of the stannous chloride. Alimarin and Podvalnaja, however, found, and the prwent authors have confirmed, that the yellow color is produced in the absence of any reducing agent and is, therefore, a compound of pentavalent niobium. The principal function of the st.annous chloride is the removal of miscellaneous oxidizing agents and elimination of the interference which will be observed with small amounts of ferric iron. Mimarin and Podvalnaja suggested a structural formula for the niobium thiocyanate compler based on the analysis of solid salts of alkaloids obtained from the aqueous solution. However, such solid derivatives may or may not have a bearing on the struct.ure of the comples which
iq extracted into ether. T h a t more than one complex is formed is suggested by the fact that unless the yellow compound is etherextracted promptly, the color in the aqueous media deepens, more yellow color is extractable, and an unextractable yellon color remains on long standing. The nature and composition of these compounds are t o be the subject of another investigation in this laboratory. ACKNOWLEDGMENT
The authors wish to thank the MIT Laboratory for Kuclear Science and Engineering and the Atomic Energy Commission for partial support of this work. LITERATURE CITED
Alimarin, I. P., and Podvalnaja, R. L., Zhur. Anal. Khim., 1, 30 (1946). Dobina, -1.A., and Platonov, hl. S., Zhur. PrikZad. Khim., 14, 421 (1941). Freund, H., and Levitt, A E., - 1 s ~CHE>I., ~ . 23, 1813 (1951). Mon'jakova, L. K,,and Fedorov, P. F.. BUZZ. Dept. Inventions Gosplan, Council OJ Sntionrrl Commissars o f C.S.S.R., p. 41 (1942). Quoted in ( I ) . Pennington. 31. E., J. A m . Chem. SOC.,18, 51 (1896). Sandell, E. B., "Colorimetric Determination of Traces of Metals," 2nd ed.. p. 292, New York, Interscience Publishers, 1930. Shapiro, RI. J., Zhur. Priklad. Khim., 11, 1028 (1938). RECEIVED for review January 20, 1952.
Accepted M a y 12, 1952.
Colorimetric Determination of N-Trci hloromethyIthiotet ram hydrophtha1imide ALLEN R . KITTLESON, Esso Laboratories, Standard Oil Development Co., Linden, II'. J . The use of chemical pesticides to protect food crops from diseases and insects has necessitated the development of methods for the quantitative determination of trace amounts of these substances that may possibly remain on the edible portion of harvested crops. These analytical methods will help to establish whether or not food products contain a sufficient quantity of these necessary chemicals to constitute a health hazard. The present paper describes a new procedure for the determination of minute spray residues (as low as 0.05 mg.) of a new organic fungicide, N-trichloromethylthiotetrahy-
T
HE !tidespread use of chemical pesticides t o protect food
crops from the ravages of diseases and insects has necessitated the development of methods for the quantitative determination of trace amounts of these substances that may possibly remain on the edible portions of harvested crops. The ultimate use of these analytical methods Tvill serve as a means of establishing whether or not food products contain a sufficient quantity of these necessary chemicals t o constitute a health hazard. The present paper describes a new procedure that has been developed for the determination of minute spray residues of a new organic fungicide, A'-trichloroniethylthiotetrahydrophthalimidc.
drophthalimide (SR-406). The method involves the formation of a chromogenic compound when SR-406 is heated with resorcinol and the light transmittance of an ethyl alcohol solution of the reaction product is measured. This compound has shown unusual promise as an agricultural fungicide during several years of greenhouse and field testing. Inasmuch as it is now approaching commercial fruition it will, in ensuing seasons, probably be widely applied to food crops. It is important, therefore, that an analytical technique become available for use by agricultural and food chemists. This chemical compound has shown unusual promise as an agricultural fungicide during several years of greenhouse and field testing by numerous agricultural institutions, experiment stations, and governmental groups in the United States, Canada, Central America, England, and Europe, under the code designation, SR-406 (also designated as Orthocide-406). These tests covered a large variety of vegetable, fruit, and grain crops and ornamental plants. Inasmuch as this fungicide is now in the final stages of experimental development and is approaching commercial fruition it will, in ensuing seasons, undoubtedly be applied widely t o food crops. An analytical technique for use by agricultural and food chemists is therefore needed. The present method is based on the reaction of resorcinol with SR-406 a t 135" t o 140" C. t o form an intense red colored compound suitable for colorimetric analysis. The structure of the chromogenic reaction product has not been precisely determined, but is undoubtedly of the phthalein type that could be
ANALYTICAL CHEMISTRY
1174 expected from the reaction of resorcinol with the dibasic acid portion of the SR-406 molecule. This is eubstantiated by the fact that the absorption curve for the resorcinol-tetrahydrophthalic anhydride condensation prodhct is almost identical with the curve obtained on heating SR-406 with resorcinol. As practically none of the 15 idely used agricultural insecticides or fungicides is a derivative of dibasic acids, it is doubtful that many interfering substances will be encountered in the examination of spray residues for traces of SR-406.
Table I.
SR-406 Content of Sprag Residues Vol. of CHCIJ Solution
CHCls Added to Resorcinol
Tomatoes Beaker
5 5
Tomato leaves Beaker Tomato leavesb Beaker remainder of Sprayer spray mix
?
2 2 2 2 2 2
Source of Residue
'M1.
5
5
+.
25
DEVELOPMENT OF TEST METHOO a
100
RESORl
$Ob
400
XZ.
Transmittancea
25
%
10
23.2 11.0 34.2 56.0
25 10
22.5
10
25
SR,-406 in Residue Mg.
3.13 1.95 2.32 0.87 3.83 1.27
17.8
1 25 20.5 Total SR-406 accounted for Original quantity of 6R-406 Original SR-406 accounted for
33.75
47.12mg. 50.00mg 94.2%
.kt 450 mp. Two spray applications.
b I n a preliminary investigation of the colorproducing reaction, a 4-mg. sample of SR-406 was dissolved in 20 ml. of chloroform. A 1-ml. sample of the chloroform solution was added to 1 gram of C.P. crystalline resorcinol in a 2 X 15 cm. test tube. The test tube was immersed to one half its length in an oil bath for 20 minutes at a temperature of 1350 to 1380 C. -4 l-gram sample of resorcinol was similarlv treated as a blank. The heated samoles were diluted with 25 ml. of absolute ethyl alcohol and placed in matched 1-cm. quartz cells. Light transmittance curves for the two samples against ethyl alcohol were obtained over a range of 300 to 600 mp with a Cary recording spectrophotometer (Model 11 PIlS Serial 54) and are sho1v.n in Figure 1. The curves show that colorimetric analysis may be made a t 410 to 440 mp or 300 to 360 mp.
GOO
MZ.
Ethyl Alcohol Added to Reaction Product
APPLICATION OF TEST
This test was applied to the determination of spray residues in the following manner:
A 0.1000-gram sample of 50% SR-406 wettable powder TTa? dispersed in 20 ml. of r a t e r in a 1-ounce atomizer sprayer. Several small tomatoes were tied in a cluster and suspended by a string in a 1-liter beaker to catch all of the spray not depositing on the tomatoes. The tomatoes were sprayed to the point of run-off and allon-ed to dry, then were washed with two 100-ml. portions of warm chloroform, an excellent solvent for pure SR-406. The composite chloroform extracts were filtered and evaporated to a small volume (ca. 3 ml.). These concentrates were then transferred to a volumetric flask and diluted to exactly 5 ml. The beaker in which the excess spray mixture was collected was nmhed a i t h 100 ml. of chloroform. This solvent v a s also evaporated and finally adjusted to a volume of 5 ml. in the same manner. Two groups of tomato leaves were similarly sprayed, using the same spray mixture as above. The v-ashing and evaporation steps were the same a8 previously described. The unused spray mixture was decanted into an evaporating dish and allowed to evaporate to dryness a t room temperature. The spray apparatus and evaporating dish were washed with 200 ml. of warm chloroform, filtered, and evaporated to a volume of 25 ml.
a00
W A V E L E N G T H M)L
Figure 1. Ultraviolet Transmittance Curve for SR-406Resorcinol Reaction Product US. Resorcinol
The range of applicability and the degree of accuracy to be expected from this method were determined in the following manner. A series of standard solutions containing 10, 5 , 2 , 1,0.5, 0.1, and 0.05 mg. per ml. of pure SR-406 in chloroform was prepared and 1-ml. aliquots of these solutions were pipetted onto 1-gram quantities of pure resorcinol contained in test tubes (approximately 2 X 15 cm.). After immersion of the tubes (to a depth of about 7 cm.) in an oil bath a t 135' to 138" C. for 20 minutes, the products were transferred to graduated flasks by washing out the tubes with absolute ethyl alcohol and diluting to a known volume. Spectrophotometric readings were made a t 450 mp on each sample a t several dilutions to give the data presented graphically in Figure 2. A similarly treated sample of resorcinol was used as a blank in this test. These data show that as little as 50 micrograms of SR-406 can be determined with a reasonable degree of accuracy. The curve obtained from these known samples forms the basis for determining the amount of SR-406 in unknown samples. The color formed in this reaction was found to be completely stable after standing in the ethyl alcohol diluent for 24 hours and showed only slight increase in intensity on standing 72 hours.
YI '0
0.2 0,4 MG. S R - 4 0 6 /
0.6
IO ML.
0.8
1.0
I2
ETHYL ALCOHOL
Figure 2. Ultraviolet Transmittance Curve for Determination of SR-406
A tabulation of the results obtained on analyzing the various spray residues for SR-406 content is given in Table I. As it !vas possible to account for 94.2% of the theoretical quantity of SR406 added, it is readily seen that the present analytical procedure can be satisfactorily employed to give quantitative data on the amount of SR-406 in spray residues.
V O L U M E 2 4 , NO. 7, J U L Y 1 9 5 2 The test has also been successfully applied to a variety of apples, peaches, grapes, and beans. ANALYTICAL PROCEDURE
The recommended procedure for applying this test to the determination of SR-406 residues on foliage or fruit involves the following stpps; 1. Residue Removal. For best results the size of sample (fruit or foliage) should be such as to yield a total SR-406 residue of a t least 0.1 mg.-for example, a 100- to 2OO-gram sample is satisfactory for estimated residues in the range of 1 to 10 p.p.m. The sample is washed with two 150-ml. portions of warm chloroform, the chloroform solutions are filtered, and the filter paper is rinsed with an additional 10 ml. of solvent. The combined filtrates are evaporated to a volume of 3 ml., which is transferred to a graduated cylinder. The container used for evaporation ip washed with sufficient fresh chloroform to give a total volume of 5 ml. For higher residue values this final volume may be correspondingly increased, 2. Resorcinol Reaction, One-gram samples of resorcinol are placed in each of two 2 x 15 cm. test tubes. A 1-ml. sample of the chloroform solution from Step 1 (for residues of less than 5
1175 p.p,m. on a 100-gram sample 2 ml. of chloroform should be used) is pipetted onto the resorcinol in one of the test tubes. To ensure contact of all of the SR-406 with resorcinol, the chloroform solution should not be permitted to run down the sides of the test tube. Both test tubes are inserted to a de t h of about 7 cm. for 20 minutes in a constant temperature oil %ath maintained at 135" to 138" C. The test tubes are removed and 5 ml. of absolute ethyl alcohol added. After the reactants have completely dissolved, the alcohol solution is transferred to a volumetric flask. The test tubes are then washed with additional quantities of alcohol to remove all the reactants. The total volume of alcohol is adjusted t o a known volume sueh that the per cent transmittance is greater than 20 but less than 85. 3. Spectrophotometric Analysis. The spectrophotometric determination may be made with a Beckman spectrophotometer using a wave length of 425 mp. Portions of the alcohol solutions from Step 2 are placed in each of two matched 1-cm. quartz cells and the instrument is adjusted to 100% transmittance with the blank sample. The reading obtained on the unknown sample may be translated into parts per million of SR-406 by referring to a semilog graph as in Figure 2 prepared from known concentrations of SR-406. RECEIYED for review August 24, 1951.
Accepted April 26, 1952.
Volumetric Determination of Milligram Quantities of Uranium CLAUDE W. SILL1 AND HEBER E. PETERSON Bureau of Mines, Salt Lake Experiment Station, Salt Lake City, Utah Because most of the uranium mined in the United States comes from low-grade ore deposits, a method was needed that would accurately determine both small and large amounts of uranium in ores and metallurgical products in a routine manner. Generally, methods have been developed for the determination of either very large or very small quantities of uranium. The accuracy attainable with the method described ranges from about 2% with 1 mg. of uranium to within =kO.ly~ with 20 mg. or
A
LTHOUGH the Jones reductor is used extensively in the determination of uranium, the conditions under which
errors nil1 or will not be produced are rather vaguely known and information regarding them varies with the source. This is especially true in the volumetric determination of small quantitiee of uranium in ores. The present paper s h o w under what conditions and to what e\tent errors are produced, and recommends a combination of procedures that has proved most satisfactory for ore analysis in over 5 years of practical application in this laboratory. The methods described differ in one or more of the following respects from similar procedures already in the literature ( 1 1 ) : A lead. reductor is used in place of the conventional Jones reductor; sufficiently large volumes of solution are used to accommodate the salt3 from relatively large samples: ferroin is used as indicator because of its sharper color change, more reproducible blanks, and independence of volume in comparison with diphenylamine sulfonic acid; the use of ferrir sulfate or phosphoric acid allows the titration to be made with sulfatoceric acid at room temperature in an essentially colorless solution; and no encumbrances such as inert atmosphere, elevated temperature, special microapparatus, or techniques or expensive equipment are required. 1 Present address. U. S. Atomic Energy Commission, P. 0. Box 1221. Idaho Falls, Idaho.
more. Separations are given for essentially all interfering elements. The method is well adapted to routine application to large numbers of samples and no expensive or unusual equipment is required. This method is designed to cover the intermediate range w-ith the greatest accuracy, but is also applicable to larger quantities. The analyst is provided with an accurate procedure to be followed, and, of equal importance, with information as to the sources of error and how they are produced. Standard solutions of uranium were prepared by dissolving
Uno8 (99.96%) in boiling perchloric acid and by diluting a stock solution of uranvl sulfate that had been standardized gravimetrically according to the method of Lundell and Knowles (9). Solutions of sulfatoceric acid were prepared according to the directions of Smith (15) and were standardized against arsenic trioxide in the presence of osmic acid according to the directions of Gleu (7). The cerium solutions were stored in 18-liter carboys fitted with siphon arrangements, so that the bottles did not need to be opened, and were protected from light by storing in a dark cupboard. Under these conditions, even the 0.01 N solution changed by less than 1 part per thousand in about 6 months. A 25-ml. buret was used in all tests to facilitate accurate reading, except where 50 ml. was used. TITRATION PROCEDURE USING JONES REDUCTOR
The solution to be titrated for uranium should have a volume of about 75 ml. and contain 6 ml. of concentrated sulfuric acid. h f t e r being cooled to 15" t o 20" C., the solution is reduced by passing it through an air-free Jones reductor having a bore of 12 mm. and a 30-cm. column of 20- to 30-mesh amalgamated zinc (1%mercury). The flask and reductor are then Jvashed with 50 ml. of cold 2 S sulfuric acid followed by 50 ml. of cold water, added in small portions to obtain maximum washing efficiency. Each solution should be poured down the side of the reductor tube to prevent the formation of air bubbles that might be drawn into the zinc column. It is extremely important to keep the column free of air, to prevent formation of significant amounts of hydrogen peroxide. The solution is then aerated by passing a
