Colorimetric Estimation of Tetrachloronitro benzene 31. E . AUERBACII Sterling- Winthrop Research Institute, Rensselaer, N. I-.
A sensitive colorimetric method is described for the determination of tetrachloronitrobenzene, the active ingredient of an agricultural dust. Color formation probably depends upon the interaction of tetrachloronitrobenzene with acetone, in the presence of tetraethylammonium hydroxide.
X P E R I M E S T S conducted within recent years indicate t h a t 2,3,5,6-tetrach1oronitrobenzene ( T C T B ) may become an important agent for the control of dr>- i'ot (Fiisari'uni coeruleum) in potatoes ( I ) and for the siniultaneous suppression of sprouting in stored potatoes ( 2 , 3 ) . I n field tests, the material has been applied in the form of a dust containing 3% of the active substance mixed with an inert carrier, such as kaoliti. I t would be of obvious value to food supply or health authorities t o have available a sensitive test method for the pure chemical which might event.ually be adapted to the assay of residual tetrachloronitrobenzene on potatoes intended for human consumption. A quantitative colorimetric test has been worked out in these laboratories. Pure 2,3,5,6tetrachloronitrobenzene is a colorless (or very faintly yellow) odorless crystalline material which melts a t 99' to 101' C., corrected. It is practically insoluble in water, nioderately soluble in alcohol or petroleum ether, and readily soluble in ethyl ether, chloroform, and benzene.
Tetraethylammonium h?-tlroside working solution (2yob).Dilute 2 ml. of the 10% tetraethylammonium hydroside with 8 ml. of reagent grkde acetnnc. The dilution should tw u s t d withiii 4 o r .j hours after prepax,atio:i. Standard tetrachloronitrobenzene solution. Dissolve esactly 100 mg. of tetrachloronitrobenzene in exactly 100 ml. of reagent grade acetone. From this stock solution, by appropriate dilution, prepare a working stltn(iard which will corit:iiri 10 n1ici.ograms of tetrachlororiitrobeiizeiie per ml. of acetone. PREPARATION OF STANDARD CURVE
I n t o sis test tubes accurately graduated with a mark a t 10 nil., transfer, respectively, 0, 1, 2 , 3,4, and 5 ml. of the standard solution of tetrachloronitrobenzene (10 micrograms per m].), Dilute the contents of each tube to exactly 10 ml. with reagent grade acetone, and mix. Add esactly 0.1 ml. of the 2% tetraethylammonium hydroxide to each tube, and mix. After 10 minutes, b u t before 20 minutes have elapsed after the addition of the quaternary base, read the standard series in a suitable photoelectric colorimeter, using a glass color filter which transmits a band of light in the range 540 to 560 mg. When the colorimeter readings are plotted on semilog paper, a linear graph results. T h e spectral characteristics of the purplish-red color developed in t h e test were determined with a Becknian spectrophotometer, and are shown in Figure 1. A broad absorption band occurs in the range 536 to 556 mp, with t h e peak a t about 548 mp. GENER.IL CONSIDERATIONS
,4s indicated, the color develops gradually, reaching maximum intensity after 7 or 8 minutes, and begins t o fade after about 20 minutes. An excess of quaternary base decreases t h e sensitivity of the test and the purity of the color, and also hastens fading. Precautions must be taken to exclude more than minute amounts of water. T h e effect of water in the acetone solvent is shown in Table I.
Table I. Effect of Added Water on Color Debeloped i n 10-311. Portions of Acetone Containing 20 .\Iicrogranls of Tet rachloronitrobenzene % Water bq Voluine
0.001 500
,
I
I
I
1
540 560 580 600 WAVELENGTH IN MILLIMICRONS
520
0 (negligtl,lr, 0 5.5 1 10 1 6.5
620
Figure 1. Absorption Spectrum of Colored Deri\ati\-e of Tetrachloronitrobenzene 60 micrograms in 10 ml.
T h e labile nature of the clilorine ortho t o the nitro group, and the presence of the nitro group itself, suggested t h e possibility of a color tevt based on the intense color of the nitroquinoid ion, as developed, for example, in the well-known Janovsky or Zinimermann test using vi-dinitrobenzene ($1, This turned out to be a fruitful approach. IIlATERIALS REQUIRED
Tetrarthylammonium hydrouide, lo%, available froin Eastman Kodnk Company, Catalog KO.2078.
rOT 71 7
78 7 90 2 9.5 9
Generally speaking, the admixture of other common solvents with the acetone should be avoided. An esception is benzene. A mixture of 4 volunies of reagent grade benzene and 6 volumes of acetone is entirely suitable, although with this mixture the color develops somen.hat more slowly than in straight acetone. The mixture is importitiit, because it' would enable one to wash treated potatoes with a w.ter-immiscible solvent (benzene) in which tetrachloronitrobenzene is extremely soluble. -2fter filtering, t h e benzene wash niight be diluted with 6 volumes o i acetone, and assayed for tetrachloronitrobenzene directly. Acetone itself is not the most suitable wash solvent for treated potatoes, inasmuch as it would probably become hydrated in the process, and thus yield fulsel?.low results. 1287
1288
ANALYTICAL CHEMISTRY
The constitution of the colored derivative of tetrachloronitrobenzene is thought to be:
active hydrogen of the acetone reagent. However, no attempt has been made t o prove the mechanism of color formation. ACKYOWLEDG\IE?IT
The author is indebted to June 31. Olson for technical assktance.
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LITERATURE CITED
H I 1 C H ? CI
(1) Bayer Products, Ltd., London, Fusarex Technical LIemorandum 3. (2) Brown, W., Ann. Applied Biol., 34, 422 (1947). ( 3 ) Luckwill, L. C., "Suppression of Sprouting in n'are Potatoes,"
C=O I
Another possihilit>. is the bis compound, in which both chlorilles ortho to the nitro group :ire eliminated through reaction with
-4nnual Report, Agricultural and Horticultural Research Station, Long ilshton, Bristol, England, 1948. (4) Zimmermann, W., 2. physiol. Chem., 245, 47 (1936). RECEIVED April 27, 1950.
Radiometric Titration with Radioactive Silver as End-Point Indicator ALOIS LANGERI r e s tinghouse Research Laboratories, East Pittsburgh, P a . Radiometric titrations were extended to include some argentometric titrations using radioactive silver as the indicator. A simplification of the titration procedure is described in which only two points on a branch of the titration curve are determined. The possibility of a radioactive exchange method for determining the amount of a given precipitate is discussed.
I
T HAS been demonstrated ( 3 ) t h a t it is possible to determine
the end point of a precipitation react,ion carried out stepwise as a titration by radioactivity measurements of the solution if i t contains a proper radioactive element as an indicator. The substance t o be titrated, the reagent, or both can be tagged b y adding t h e radioactive element in tracer amounts directly t o the solution or b y incorporating it as a constituent-for example, in t h e reagent. Depending upon how the volumetric procedure is performed, different titration curves can be obtained which are quite similar to the ones known from conductometric, amperometric (polarographic), and other titrations, where the readings of the end-point indicator are directly proportional to the concentration of the ion in solution. Although generally limited to reactions in \
