'ATE PLANT BOARD
November 1944 FT-211_
United States Department of A;ri:culture
Agricultural Research Administration
Bureau of Entomology and Plant Quara:ntiL
CHEMICAL DETERMINATION OF DTfiL/
By S. A. Hall, Division of Insects Affecting Man and
Animals, Milton S. Schechter, and Elmer E. Feck,
Division of Insecticide Investigations
In the absence of a specific color test for 2,2-bie(p-chlorophenyl)-
101,1-trichloroethane, commonly known as DDT, the high chlorine content
(50.01oi percent) of this compound can be utilized as a basis for its in-
direct chemical analysis. Of the Various known methods applied to DDT
for chlorine analysis, we have found that the Carius method has given
consistently accurate results. The Carius method, however, is not only
time-consuming but also requires a painstaking technique if it is to be
used without hazard. We have therefore modified a method proposed by
Winter for the determination of halogen in organic compounds. In prin-
ciple the sample to be analyzed is volatilized and burned in a flame of
ordinary illuminating gas, the chlorine-containing combustion products
are absorbed in an alkaline solution of sodium arsenite, and this solution
is then titrated for chloride ion, using standard solutions of silver
nitrate and ammonium thiocyanate.
The apparatus (fig. 1) consists of a Pyrex glass tube, ABC, shaped
in the form of a U, to one end of which is sealed a short leRth of 1.5-am.
capillary tubing. This U-tube is heated electrically by a winding of
about 10 feet of nichrome resistance wire (No. 28 B. & S. gage) spaced at
about 10 turns per inch. The nichrome wire, W, is held in place by a
strip of braided asbestos tape, T, underlying -the wire, and by 2 or 3
single loops, L, of wire made tight by twisting the ends together. The
terminal leads-of the nichrome wire are connected to a variable transformer
(Tarliac), V. The chimney, D, is made from a short piece of 20-mm. Pyrex
tubing Joined to apiece orf-9-mm.. tubing. This is connected to the inlet,
M, of the gas-washing bottle, G, by a short length of rubber tubing, F.
The fritted glass disc, H, is covered by the absorbing solution, S. The
outlet, K, of the gas-washing bottle is connected by rubber tubing, R, to
a water aspirator (not shown). Only a moderate suction is needed to draw
the combustion products from the chimney, through the absorbing solution.
A pinch clamp, P, over the rubber tubing, R, is used to regulate the amount
1_ Some of the work reported was done under a transfer of funds recom-
mended by the Comittee on Medical Research from the Office of Scientific
Research and Development to the Bureau of Entomology and Plant Quarantine.
9- Winter, P. K. Determination of halogen in organic compounds. Indus.
and hngin. Chem., Analyt. Ed. 15: 571-574. 1943.
Reagents and Solutions
Absorbing Solution.--Five grams of arsenic trioxide is dissolved in
100 ml. of a solution containing 8 grams of sodium hydroxide, and made up
to 1 liter with distilled water.
Silver Nitrate Solution.--4.791 grams of silver nitrate is dissolved
in distilled water and made up to 1 liter in a volumetric flask. This
gives a calculated normality of 0.0282 so that 1 ml. of solution is
equivalent to 1 mg. of chloride ion.
Ammonium Thiocyanate Solution.--2.147 grams of ammonium thiocyanate
is dissolved in distilled water and made up to 1 liter in a volumetric
flask. This is calculated for a normality of 0.0282 equivalent to that
of the silver nitrate solution.
Other Reagents.-Powdered ferric sulfate, concentrated nitric acid
(halogen-free), and nitrobenzene are also needed.
A suitable aliquot of the silver nitrate solution is titrated with
the ammonium thiocyanate solution, about 3 ml, of nitric acid added and
0.5 gram of ferric sulfate being used as the indicator. The strength of
the thiocyanate solution must then be adjusted so that it is exactly
equivalent to that of the silver nitrate solution; otherwise it becomes
necessary to use a correction factor. The silver nitrate solution may be
standardized directly against the combustion of a weighed amount of pure
DDT or against the combustion of an aliquot of an accurately made-up
solution or emulsion of known DDT concentration. The material for standard-
ization should be of the same nature and form as the material being ana-
lyzed, i.e., whether a pure or technical grade of DDT, and whether it in
in the form of a solution, emulsion, or dust.
With 50 ml. of the absorbing solution in the gas-washing bottle and
with the apparatus connected up as shown, exactly 1.00 ml. of the solution
or emulsion containing the DIDT under determination is introduced by means
of a 1-ml. precision-quality transfer pipette inserted through the opening
A, into the B portion of the U-tube. The U-tube is then connected at A
by means of rubber tubing (not shown) to an outlet of illuminating gas.
Before the gas is turned on, the U-tube is lowered and removed from the
chimney. With the Variac set at 15 volts, the gas is then ignited at the
capillary opening, C, and the flame is adjusted to a height of about 15 ncm.
After 10 minutes, when the solvent carrying the DDT is largely burned,
the height of the flame is reduced to about 1 cm. The U-tube is raised
to insert the flame in the chimney, and by manipulating the pinch clamp,
P, the suction from the water aspirator is adjusted so that sufficient
air is drawn through the system to support the flame. During the course
of about 45 minutes the heat input is gradually increased by raising the
voltage from the Variac. A sooty flame should be avoided by increasing
the heat gradually or having sufficient air drawn through the system.
It will be observed that the flame becomes quite luminous as the DDT is
volatilized. For the last 10 minutes the setting of the Variac should
be about 100 volts. This setting should deliver current to impart a dull
red glow to the nichrome wire.
The gas is then turned off, the inlet tube, M, removed from G, and
adhering solution quantitatively rinsed with distilled water through the
disk H by means of a wash bottle into container G. The solution in G is
than quantitatively transferred to a 500-ml. Eripnmeyer flask. Likewise
the chimney, after cooling, is rinsed with a little distilled water into
the flask. Silver nitrate solution in excess of the amount necessary to
precipitate all the chloride ion is added from a burette to the alkaline
absorbing solution in the flask. There is then added in succession approx-
imately 3 ml. of concentrated nitric acid, 0.5 gram of ferric sulfate, and
5 ml. of nitrobenzene. The flask is swirled to coagulate most of the pre-
cipitate. Then the excess of silver nitrate is back-titrated with the
thiocyanate solution until a faint pink color appears. As recommended by
Winter, it has been found preferable to cross-titrate with both standard
solutions, crossing the end point in each direction. The end point, which
is not too sharp, is more easily perceived in this way. Since the two
standard solutions have been made exactly equivalent, no calculation is
involved other than subtraction of the amount of thiocyanate solution
used from the amount of silver nitrate solution to obtain the quantity of
silver nitrate solution consumed. Each milliliter will be approximately
equivalent to 2 6, of EDT. The exact figure is determined, as previously
mentioned, by standardization of the silver nitrate solution against the
combustion of the DDT being used. The technical grade of DMT may contain
48 to 51 percent of chlorine. It may be assumed, however, that the chlor-
ine content of a given batch of DDT is sufficiently uniform to justify the
use of this method. Where the purified grade of DDT is used, the silver
nitrate solution should of course be standardized against. the purified
DMT and not against the technical grade.
Determination of DDT in F&ulsions
We first applied this method to the analysis of samples taken from
large batches of an emulsion reported to contain 2 percent of technical
DDT, used in the impregnation of clothing. To control this batch-processing
operation it was necessary to check the percentage of DIDT contained in
the emulsion from time to time during the processing. A 5-percent emulsion
was made up using a technical grade of DDT. A portion of this material
was then diluted to a strength of 1 percent, and both samples were analyzed
for chlorine content. Upon titration with silver nitrate solution, which
had been standardized against constant-boiling hydrochloric acid, the re-
sults in both cases showed the chlorine content of the DDT to be 44 per-
cent. Since these low results were found to be consistently reproducible,
and on the assumption that technical DDT contains 50 percent of chlorine,
a correction factor was introduced to give the results in percentage of
Table 1 shows the calculation of this correction factor. Table 2
gives the results obtained in the control analyses on 2-percent emulsions
used for impregnating clothing. The use of a correction factor is obviated
by the standardization of the silver nitrate solution against the combus-
tion of the DDT being used as already recommended under Procedure. The
titer is then expressed directly in milligrams of DDT rather than in
milligrams of chloride ion per milliliter of silver nitrate solution.
Application of Method to Spray Deposits or Residues
In determinations of. sprav deposits or residues of DDT on plants3,
the plant material may be extracted with a suitable solvent such as Vlene,
cyclohexanone, or benzene, and made up to a definite volume. A l-ml.
aliquot of this extract can be run by this method in precisely the same
way as described for the DDT emulsions. Where only minute amounts of
DDT are encountered, sufficient plant material should be extracted or the
extract should be concentrated by evaporation of solvent, so that a 1-ml.
aliquot contains a minimum of 1 mg. and preferably about 50 mg. of DIDT.
In case the DDT concentration in the solvent is very low, several 1- or
2-ml. aliquots may be concentrated successively in the combustion tube,
with the Variac set at 15 volts. The aspirator is then adjusted and the
combustion carried out.
It should, of course,. be borne in mind that the presence of other
halogen-containing compounds may interfere with the determination of DDT
by this method.
A modification of Winter's method for the determination of halogen
in organic compounds has been adapted to the determination of DDT. About
1 hour is required for an analysis te be carried out, and the method is
sufficiently sensitive for the determination of DDT in spray residues.
/ Fahey, J. Z. The Determination of DDT Spray Deposits on Apples.
Submitted for publication.
Table 1.--Analyses of DDT emulsions, showing calculation of the correction factor
DDT in sample
Percent (wt rol.)
Correction factors: For 5-percent emulsion, 50.0/44.0 = 1.14;
for 1 percent emulsion, 10.0/8.8 = 1.14.
Table 2.--Control analyses on 2-percent DDT emulsions used for impregnating clothing
Description of sample Cound Cor. value DDT (cor. value)
Dipping solution (1-ml. samples):
Before dipping of first batch 8.6
End of first batch 8.1
End of second batch 8.4
End of tenth batch 6.9
First sample on following morning 7.9
New lot before dipping 8.9
End of tenth batch 8.5
Composite, drums 1 and 2 26.4
Composite, drums 3 and 4 25.7
-/Containing 20 percent of DDT (technical
an organic solvent.
grade) plus a wetting agent and
UNIVERSITY OF FLORIDA
3 1 262 09240 895111 l Ini I 111111111
3 1262 09240 8995
-- -*-Io0 mfn. O.D.
Figure 1.--Apparatus used for chemical determination of DDT.