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November 1951 ET-296
United States Department of Agriculture
Agricultural Research Administration
Bureau of Entomology and Plant Quarantine
A METHOD FOR CALCULATING AND EXPRESSING THE
CONCENTRATION OF INSECTICIDAL CHEMICALS IN SOLUTIONS
By R. D. Chisholm and L. Koblitsky
Division of Insecticide Investigations
The concentration of an insecticidal chemical in a solution may be
expressed in several ways. In formulations sold in interstate commerce
under the Federal Insecticide Act it is expressed on a weight/ weight
(percent) basis. In test formulations other bases sometimes used are
volume/volume, weight per volume of solvent, and weight per volume of
formulation. Often the basis on which the formulation is expressed is
not made clear. Units from both the U. S. and metric systems are some-
times used in a single formulation.
Volumes of insecticidal solutions are often measured in metric units
for laboratory tests and in U.S. units for field use. When the effective-
ness of several chemicals or of a given chemical in various formulations
is compared, it is helpful if the concentrations of sprays can be expressed
in convenient units of both systems. A lack of clearness or of a uniform
basis of expression may be a source of error, or it may be difficult to
measure predetermined doses for such comparisons. A method for cal-
culating and expressing the concentration of an insecticidal chemical on
both weight per volume of formulation and weight/weight (percent) bases,
in convenient metric or U.S. units, is described in this paper.
A test formulation in the developmental stage is prepared in a tared
100-ml. graduated cylinder at room temperature (68 F.). The insecti-
cidal chemical and the other components, except the solvent, are trans-
ferred to the cylinder, which is weighed in grams after each transfer.
Solvent is added, and when all the components are in solution more
solvent is added to make up to the 100-ml. mark. The filled cylinder is
weighed and the weight of the solvent obtained by difference.
Weights in grams per 100 ml. may be converted to pounds per gallon
by dividing by 12. The factor is derived by dividing 453.6, the number
of grams in a pound, by 3785.3, the number of milliliters in a gallon,
multiplying by 100, the number of milliliters in the test formulations,
and rounding the product slightly. The density in grams per milliliter
and percent by weight are obtained by calculation.
UNIVERSITY OF FLORIDA
3 1262 09242 9421
When a formulation is ready for large-scale use and specifications on
the weight/weight (percent) basis are required, the percentages derived
from the test formulation are rounded to convenient units.
A formulation for an ethylene dibromide-chlordane dip developed as
outlined above and expressed on several bases follows:
Ingredient Quantities Calculated values formulation,
Weight/volume Weightweigh w tWeight
Grams/100 ml. Pounds/gallon Percent Percent
Ethylene dibromide 12 1.00 13.08 13.0
Chlordane 6 .50 6.54 6.5
Cellosolve 6 .50 6.54 6.5
Tween 20 6 .50 6.54 6.5
Isopropyl alcohol 61.7 (by diff.) 5.14 67.28 67.5
The specification formulation weighed 91.5 grams per 100 ml., or 7.63
pounds per gallon, at 68 F.
When conversions are made from one basis of expression to another
and figures are rounded, slight discrepancies are unavoidable, but they
are not considered important with reference to the effectiveness of the
formulation for insect control.
It is believed that the adoption of this method as a standard for expressing
the concentration of insecticidal chemicals in solutions will facilitate labora-
tory testing, the establishment of dosage schedules in large-scale use, and
the preparation of specifications on several bases, including weight/weight
(percent). For laboratory testing the method provides for the measure-
ment by volume of predetermined weights of a chemical expressed in
metric units selected with reference to equivalent U.S. units convenient
for large-scale use. For example, a formulation may contain 24 grams
of an insecticidal chemical per 100 ml., or 1.2 grams per 5 ml. This is
equivalent to 2 pounds of the chemical per gallon, or 1 pound per 1/2
gallon. When 5 ml. of such a formulation is diluted to make 1 liter of
spray or when 1/2 gallon is diluted to make 100 gallons of spray, the con-
centrations of the sprays are equivalent.
When specifications on a weight/weight (percent) basis are required,
the method provides, with a little rounding, all the data necessary for
their preparation. It is suggested that such specifications include the
approximate weight in pounds of insecticidal chemical per gallon or the
weight of 1 gallon of the formulation at 680 F.