374 
Journal of Agricultural Research 
Vol. XXVI, No. 8 
10 liters. These are then mixed by slowly pouring the solution of lead 
nitrate into the solution of sodium arsenate and gelatin, stirring con¬ 
tinuously. During this operation the mixture should be tested oc¬ 
casionally with potassium iodid test paper, to determine when the lead 
salt is slightly in excess. This will be indicated by the paper turning 
yellow. 3 A large excess should be avoided, for the nitrate may injure 
the foliage. 4 
CONCENTRATION 
The physical properties of colloidal lead arsenate depend upon the 
concentration of the solutions of lead nitrate and sodium arsenate at 
the time of mixing. The following table shows the results of different 
concentrations upon the physical properties of the resulting arsenate: 
Concentration. Properties. 
1/2 molar solution.Curdy precipitate, particles large; settles 
rapidly. 
4/10 molar solution.Precipitate viscous and cheesy; settles 
rapidly. 
3/10 molar solution.Precipitate viscous; particles seem to be large 
and flaky. 
2/10 molar solution.Precipitate not curdy, particles large; settles 
slightly on standing. 
1/10 molar solution.Precipitate fine and creamy; settles only 
slightly after standing several days. 
1/100 molar solution. . . . Very minute particles. 
It will be noted from this table that very concentrated solutions 
result in a curdy precipitate of lead arsenate, which is undesirable from 
a spraying standpoint, as it will form large, hard particles upon the 
foliage and will not spread evenly. Very dilute solutions tend to form 
small, needlelike crystals. These may be seen under the high power 
of a compound microscope, and the suspension of the arsenate in water 
has a silky, crystalline appearance. The results indicate that 1/10 
molar is the most desirable concentration to use in the preparation of 
colloidal arsenate. 
PHYSICAL PROPERTIES 
The arsenate of lead prepared by the formula mentioned, using 1/10 
molar solutions, is a colloid; it will pass through filter paper and remain 
in suspension several days. The “Brownian movement” of these small 
particles (1 micron or less in diameter) may be seen under high power 
of a compound microscope. The material, when sprayed upon the leaf, 
forms a very thin film over the entire surface, which adheres closely and 
is not easily washed off by rains. 
SUMMARY 
A colloidal lead arsenate may be prepared by precipitating lead arse¬ 
nate in the presence of a protective colloid, such as gelatin, by the chemical 
action between lead nitrate and disodium arsenate. 
The colloidal arsenate is composed of very fine particles which will 
remain in suspension for several days. When sprayed upon a leaf the 
material forms a thin, smooth film over the entire surface, which is not 
easily washed off by rains. 
3 The test paper may be prepared by soaking filtering or blotting paper in a concentrated solution of 
potassium iodid and allowing it to dry. 
4 To make approximately 1 pound of lead arsenate, use 14.03 ounces of disodium arsenate, 14.91 ounces 
of lead nitrate and 1 ounce of gelatin. The salts should be mixed and dissolved as before, except that each 
salt should be diluted to make 3.5 gallons. The resulting lead arsenate may be diluted to any desired 
strength for spraying. 
