28 BULLETIN 1452, U. S. DEPARTMENT OF AGRICULTURE 
In deternrning the quantity of methylene blue 19 required to render 
each colloid isoelectric, the procedure was as follows: To each of 
about 10 tubes, 25 cubic centimeters of a suspension containing 10 
milligrams of a freshly prepared colloid was added. Varying quan- 
tities of methylene blue dissolved in 25 cubic centimeters of water 
were then added to each tube, and after mixing, the tubes were al- 
lowed to stand 24 hours. The following day the tubes were shaken 
and the cataphoretic movement of the particles or floes was observed. 
The apparatus used for measuring the cataphoretic movements was 
essentially of the same construction as the one already described by 
one of the writers, (36), the only important changes being that in 
this experiment the tube was provided with glass stop cocks and that 
the plane surface under the microscope objective was ground to with- 
in 0.2 millimeters of the inner wall, leaving the natural curvature 
of the latter intact. The optical arrangement was of the Siedentopf- 
Zsigmondy type. The microscope was focused on a point at a dis- 
tance of 0.7 of the radius above the axis of the tube. This point lies 
in an annular layer in which the electrosmotic movement of the 
liquid in the close tube is theoretically zero, so that observed veloci- 
ties of the particles in this layer should be their true velocities. 
The effect of increasing concentrations of methylene blue on the 
migration of bentonite and eight colloids is shown in Figure 1. The 
velocities of the particles are plotted on the ordinate in microns per 
second for a potential gradient of 1 volt per centimeter. The ordi- 
nates also represent the electro-kinetic potential of the particles, since 
there is a direct proportionality between velocity and charge. 20 
The range over which the colloids were completely flocculated is 
shown by the broken parts of the curves. There was, of course, 
partial flocculation preceding this. 
Each soil colloid has a characteristic curve, but all the curves are 
similar in form. After a certain concentration of dye is reached, 
slight increments affect the charge or movement of the particles pro- 
foundly. Flocculation also becomes complete at this point. In the 
case of bentonite an increased velocity of the particles is produced 
by a concentration of dye somewhat less than that needed to bring 
about complete flocculation. The flattening of the curves of the 
other colloids at this point may be evidence of a similar but less 
pronounced effect. This phenomenon is common in the case of other 
colloids but has not been thoroughly explained. 
Table 16 shows the migration velocity of the particles in distilled 
water without any methylene blue, the quantity of methylene blue 
required to render each colloid isoelectric, and the exchangeable 
monovalent and divalent bases as determined with an NH 4 C1 solu- 
tion, these last values being taken from Table 12. The quantity of 
methylene blue required to render 1 gram of colloid isoelectric is 
expressed in both grams and milliequivalents. Duplicate determina- 
19 The methylene blue used was U. S. P. Medicinal. It contained 13.6 per cent water 
and 0.48 per cent ash. Calculations were based on the assumption that the balance 
consisted of tetramethylthionine chloride. 
20 The electro-kinetic potential is calculated from the velocity by the Helmholtz-Perrin 
formula, £= ^ g p , in which D represents the dielectric constant and n the viscosity 
of the dispersion medium, V the velocity in centimeters per second, H the potential 
gradient in volt/centimeters and 30O 2 is a constant factor to convert H and t to abso- 
lute units. Assuming a value of 80 for the dielectric constant and 0.01 for the viscosity 
of water, f, in millivolts, would be obtained by multiplying the microns/second velocities 
by 14.1. 
