Title: Contributions from the Botanical Laboratory, vol. 11
Identifier: contributionsfro11univ (find matches)
Year: 1934 (1930s)
Authors: University of Pennsylvania. Botanical Laboratory; University of Pennsylvania. Morris Arboretum
Subjects: Botany; Botany
Publisher: Philadelphia : (s. n. )
Contributing Library: Penn State University
Digitizing Sponsor: Lyrasis Members and Sloan Foundation

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296 AMERICAN JOURNAL OF BOTANY (Vol. 21, (with the latex in it) by the quinhydrone method (Michaehs, 1926; van Harpen, 1929, 1930- A "type-K" potentiometer and a saturated calomel cell were used in each case. The standards proposed by Clark (1928) were used to convert voltage to pH. Measurements of electrophoretic velocity were made by the microscopic method with a modified Northrup-Kunitz (1925) apparatus after the design used by Mudd, Lucke, McCutcheon, and Strumia (1928). Three radio B batteries, giving approximately 135 volts, were connected at each end of the cell to non-polarizable electrodes of zinc in saturated ZnSO,. The electro- phoresis cell was mounted with oil immersion contact over a Zeiss Wechsel- condensor. A single cell was used throughout. A 28 X Zeiss ocular and a Bausch & Lomb 8 mm. objective combined working distance with sufficient magnification. The apparatus is shown in figure i.
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\ Fig. I. The Northrup-Kunitz electrophoresis apparatus. Since velocities are given in ^/sec./volt/cm., the potential drop per cm. must be found. Following Ohm's Law, where E is the potential across the chamber itself; L, the length of the cham- ber- Q, its cross section area; R, the specific resistivity of the fluid filling it; / tiie current; and K, a constant dependent upon shape. For calibration,^ the cell is filled with mercury and placed in series with a dry cell, a rheostat, and a Weston standard ammeter. A potentiometer is connected across the cell by small platinum electrodes led into it. The P.D. and current are read 1 Thanks are due to Professor Charles Weyl for suggesting this method of caHbration. »' J .... If X 4 ^IV June, 1934I MOVER — EUPHORBIA 297 simultaneously and the resistivity of the mercury at room temperature is found from tables. Having found K, it is used, in practice, in the equation H = KRI where H is the P.D. per cm. R is measured by a Wheatstone Bridge; /. by a milliammeter, which can be introduced into the circuit; and L is meas- ured directly. The cell was flushed several times with part of the buffer before the latex suspension was introduced. Care was taken to prevent the mclusion of air bubbles. The current and resistivity were measured at the start. Velocity was measured with a stop-watch by determining the time required for a latex particle in sharp focus to travel between two lines of the ocular micrometer and back again; a Pohl mercury commutator was used to change the direction of migration when the particles reached the second line. Due to the charge which the glass wall assumes against the water and the consequent electroendosmotic streaming when the circuit is closed, it is neces- sary to measure the particle velocity at definite depths to eliminate this source of error. Smoluchowski (1921) has formulated these levels for flat cells. These " stationary " levels lie at 0.21 and 0.79 of the total depth of the cell. At least five readings were made at each level, and the mean of these was taken to calculate the velocity in />t/sec./volt/cm. Measurements were made at temperatures from 21° to 28° C. The tem- perature coefficient of velocity was found to be approximately 2 per cent per degree centigrade. This same coefficient has also been used by Abramson (1929, 1932) for protein-coated particles. To aid in comparison, all data were recalculated to a temperature of 25° C. Variations in the coefficient ap- pear to lie outside its experimental error, so that the same value was used throughout. Velocities were plotted against pH, all curves being drawn to the same scale. As shown by Abramson (1931), Freundlich and Abramson (1928), Henry (1931), and Sumner and Henry (1931), the Helmholtz-Lamb equa- tion, C HD (where H is P.D. per cm., D, dielectric constant, f, the electrokinetic poten- tial, rj, coefficient of viscosity, and V, the velocity—all units being electro- static), is valid for insulating particles. Using this equation and making certain assumptions for r; and D, the zeta potential can be calculated in milli- volts by multiplying the observed velocity by 12.6 (Northrup and Cullen, 1922). It was thought better to express the results simply in terms of mobility. Near the isoelectric point measurements of time are not very accurate, since the time needed to travel the required distance approaches infinity as

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