Electroosmotic velocities of buffered solutions are shown for a bare silica capillary and one with aminopropyl groups (silicaSiCH2CH2CH2NH2) covalently attached to the wall. A positive sign means that flow is toward the cathode. Explain the signs and relative magnitudes of the velocities. Electroosmotic velocity (mm/s) for E 4.0 104 V/m Capillary wall pH 10 pH 2.5 Bare silica 3.1 0.2 Aminopropyl-modified silica 1.8 1.3 SOURCE: K. Emoto, J. M. Harris, and M. Van Alstine, Grafting Poly(ethylene glycol) Epoxide to Amino-Derivatized Quartz: Effect of Temperature and pH on Grafting Density, Anal. Chem. 1996, 68, 3751. 25
1.4 | FORMAL CHARGES Formal Charge: Is associated with any atom that does not exhibit the appropriate number of valence electrons. ✦ How to Identify if a formal charge is required 1. Determine the appropriate number of valence electrons for an atom. 2. Determine whether the atom exhibits the appropriate number of electrons. • Group number indicates the appropriate number of valence electrons for each atom. • After identifying the appropriate number of electrons for each atom in a Lewis structure, the next task is to determine if any of the atoms in the Lewis structure exhibit an unexpected number of electrons.