  Large fluctuations in the electron column density can occur during the
reionization process. We investigate the possibility of deriving the electron
density fluctuations through detailed mapping of the redshifted 21-cm emission
from the neutral medium during reionization. We find that the
electron-scattering optical depth and 21-cm differential brightness temperature
are strongly anti-correlated, allowing optical depth estimates based entirely
on redshifted 21-cm measurements. This should help isolate the CMB polarization
fluctuations that are due to reionization, allowing both cleaning of the patchy
reionization polarization signal as a contaminating source of confusion to
other signals and a measurement of the primordial quadrupole that would be
measured at various locations in the universe at the epoch of reionization.
This latter application in principle allows mapping of the primordial density
field at z~1100 over a large fraction of the Hubble volume.
