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Adding a few drops of D2O to an NMR sample containing acidic protons causes them to be replaced by deuterium. A possible mechanism is shown above. The vast number of water molecules in one drop of D2O drives the equilibrium so that the ethanol OH is completely replaced by deuterium.
The yellow deuterium atom replaces the white hydrogen atom on the alcohol group. The new O-D group would not appear on a HNMR spectrum.
Ethanol equivalent protons in NMR.