We show a possible way to measure the column density of free electrons along the light path , the so-called Dispersion Measure ( DM ) , from the early [ \sim 415 ( \nu / 1 { GHz } ) ^ { -2 } ( { DM } / 10 ^ { 5 } { pc } { cm } ^ { -3 } ) { % s } ] 
radio afterglows of the gamma-ray bursts .
We find that the proposed Square Kilometer Array can detect bright radio afterglows around the time \sim 10 ^ { 3 } ( \nu / 160 { MHz } ) ^ { -2 } s to measure the intergalactic DM ( \gtrsim 6000 pc cm ^ { -3 } at redshift z > 6 ) up to z \sim 30 , from which we can determine the reionization history of the universe and identify the missing warm-hot baryons if many DMs can be measured .
At low z , DM in the host galaxy may reach \sim 10 ^ { 5 } pc cm ^ { -3 } 
depending on the burst environment , which may be probed by the current detectors .
Free-free absorption and diffractive scattering may also affect the radio emission in a high density .