Use an amount of chloramphenicol to have a final concentration of 0.05 g/L (filter sterilize it and add it to the medium after cooling down to 45-50ºC). although some references say chloramphenicol is heat stable, but the optimal way to add any antibiotic to the any medium is by filter sterilization. If you are not able to use filter sterilization you can add it before autoclaving.