The azide is a small molecule, and will pass through the pores of the dialysis membrane just as buffer salts would. Use a standard dialysis membrane of about 12,000 to 14,000 molecular weight cutoff, the azide will pass through the membrane while the proteins will remain inside. This will give effective buffer exchange. Dialyze a few ml of serum against 1-2 liters of azide free buffer (PBS is OK). Dialyze at 2-80C for about 24-48 hours. The buffer should be changed three times during this period. The exact timing is not critical. Each change of buffer dilutes the azide level further.