Heating NaOH and S Na2S2O3 is sodium thiosulfate, also called hypo. In the laboratory it is prepared by heating an aqueous solution of sodium sulfite with sulfur or by boiling aqueous sodium hydroxide with sulfur. The reaction can be represented as 6NaOH + 4S → Na2S2O3+2Na2S+3H2O Upon cooling sodium thiosulfate crystallizes out.