Scrape outer skins of oranges with blade of a knife to release citrus oils. Peel oranges, and grind rind; reserve pulp. Combine ground orange rind, 3 cups water, and soda in a medium size stainless steel saucepan; bring to a boil. Reduce heat; cover and simmer 15 minutes. Rinse well; drain.
Add remaining water and 1/2 cup lemon juice to prepared orange rind in saucepan; bring to a boil. Reduce heat; cover and simmer 15 minutes. Let stand in cooking liquid overnight.
Cut reserved orange pulp into quarters; remove and discard seeds. Place pulp in container of an electric blender; process until smooth. Add to orange rind mixture; stir until well blended.
Place 3 cups orange mixture (reserving remainder for other uses) in a flat-bottomed kettle. Add remaining lemon juice and sugar, stirring well. Slowly bring to a boil, stirring until sugar dissolves. Boil, stirring occasionally, until mixture registers 220° on a candy thermometer or until mixture sheets from a cold metal spoon. Remove from heat, and skim off foam with a metal spoon.
Quickly ladle marmalade into hot sterilized jars, leaving 1/4- inch headspace. Cover at once with metal lids, and screw bands tight. Process marmalade in boiling-water bath 10 minutes.