Remove skins in quarters from grapefruit and lemon. Set fruit aside. Lay quarters
flat; shave off and discard almost all white part. With a sharp knife or scissors,
slice rind very thin. Combine rind, water and baking soda in large saucepan. Bring
to a boil and simmer, covered, 20 minutes, stirring occasionally.
Chop peeled grapefruit and lemon; discard seeds. Add to cooked rind and continue
simmering 10 minutes.
Measure 3 cups fruit mixture into large saucepan. Add sugar to fruit in pan;
mix well. Place over high heat; bring to a full, rolling boil, stirring constantly.
Boil hard 1 minute. Remove from heat and stir in pectin at once. Skim off any foam.
Stir and skim 7 minutes.