Use a large over-sized stainless steel or enamel pot for stirring ingredients, not aluminum. Place the tomato juice or puree into the pot.
In blender, put the chopped onion, celery, flour, sugar, salt and pepper. Remove 2-3 cups of the puree from your stockpot and add the the ingredients in the blender and process until smooth and thick. You may have to do this in two or three batches.
Add the thick liquid from the blender to the remaining tomato puree in the stockpot.
Cook over medium heat and stir constantly to avoid sticking. Cook until thick and bubbly and it comes to a boil.
Remove from heat and stir in baking soda gradually. Mixture will foam, using a wire whisk helps stir it up and dissipate the foam. Stir slowly until foaming ends, this may take awhile.
Bring to a boil again, stirring constantly. Boil one minute and then fill pint jars, adjust sterilized lids, and process for 30 minutes at 10 lbs pressure in your pressure canner.
To serve, mix equal parts soup with water or milk. Serves 56 people... or 14 pints.