In a large saucepan, saute onion and celery in butter until tender. Add broth and baking soda. Combine cornstarch and water until smooth; gradually add to pan. Bring to a boil; cook and stir for 2 minutes or until thickened.
Reduce heat. Add sauerkraut, cream and corned beef; simmer and stir for 15 minutes. Add cheese; heat until melted. Add salt and pepper. Garnish with croutons if desired.