Although most people store celery in their fridge and consume it fresh, many times the celery is used in casseroles or soups. So just like spinach, when the celery is once heated, it releases nitrates when we cool it and then reheat again.
However, when you are reheating the soup, make sure you take out celery since the nitrate contents of celery can make you a big health problems, if it goes under reheating for the second time.
Generally speaking, any healthy and good nutritional vegetable that comes around the Celery family or carrot family, can turn into toxic food if reheated on a continuous basis. If you put celery in a soup or casserole dish, consume your meal instantly and throw the leftovers in the trash can.