empty 30 Gal. barrel of Potassium Hydroxide and Sodium Hypochlorite - safe for collecting Maple syrup?
While I also consider the question somewhat off-topic, I would still like to answer here. In a time in which people eat tide pods for fun it seems kind of necessary...

So, for general information, regardless of what chemical substances were inside the container or what you plan to put into, you should never (re)use containers for chemicals to store food!

Here are some reasons why:


Standard food containers are readily available, do not cost much, and are much better suited to store and conserve food
you are sending a disastrous message to children if you are taking food from a container with chemical warning symbols on it
there is a chance that you mix up your food container with one that contains what is actually on the label
you might not be able to remove all the previous content from the container by rinsing it with water, e.g. if the previous substance is non-soluble in water or if it is greasy. Minuscule quantities of the remaining substance may be toxic or hazardous
the substance may have changed over time if it was stored for a prolonged time or under inadequate conditions, like exposure to heat, moisture or sunlight. Products may precipitate or form films on the inside of the container.
you do not know the history of the container. Maybe the previous owner used up the original content and then used the empty container to store chemical waste. This is also not a good practice, but still more common than to store food!