One word of warning about using mystery vessels. You never know what other purposes the drum was used for after the original product was finished. Some things are actually hazardous to human health and all plastics will absorb some and then leach into your beer.
The suggestions of cleaning with alkaline is a good start. I don't know how you would go about doing a chemical analysis.
Your best bet is stick with known drums and those used for food and marked food grade.
The U.S. Food & Drug Administration (FDA) requires that plastics used in food packaging be of greater purity than plastics used for non-food packaging. This is commonly referred to as food grade plastic. Plastics used to package pharmaceuticals are held to an even higher standard than food grade.
Food grade plastic does not contain dyes or recycled plastic deemed harmful to humans. However, this does not mean that food grade plastic cannot contain recycled plastic. The FDA has detailed regulations concerning recycled plastics in food packaging.
Another aspect of food grade plastic is matching the appropriate type of plastic to the food in question. Foods that are highly acidic or that contain alcohol
or fats can leach plastic additives from the packaging or container into the food. As a result, you should only use plastic containers that are FDA approved for the particular type of food the plastic will come into contact with.
Finally, it should be noted that a plastic container can no longer be considered food grade if it has been used to store non-food items like chemicals, paint, or detergent.
check out this site to learn more about plastics. http://www.virtualweberbullet.com/plastics.html
I bought a 15 gal container that used to hold Hawaiian punch. Try to find one of those.
Your container, while not portable water, is still very handy for using at picnics and bbq etc. A group I belong to has two of these set up on stands laying sideways with faucets attached. We can then wash our hands before eating and after using the porta potty.
have a home brew.