Thanks for the advice but i had no intention at all to open the tank other
that unscrewing the top plug and insert a cotton swab impregnated with
copper sulfate to touch the wall of the tank.

The tank has been emptied from gasoline and is now 1/2 full of diesel fuel
already. There is no remaining smell of gasoline, just that nasty diesel
smell !

"Dave Hinz" wrote in message
...
On Thu, 27 Apr 2006 11:06:05 -0400, Andre Langevin
wrote:
I have found this:

5% Cupper sulfate CuSO4 is a good solution for that, where it forms black
color with zinc and red with steel.

So i will test with some on a cotton swab and check which color it turns,
black is zinc.

You're aware, I hope, that gas tanks that are "empty" can have enough
residue in them to stay explosive, sometimes for years, right? I went
to an emergency call a few years back where a fellow was cutting an old
gasoline tank, had been empty for as long as anyone could remember.
Took him a couple days to die of his injuries. This isn't the time for
random experimentation. Take it to someone who knows how to handle it.