Chemical Ratio Tester

For a mix of hydrofluoric and nitric acids, after one week the tank will be gone, as will the contents - unless the tank is made of something like borosilicate glass. Beware the exotherms in the drainage system!

Check-out the Cole-Palmer chemical resistance database on the 'net. Choose an "excellent"-rated material-of-construction for the tank, and the ratio of materials in the tank after one week will be the same as it was seven days ago, other influences ignored, thereby obviating the need for such an instrument, the like of which does not exist at present.

Not even borosilicate glass:

http://www.pegasus-glass.com/pyrex.asp