Patrick further reading of my notes indicates that Rodinal (current variety) does have a Specific Gravity of 1.383 so that's a very clear indication that the Sulphite is up around the 344g/litre mark, Bayer quote a total solids content of 432g/litre.

The Potassium Sulpjite is the major ingredient in Rodinal. I've had Sodium sulpite that wouldn't dissolve and it something must happen to prevent it.

Ian