I use 100 mL of  1% formaldehyde in 20 g of papilloma, regards,

CANAL

We use 0.2% formalin in 100 ml virus suspension (White spot virus). U need to standardize the formalin percentage required for complete inactivation of  your virus. After inactivation u can check the infectivity of the inactivated virus. 

For the detoxification of Diphtheria, Tetanus proteins we are used 0.7 and 0.6% respectively, in case of virus the range of formaldehyde is 0.2-0.5 (for the production of neural tissue culture rabies vaccine  8mM ), and also the infectivity titre of the3 virus per ml, total protein and the purity these are the limiting factor for the inactivation condition, 

To optimize the inactivation of virus by formalin for preparation of a vaccine, you need to evaluate the inactivation curve of the virus. Inactivation of viruses by formalin depends upon a number of factors, including conc. of virus preparation, amount of impurities or other organic matter, conc. of formalin, temperature, time and pH of inactivation. Formalin conc depends upon a number of factors mentioned. Once you evaluate an inactivation curve, then you have to use a statistical model to ensure that the virus gets inactivated with a high degree of confidence. As you probably know that the polio vaccine inactivated by formalin in late 1950s or early 1960s had live virus, that caused polio in many recipients with several deaths (Cutter Incidence). A robust inactivation method and a sound and sensitive method to detect residual live virus are very important in preparing safe vaccines inactivated by formalin.

Just to make it simple, a suitable formalin concentration for inactivation of a particular virus has to be determined by trying out different concentrations for different holding periods while all other experimental parameters remain constant. Complete inactivation has to be ensured.