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What is meant by "molten" and "in solution" when describing extremely hazardous substances?

To assist state and local officials in the development of emergency response plans, EPCRA requires the owner or operator of each facility at which an extremely hazardous substance (EHS) is present in an amount equal to or exceeding its threshold planning quantity (TPQ) to notify the State Emergency Response Commission (Section 302). The list of EHSs (found in 40 CFR Part 355, Appendices A and B) whose presence may trigger an emergency planning notification indicates each chemical's threshold planning quantity.

Non-reactive EHSs which are in solid form under standard conditions have two TPQs: a lower threshold, which applies to powders with a particle size less than 100 microns, chemicals in molten form, and solids in solution; and an upper threshold, which applies to all other forms of the chemical (40 CFR 355.15).

What does EPA mean by the terms "molten" and "in solution" when used to describe extremely hazardous substances, and how are these forms quantified for comparison to the appropriate threshold planning quantity?

The term “molten” denotes the liquid form of a solid EHS at elevated temperature, which is normally in solid form at standard temperature and pressure. EPA requires facilities to account for the potential volatility of molten chemicals by applying the lower of the two TPQs listed in 40 CFR Part 355, Appendices A and B, to non-reactive solid EHSs present in molten form. Facilities need not, however, compare the entire weight of a molten chemical to the lower TPQ. The Agency examined the fraction of volatilization expected for the molten solids on the list and found that it ranges from 0.3 to 0.008 pounds/minute per pound spilled. Since data were not available for all solids and to be conservative, the Agency chose to incorporate the 0.3 fraction into the reporting requirements (59 FR 51819; October 12, 1994). To determine if the presence of a molten solid EHS triggers an emergency planning notification, the facility owner or operator should therefore multiply the weight in molten form by 0.3 and compare the resulting figure to the lower TPQ for the chemical in question (40 CFR 355.16). On March 22, 2012 (77 FR 16679), EPA clarified that the application of the reducing factor of 0.3 is to be applied for only non-reactive molten solid EHSs (40 CFR 355.16(c)).

A solid EHS is present “in solution” when dissolved, suspended or emulsified in a liquid. Solutions include aqueous or organic solutions, slurries, viscous solutions, suspensions, emulsions, and pastes (40 CFR 355.61). When determining if the presence of a solid EHS in solution triggers an emergency planning notification, the facility owner or operator should multiply the weight of the solid in solution (rather than the entire weight of the solution) by 0.2 and then compare this amount to the lower TPQ for the chemical in question (40 CFR 355.16(b)). The 0.2 reducing factor is used because experimental data for accidental releases of liquid aerosols shows that no more than 20% of the release is expected to remain airborne. This change in reporting for non-reactive solid EHS in solutions was published in a March 22, 2012 final rule (77 FR 16679).