Yes, sodium tribasic phosphate (also referred to simply as sodium phosphate) is remarkably safe, especially in the relatively small amounts found in Race Day Boost and Perpetuem. Sodium phosphate enjoys FDA 'GRAS' (Generally Recognized As Safe) status as an emulsifying agent, Nutrients and Dietary Supplement, a sequestrant (a food additive whose role is to improve the quality and stability of the food products), and for miscellaneous use.
The LD50 of orally dosed sodium tribasic phosphate for rats (the amount of an agent that is sufficient to kill 50 percent of the rats) is 7,400 mg/kg. Translated for a 75 kg/165 lb. athlete, that would mean a dose of nearly 555 grams (555,000 mg), an astronomically high amount that no one would even conceive of trying to take. By comparison:
The LD50 of sodium bicarbonate (baking soda) for rats is 4,220 mg/kg
The LD50 for rats consuming acetic acid (vinegar is a dilute aqueous solution of acetic acid) is 3,310 mg/kg
The LD50 of sodium chloride (table salt) for rats is 3,000 mg/kg
More information can be found in the article 'Alleviating Sodium Phosphate Concerns' in Endurance News #69, beginning on page 48.