Phosphoric acid is a relatively weak acid used in a surprising number of applications: rust removal, dentistry, disinfection, pharmaceuticals, food (just think of the bite in coca cola -- it's due to phosphoric acid) and antifreeze, to name but a few. (Ref 1 pg. 6) A spill of phosphoric acid is hazardous to both your health and the environment, however, and even used carefully it must still be neutralized and cleaned. By nature, acids are neutralized with bases. Using a suitable and convenient base substance and observing a few precautions will make clean up safe and fairly simple.
Clear the area of other people. Fumes from the phosphoric acid alone are hazardous and the the neutralization process may cause the release of corrosive irritants such as chlorine gas. (Ref 2 - 4th parag. from bottom)
Ventilate the area thoroughly. Open windows and turn on fans to increase the air flow. Concentrated fumes from the acid and clean up are more toxic than diluted vapors. (Ref 2, par 4)
Sprinkle spills with either baking soda or powdered garden lime (both bases). (Ref 2, 7th par. from bottom) The amount to use will depend on the volume of the spill; use generously and work carefully to avoid splattering the phosphoric acid. If you are neutralizing a controlled use of acid, you don't have to sprinkle it with a base before cleaning, but it is still a good precaution.
Allow the base to sit on the acid, absorbing as it neutralizes. You should notice some fizzing, indicating it is working. (Ref 2, 4th par. from bottom) Give it several minutes to work and do not stand over it while you wait.
Sweep the sodden base and acid mixture up using a broom and dust pan. Dispose of the waste in a plastic trash bag, sealing it well before discarding. (Ref 3)
Mix a gallon of water, in a bucket, with either a cup of household ammonia (Ref 2 and 4 ) or enough of an alkaline dishwasher or dish detergent to create an abundance of suds. (Ref 5. pg. 4) All dishwasher and dish detergents are alkaline -- it makes them able to clean better and helps neutralize acids. However, detergents based on oxygen bleach or enzymes are less alkaline. Use a plain, higher alkalinity detergent for best results. (Ref 6)
Flood the area where the acid was with enough of the water to saturate the spot. Use a mop or old cloths, depending on the amount of acid that was involved and the location of the clean up, to swab the area. (Ref 2,5)
Rinse the area with fresh, clean water. Continue rinsing until the area is completely clean. Air-dry as desired. (Ref 2,5)
Flush your broom, dustpan and mop with plenty of water after neutralizing the surfaces. Throw cloths away along with the acid and base mixture. (Ref 3)