Acidulation of phosphate rock: Reaction of orthophosphoric acid (H3PO4) with phosphate rock primarily produces which commercial fertilizer product?

Introduction / Context:Phosphate fertilizers are made by treating phosphate rock (largely fluorapatite) with acids. The choice of acid determines the product: sulfuric acid produces single superphosphate with gypsum byproduct, whereas phosphoric (orthophosphoric) acid produces a more concentrated, largely water-soluble product known as triple superphosphate (TSP).

Given Data / Assumptions:

• Feed: phosphate rock, chiefly Ca10(PO4)6F2 variants.
• Acid: orthophosphoric acid (H3PO4), not sulfuric acid.
• Goal: identify the primary fertilizer produced.

Concept / Approach:Acidulation with H3PO4 converts the insoluble rock phosphates into monocalcium phosphate, Ca(H2PO4)2, with minimal gypsum formation. This results in triple superphosphate, a high-analysis P fertilizer (typically about 44–46% P2O5) with good water solubility, suitable for various crops and soils when properly placed.

Step-by-Step Solution:

Verification / Alternative check:Process descriptions of TSP plants show digestion of rock with merchant-grade phosphoric acid, granulation, and drying to yield high P2O5 product.

Why Other Options Are Wrong:

• SSP results from sulfuric acid treatment and contains gypsum.
• Metaphosphoric acid is a condensed phosphate not directly produced by this route.
• MAP and DAP require neutralization of phosphoric acid with ammonia, not rock acidulation alone.

Common Pitfalls:Conflating “superphosphate” generically with SSP; the type of acid matters for the final product composition and analysis.

Final Answer:Triple superphosphate (TSP).