14 BULLETIN 144, U. S. DEPARTMENT OF AGRICULTURE. 
QUANTITY, STRENGTH, AND TEMPERATURE OF SULPHURIC ACED. 
The quantity and strength of sulphuric acid which should be used 
in treating phosphate rock is a perplexing problem, yet many manu- 
facturers give it little consideration, proceeding in a "rule of thumb" 
manner without regard to the composition of the rock. The^ reason 
why many of these latter obtain such good results may be explained 
by the fact that they have been using one grade of rock for years 
and have thus learned by actual experience the proper proportions 
of acid and rock to use. A sudden change in the composition and 
grade of the phosphates often results in a loss of both acid and rock 
in attempts to find the quantity of acid required for this new mate- 
rial. While actual trial mixings should be made when testing out a 
new grade of phosphate, these tests should be intelligently con- 
ducted with due regard to the composition of this rock. Take, for 
example, a sample of high-grade Florida hard rock phosphate having 
the following composition : 
Per cent. 
Moisture 0. 5 
Calcium fluoride (CaF 2 ) 4. 5 
Tricalcium phosphate (Ca 3 (PO,) 2 ) 80. 
Calcium carbonate (CaC0 3 ) 3.5 
Aluminum phosphate (AlPOj 6. 
Silica (Si0 2 ) 5. 5 
Total 100. 
The problem is to convert the phosphoric acid of this rock into a 
soluble form and yet obtain a dry product which can be uniformly 
spread on the soil or readily mixed with other ingredients to make 
up a complete fertilizer. The reactions which are desired may be 
represented in their simplest form, thus : 
(1) Conversion of tricalcium phosphate to monocalcium phos- 
phate and calcium sulphate: 
1 molecule 2 molecules 1 molecule 2 molecules 
Ca 3 (P0 4 ) 2 + 2H 2 S0 4 = CaH 4 (P0 4 ) 2 + 2CaS0 4 
Wt. 310 Wt. 196 Wt. 234 Wt. 272 
(2) Conversion of calcium fluoride to hydrofluoric acid and calcium 
sulphate : 
1 molecule 1 molecule 2 molecules 1 molecule 
CaF 2 + H 2 S0 4 = 2HF + CaS0 4 
Wt. 78 Wt. 98 Wt. 40 Wt. 136 
(3) Conversion of iron phosphate to iron sulphate and phosphoric 
acid : 
2 molecules 3 molecules 1 molecule 2 molecules 
2FeP0 4 + 3H 2 S0 4 = Fe 2 (S0 4 ) 3 + 2H 3 (P0 4 ) 
Wt. 302 Wt. 294 Wt. 400 Wt. 196 
