16 BULLETIN 144, U. S. DEPARTMENT OF AGRICULTURE. 
It has been found, however, that the quantity of sulphuric acid 
theoretically required to convert phosphate rock into acid phosphate 
does not always produce the best results. The physical condition of 
the acid phosphate is just as important as its chemical composition, 
and sometimes the proper mechanical condition can not be obtained 
except by sacrificing some of the water-soluble phosphate. In factory 
practice it is often well, therefore, to add a little less sulphuric acid 
than is necessary to satisfy the equations outlined above. 
The strength of sulphuric acid used is another detail of great impor- 
tance in the production of acid phosphate. The quantity of strong 
acid sufficient to bring about the desired chemical reactions is of such 
small bulk and has such a viscosity that it is difficult to obtain an 
intimate mixture with the ground phosphate ; moreover, the calcium 
sulphate produced, being much less soluble in strong than in weak 
sulphuric acid, 1 forms a relatively insoluble coating over the phos- 
phate, preventing further action by the acid. 
On the other hand, if very dilute acid is employed, the amount 
required to bring about the necessary chemical reactions is so great 
and so much water is contained therein that it is almost impossible to 
obtain a product in good mechanical condition. The strength of acid 
with which the best results are ordinarily obtained ranges from 50° 
to 55° B., though phosphates very high in iron and aluminum com- 
pounds sometimes yield better to slightly stronger acid. 
There is considerable difference of opinion concerning the tempera- 
ture at which sulphuric acid should be added to phosphate rock. 
Some manufacturers, however, give little heed to this important 
point, mixing their acid and rock in the same proportions winter and 
summer at whatever temperature the air happens to be. Others 
believe in heating the acid to 50° to 55° C. before using, while many 
others claim it is bad practice to use acid at a temperature below 25° 
or above 30° C. 
It is obvious that this matter should not be disregarded entirely, 
for in mixing acid and rock either very low or excessively high tem- 
perature may seriously affect the product. No definite rule, however, 
can be prescribed, for here again the composition of the phosphate 
used is the controlling factor. 
Phosphates containing large quantities of carbonates heat up 
rapidly when mixed with sulphuric acid. If the mixture becomes 
very hot violent frothing occurs and the mass is apt to overflow from 
the pan. Rocks of this type should not be treated with hot acid. 
When dealing with phosphates high in compounds of iron and alumi- 
num, however, it often saves time to use acid heated to a temperature 
of 50° to 55° C. The reactions then begin promptly in the pan and 
i Bui. 33, Bureau of Soils, U. S. Dept. of Agr., pp. 41-42 (1906). 
