APPENDIX. 
THE DETERMINATION OF PHOSPHORIC ACID IN HAWAIIAN SOILS. 
During the course of a great many soil analyses in this laboratory, 
it has been found that influences of an inhibitory nature seriously 
affect the determination of phosphoric acid in the hydrochloric acid 
extract of Hawaiian soils. The error resulting has always been 
attributed to the presence of titanium, although no work has been 
done to establish this theory definitely. 
The nature of the error is indicated by the appearance of a white 
precipitate upon dissolving the yellow phosphomolybdate precipitate 
in ammonia. A part of this white precipitate may pass through the 
filter paper and appear in the filtrate in a flocculent form which settles 
out on standing overnight, or it may be precipitated before passing 
through the filter paper and hence mask the appearance of the error. 
In many soil extracts it is present in appreciable amounts, produc- 
ing seriously misleading results. On dissolving the yellow precipi- 
tate on the paper by treating first with hot water and then making 
alkaline on the filter with ammonia, a larger precipitate is obtained 
than if it were dissolved by a hot solution of ammonia. 
The size of the error due to this white precipitate is indicated by 
the data in Table XV showing the difference in phosphate content 
of f 12 Hawaiian soils as determined by the official method. In one 
series the precipitate was removed before adding the magnesia 
mixture, in another the magnesia mixture was added directly. 
Table XV. — Variation in phosphoric acid content as determined by different methods. 
Method. 
Soil 
No. 1. 
Soil 
No. 2. 
Soil 
No. 3. 
Soil 
No. 4. 
Soil 
No. 5. 
Soil 
No. 6. 
Soil 
No. 7. 
Soil 
No. 8. 
Soil 
No. 9. 
Soil 
No. 
10. 
Soil 
No. 
11. 
Soil 
No. 
12. 
Filtered 
P.c. 
0.299 
.395 
.175 
P.c. 
0.311 
.399 
.188 
P.c. 
0.346 
.419 
.224 
P.c. 
0.304 
.349 
.227 
P.c. 
0.412 
.596 
.204 
P.c. 
0.330 
.495 
P.c. 
0.280 
.446 
P.c. 
0.526 
.733 
.569 
P.c. 
0.515 
.824 
.514 
P.c. 
0.277 
.445 
.219 
P.c. 
0.302 
.402 
.240 
0.44^ 
Not filtered 
.607 
.382 
As indicated in the table, the error varies from 0.073 per cent to 
0.309 per cent, depending on whether the white precipitate is removed 
by filtration or weighed as magnesium pyrophosphate. The volumet- 
ric method of titrating the yellow phosphomolybdate with standard 
alkali does not eliminate the error, but for comparison, the deter- 
mination by this method is given in the table. 
(42). 
