THE DIRECT DETERnINATION OF AVAILABLE PHOSPHORIC ACID. Ds F , P. V I X T C H . K e c e l i d S c p t e n l l h 4 , ,899.
T
HE direct determination of available phosphoric acid is not
nen7, being official in several of the European countries. I n this country, however, it has not met with favor, probably because the citrate method is not official here. T h e necessity of destroying the orgaiiic inatter before precipitating with molybdate solution precludes the use of the molybdate method. I n 1893 Ross presented a method' for the direct determination of the rcverted phosphoric acid. TT'hile the aim of this method met with hearty approval from the official chemists, the method itself did not, owing to sonie difficulties met with in the manipulation, and more particularly to tlie fact that it did not give results agreeing with the official method.' Agreement could hardly be expected seeing that the method did not account for that phosphoric acid removed in t h e water used in washing t h e citrate insoluble. Believing that this last-mentioned fact, zv'z., that the Ross method did not account for the phosphoric acid contained in the wash.water of the official niethod, is the principal cause of the difference in the results by the two methods, in I89j, the writer did some work with the view of proving his hypothesis. T h i s work consisted of the determination of the water-soluble phosphoric acid by the volumetric method, as modified and carried out by- the writer ;" the direct determination of the citrate-soluble by tlie citrate method, i n fifty cc. of the citrate filtrate, and the determination of that removed by washing the citrate-insoluble residue. using the modified volumetric method. T h e sum of these three results should equal the available phosphoric acid by the officiai method. I t is perhaps sufficient to say that the citrate method at that time and later4 gave satisfactory results on sixty or seventy samples. T h e results of the comparison are given in the table. Department of Agriculture, Division of Cheinistry, Bulletiii 38. p . 17. U. S. Department of Agriculture, Division of CIiemislry. Bulletin 13.p. 72 ; a n d Bulletin 47, p . XI. 3 This Journal, 18,389. and U . S. Departineut of Agriculture. Division of Chemistry, Bulletiii 49, p . 61. 4 U. S.Department of .4grictllture, Division of Chemistry, Bulletiu 49. p p . 61. et seq. 1 U. S.
2
AVAILABLE PHOSPHORIC ACID.
1091 Official method.
Direct method. 7
7-
Soluble. P e r cent.
Reverted P e r cent.
I1
13 12.46 10.88 4.80 8.25 7.56 8.08 7.9.03 1.18 1.08
I2
3.10
3.60 2.96 3.68 5.13 2.65 3.61 2.24 4.76 3.12 6.22 6.35 4.88 1.73
I 2
3 4 5 6 7 8 9 IO
IO.
Wash. P e r ceut. 0.24
0.17 0.21
0.29 0.47 0.26 0.25 0.37 0.28
0.68 0.99 1.48
Available P e r cent.
Total. P e r cent.
13.97 15-59 14.77
16.15 16.09 16.10 11.70 12.17 14.48 12.79 16.48 13.86 12.74 12.96 15.41 8.16 '1.39 8.84 10.72 12.53 13.28 14.48 11.92
10.22
11.37 11.45 10.57 12.08 '2.43 8.08 8.42 9.46 7.04
-
h
-
7
Available. Total. P e r ceut. P e r cent.
14.00 15.49 14.79 IO.
14
11.24 11.58 10.64 12.21
12.31 8.03 8.32 9.50 7.03 10.86 7.45 8.44 11.82 11.23 9.70 9.70
16.18 15.99 16.12 11.62 11.97 14.64 12.75 16.54 13.74 12.69 12.86 15.45 8.35 11.19 8.85 10.83 12.55 13.40 14.64 11.90
0.21 5. IO 0.09 8.87 2. I O 11.06 2.30 0.39 4.75 15 7.44 16 2.98 0.38 4.97 8.33 iI.80 0. rg 2.34 4.27 I7 0.30 11.11 I8 3.34 7.47 0.40 9.54 5.55 3.59 19 20 2.06 0.16 7.50 9.72 Nos. I, 2 , and 3 were dissolved S. C. rock. Nos. 4, 5, and 6 were dissolved S. C. rock and potash. Nos. 7, 8, and g were dissolved animal bone. Nos. IO, 11, and 12 were raw bone. Nos. 13, 14, 15, 16, 17, 18, 19 and 2 0 were compounded for this w o r k a n d were complete fertilizers.
I3
I4
T h e two methods gave practically the same results on availables and on totals. T h e work also shows very plainly why the Ross method differs from the official, from 0.09 per cent. to 1.48 per cent. being removed and accounted for in the wash-water of the official method that could not be accounted for by the Ross method. Of course the amount removed by the wash-water will vary somewhat in the hands of different analysts, according as they wash the citrate-insoluble much or little. I t is the practice of the writer to wash until the filtrate and washings amount to about 250 cc. A t this point pressure of other work necessitated the dropping of this, and nothing more was done until the present summer, when it was determined to extend the work somewhat. A comparison of the official method with the citrate and the molybdate methods, precipitating with magnesia mixture and
F. P. V E I T C H .
I092
with molybdate solution, respectively, in the mixed filtrates, containing the water-soluble and the citrate-soluble, was undertaken. T h e method pursued was a s follows : the water-soluble, extracted a s usual, was received in a 500 cc. flask, graduated roughly at 250 cc. and containing five to ten cc. nitric acid. T h e citrate-soluble was then extracted as usual, the filtrate and washings received in the flasks with the water-soluble. After cooling, the volume was completed, shaken, filtered, and in aliquots of IOO cc. the phosphoric acid was determined by each method, the molybdate solution being added directly to the solution without destroying the organic matter, but the precipitates were allowed to stand over night before filtering. T h e deterniinations were completed as usual. The samples were carefully selected to represent most classes of goods found on our markets. Most of them contained organic matter, 5ome in considerable quanti ties. Available official method. Per cent. 21
Dissolved S. C. rock
22
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23 24
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25
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26 27 28 29 30 31 32
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L i
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and potash
c
