I N D U S T R I A L A N D ENGINEERING CHEMISTRY
March, 1925
Free Acid in Acid Phosphate-Its and Value’
269
Determination
By P. McG. Shuey SHUEV&
Co., SAVANNAE, GA.
T T H E Milwaukee meeting of the American Chemical
A
Society the writer presented a paper on the comparison of various methods for the determination of free phosphoric acid, in which he pointed out that the accuracy of the determination by titration depends upon securing an indicator that tells when the first or the second stage of the reaction has been completed. For instance, in the titration of free Hap04 with standard alkali, the first stage of the titration is represented by the equation HsP0,
+ NaOH
= NaHzPO,
f HzO
and the second stage by
+ NaOH
f H20 It is clearly seen that the same amount of alkali is employed in NaHZP04
= NaeHPOd
completing each reaction, and that the proportion of alkali to this case, 40 parts of NaOH to 142 parts of PzOs. For many years free phosphoric acid has been determined by titrating the first stage of the reaction in an aqueous solution, after the free acid has been washed out with water; but it has been found by experiment that methyl orange does not show the completion of the first stage, but gives an end point about 0.5 cc. 0.1 N alkali beforehand on the basis of a 5.5-cc. titration. This was determined by an experiment on sirupy phosphoric acid. Alcohol, although a suitable solvent for free acid, is not suitable as a titrating medium, probably largely owing to its tendency partially to oxidize into aldehydes with splitting off of carbon dioxide and regular results could hardly be obtained by its use. Acetone is equally as good, if not a better solvent than alcohol, and it has the advantage that it may be readily procured in a neutral condition. It does not change appreciably in composition and therefore does not have to be neutralized before use. It has the further advantage that any free sulfuric acid may be determined during the same titration. More than a year of continuous use of acetone as a solvent has shown that it is far superior to any other known method of extraction, and results obtained are most concordant and accurate. Until recently there was no uniformity of results by various laboratories with respect to the determination of free phosphoric acid, but with the acetone method, which is being used by some of the larger companies and laboratories, much more regular results are obtained, and this regularity will continue to increase in direct proportion to the adoption of one uniform, accurate method. The accuracy of the method under discussion is shown by the very close agreement usually obtained between the titration of the first stage with methyl red and the second stage with phenolphthalein. This further shows that free sulfuric acid, which would be found in addition to the first stage of the reaction, is normally only present in very minute quantities, if a t all.
PzOsis the same in each instance-in
* Presented under the title “The Determination of Free Phosphoric and Sulfuric Acids hy the Acetone Method and Their Application to Manufacture” before the Division of Fertilizer Chemistry a t the 68th Meeting of the American Chemical Society, Ithaca, N. Y., September 8 to 13, 1924.
Acetone Method U. s. P. acetone; should be transparent and colorless; specific gravity 0.788 to 0.790 a t 25” C.; neutral or small blank to indicators used. S o d i u m hydroxide: 0.1 N , free from carbonates. Indicators: methyl red and phenolphthalein. REAGENTS-Acetone:
PROCEDURE-weigh 2 grams of substance (acid phosphate, acidulated base, or mixed fertilizer) into a dry 100-cc. volumetric flask, fill to mark with acetone, and stopper tightly. Shake every 12 minutes for 2 hours. Filter, dilute 50 cc. of the clear solution to about 250 cc. with cold, boiled distilled water, and titrate with carbonate-free 0.1 N sodium hydroxide, using methyl red as the first indicator. When the neutral point is reached, note the number of cubic centimeters used, add phenolphthalein, and continue the titration, without refilling the buret, to the neutral point, indicated by phenolphthalein. The total titration from zero to the end point indicated by phenolphthalein represents the H3P04 HzS04. The difference between the total titration and that indicated by methyl red represents one-half of the H3P04. Per cent H3P04is found by doubling this titration and multiplying by the factor 0.4903. The difference between the total number of cubic centimeters used and the total number required by the HaPo4 represents the &So4 titration. Per cent H2S04 is found by multiplying this titration by the factor 0.4904. In some localities it has long been the custom t o report free phosphoric acid as H3P04; in other localities it is reported as PzO:. Therefore, one should state the form of phosphoric acid when reporting results. The acetone method shows calculations to H3P04. The factor for converting &PO4 to PzOcis 0.7245.
+
Value of Free Acid Determination The value of the determination of free phosphoric acid to the manufacture of acid phosphate and fertilizers is primarily to show the potential acid-activity and curing qualities of acid phosphate. Its potential acid-activity is not only the extent to which the “insoluble” may be expected to become lower, but it also tells one what may be expected when mixed with other substances-as, for example, other phosphatic material high in “insoluble,” and cyanamide. Ordinarily, only a limited amount of Cyanamid is used in dry mixed goods, but the amount may be increased in direct proportion to the free-acid content of the acid phosphate. There has been added as much as 480 pounds of Cyanamid to acid phosphate, the latter having been made by acidulating with twice the usual amount of sulfuric acid, and the maximum reversion to insoluble was only 2.10 per cent after 48 hours, and in one month it had dropped to 0.69 per cent. The acid phosphate used contained 11.39 per cent free Pz05. It is also noteworthy that in an experiment in which 160 pounds of Cyanamid were added to a ton of the same overacidulated acid phosphate the reversion was only 0.09 per cent in 24 hours. To show the curing qualities of acid phosphate with respect to the free acid, there is given below the result of some experiments on acid phosphate made from 68 per cent rock,
INDUSTRIAL AND ENGINEERING CHEMISTRY
270
using about the usual amount of sulfuric acid, the acid phosphate having been made a t a factory, and the samples having been drawn under factory conditions:
AGE
Insoluble phosphoric acid Per cent
Free PzOa Per cent
Free HaPOi Per cent
4 hours 19 hours 48 hours 3 days 5 days 8 days 2 weeks
2.72 1.82 1.52 1.39 1.58 1.38 1.11
8.67 5.32 4.70 4.08 4.35 4.22 3.46
11.96 7.34 6.49 5.63 6.00 5.82 4.77
Free HzSOi Per cent None None None None None
0.07
None AVERAGE..
. . . . . . ..
Compensation 1% free PzOa Per cent
...
0.27 0.30 0.29 0.26 0.30 0.31 0.29
rl sample of well-cured acid phosphate that was taken a t random from a pile 7 months old, and manufactured by the same formula, showed: insoluble phosphoric acid 0.46 per cent, and free P20a,3.01 per cent. It would appear from the foregoing that the free phosphoric
Vol. 17, No. 3
acid acts regularly upon the insoluble over a certain period, after which the action only of monocalcium phosphate is brought into play in lowering the insoluble while curing. This means that if one anticipates that, on the basis of 3 per cent free P205 (calculating that 1 per cent equals 0.30 per cent insoluble), the insoluble will be 1.00 per cent, then after good curing for several months the insoluble will be nearer 0.50 per cent. The only objection in running too close on acid in acidulation is that a state of equilibrium might be struck and when it does, reversion as well as conversion might place take. In conclusion, therefore, free phosphoric acid probably plays a more important part in scientific manufacture than has generally been appreciated. It may not only be considered an index of proper and uniform mixing in the manufacture of acid phosphate, but it gives the exact state of acidity, and over-acidulated material may be used to advantage in some instances.
Notes on Continuous Vertical Retorts' By N. H. Memory ISBELL-PORTER C o . ,NEWARK, N. J.
ARBONIZATION of coal to produce supplies of manufactured gas for distribution in cities has been more generally the practice abroad than in the United States. One explanation of this has been the relatively abundant supplies of cheap anthracite and natural gas for domestic fuel in this country. Conditions here have changed and the use of coke as a smokeless domestic fuel is rapidly developing and is providing the market for coke which is essential to the development of coal-gas manufacture. The alternative method of gas manufacture in common use is the generation of blue water gas, which, generally speaking, under present B. t. u. standards calls for enrichment with oil. The present trend toward coal-gas manufacture is based in part on the belief that cheap gas oil is a thing of the past. Most large modern works in this country are being equipped with both coal-gas and water-gas manufacturing facilities. The water-gas plant uses the coke produced in the coal-gas plant as fuel and is independent of the coke market. The use of blue water gas or carbureted water gas to carry peak loads will undoubtedly continue on account of the relatively low cost of the type of apparatus used, even though the gas produced may be expensive. I n the continuous vertical system the coal is elevated to overhead bunkers and from there flows by gravity through the retorts as the coke is extracted continuously and automatically a t the bottom. The coke is delivered a t a temperature below the ignition point and elevated to a sizing plant, and from there is marketed or put in storage. The result of this method of handling materials of carbonization is to bring the operating labor charges to a minimum. The advent of steaming of continuous vertical retorts has cut down the capital charges to a point where the continuous vertical compares favorably with any type of carbonizing plant. By the admission of steam through calibrated orifices a t the base of the continuous vertical retort and consequent formation of blue water gas through the reaction with the incandescent coke charge, an increase of a t least 25 per
C
1 Presented before the Section of Gas and Fuel Chemistry at the 68th Meeting of the American Chemical Society, Ithdca, N. Y.,September 8 to
13,1924
cent in the thermal yield from a ton of coal in the form of gas may be obtained without exceeding the economic limit where it becomes cheaper to generate blue gas in outside generators. I n using continuous vertical plants to furnish gas to mix with natural gas in cases where the natural gas supply is failing, it is essential that the B. t. u. value per cubic foot and the specific gravity of the mixed gas vary as little as possible in order to insure against troublesome burner conditions a t the consumers' appliances. The accompanying table indicates what variations in this respect might be expected under operating conditions assumed for a plant now under construction. Mixtures of Manufactured Gas w i t h Natural Gas, a n d Resulting Values of Specific Gravity a n d B. t. u . Specific B.t.u. per gravity cubic foot DATA: 0.40 580 (1) Coal gas 0.48 435 (2) Steamed coal gas 0.52 363 (3) Complete gasification 0.53 342 14) .~ . . Maximum ulant capacity 0.66 1100 Natural gas Blue water gas 0.56 290 CALCULATIONS: For 860 B . t . u. mixture value: Let X = proportion of manufactured gas in 1000 cubic feet A = B . t . u. value in manufactured gas per cubic foot AX (1000 - X) 1100 = 1000 X 850 X = 2A0,000 1100 A (1) A = 5SO X = 481 = 376 (2) = 435
+
-
(3) (4)
= 340 = 330
= 363 = 342
For 900 B. t . u. mixture value:
x
= 200,000 1100 A
-
580 435 363 342
X
-
-
385
= 301 = 272
265
VARIATION OF MIXEDGASSPECIFIC 860 GRAVITIES 900 Manufactured eas Natural zas B. t . u. B. t. u. "~ 0.481 X 0.40 f 0.519 X 0.66 = 0.534 0.570 0.385 X 0.40 -t 0.615 X 0.66 = 0.376 X 0.48 4- 0.624 X 0.66 = 0.593 0.606 0.301 X 0.48 -I-0.699 X 0.66 = 0.340 X 0.52 0.660 X 0.66 = 0.613 0.621 0.272 X 0.52 -t 0,728 X 0.66 = 0.330 X 0.53 f 0.670 X 0.66 = 0.618 0.626 0.265 X 0.53 0.735 X 0.66 =
+ +
