Sampling and Analysis of Phosphorus J. W. H. ALDRED, Tennessee Valley Authority, Wilson Dam, Ala.
A
N IMPROVED apparatus and a procedure for sampling
phorus but it increases rapidly as the phorus becomes poorer,
and analyzing phosphorus have been developed for routine control of the phosphorus condensing plant of the TVA Fertilizer Works located a t Wilson Dam. Ala. (8). The
phos-
Sampler
a
equal part; by volume of heavy d white lead-in-oil paste is used for n. If
Sampling Procedure sed samuler. attached to
a
handle of
quartetturn ana 1s swung a fewtimes short arc to clear the bore of air oraddge from a higher level. The sampler ised and withdrawn from the tank and ir is washed with a stream of water or rhe handle is detached from the sampler peration is repeated with another samother level in the tank. ore of the sampler holds approxigrams of phosphorus. A few unit will represent accurately the quality of high-grade phosphorus, but many
The problem of determining the quality of phosphorus in process or in storage jnvolves removal and preservation of representative samples from large-scale equipment, transfer of the samples t o the analytical apparatus without either loss or oxidation, extraction of the phosphorus with an appropriate solvent, and determination of the phosphorus in the extract. Brown, Morgan, and Rushton (1)reported the initial study made in this laboratory on the procedures for sampling and analyzing phosphorus. After these procedures had been used in plant control, certain improvements were made from time to time and are embodied in the apparatus and procedure described in this paper. In the initial procedure, samples of phosphorus withdrawn from a storage tank by means of the usual types of thief or bottle samplers contained some of the seal water as well as the water dispersed in the phosphorus. To obtain representative samples with these samplers, a large sample was taken, placed under a seal of water, and later subdivided to obtain the sample used for analysis. Subdivision of the sample by the granulating technique described washed away a portion of the salts, dispersed the silica and fine particles of phosphorus, and modified the amount of dispersed water and the resultant granules, so that the sample taken for analysis was not representative of the material initially sampled. The error is slight when this technique is applied to pure phos-
FIGURE 2. EXTRACTIONJUBE 390
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ANALYTICAL EDITION
June 15, 1941
samples are necessary in determining the grade of phosphorus sludge. The extraction apparatus described below permits combination of a maximum of ten 3-gram samples for one analysis. One such analysis has been found to represent accurately a body of the higher grades of phosphorus. The unit samples of the composite sample should be taken so that each unit sample represents au equal volume of the phosphorus in the container.
391
faceof the'top. joint . of theextra& bv insertionof ac&dmetal shield. After introduction of a sutficientnmher of slugs of phosphorus, the extractor is moved downward without stopping the flow of nitrogen, the upper stopper inserted, the adapter removed from the lower end, and the lower cap attached. The weight of the extractor and sample is determined by means of a bdanee aecurate t o 0.5 mg.
Extraction of Sample
FIGURE3.
SAMPLETRANSFER APPAamuS EXTRACTION TUBEIN PLACE
SAXFLEE AND
WITH
Since the entire contents of the samplers are used for analy sis, duplicate samples, when required, should be taken sepa rately. The duplicate sample can he left in the sampler in definitely without troublesome corrosion of the sampler. I the moisture content of the sample can be ignored, the uni samples can be transferred to a vessel containing water an( later analyzed for totel phosphorus and henzeneinsolubl, material.
After weighing the extractor and sample, the cap is removed, the flask containing benzene is attached, the stopper is removed, and the extractor is connected t o the water trap. The complete assembly of the extraction apparatus is shown in Figure 4. The volume of the flask should be adequate to contain the benzene required at room temperature to dissolve rtt least one and onehalf times the quantity of phosphorus in the sample, so that the beniene solution will not be more than two-thirds saturated. The solution should not contain over 2.4 grams of phosphorus per 100 ml. For routine work, a minimum amount of benzene for extraotion of one 3-gram sample is 200 ml. in a 500-ml. flask. After completion of the extraction, the benzene solution of phosphorus is transferred to a volumetric flask of appropriate size (250 ml. for a 3-gram sample) and made t o volume. If, far any reason, the above ratio of phosphorus to benzene has been exceeded, so that the benzene solution is more than twothirds saturated, there is danger of spontaneous ignition of the phosphorus solution when it is transferred t o a volumetric flask. In this case the beneene solution may he left in the boiling flask, oooled, and diluted t o mark. It is convenient to use the lower edge of the ground surface as a calibration mark on the boiling flask. To facilitate mixing, the neck of the boiling flask should be extended by mean8 of a glass-stoppered adapter tube, thus simulating the usual volumetric flask. The volume of water in the trap is read. The extractor is heated at 105' C. for an hour to remove the benzene; the extractor is then stop] Typical results a Weight of extrho weight of extrae Weight of sampl volume of lYater Weight of eatisator benzene-insoluble residue. grams Weight of benzencinsoluble residue, gram Benzene-insoluble. % of dry sample
+
Extractor The extraction apparatus described by Brown, Morgan, an( Rushton has been modified by replacing the crucible an( flask with an extractor tube (Figure 2) and boiling flask. f sintered-glass disk of medium porosity supports the samplt and retains the benzene-insoluble material. A disk witl smaller pores will retain water from the sludges so tenacious12 that the flow of benzene through the disk is impeded and th, benzene overflows through the side arm, carrying with i insoluble material. The extractor tube is provided with i stopper for the large opening, and a cap and an adapter fo the smaJ opening. The apparatus, shown in Figure 3, i; used to transfer the sample from the sampler to the extractor
Determination of Phosphorus in Benzene Solution The determination of phosphorus in the benzene solution involves three steps: (1) transfer of phosphorus from the benzene solution to an aqueous medium by reaction with copper nitrate solution, (2) conversion of the copper phosphide to orthophosphate, and (3) determination of orthophosphate by either volumetric or gravimetric method.
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The phdsphorus ~Glutianshodd bi &aGn into t6e trinsfeer pipet by a vacuum pump, an aspirator
Transfer of Sample to Extractor The extractor with stopper and cap is dried and weighed The extractor is clamped in position, as shown, and then con nected to a supply of nitrogen. The dry sampler is fsrtened ii1 the holder with pins through the wing and placed in position ove the extractor. A stream of nitrogen is passed through the extractor at the rat13 of about 1 liter per minute for 2 to 3 minutes to clear the systen1 of oxygen, and the lever is turned to open the sampler. If thi3 i films that adhere t i t h e walls of the samiler complicate the tians
t
fer of low-grade sludges. The material lost in transfer is chiefl) water. Although the weight of water lost in the sampler witl
The flask 18 s t o p p e d ana shiken mioro1;dY at about 2-mmute intervals for 10 65 15 minutes; thestoppers and sides of the flask me washed down with water, and the mixture is heated on R water bath to volatilize the benaene. A few elass beads are added~toprevent bumping. About 5 ml. of concentrated nitric acid are added cautiously, and the heating is continued until the precipitated phosphides are dissolved. At this point, the clear solution is made up to 250 ml. with distilled water, and an aliquot, representing not more than 15 mg. of phosphorus, is transferred to a 500-ml., widemouthed Erlenmeyer flask. About 10 ~~~
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 33, No. 6
Acknowledgment The author wishes to acknowledge the work of other members of the TVA chemical engineering staff in the develop ment of the procedures described in this paper.
Literature Cited (1) Brown, Morgan, and Rushton, IND. ENS. C~EM.. Anal. Ed., 9, 524-6 (1937). (2) Curtis, Miller, and Newton. Chem. Met. Eng., 45, 193-7 (1938).
P n ~ a n x ~ ebefore o the Dividon of Physical and InorgsniC Chemistry at the lCOth Meeting of the American Chemical Sooiety. Detroit, Mich.
Oxidation of Graphite in the Analysis of Ferrous Metals FIGURE 4. EXTRACTOR TJNITS Metal cans are electriaally heated water bat1hs. Conioal oover on third bath from left aids in heating sinimed-glass disk
about 2.5 to 3.25 em. (1 t o 1.5 inches) beLuwuuc uup. I L ~ ~ ~ U L too vigorously will cause the acid t o creep over the edge, while heating too gently will fail to oxidize the phosphorus completely. The heating is continued until the solution volume is 2 to 3 ml. The solution is then cooled and neutralized, and the orthophosphate content is determined by a standard method. When the method given hy Brown, Morgan, and Rushton was used by the analysts in the routine control laboratory, considerable difficulty was encountered in obtaining check results from aliquots of the same benzene solution. The time allowed for completion of the action of the initial addition of the nitric acid-bromine solution and the subsequent rate of boiling the mixture apparently influenced the degree of oxidation of the phosphorus and the hydrolysis of the resultant oxides. The use of perchloric acid after decomposition of the copper-phosphide precipitate gives uniformly reproducible results. Table I shows the results obtained by two analysts using the modified procedure. Both analysts used aliquots of the same benzene solution of phosphorus. TABLEI. ANALYSISOF PHOSPHORUS Analyst
Phosphorus %
,
RALPH H. STEINBERG AN0 FRED WILSON SMITH Carnegie-Illinois Steel Corporation, South Works, Chemioal Laboratory, Chicago, Ill. Y ~
I.
N THE determination of manganese and phosphorus in
irons containing graphitic carbon it has been necessary to filter off the graphite after the sample is in solution. During solution water must he continually added to prevent the separation of gelatinous silica which would render the grsphite filtration difficult. With many irons these operations may require an hour or more. While perchloric acid will dissolve the sample rapidly, it oxidizes graphite only slightly. However, if 0.2 gram of sodium dichromate (or other soluble chromium salt) is added to the sample and the contents of the flask are fumed strongly, the graphite will be completely and rapidly oxidized. The chromium is subsequently volatilized quickly (2)by the addition of sodium chloride. A single determination for manganese can he made in 25 minutes, and 12 to 18 determinations in an hour. The method can also be adapted to the determination of phosphorus with at least two advantages. The use of nitric acid in conjunction with the usual hydrochloric-perchloric acid mixture has been found to eliminate the loss of phosphorus as phosphorus hydrides, and the rapid oxidation of the graphite saves much time as compared with prior procedures.
Procedure for Manganese
The sum of the benzene-soluble phosphorus and benzeneinsoluble residue seldom equals 100 per cent. The sample used to obtain data given in Table I contained 0.6 per cent benzeneinsoluble material. The difference of 0.4 per cent between 100 per cent and the sum of the benzene-insoluble material and phosphorus covers the inaccuracies of the method and unaccounted for substances such as silicon tetrafluoride and other compounds of flnorine soluble in phosphorus and benzene.
To a 1.000-gram sample (0.5 gram for over 1.25 per cent of manganese) in 8 500-ml. Erlenmeyer flask add 0.2 gram of solid sodium dichromate, 5 ml. of dilute hydrochloric acid (1to l),and 25 ml. of 72 per cent perohlano acid. Heat until dense white fumes issue from the mouth of the flask; then volatiliae the chramium by adding 0.5 gram of sodium chloride at short intervals until the red fumes cease to be evolved upon the addition of the salt. Remove the flask from the hot plate, let it cool somewhat, wash down its neck and sides with water, and again heat until its contents fume. When the flask cleam of white fumes (fumes will still form at the mouth), remove it from the hot plate. Add 3 ml. of 85 per cent phosphoric acid, 10 ml. of 0.8 per cent silver nitrate solution, 100 ml. of warm water, and 2 ml. of 25 per cent ammonium persulfate solution. Bail for 2 to 3 minutes to decompose peroxides. add 8 ml. of 25 per cent ammonium per-
