ANALYTICAL EDITION
366
It is worth mentioning here that dimetol gives no precipitate with ketones, so that it may be used to determine aldehydes in the presence of ketones. Sensitivity of Test A solution containing 4 mg. of formaldehyde Per liter gives a definite precipitate in 15 minutes, using salt. This degree of sensitivity was determined as follows: A number of formaldehyde solutions properly neutralized and of various strengths were prepared, sodium chloride was added to each, and they were all kept at 25" C. To a portion of each solution a few drops of the reagent were added, and the weakest concentration of formaldehyde that gave a definite precipitate within 15 minutes was taken as the limit of the sensitivity of the test. The time limit was set because it was desired that the test be used as a rapid test and in routine analysis. Long waiting would, obviously, have made it unfit for such a purpose. Under the conditions stated, formaldehyde was definitely detected in a $ohtion containing only four parts of formaldehyde per million. Because of its extreme sensitivity, the definiteness of the precipitate, and the very simple technic required, this test should rank high among the classic tests for aldehydes. That
Vol. 3, No. 4
i t may be put to a wider use, notably in differentiating between and in confirming the presence of certain specific aldehydes, the melting points of the condensation products formed by the reaction of dimetol and certain aldehydes are described (Table I). It is hoped that this list may be expanded in time. T a b l e I-Melting
Points of C o n d e n s a t i o n P r o d u c t s of Dimetol and
Various Aldehydes
ALDEHYDE
MELTINGPOINT
- L. A
Formaldehyde Acetaldehyde Anhydride of acetaldehyde Citral Citronellal Anisic aldehyde Acetylvanillin Piperonal Vanillin Salicylic aldehyde
"
187 138 to 140 (4) 173 t o 175 (4) approx. 25 7 0 t o 71 (1)
% 8 E: $1 208 to 209
Literature Cited (1) Bernardi, ilnn. chim. afipiicata, 17, 163 (1927). (2) Bernardi and Tartarini, Ibid., 16, 132 (1926). (3) Leffman and Pines, Ball. Wagner Free Inst. Sci. Phila , 4, 15 (1929).
ii;
:;,4"~,$~~'
~ ~ ~ ~ n ~ 'l4$~ 301 ,(lg21). ~ (6) Vorlander, Ibzd., SSB, 2656 (1925). (7) Vorlander and Kalkow, Ann. chim., 39, 366 (1899).
~
~
~
Method for Determination of Fluorine in Phosphate Rock and Phosphatic Slags' D. S. Reynolds and K. D. Jacob FERTILIZER AND FIXED NITROGEN INVESTIGATIONS, BUREAU OF CHEMISTRY A N D SOILS, WAFHINGTON, D
c
The fluorine in highly phosphatic, calcareous matealkali fusion in order to deN AN investigation of the rials, such as phosphate rock, cannot be brought into termine fluorine ia the slags. volatilination of fluorine the water-soluble condition by a single fusion with At the same time it was deduring the smelting of alkaline fluxes. Three fusions usually fail to convert sired to obtain, if possible, a phosphate rock it was necesmore than about 90 per cent of the fluorine into the method that would be apsary to determine the fluorine water-soluble condition. The failure of alkaline plicable also to the determicontent of slags containing fusions to effect complete decomposition of such nation of fluorine in phosvarious quantities of phosmaterials is due to the setting u p of equilibrium rephate rock, inasmuch as the phate. Preliminary studies actions involving insoluble fluorphosphates of the volatilization method, under showed that the volatilization fluorapatite type. the m o s t f a v o r a b l e condim e t h o d (1.2, 16), which is A method for the determination of fluorine in phost i o n s , seems to give a rewidely usedfor the determinaphate rock and phosphatic slags is described. The covery of only about 92 to 94 tion of fluorine in phosphate method involves a single fusion of the sample with per cent of the fluorine presrock, gives very low and ersodium carbonate and silica, followed by extraction ent in this material (19). ratic results when applied to of the water-insoluble residue with dilute nitric acid. The lead c h l o r o f l u o r i d e the analysis of fluorine-bearAfter removal of the dissolved calcium and phosphoric method; first s u g g e s t e d by ing slags, such as those obacid, the fluorine is determined by the lead chloroStarck (24) and later studied tained in the manufacture of fluoride method. The method gives satisfactory rein some detail by Hawley (4). phosphoric acid by furnace sults on fluorine-bearing phosphatic slags and on the has recently been applied by processes, The low results domestic types of phosphate rock with the exception Hoffman and Lundell(6,7) to obtained with this method of Tennessee blue-rock phosphate. Comparative rethe determination of fluorine may be due in part to failure S U h are given for fluorine in various phosphatic matein glasses and enamels, with of the sulfuric acid to effect rials, as determined by this method and by the volaexcellent r e s u l t s . Hoffman complete decomposition of tilization method. and Lundell state, however, the fluorine compounds prest h a t t h e method a s deent in the slag. The principal source of error seems to lie, however, in the formation of a veloped by them is not suitable for determining fluorine in non-volatile oxyfluoride, probably SiOF2 ( I ) , by the action phosphate rock, This was confirmed by the writers in sevof hydrofluoric acid on the silicic acid resulting from the action eral experiments, only about one-half of the fluorine being of sulfuric acid on silicates present in the slag.2 It seemed recovered. It seemed desirable, however, to make a further necessary, therefore, to resort to some method involving study of the method to determine whether it might be modified to give satisfactory results on phosphatic materials. 1 Received M a y 14, 1031. Presented before the Division of Fertilizer After considerable experimental work a modification was deChemistry a t t h e 80th Meeting of the American Chemical Society, Cinveloped which seems to give fairIy satisfactory results for cinnati, Ohio,September 8 to 12, 1930. fluorine in phosphate rock and phosphatic slags. The results 2 See the immediately following paper, "Effect of Certain Forms of of this study are given in the present paper. Silica on Determination of Fluorine by Volatilization Method."
I
;
,
October 15, 1931
I N D U S T R I A L A N D ENGINEERING CHEMISTRY Preliminary Experiments
367
it contains between 3.45 and 3.55 per cent fluorine. For the The method as outlined by Hoffman and Lundell is de- purpose of the present paper a fluorine value of 3.45 per cent signed for the determination of fluorine and silica on the same is used. Several experiments were also made with synthetic mixtures of tricalcium phosphate and Bureau of Standards sample. Briefly it is as follows: fluorspar, standard sample No. 79, containing 46.19 per cent The sample is fused with sodium or potassium carbonate and fluorine. The carbonate fusions were made in platinum the insoluble residue remaining from the disintegration of the dishes in a muffle furnace at 950" C. for one hour. The melt with hot water is filtered off. Silica is removed from the sodium hydroxide fusions were made in nickel crucibles a t filtrate by two precipitations, first with acid zinc nitrate solution, and second with ammoniacal zinc oxide solution. The filtrate 600" C. for one hour. The sodium peroxide-carbon fusions from the second silica precipitation is treated with hydrochloric were made in nickel crucibles according to the recommendaacid, lead nitrate, and sodium acetate to precipitate lead chloro- tions of Muehlberg ( I O ) , and Marvin and Schumb (8). fluoride (PbClF), and the chlorine in the latter is determined The results given in Table I show that only 50 to 60 per volumetrically by means of standard silver nitrate and potassium cent of the fluorine was recovered from phosphate rock by a thiocyanate solutions. single fusion with sodium carbonate, sodium carbonate plus Preliminary experiments showed that the low results ob- potassium carbonate, or sodium hydroxide. The sodium pertained on phosphatic materials by the Hoffman and Lundell oxide-carbon fusions also gave poor results. Addition of procedure are due to interference of the phosphate. It was silica had a beneficial effect, but even then a maximum of thought that the trouble lay either in the lead chlorofluoride only about 82 per cent of the fluorine was recovered. Similar precipitation or in the alkali fusion. results were obtained on mixtures of fluorspar and either I n order to obtain information on the effect of phosphate dicalcium or tricalcium phosphate. on the lead chlorofluoride precipitation, various quantities of phosphoric acid in the form of potassium dihydrogen phosTable I-Recovery of Fluorine b y Single Fusion w i t h Various Fusion Mixtures phate were added to solutions containing 8 grams of sodium MATERIAL FUSION MIXTURE FLUORINE nitrate (equivalent to 5 grams of Podium carbonate) and 0.040 Present Recovered gram of fluorine as sodium fluoride. The Hoffman-Lundell Mg. Mg. 3'% procedure was followed in precipitating the lead chlorofluoride Tennessee brown-rock phosphate,Q 1 g. and determining the fluorine equivalent of the chlorine. I n the absence of phosphoric acid, 0.0401 gram of fluorine was found, whereas in the presence of 0.005 to 0.06 gram of phosthe quantities of fluorine found increased phoric acid (P205), progressively from 0.0407 gram to 0.0467 gram. As pointed out by Hawley (4), the high results in the presence of phosphates are probably due to the fact that when lead phosphate is precipitated in the presence of chlorides, i t carries with it some lead chloride. 3 4 . 5 24 9 72.2 Since it is evidently necessary to remove phosphate prior 6 g. NazC03 + 0.75 g. feldspar 17.26 14.1 81.7 to the lead chlorofluoridg precipitation, experiments were 2 g. NazC03 + 0.5 g. Si02 37.9 23 8 6 2 . 8 carried out to determine whether the two zinc treatments 2 g. NazCOa + 0.5 g. Si02 37.9 27.4 72.3 used by Hoffman and Lundell for precipitating silica3 are 6 g. NazCOa f 0.75 g. feldspar 2 3 . 1 15 3 6 6 . 2 effective also in removing the phosphate. A solution containing 0.040 gram of fluorine as sodium fluoride, 0.35 gram of 6 g. NazC03 35.7d 3 1 . 0 86.8 phosphoric acid (P206), and 5 grams of sodium carbonate was a Bureau of Standards standard sample No. 56, containing 9.26 per carried through the zinc precipitations and the subsequent cent bofInSiOz. all cases SiOz in fusion mixture is in addition t o t h a t present in material. operations outlined by Hoffman and Lundell, with a recovery phosphatic Fluorspar, Bureau of Standards standard sample No, 79. of 0.0402 gram of fluorine, Only about 0.005 gram of phosd Approximate figure. phoric acid remained in the solution after the zinc treatments. I n a second experiment, 1 gram of silica was fused with 10 Fairchild (3) states, in a recent paper, that the fluorine in a grams of sodium carbonate and the melt was dissolved in a mixture of dicalcium phosphate and fluorspar is completely solution containing 0.040 gram of fluorine and 0.35 gram of converted into the water-soluble condition by fusion with a phosphoric acid. About 0.008 gram of phosphoric acid re- mixture of sodium carbonate and feldspar. The writers were mained in solution after the zinc treatments and 0.0398 gram unable, however, to confirm Fairchild's statement. The of fluorine waR recovered. figures given in Table I show that sodium carbonate-feldThese results show that the zinc treatments correct the spar fusions gave no better results on either phosphate rock error arising from the presence of phosphates in solution. or mixtures of dicalcium phosphate and fluorspar than those They indicate also that the low results obtained on phosphatic obtained by sodium carbonate-silica fusions. (The phosphate materials are due to failure of the alkali fusion to convert all rock and feldspar were ground to 200 mesh.) Fairchild's the fluorine into a water-soluble condition. This was shown conclusion that all the fluorine is brought into the waterto be the case in a number of experiments with various fusion soluble condition by a single fusion with sodium carbonate mixtures. and feldspar is based upon his failure to find fluorine in the insoluble residue by the etching and hanging drop tests. Alkali Fusion of Fluorine-Bearing Phosphatic It is known, however, that these tests ( I , 15) are not reliable Materials in the presence of amorphous silica, silicic acid, or silicates Bureau of Standards standard sample No. 56, Tennessee which are easily decomposed by sulfuric acid. The insoluble brown-rock phosphate, was used in the majority of the ex- residue from the sodium carbonate-feldspar fusion of the periments with different fusion mixtures. Previous analyses mixture of dicalcium phosphate and fluorspar did not give a (11) of this material by the volatilization method, together test for fluorine by the etching test, and the results by the with analyses during the present investigation, indicate that hanging drop test were questionable, despite the fact that the residue actually contained approximately 0.008 gram of According t o Hawley, silica itself does not interfere in the d e t e f h a fluorine. tion of fluorine b y the lead chlorofluoride method.
AiVALYTICAL EDITION
368
The figures given in Table I1 show that, in general, three successive fusions did not bring into the water-soluble condition all the fluorine present in phosphate rock and in synthetic mixtures of tricalcium phosphate and fluorspar. It seems that, in general, less than 90 per cent of the fluorine can be recovered in this way, although a 99 per cent recovery was obtained in one case. The results give further evidence of the beneficial effect of silica in the fusion mixture. of Fluorine by Three Successive Fusions with Various Fusion Mixtures
Table 11-Recovery MATERIAL
FUSIONMIXTURE
Tennessee brown-rock phosphate,a 1 g.
FLUORINE Total Present recovered M g . Mg. %
2 6. NarCOa 5 5 g , KzCOa 4 5 g. NazCOa 2 g NanCOa -I0 5 g SlOzb
+
34 5 2 6 . 0 7 5 . 4 34 5 22 9 66 4 34.5 3 4 . 4 99 7
Florida pebble phos2 g. iiazCOa 0 5 g. Si02 37 8 d 3 2 . 8 86 8 phate,c 1 g Caa(PO4)r, 0 83 g. CaFr,e 0 OS36 g. 2 g NarCOa -I- 0.5 g Si02 38 6 33.9 87.8 Ca3(P04)z, 0 73 g. CaFz,* 0 0821 g. 2 g. NarCOs 0 5 g. Si02 37 9 33 3 8 7 . 9 a Bureau of Standards standard sample No. 56 b In all cases, Si02 in fusion mixture is in addition to that present in phosphatic material. C Representative sample of commercial material. d Approximate figure e Fluorspar, Bureau of Standards standard sample No. 79.
+ +
+
Reactions Occurring in Alkali Fusion of Phosphates
When identical fusion mixtures were used, for example, 2 grams of sodium carbonate and 0.5 gram of silica, duplicate determinations on a particular sample of phosphate rock, or mixture of tricalcium phosphate and calcium fluoride, usually checked within 0.001 gram of fluorine, despite the fact that, in general, complete recovery of the fluorine was not obtained even with three successive fusions. Almost identical results were obtained when the determinations were repeated a t a later date. Calcium fluoride alone is completely converted into the water-soluble condition by a single fusion with sodium carbonate and silica, but mixtures of calcium fluoride and calcium phosphate behave like phosphate rock. These facts indicate that in the alkali fusion of fluorinebearing phosphatic materials, failure to obtain all the fluorine in the water-soluble condition, even after repeated fusion, is due to equilibrium reactions in the melt involving a waterinsoluble fluorine-phosphate compound. It seems to be definitely established that the fluorine in phosphate rock is present principally in the form of a complex calcium fluorphosphate having the same empirical formula, 3Ca3(P04)2.CaF2, as crystalline fluorapatite. A number of investigators (9) have prepared this compound by heating mixtures of calcium fluoride and calcium phosphate in the presence of alkali salts. A further study of the reaction showed that alkali fusion, with or without addition of silica, does not convert the same percentages of the total phosphoric acid and total fluorine into the water-soluble condition. I n one experiment, fusion of 1 gram of phosphate rock with 10 grams of sodium carbonate converted 60 per cent of the fluorine and only 32 per cent of the phosphoric acid into the water-soluble condition. A second fusion solubilized 28 per cent of the remaining fluorine and 11 per cent of the remaining phosphoric acid. These inequalities may result from the formation of water-insoluble calcium sodium phosphate, CaNaPOd (2, IS), but further investigation of the conditions under which this compound is formed are necessary before a definite statement to that effect can be made. With these facts in mind, the principal reaction occurring when phosphate rock is fused with pure sodium carbonate, under the customary conditions, may be represented, for the present, by the equation
+
3Ca3(P04)2.CaF2 10NazCOs e 10CaC03
+ 6Na3P04 + 2NaF
VOl. 3,
KO.
4
I n the presence of silica, the reactions may be represented by the equation 3Cst3(P0&CaFz
+ lOSiOz + 10Na2C03 2 10CaSiOa + 6Na3POh + 2NaF + locoz
Similar reactions occur in the fusion of fluorine-bearing phosphatic slags and of synthetic mixtures of calcium phos. phates and calcium fluoride. Acid Extraction of Fluorine from Fusion Residues
Inasmuch as satisfactory extraction of the fluorine did not seem to be possible by alkali fusion alone, experiments were made to determine whether the fluorine remaining in the insoluble residue from a single alkali fusion could be extracted by treatment with dilute acid. It was found that complete extraction of the fluorine could be obtained by treating the residue with nitric acid (1to 10). The acid treatment introduces, however, several complications. In the first place, the calcium that is brought into solution simultaneously with the fluorine must be removed in order to prevent loss of fluorine as calcium fluoride during the zinc precipitations. This may be accomplished by precipitation as calcium oxa. late under carefully controlled conditions. It is very difficult to obtain fluorine-free precipitates of calcium oxalate if more than about 20 mg. of fluorine are present. It is then necessary to remove the excess of oxalic acid remaining in the filtrate from the calcium oxalate precipitate. If this is not done, the oxalate is carried into the lead chlorofluoride precipitate as lead oxalate and makes it impossible to obtain a satisfactory end point in the thiocyanate titration. This is effected by oxidation with potassium permanganate in slightly acid solution, and the manganese is precipitated as a mixture of the phosphate, carbonate, and dioxide by treatment with sodium carbonate. The filtrate from the manganese precipitate is then added to the solution obtained by water extraction of the melt resulting from the alkali fusion of the original material. The method used for the final determination of fluorine is essentially the same as that described by Hoffman and Lundell. Direct treatment of phosphate rock with dilute nitric acid was quite effective in bringing the fluorine into solution, but the final results were erratic and usually much lower than those obtained by the combination of alkali fusion and acid extraction. Recovery of Fluorine in Presence of Pyrite and Calcium Sulfate
According to Hawley (4,the presence of 0.1 gram of iron sulfide (FeSz) causes a serious reduction in the recovery of fluorine from fluorspar by fusion methods. Hawley’s results indicate that this loss is reduced but is not entirely prevented by oxidizing the sulfide with a small quantity of sodium peroxide or potassium nitrate.4 Hoffman and Lundell ( 7 ) , on the other hand, did not find a significant loss of fluorine when 10 per cent of iron sulfide (FeS2) was added to an opal glass containing 5.75 per cent fluorine. Some types of phosphate rock, partioularly Tennessee bluerock, contain appreciable quantities of iron sulfide, principally pyrite, and acid-soluble sulfate, principally gypsum. Inasmuch as the fusion-acid extraction method in the absence of oxidizing agents gave results appreciably lower than those obtained by the volatilization method on several samples of Tennessee blue-rock phosphate (Table IV) experiments were made to determine whether better recovery could be obtained in the presence of oxidizing agents such as potassium nitrate and sodium peroxide. I n making these experiments the 200-mesh materials were thoroughly mixed with 4 Hawley does not state whether the oxidation of the sulfide was carried out prior to or simultaneously with the fusion of the sample.
INDUSTRIAL A N D EhTGINEERING CHEMISTRY
October 15, 1931
the fusion mixture and oxidizing agent, and the oxidation and fusion were carried out simultaneously. The fusion-acid extraction method, as outlined later, was used. The results given in Table I11 show that when no oxidizing agent was used, addition of 0.1 gram of pyrite or of gypsum had no pronounced effect on the recovery of fluorine from mixtures of tricalcium phosphate and fluorspar. The addition of 0.1 gram of potassium nitrate had no effect on the recovery of fluorine from mixtures containing only tricalcium phosphate and fluorspar, but in the presence of 0.1 gram of pyrite, the recovery from synthetic mixtures was seriously reduced by the use of either potassium nitrate or sodium peroxide as oxidizing agents. I n the case of the Tennessee blue-rock phosphate No. 930, however, addition of 0.1 gram of oxidizing agent gave, in general, a significant increase in the recovery of fluorine, but the results were still lower than those obtained by the volatilization method. This particular sample contained 5.18 per cent of 803 in the form of acidinsoluble sulfide. The addition of 0.1 gram of potassium nitrate did not improve the recovery of fluorine from Tennessee brown-rock phosphate, Bureau of Standards sample No. 56, which contained 1.18 per cent of SO3as acid-insoluble sulfide.
369
with the possible exception of Tennessee blue-rock phosphate, the recovery of fluorine from phosphatic materials is not improved by the addition of an oxidizing agent to the fusion mixture. Special Reagents Used in Method
SILICA-Any type of anhydrous fluorine-free silica may be used. It should be ground to 200 mesh. ACID ZINC NITRATE-DiSSOlVe 5 grams of zinc oxide in 100 ml. of dilute nitric acid ( I to 9). AMMONIACALZINCOXIDE-Dissolve 10 grams of ammonium carbonate in 100 ml. of water and 10 ml. of ammonium hydroxide (sp. gr. 0.90). Add 5 grams of zinc oxide and heat on the steam bath until a clear solution is obtained, adding more ammonia if necessary. BROMOPHENOL BLUEINDIcAToR-Prepare a 0.4 per cent solution by grinding the dry powder with sodium hydroxide solution (1.5 ml. of 0.1 N sodium hydroxide to 0.1 gram of powder) and diluting to the proper volume. LEADNITRATE-It is the writers' experience that the use of the so-called c. P. grades of lead nitrate may lead to low results for fluorine, particularly if the salt has been dried a t too high a temperature or for too long a time a t a lower temperature. It is advisable t o test the salt by using it to precipitate lead chlorofluoride from a solution of sodium fluoride of known purity. If the results for fluorine are too low, the lead nitrate should be Table 111-Recovery of Fluorine in Presence of Pyrite a n d recrystallized from a solution containing 2 ml. of concentrated C a l c i u m Sulfate nitric acid to 100 ml. of water. Dry the recrystallized salt a t a OXIDIZING FLUORINE temperature below 100" C. and determine its behavior towards a MATERIAL AGENT= RECOVERED solution of sodium fluoride of known purity. Mg. LEAD CHLOROFLUORIDE WASH SOLUTION-(a) Dissolve 10 Caa(PO4)?,0.7 g . CaFz,b 0.0821 g. (37.9 mg. None 36.9 grams of lead nitrate in 200 ml. of water. ( b ) Dissolve 1 gram F) 0 . 1 6. KNOa 37.6 of sodium fluoride in 100 ml. of water and add 2 ml. of con0 . 1 g. KNOs 36.5 centrated hydrochloric acid. Mix solutions a and b, allow the 0 . 1 g. KNOs 36.2 precipitate to settle, decant the supernatant liquid, and wash the Caa(PO4h 0.7 g. CaFz, 0.0821 g. + gypsum, None 35.9 0.1 g. (37.9 m g . F) None 36.3 precipitate 4 or 5 times with 200 ml. of water by decantation. Cas(Pbr)z, 0.7 g . - + CaF?, 0.0821 Fe&,c Transfer the precipitate to a large beaker or flask, add about 1 0.1 g. (37.9 mg. F) liter of cold water, and allow to stand for 1 hour or longer with occasional stirring. Filter and keep the clear filtrate for use as a wash solution. Save the remainder of the precipitate for the preparation of further quantities of wash solution as needed. Caa(P01)z, 0.7 g. CaFz, 0.0821 g. + FeSz, None 31.7 0.1 N SILVERNITRATE-It is preferable to standardize this None 35.4 0.050 g. 4- gypsum, 0.050 g. (37.9 mg. F) solution by precipitating and weighing the chlorine as silver 0 . 1 g. KNOa 34.8 0 . 1 g. KNOa 36.0 chloride. One milliliter of 0.1 N solution is equivalent to 0.00190 Tennessee blue-rock phosphate,d KO.930, 1 g. gram of fluorine. (> 35.3 mg. F) FERRIC ALUMINDIcAToR-Prepare a saturated solution (free from chlorides) and add a few milliliters of concentrated colorless nitric acid to bleach the brown color. 0.1 N POTASSIUM THIOCYANATE-Standardize this solution against the silver nitrate solution.
+
+
+
+
Tennessee brown-rock phosphate,* Bureau of Standards N o 56, 1 g. (34.5 mg. F)
None 33.4 None 32.9 0 . 1 g. KNOa 30.3 32.8 0 . 1 g. KNOs Florida pebble phosphate,f 1 g. (> 36.8 mg. F) None 36.4 None 36.0 0 . 1 g. KNOa 36.5 0 . 1 g. KNOs 36.5 a Fusion mixture of 2 grams of NazCOa 0.6 gram of SiOz used in all experiments. b Fluorspar, Bureau of Standards standard sample No. 79. c Crystalline pyrite. d Containing total SOa, 6.61 per cent: acid-soluble sulfate, 1.25 per cent SOa; and acid-insoluble sulfide, 5.18 per cent SOa. Containing total SOa, 2.82 per cent; acid-soluble sulfate, 1.23 per cent SOa; and acid-insoluble sulfide, 1.18 per cent SOa. f Containing total SOa, 0.30 per cent, acid-soluble sulfate, 0.20 per cent sea; and acid-insoluble sulfide, 0.00 per cent SOa.
+
Experiments were also made with synthetic mixtures of tricalcium phosphate, calcium fluoride, pyrite, and gypsum. I n the absence of an oxidizing agent, low and erratic results were obtained. When 0.1 gram of potassium nitrate was added to the fusion mixture, the results were more consistent but were still lower than those obtained by the volatilization method. This indicates that the low figures obtained on Tennessee blue-rock phosphate are caused by a reaction, or reactions, involving the gypsum, pyrite, and calcium fluoride, or calcium fluorphosphate, present in the rock. Roasting a t temperatures up to 800" C. prior to fusion with sodium carbonate did not improve the recovery of fluorine from Tennessee blue-rock phosphate. The results, as a whole, indicate that,
Procedure
Grind the sample to a fineness of about 100 mesh. In the case of phosphate rock, fuse a 1-gram sample in a platinum crucible or dish with 2 grams of sodium carbonate and 0.5 gram of silica6a t a temperature of 900" to 950" C. for 1 hour. I n the case of slags containing a considerable quantity of silicates, fuse a 1-gram sample with 5 grams of sodium carbonate. Transfer the cooled melt to a beaker, digest overnight on the steam bath with 50 to 75 ml. of water, and decant the solution onto a filter. Break up the lumps thoroughly with the stirring rod and digest with 50 ml. of a 1or 2 per cent sodium carbonate solution on the steam bath for about 15 minutes with frequent stirring. Filter into a 400ml. beaker, wash the residue 5 or 6 times with hot water, and evaporate the combined filtrates to a volume of 50 to 75 ml. This solution, A, which contains the greater portion of the fluorine, is reserved for further treatment. By means of a jet of warm water return the residue to the same beaker in which the water and carbonate digestions were made, using a total volume of about 50 ml., add 3 ml. of concentrated nitric acid, and allow the mixture to stand for 0.5 6 In the case of Tennessee blue-rock phosphate somewhat higher results are obtained by the addition of 0.1 gram of potassium nitrate or sodium peroxide t o the fusion mixtures. The nitrate is preferable because it is easier to handle
e
370
*
ANALYTICAL EDITIOX
to 1.0 hour with frequent stirring. This treatment will usually bring all the fluorine into solution, but in case considerable insoluble material remains, it is advisable to filter i t off, fuse with a small amount of sodium carbonate, and add the water extract of the melt to solution A . Add 50 ml. of a 5 per cent oxalic acid solution to the cold nitric acid solution and precipitate calcium oxalate by adding 10 per cent sodium carbonate solution drop by drop until the solution is neutral to methyl orange. Boil the mixture 1 minute, stirring continuously to prevent bumping, and after cooling, filter and wash the precipitate four or five times with cold water. Make the filtrate acid to methyl orange, add 4 ml. of concentrated nitric acid and 10 ml. of saturated potassium permanganate solution, and warm on a hot plate or steam bath. When the color disappears, add more permanganate solution drop by drop until the solution becomes permanently colored, or a brown precipitate forms. Neutralize the excess acid by adding solid sodium carbonate in small quantities until frothing ceases, and then add about 2 grams of carbonate in excess. If the precipitate is light colored, add permanganate solution drop by drop until it becomes dark brown, and bring the mixture to boiling, stirring continuously. Filter the hot solution through a rapid-filtering 12.5- or 15-em. paper and wash the precipitate four or five times with a hot 1per cent sodium carbonate solution. Catch the filtrate in the beaker containing solution A and adjust the volume to about 250 ml. The procedure from this point is essentially the same as that recommended by Hoffman and Lundell. Heat the solution to boiling, add 25 ml. of acid zinc nitrate solution, and bring to a boil, stirring continuously to prevent bumping. The precipitate is very heavy and loss of the determination is almost certain to occur unless bumping is prevented. Filter through a rapid-filtering 15-cm. paper into a 600-ml. beaker, wash to a volume of about 400 ml., and discard the precipitate. Neutralize the solution with nitric acid, using methyl red as an indicator and heating almost to boiling before the final end point is reached, add 25 ml. of ammoniacal zinc oxide solution, and evaporate to a volume of 50 to 75 ml. Wash down the side of the beaker with 50 ml. of warm water, allow to stand until the precipitate settles, and then filter and wash with cold water to a volume of 250 ml. Discard the precipitate. Add 2 drops of bromophenol blue indicator to the solution, make slightly acid with dilute nitric acid and then just alkaline with dilute sodium hydroxide, and add 3 ml. of a 10 per cent sodium chloride solution and 2 ml. of dilute hydrochloric acid (1 to 1). Add 5 grams of solid lead nitrate and heat on the steam bath. After the lead nitrate has completely dissolved, add 5 grams of solid sodium acetate, and heat on the steam bath for 0.5 hour with occasional stirring. Allow the solution and precipitate to stand for 4 hours or overnight at room temperature, and filter through a paper of close texture. Wash the precipitate, beaker, and paper once with cold water, then four or five times with a cold saturated solution of lead chlorofluoride, and finally once more with cold water. Transfer the precipitate and paper to the beaker in which the precipitation was made, wash the precipitate from the paper with 100 cc. of dilute nitric acid (1 to 19), warm until the precipitate is dissolved,6 and then pulp the filter paper in the solution. Add a slight excess of 0.1 N silver nitrate (a total of 20 ml. is usually sufficient for samples of phosphate rock), heat on the steam bath for 0.5 hour, stirring occasionally, cool to room temperature in a dark place, and then filter and wash with cold water. Add 5 ml. of ferric alum solution to the filtrate and determine the excess silver by titration with 0.1 N potassium thiocyanate. Subtract the amount of silver found in the filtrate from that originally 0 If large quantities of lead sulfate are present the precipitate dissolves slowly, but this does not affect the results.
Vol. 3, No. 4
added. The difference represents the silver required to combine with the chlorine in the lead chlorofluoride and from this the fluorine content is calculated, 1 ml. of 0.1 N silver nitrate being equivalent to 0.00190 gram of fluorine. Results Obtained by Fusion-Acid Extraction and Volatilization Methods on Various Phosphatic Materials
Table I V gives a comparison of results for fluorine in various phosphatic materials as determined by the fusion-acid extraction and the volatilization methods. With the exception of one sample, No. 930, oxidizing agents were not used in the fusion mixtures. I n all cases the figures actually determined are given, no corrections being made for failure of the methods to give complete recovery of the fluorine. Table IV-Comparison of Recovery of Fluorine by Combined Fusion a n d Acid Extraction Method a n d by Volatilization Method SAMPLE MATERIAL^ FLUORINE RECOVERED BY: Fusion and Volatiliacid extraction zation method method Mg. Mg. , Cas(P04)z, 0.7 g. 4- CaFa, 0.0821 g.b 36.9 35.4 ,. , Tennessee brown-rock phosphateo 33.3 33.3 912 Florida pebble phosphate, high grade 36.8 36.2 947 Florida pebble phosphate. low grade 36.4 37.5 932 Florida hard-rock phosphate 36.8 36.4 726 Florida waste-pond phosphate 20.1 16.9 727 Florida waste-pond phosphate 19.4 13.5 930 Tennessee blue-rock phosphate 32.0 35.3 930 Tennhsee blue-rock phosphate 33.9d 35.3 448 Tennessee blue-rock phosphate 32.5 34.3 449 Tennessee blue-rock phosphate 36.1 36.9 983 Electric phosphoric acid furnace slag 35.7 11.6 15.0 1005 Electric phosphoric acid furnace slag 35.1 1006 Electric phosphoric acid furnace slag 35.7 18.7 999 Experimental phosphoric acid blast furnace slag 6.9 1.1 ’1141 Fluorspar basic slag 11.9 9.4 1142 Fluorspar basic slag 10.9 9.2 a One-gram samples of all materials except Cas(POi)t-CaFz mixture. b Contained 37.9 mg. of duorine. c Bureau of Standards standard sample No. 56. Contained approximately 34.5 mg. of fluorine. d 0.1 gram KNOs added to fusion mixture.
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Although it seems quite certain that neither method, as developed at present, gives complete recovery of the fluorine in phosphate rock, they both give about the same results on commercial grades of Tennessee brown-rock, Florida pebble, and Florida hard-rock phosphate, while the fusion-acid extraction method gives decidedly lower results than the volatilization method on Tennessee blue-rock phosphate. The volatilization method is, therefore, recommended for the analysis of the usual commercial grades and types of phosphate rock. On the other hand, the fusion-acid extraction method gives much higher results than the volatilization method on Florida waste-pond phosphates (5) and slags, which contain considerable quantities of silicates, and is recommended for the analysis of these and simiIar materials. Literature Cited (1) (2) (3) (4) (5) (6) X7) (S) (9)
(10) (11) (12) (13) (14) (15) (16)
Daniel, 2. anorg. Chem., 88, 299 (1904). Ditte, Compt. rend., 94, 1592 (1882). Fairchild, J . W ~ s h i n g l o nAcad. Sci., 20, 141 (1930). Hariley, IND. ENC.CHEM., 18, 573 (1926). Hill, Jacob, Alexander, and Marshall, Ibid., 22, 1392 (1930). Hillebrand and Lundell, “Applied Inorganic Analysis,” p. 806, Wilev 1929. Hoffman and Lundell, Bur. Standards J. Research, 3, 581 (1929). Marvin and Schumb, J. A m . Chem. SOL.,52, 574 (1930). Mellor, “A Comprehensive Treatise on Inorganic and Theoretical Chemistry,” Vol. 111, p. 897, Longmans, 1923. Muehlberg, IND. ENG.CHEM.,l T , 690 (1925). Reynolds, Jacob, and Hill, I b i d . , 21, 1253 (1929). Reynolds, Ross, and Jacob, J . Assocn. Oficial Agr. C h e m , 11, 225 (1925). Rose, Pogg. Ann., 17, 292 (1849). Starck, Z. anovg. Chem., 70, 173 (1911). Treadwell and Hall, “Analytical Chemistry,” Vol. I , p. 408, Wiiey, 1916. Wagner and Ross, J. I N D . ENG.C R E W .9, , 1116 (1917).
