SEPTEMBER 15, 1933
ANALYTICAL EDITION
2. Separations are unnecessary, as hexavalent chromium is reduced by boiling with hydrochloric acid, and the iron may then be determined by direct reduction with standard titanous chloride solution, and ammonium thiocyanate as indicator. If desired, iron may first be separated from hexavalent chromium by 'precipitation with ammonia and then determined with titanous chloride. 3. Fifty-seven determinations of nine different leathers, both chrome- and vegetable-tanned, show that the new method is accurate within *0.01 per cent of the Fez03 present in the case of chrome-tanned leathers, and within *10 per cent of the iron present in vegetable-tanned leathers for 0.01 to 0.03 per cent total Fe203content.
305
4. Hazardous reactions following the new method were not encountered and the speed of analysis by comparison with existing methods is improved without sacrificing accuracy.
Literature Cited (1) Bergman and Mecke, Collegium, 762, 609 (1933); IND. ENQ. CHEM.,30, 48 (1934). (2) Ibid., 773, 451 (1934). (3) Merrill and Henrich, Ibid., 25, 270 (1930). (4) Smith and Gets, Ibid., Anal. Ed., 6, 252 (1934). (5) Smith and Sullivan, J . Am. Leather Chem. Assoc., 30 (1935). (6) Zintl and Rienacker, 2. unorg. allgem. Chem., 155, 84 (1926). RECEIVEDJuly 8, 1935. Presented before the Division of Induatrial and Engineering Chemistry at the 90th Meeting of the American Chemical Society, San Francisco, Calif., August 19 to 23, 1935.
Loss of Water-Soluble Potash in Fertilizer Mixtures WILLIAM H. ROSS, KENNETH C. BEESON, LAWRENCE M. WHITE, AND ALBERT R. MERZ Bureau of Chemistry and Soils, Washington, D. C.
T
HE fertilizer materials that are used as sources of nitrogen and phosphoric acid may be divided into two classes according as they are soluble or insoluble in water. Those that are soluble in water are recognized to be available to plants, and methods based on treatments other than water-extraction have been adopted for determining the availability of those that are water-insoluble. Since all the potash materials used are readily water-soluble, they too have been recognized as available. A method based on the extraction of the sample with water was therefore adopted for determining the availability of potash in all types of mixed fertilizers, and with slight modifications has been used in this country during the past 50 years. The claim has frequently been made, however, that extraction with water does not recover all the soluble potash incorporated in mixed fertilizers and that any method based on water-extraction does not give a true measure of the availability of the potash in fertilizer mixtures. This view was first suggested by Carpenter in 1901 in a report to Hare (IS), referee on potash for the Association of Official Agricultural Chemists. Experimental data in support of this view have been presented by different investigators (4, 6, 6, 11, 18, 19, 20) and various modifications of the method for determining potash availability have been suggested (4, 6, 6, 11). The failure to recover all the soluble potash included in a fertilizer mixture has been explained on the assumptions that a portion of the potash is fixed by siliceous or other constituents of the superphosphate with which it is mixed (2, 4 , 6 , 6, 18, 20); and that the loss is apparent rather than real because of errors in the analytical procedure following extraction (3, 4,10,16, 18, $1). In a recent investigation of the subject, Thornton and Kraybill (8s) found that the residues obtained after leaching mixed fertilizers with water as directed in the official method for determining potash availability, all contained considerable amounts of potash and that this residual potash is available to plants as determined both by the Neubauer method and by pot tests. The present paper gives the results of a study of the problem that was undertaken a t the suggestion of the Chemical Control Committee of the National Fertilizer Association.
The work was limited to an investigation of the cause of the loss of water-soluble potash in fertilizer mixtures and of the extent to which this loss takes place.
Experimental Consideration of the problem suggested that loss of potash might occur (1) by reaction with constituents of the mixture to precipitate a water-insoluble compound, (2) by occlusion within caked particles of the mixture, (3) by base-exchange reaction with siliceous or claylike constituents in the superphosphate component of the mixture, and (4)by adsorption by the gel-like or amorphous ingredients of the mixture. INSOLUBLE POTA~H SALTS. The best known of the more insoluble potash salts are syngenite, KzS0&aSO4.H2O, potassium fluosilicate, KzSiFel potassium fluoaluminate, KaAlFB, and potassium metaphosphate, KP03. When pure syngenite is brought into contact with water at 100' C. decomposition takes place with deposit,ion of gypsum until the solution contains 1.05 per cent of potassium sulfate (14). The potash in this material would therefore be expected to be entirely available as determined by the official method and this was found to be true both in tests made with the c. P. product and with a sample of material supplied by the U. S. Bureau of Mines. The latter sample was obtained as a by-product in the extraction of potash from polyhalite. Potassium fluosilicate has a solubility of 0.546 gram per 100 grams of water a t 100" C. (7). When a 2.5-gram sample was washed as described in the official method it was found that about 55 per cent remained undissolved. The potash in this material is therefore not completely available as determined by the official method. The maximum quantity of potassium fluosilicate that could be present in a 2.5-gram sample of a mixed fertilizer if all the fluorine in the mixture were combined in this way would not exceed 0.1 gram. This quantity will dissolve completely when washed with 200 cc. of hot water, as shown by tests with the c. P. product and with a sample of impure material supplied by the International Agricultural Corporation. The impure material was collected as a crystalline product in the bottom of phosphoric acid storage tanks.
INDUSTRIAL AND ENGINEERING CHEMISTRY
306
Potassium fluoaluminate has a solubility of only 0.158 gram per 100 grams of water (8). It requires an alkaline medium for its preparation and could therefore not be formed in the ordinary fertilizer mixture. Potassium metaphosphate that is formed by double decomposition is entirely water-soluble as determined by the official method but the ignited product is only partially water-soluble when slowly cooled. There is no possibility, however, that the latter would be formed in fertilizer mixtures of the ordinary type. No evidence has thus been obtained to indicate that there is any loss of potash as a result of the formation of any of the compounds listed, but this does not preclude the possibility that loss of potash may occur by precipitation in the form of some unknown compound.
VOL. 7 , NO. 5
concentrate the clay constituents of the superphosphate as much as possible without destroying their base-exchange capacity. The citrate-insoluble residues obtained were washed and dried. The compositions of the residues are given in Table I. The analysis of the residues Shows that the combined iron oxide and alumina content does not exceed 3 per cent in any case. The sesquioxides in a soil colloid usually range between 25 and 45 per cent (88). It is improbable that the sesquioxides in the residues are present entirely as soil colloidal material, but if this is assumed to be the case the soil colloids in any of the residues should not exceed 12 per cent, or 1 per cent on the basis of the original superphosphate, The quantity of potash that a soil colloid will adsorb varies greatly with different colloids, but the maximum that any soil colloid will retain after washing is not likely to exceed 2 per cent. It may be conTABLE I. ANALYSISOF CITRATE-INSOLUBLE RESIDUES cluded therefore that the maximum Residue in Composition of Residues amount of potash that any of the superphosphates used in the tests can retain Percentage of Original Loss ignition on Material Material a t 1000° C. KzO CaO PZOS FenOs AlgOa SiOn F by base exchange will not exceed 0.02 7% 7% 7% 7% 9% 7% 7% % ' per cent, or 0.10 per cent onthebasisof Ammoniated Florida the Pz05in the superphosphate. pebble superphosphate 12.9 6.3 0.06 27.1 18.5 1.60 0.19 43.9 3.10 A weighed portion of each of the TennesEee BU erphosphate 9.8 4.6 0.08 19.9 13.0 0.83 0.24 5 9 3 2.85 Florida pebile super1.6" 23.3 0.24 11.6 4.3 2.30 0.50 47.4 0.72 residues was to stand in a lo phosphate per cent potassium chloride solution for Most finely divided portion of residue which amounted t o 6.7 per cent of the original material. 3 days, washed with boiling water as d i r e c t e d in the official method, and analyzed for potash by the J. Lawrence OCCLUSION IN CAKED PARTICLES. In studying the possible Smith method. The potash found was the same as before loss of potash by occlusion in caked particles, quantities of treatment with the potassium chloride solution. The tests potassium chloride and potassium sulfate were mixed with indicate that any base-exchange capacity which phosphate separate portions of plaster of Paris and sufficient water was rock may possess is destroyed in the process of converting it added to hydrate the entire mass. The hard cakes obtained into superphosphate, on standing for a month were crushed to pass a 20-mesh screen ADSORPTIONIN AMORPHOUSINGREDIENTS. It has long and 2.5-gram samples of the screened materials were washed been known that amorphous compounds will remove more or with 200 cc. of boiling water. The residues were analyzed less soluble salts from the solution in which they are precipifor potash but none was found. tated. Gordon (1%')has shown, for example, that when aluIn a second experiment one part of potassium chloride was mina and ferric oxide gels are treated with a 0.1 N potassium mixed with 5 parts of Florida pebble superphosphate. A corresulfate solution the former will take up 1.1 per cent of its sponding mixture was also prepared with potassium sulfate. weight of potash from the solution and the latter 0.5 per cent. The mixtures were then moistened with water, dried to induce The removal of electrolytes from solution by colloids of this caking, and the caked mass in each case was crushed to pass a nature may take place both by occlusion and adsorption. 20-mesh screen. Portions of each of the resulting products Amorphous materials may form a protective coating over ma(2.5 grams) were then washed with boiling water as in the preterials with which they are associated and the removal by ceding test, and potash was determined in the residue by the washing of soluble salts occluded in this way may be difficult. J. Lawrence Smith method. No greater amount of potash The mechanism of adsorption of electrolytes by colloids is not was found, however, than in the residues from similar superclearly understood, but it is known that the degree of adsorpphosphate-potash salt mixtures that had not been dried so tion may vary with the concentration, acidity, and nature of the electrolyte and that an element may be removed from as to induce caking. solution in the ionic form or in molecular combination. BASEEXCHANGE REACTION.It is known that potash is absorbed by the colloidal silicates in the soil and that the Superphosphate contains such amorphous ingredients as potash occurring in a combination of this kind cannot all be silicic acid, organic matter, and various simple and complex recovered by the degree of washing specified in the official combinations of iron and aluminum. Of these amorphous method for water-soluble potash. It is also well known that ingredients the phosphates of iron and aluminum are present in much the largest proportions. With a view to determining phosphate rock matrix contains soil colloids or claylike conthe capacity of these materials for retaining potash, quantistituents. I n washing the rock an attempt is made to reties of each were prepared by treating solutions of ferric chlomove as much of these constituents as possible but it is impractical to remove them entirely, and there is therefore the ride and of aluminum chloride with equimolecular proportions of monoammonium phosphate, the solutions were made possibility that constituents of this kind may be contained in alkaline with ammonia, and the precipitates that formed we1e superphosphate made from the rock. A study was aceordfiltered off and washed, Known quantities of the potash-free ingly undertaken of the extent to which the water-soluble products prepared in this way were treated with potassium potash in a fertilizer mixture may be retained by base-exchloride in various ways. The treated samples were then change reaction with the siliceous or claylike constituents of leached with boiling water (1) and potash was determined in the superphosphate present. I n making these tests, 1000 the residues. It was found, as shown in Table 11,that when grams of each of three superphosphate samples were washed dry iron or aluminum phosphate was soaked for one day in a with water and then digested with neutral ammonium citrate 10 per cent potassium chloride solution, filtered, and washed, solution as directed in the official method for determining more or less potash always remained in the residue. The citrate-insoluble P205. The purpose of this treatment was to I
0
ANALYTICAL EDITION
SEPTEMBER 15, 1935
307
results obtained, as given in Table 111, show that the loss of potash varies greatly with superphosphate8 made from different types of rock; that the loss is greater when the mixture contains potassium sulfate than when it contains potassium chloride; and that there is a close correlation between the potash retained by a superphosphate and its total iron TABLE11. RETENTION OF POTASH BY IRON AND ALUMINUM PHOSPHATES and aluminum content. Potash Retained Table IV shows the potash that is rendered insoluble when in Percentage of potassium chloride is mixed with a cured superphosphate in PzOs in phosdifferent proportions. The results indicate that the loss of Treatment Phosphate phate Phosphate potash when expressed in per cent of the P205 in the superFerric phosphate, dry phosphate undergoes little change with change in the propor0.73 2.26 Aluminum phosphate, tion of potash in the mixture. It follows, therefore, that the 0.15 0.37 dry Ferric phosphate, moist, loss of potash, expressed in percentage of the total potash in 3.41 7.19 solution for 1 day and washed freshly precipitated the mixture, will increase as the proportion of K10 to P20Ein Soaked in 10 per cent KC1 Aluminum phosphate, 6.44 3.73 solution for 1 day and washed moist, freshly prethe mixture decreases. cipitated Precipitated in a 10 per cent The results given in Table V show that the loss of potash in Ferric phosphate KC1 solution containing FeCla 1.01 2.15 mixtures that have been allowed to stand for a month before and NH4HgPOa by adding NHa Precipitated in a 10 per cent Aluminum phosphate washing is greater than in day-old mixtures and that a loss KCl solution containing AlCla 1.44 2.49 and NHaHzPOa by addingNHa of potash occurs in mixtures with old superphosphate as well as in those that contain fresh superphosphate. The values in the last three columns of the table represent the means of TABLE111. VARIATION I N LOSS O F WATER-SOLUBLE POTASH IN CURED SUPERPHOSPHATE-POTASH SALTMIXTURESWITH IRON closely agreeing results obtained with several samples of superphosphate. AND ALUMINUM CONTENT OF SUPERPHOSPHATE
quantity retained in this way was increased when moist freshly precipitated iron or aluminum phosphate was substituted for the dry product, or when the phosphates were precipitated in a 10 per cent potassium chloride solution.
(KzO/P*Os in mixture = 1 t o 2)
phosphate No. 3 146 147 10 16
Total Potash Rendered Insoluble by Superphosphate in Mixture super- Composition of Superphosphate with: phosphate Kz0 Fez03 AlzOa PzOs KC1 KzSOa Curaqao Bone Morocco Floridapebble Tennessee
%
%
%
0.00 0.03 0.01 0.10 0.16
0.26 0.45 0.26 0.64 1.70
0.00 0.12 0.36 0.64 1.76
% 23.2 20.4 21.5 20.4 19.8
%
%
0.01
0.03 0.78 0.74 1.10 2.80
0.32 0.36 0.96 2.44
The phosphates of iron and aluminum form a series of compounds of which some are acidic and may be crystalline (9), while others are basic and are noncrystalline. The former undergo decomposition in contact with hot water to form the amorphous basic compounds. Superphosphate is an acid material and the iron and aluminum phosphates which it contains must therefore be present in acidic combination. When a mixture of superphosphate and a potash salt is washed with hot water, iron and aluminum phosphates present will therefore decompose and be precipitated in an amorphous state in contact with the potash in the mixture. In this way the conditions are duplicated which were found to be effective in bringing about a retention of potash. Iron and aluminum may occur in superphosphate in various combinations, some of which undergo no change on treatment with hot water, but the greater portion of each element is ordinarily present as the phosphate (16, 17). If the loss of potash in superphosphate mixtures is due to the iron and aluminum phosphates in the superphosphate, a greater loss would be expected with superphosphates high in iron and aluminum than with those that are low in these constituents. In making tests to determine this relationship, the required quantities of potassium chloride and potassium sulfate were mixed with separate samples of cured superphosphates from different sources to give mixtures containing one part of K20 to two parts of Pz06. The mixtures were adjusted to a moisture content of 7 to 8 per cent and stored in closed containers to prevent loss of moisture. After standing for one month the mixtures were washed with hot water (1) and the washed residues were analyzed for total potash by the J. Lawrence Smith method. The increase in the potash found in each residue over that in the washed residue of the corresponding superphosphate before treatment gives the portion of the potash that has been retained by the superphosphate. The
TABLE IV. Loss OF WATER-SOLUBLE POTASH IN SUPERPHOSPHATE-POTASSIUM CHLORIDE MIXTURES (Exoressed in Der cent of PzOs in suoerDhosahate) . . - - . Potash in Superphosphate Residue Potash Rendered Before After KzO/PzOa treatment treatment' Water-Insoluble in Mixture 1:0.5 1:l 1:2 1:3 1:10 1:o.d
1:l 1:2 1:3 1:lO (I
Florida Debble superphosphate No. 54 0.41 0.41 0.41 0.41 0.41 Tennessee superphosphate No. 56 0.31 0.81 0.31 0.94 0.31 0.98 0.31 0.96 0.31 1.21
0.50 0.63 0.67 0.65 0.90
Superphosphate mixed with potassium chloride and leached after 30 days.
TABLE V. VARIATIONIN Loss OF WATER-SOLUBLE POTASH IN SUPERPHOSPHATE-POTASSIUM CHLORIDEMIXTURESWITH AQE OF SUPERPHOSPHATE (KzO/PzOs in mixture = 1 to 2) Age of Superphosphate Used Days
of Mixture when Leached Day8
Potash in Washed Residuea Before After treatment treatment b
%
%
potash Rendered Insolublec
%
Florida pebble superphosphate
0
30
^^^
30 600 4 30 600 5
b 0
1 1 30 30 30
Tennessee superphosphate 0.57 0.80 0.84 1.28 0.17 0.85 1.35 0.57 2.06 0.84
0.46 0.88 1.36 1.56 2.44
Expressed in per cent of P z O in ~ superphosphates. Superphosphate mixed with potassium chloride and washed after 30 days. Per cent of total in mixture.
Table VI gives some results obtained when superphosphates were treated with a potash salt in various ways. The maximum loss of potash in these tests, amounting to 3.54 per cent of the total potash present, occurred when a mixture of a superphosphate with a saturated solution of potasssium sulfate was allowed to stand for 30 days, and then evaporated to dryness before leaching with water. A superphosphate is allowed to cure for about a month on
308
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INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE VI.. Loss
OF
WATER-SOLUBLE POTASH IN DIFFERENT TREATMENTS OF A CURED SUPERPHOSPHATE WITH
A
POTASH SALT
(KsO/PzOa in mixture = 1 to 2)
Phosphate Sample
10 16 10 16 12 13 56 56 15 16
Phosphate
Total Potash Potash in Washed Rendered WaterResiduea Insoluble by Before After treat- Superphosphate in treatment with: Mixture with: ment KCl K&Oa KCl KzSO~
Treatment
Florida pebble su erphosphate Tennessee superpKosphate Florida pebble superphosphate Tennessee super hosphate Florida Debble d k b l e SUDerDhOSDhate Tennessee double superphosphat’e Tennessee superphosphate Tennessee superphosphate Tennessee double superphosphate Tennessee superphosphate
Mixed with solid salt, washed at once Mixed with solid salt, washed a t once Mixed with solid salt washed after 30 days Mixed with solid salt: washed after 30 days Mixed with solid salt, washed a t once Mixed with solid salt washed a t once Soaked in 10 per cend salt solution for 1 day and washed Soaked in 10 per cent salt solution for 30 days and washed Soaked in 10 per cent salt solution for 1 day and washed Mixed with concentrated salt solution, evaporated to dryness after 1 day and washed Mixed with syngenite, evaporated to dryness after 1 16 Tennessee superphosphate dav ” and washed a Expressed in per cent of PzOs in superphosphate.
an average before being used in fertilizer mixtures, and it is probable that fertilizer mixtures as a rule are a month old before being analyzed by the state control chemist. The average fertilizer mixture for the past 10 years contained a ratio of KzO to PZOSof 1 to 2. It may be concluded, therefore, from this investigation that the potash retained in the extraction of mixed fertilizers by the official procedure amounts on an average to about 1.5 per cent of the total potash present. The results further indicate that the loss of potash occurs for the most part as a result of the reactions taking place during the extraction of the sample for analysis and that there is no actual loss in the fertilizer value of the potash. The failure to recover all the available potash in fertilizer mixtures has meant a loss to the fertilizer industry of about 3200 tons of potash (K20) per annum, valued a t $220,000.
Summary The extraction of mixed fertilizers containing superphosphate as directed in the official procedure for determining available potash fails to recover all the water-soluble potash incorporated in the mixture as a result of occlusion or adsorption of a small portion of the potash in the basic iron and aluminum phosphates formed during the extraction. The loss of potash varies with the iron and aluminum content of the superphosphate in the mixture and is greater when the mixture contains potassium sulfate than when it contains potassium chloride. A loss of Dotash occurs in mixtures that contain an old as in those that contain a superphosphate as prepared superphosphate and the loss of potash in mixtures that have been allowed to stand for a month is greater than
%
%
%
%
%
0.50 0.80 0.50 0.80 0.30 0.46 0.31 0.31 0 33
0.90 1.25 0.72 2.05 0.32 0.79 0.62 1.68 0.69
0.95 1.45 0.95 2.20 0.33 1.02 0.74 2.08 0.87
0.80 0.90 0.44 2.50 0.04 0.66 0.62 2.74 0.72
0.90 1.30 0.90 2.80 0.06 1.12 0.86 3.54 1.08
0 84 0.80
1.73
2.65 3.05
0.89
1.81 2.25
.. . .
.. ..
in day-old mixtures. For mixtures that are a month or more old the loss of potash amounts to about 1.5 per cent of the total potash present.
Literature Cited (1) Assoc. Official Agr. Chem., Official and Tentative Methods, 1930. (2) Baker, Bur. Chem., Bull. 152, 28 (1912). (3) Bible, J . Assoc. Oficial Agr. Chem., 8, 420 (1925); IND.ENQ. CHEM.,Anal. Ed., 4, 234 (1932). (4) Breckenridge, J. IND.ENQ.CHIM., 1, 409, 804 (1909). ( 5 ) Carpenter, Bur. Chem., Bull. 81, 125 (1904); 90, 107 (1905); A m . Fertilizer, 79, No. 12, 11 (1933). (6) Carpenter and Powell, IND.ENQ.CHEM.,Anal. Ed., 6, 62 (1934). (7) Carter, IND. ENQ.CHEM.,22, 886 (1930). (8) Carter, Ibid.,22, 888 (1930). (9) Carter and Hartshorne, J . Chem. Soc., 123, 2223 (1923). (IO) Donk, Bur. Chem., Bull. 73, 35 (1903). (11) Fraps, Ibid.,99, 134 (1906). (12) Gordon, J . Assoc. Oficial Agr. Chem., 6, 407 (1923). (13) Hare, Bur. Chem., Bull. 67, 17 (1902). (14) Intern. Critical Tables, 4, 353. (15) Jacob, Rader, and Ross, J. Assoc. OficiaE Agr. Chem., 15, 146 (1932). (16) Kerr, Ibid., 8, 419 (1925). (17) Marshall, J . Agr. Research, 49, 71 (1934). (18) McDonnell, Bur. Chem., Bull. 73, 28 (1903). (19) Munson and Hare, Ibid., 62, 13 (1901). (20) Porter and Kenny, J. IND.ENQ.CHIM.,1, 304 (1909). (21) Robinson, J. A m . Chem. Soc., 16, 364 (1894). (22) Robinson and Holmes, U. S. Dept. Agr., Bull. 1311 (1924). (23) Thornton and Kraybill, J . Assoc. Oficial Agr. Chem., 18, 281 (1935). R E C B I V ~July D 6, 1935. Presented before the Division of Fertiliser Chemistry a t the 88th Meeting of the American Chemical Society, Cleveland, Ohio, September IO to 14,1934.
Chemical Microscopy IN THE Symposium on Recent Advances in Microanalysis at CHEMICAL SOCIETY, the New York Meeting of the AMERICAN C. W. Mason, of Cornell University, speaking on “Chemical Microscopy,” limited his paper to the microscopical study of crystals and related physico-chemical phenomena, emphasizing that this field is of the greatest potential value to chemists who deal with solids in any way, since the microscope can be used as an instrument for determining numerous optical constants and also for the direct observation of “phase-rule” phenomena in one- or multi-component systems. The optical tests used by microscopists in crystal identifications were demonstrated by microprojection with polarized light, and it was pointed out that the growing ampunt of available data on optical characteristics makes possible highly positive identifications or differentiations by using properties that are obtain-
able in addition to any chemical information that is a t hand The application of such phenomena in the study of micellar aggregates was also illustrated; materials of this character often show marked variations in microscopical features with chemical or physical treatment, and excellent control tests are possible for studying chemical derivatives, swelling, stretching, or permeation of fibers or other substances of high molecular weight. The physical chemistry of processes involving crystals wm stressed as an important field for microscopical study, not only for the identification of the actual solid phases present in the system at any time, but also because the type of equilibrium diagram is often predictable or confirmable by a simple examination. Examples of solid solution, adsorption phenomena, freezing, allotropy, salt systems, and eutectics were shown, indicating how the microscope is an essential tool of the physical chemist.
