ANALYTICAL EDITION
June, 1944
Murray and Ashley reported serious interference with high (4%) silicon, but in the usual range of this element no interference wab found in this work. More than 0.4% chromium gives a positive error. I n general, the accuracy equals that of the common titrimetric method, which is less rapid and requires more skill. The molybdenum blue method of Hague and Bright ( 2 ) is also 1e.w rapid. LITERATURE CITED (1) Getzov, Zovodskayu Lab., 4, 349 (1935).
(2) Hague and Bright, J . Research Natl. Bur. Standardst 26, 405 (1941). (3) Kitson and Mellon, IND.E m . CHEY.,AINAL. ED.,16,42 (1944).
383
(4) Koenig and Johnson, Ibid., 14, 155 (1942). (5) Lundell, Hoffman, and Bright, “Chemical Analysis of Iron and Steel”, p. 131, New York, John Wiley & Sons, 1931. ( 6 ) Mission, Chem. Ztg., 32, 633 (1908); Ann. chim. anal. chim. appl., 4,267 (1922). (7) Murray and Ashley, ISD. ENG.CHEY..ANAL.ED., 10, 1 (1938). (8) Schroder, StahE u. Eisen, 38,316 (1918). (9) Vosburgh and Cooper, J . Am. Chem. SOC.,63,437 (1941). (10) . , Willard and Center, IND.ENQ.CHEM.,ANAL.ED.,13, 81 (1941); Center and Willard, Ibid., 14, 287 (1942). (11) Woods and Mellon, Ibicl., 13,760 (1941). ABCITRACTED from a portion of the thesis presented by R . E. Kitson t o the Graduate School of Purdue University in partial fulfillment of the requirements for the degree of doctor of philosophy.
Determination of Potassium in Fertilizer Mixtures Removal of Ammonia and Organic M a t t e r without Ignition A. 6. JOY The Pacific Chemical and Fertilizer Co., Honolulu,
T
HE ignition step in the A.O.A.C. ( 1 ) method Cor the determination of potassium in mixed fertilizers require. c,:ci,eful attention and may sometimes r e d t in pot or insoluble residues. Thi.: paper presents a procedure which h:t- been found t u reduce errors and shorten the tinit, of tiiidysis of many fertilizers by eliminating high-teniper:iturr ignition Ivitti rulfuric acid. Kraybill and Thornton ( 4 ) have called attention to lossea during ignition which may be caused by spatbering or volatilization. St. John and llidgley (6) found that controlled temperatures tend to avoid volatilization of potassium due to localized iJi.rrheating. They also noted insoluble residues from ashing plurit materials with sulfuric acid, but usually none when using their acid digestion method. [II his 1940 Report on Potash, Ford ( 3 ) sliowed that errors From “water-insolul,le residues that are ofteu encountered” should be corrected either by filtering the potassium solution twfore adding platinic chloride, or by dissolving the weighed potassium platinic chloride with hot water and reweighing the dried crucible. In eit,her case additional work is involved which iiicrrases thr. timc of analysis. Iii 1934 this laboratory experienced difficulty in obtaining :tc.c.urate potash results by the official method of that time when analyzing fertilizers conta.ining little or no Puperphosphate and large amounts of monoammonium phosphate. Since the ignition step was found to be t,he main source of error in the analysir of mixture.. of t,his type (Z), a procedure was adopted which eliminated ignition by using a low-temperature method to remove interfering ammonium salts and organic matter. Thus it was impossible to form insoluble metasilieates or phosphates. Changes in the A.O.A.C. method were made in 1936, so that with proper manipulation and corrections for insoluble residue it was possible t o determine potassium in all types of fertilizer Fitti a greater degree of accuracy. Although these changes included better ignition technique, the low-temperature procedure as used in t’his laboratory appeared to have certain advantages. Several improvements in the method have been made recently and analytical results compared with those obtained by ignition with sulfnric acid, using t,he same solution for both drterminat ion;;.
T. H.
METHOD1 (for inorganic fertilizers). Place 5 grams of sample in a 250-nil. volumetric flask and add about 100 ml. of water and 50 ml. of saturated ammonium oxalate solution. Boil 30 minutes, cool, dilute to 250 ml., mix well, and filter or allow coarser particles to settle. Pipet a 100-ml. aliquot into a 500-ml. Kjeldahl flask containing a few glass beads. (If moisture content of the fertilizer permits grinding fine enough to prevent separation of particles, weigh a 2-gram sample directly into the Kjeldahl flask,. Add about 75 ml. of water and 20 ml. of saturated ammonium oxalate solution. Boil 10 minutes.) Add 3 or 4 drops of 1% phenolphthalein solution and 5 ml. of concentrated sodium hydroxide solution. Boil vigorously until damp red litmus paper placed over mouth of flask shows no trace of blue color when left there several minut’es. The time required to expel ammonia is usually less than 15 minutes. If red color of phenolphthalein fades during boiling, add sufficient sodium hydroxide to restore. Cool and transfer with thorough washing to a 200-ml. volumetric flask. Dilute tu mark, mix well, and pass through a close-textured dry filter. Determine KzO on a 25- or 50-ml. aliquot by t,he platinic chloride method. Treat precipitate in evaporating dish n-ith about 16 ml. of 85% alcohol and 1 ml. of concentrated hydrochloric acid. Use rubber policeman to break up residue thoroughly before transferring to Gooch cruciblr. Wash as usual with alcohol and ammonium chloride solution, but use ten 10-ml. portions of ammonium chloride solution if :t jO-ml. aliquot was taken. Adjust suction during the ammonium chloride washing so that the jet from wash bottle or 10-ml. pipet (enlarged tip opening) will about half fill the crucible and agitate the precipitate with each washing. ~ I E T H O2D(for fertilizers containing urea, cyanamide, or other interfering organic materials). Prepare a 5-gram to 250-ml. fertilizer solution as in Method 1, but after boiling 30 minute3 add 2 ml. of Concentrated ammonium hydroxide or sufficient to make alkaline. Cool, dilute to volume, mix well, and pass through a close-textured dry filter. Add 5 ml. of concentrated nitric acid to a 100-ml. aliquot in a 500-ml. Kjeldahl flask and boil vigorously to a volume of about 10 nil. Add 5.ml. of sodium chlorate solution and 5 ml. of 30y0 sulfuric acid. (If sample is high in organic matter or urea, use 10 ml. of 30y0 sulfuric acid.) Boil to dense sulfur trioxide fumes using low heat, then use high heat for 10 to 15 minutes to destroy organic matter. Cool 3 or 4 minutes and wash down neck of flask with about 100 ml. of water. Heat to obtain complete solution of residue. Add phenolphthalein and neutralize with concentrated sodium hydroxide solution. Add 2 ml. excess and boil to negative test for ammonia. Proceed as in Method 1.
METHOD
DISCUSSION
REhcxwrs (other than used in A.O.A.C. method). Concentrated sodium hydroxide solution, 30 grams of A.C.S. reagent sodium hydroxide per 100 ml. of solution. Sodium chlorate solution, 10 grams of A.C.S. reagent sodium chlorate per 100 nil. of solution. 30% sulfuric acid, 20 ml. of sulfuric acid (A.C.S. reagent, 1.84 sp. gr.) in 80 ml. of water.
The use of a 5-gram sample in this method rather than the official 2.5 grams is based upon the study of Kraybill and Thornton ( 5 ) , who found that errors may result due to difficulty in weighing uniform 2.5-gram samples. The larger sample is taken in order to minimize such errors, usually caused by separation
INDUSTRIAL AND ENGINEERING CHEMISTRY
384 Table 1.
Comparative
KzO Analyses
and Composition of Laboratory Mixtures New HzSOL IgNtlOn Method Composition of Kz0 Mixture KzO
Mixture"
Caloulated Kz0
%
%
%
L-1
22.2
22.14 22.32
22.19 22.23
22.23 21.93d 22.24
22.21 22.17 22.10
Av. L-2
22.2
L3
30.4
L-4
30.4
L-5
30.4
12-8
10. E
Av.
22.09 30.28 30.44
22.14 30.35 30.30
Av.
30.36 30.37 30.43
30.33 30.42 30.36
Av.
30.40 30.18 30.40
30.39 30.41 30.29
30.29 10.75
30.35 10.76
Av.
% 20 20 10 25 20 5 20 20
KCla KzSOLC Superphosphate ("c)zSO4
10
10 10 10 10 10
Ammo-Phos A Bone meal KClb KzSOrC Superphosphate Ammo-Phas A CaCNz Uramon Fish meal Bone meal
50 20 20 10
Superphosphate NHc)zSO4 ammo-Phos A
KCI~
.s Materials weighed on analytical balance. Duplicate analyses made on a second series of weighings. b Trona muriate. Analysis by this laboratory, 61.29% Kz0. Manufacturer's analysis 61.26% KzO. 6 Foreign poheaium sulfate. Analysis by this laboratory, 49.70% KaO. Manufaoturer's guarantee, 49.5% KzO. d Low result Drobabls caused bv sDatterinn due to larze - at start of iznition amount of orgahio matier. e Trona muriate. Analysis by this laboratory, 60.80% KzO. Manufacturer's analysis, 60.81% Kz0. I Analytical reagent, dried before using. Theoretical KaO, 54.05%.
of coarse 1-mm. particles from the fines. Many fertilizers of low moisture content, however, can be finely ground and have only traces of organic matter from phosphates and by-product ammonium sulfate. Two grams of such material may be treated directly for ammonia removal by the shortened procedure. The total time from weighing a 2-gram sample to taking an aliquot for evaporation with platinic chloride is about 40 minutes. The time of boiling a 5-gram sample with ammonium oxalate has been made to agree with the A.O.A.C. period of 30 minutes, but it has been the experience of this laboratory that 10 to 15 minutes is sufficient for most types of fertilizer, especially when the composition of the sample is known or a finely ground 2-gram sample is used. Although Method 1 does not require addition of ammonium hydroxide after boiling with ammonium oxalate, this may be done if the solution is to be used for a check analysis by sulfuric acid ignition. This does not appreciably lengthen the time of expelling ammonia from a 100-ml. aliquot. Phenolphthalein indicator is not destroyed or volatilized during the boiling period. Fading of color indicates a drop in pH and will occur only in exceptional cases where 5 ml. (1.5 grams) of sodium hydroxide are not sufficient to react with all ammonium salts present. If it is desired to check the litmus test for complete removal of ammonia, phenolphthalein may be omitted and a portion of the final filtered solution treated with Nessler reagent. The final solution of a fertilizer analyzed by Method 1 will contain a small amount of sodium oxalate. I n case the sample contains no soluble calcium the amount of sodium oxalate in a 50-ml. aliquot taken for evaporation with platinic chloride will be approximately 0.24 gram. This may be decomposed before precipitation of potassium platinic chloride by the hypochlorite reaction, or preferably allowed to remain with the precipitate,
Vol. 16, No. 6
since this amount of sodium oxalate is easily removed by acid alcohol and ammonium chloride washing. Decomposition by sodium hypochlorite is accomplished by the addition of 3 to 4 ml. of pure 5% available chlorine solution to a 50-ml. aliquot in a porcelain dish. The solution is then made distinctly acid with concentrated hydrochloric (2 ml.) and evaporated for 10 minutes, after which the necessary amount of platinic chloride is added. The final solutions of organic fertilizers will not contain oxalate since it is destroyed by the sulfuric acid treatment of Method 2 In this procedure sodium chlorate is used in addition to sulfuric acid in order to destroy other forms of organic matter, especiallj when present in large amounts. No attempt was made to use perchloric acid because of the possible hazard of explosion. Table I gives the comparative analyses and composition of six fertilizer mixtures made in the laboratory, each containing known amounts of KeO. Interfering materials such as calcium cyanamide, urea, tankage, and fish meal were used in amounts greater than those ordinarily found in commercial fertilizers One inorganic mixture (L-6) which contained no calcium was included, so that the maximum amount of sodium oxalate would be present. I n these particular mixes, with the exception of L-1, ignition with sulfuric acid according to the A.O.A.C. technique produced only traces of insoluble residue. The asbestos-padded GoocE crucibles were therefore not washed out with hot water and reweighed. Table I1 gives comparative analyses on various type& of local commercial fertilizers.
Table
Comparative KzO Analyses of Commercial Fertilizers HzSO4 I nition New Method Variation K,% KiO Kz0 Formula
II.
% 7-20.5-17 7-20.5-17" 7-20.5-17b 6-20-12 8-12.5-6 7-3.5-23 6-15-10 7-10-10 7-11-10 8-9-11 7-12-7 11-20-22 z .
%
%
18.07 16.48 17.86 13.32 5.72 23.01 9.98 10.17 10.48 10.68 7.83 26.63
Containing 25 pounds of bone mer -9er ton.
b Containing 50 pounds of Uramon per ton.
SUMMARY
A method presented for the determination of potassium in mixed fertilizers involves the usual ammonium oxalate solution and chloroplatinic acid precipitation, but employs a rapid volatilization procedure for removal of ammonia, and an oxidizing method for destroying interfering organic compounds. This means of preparing a solution from which potassium may be precipitated makes it unnecessary to ignite a t a high temperature with sulfuric acid, which may result in the formation of insoluble residue or the loss of potassium by spattering or volatilization. A large number of analyses by this method have been made on inorganic and organic fertilizers containing salts of known potahsium content, and the results are in close agreement with t h e calculated values of these mixtures. LITERATURE CITED (1) Assoc. Official Agr. Chem., Official and Tentative Methods i d
Analysis, 5th ed. (1940). Bible, C . M., IND.ENO.CHEM.,ANAL.ED., 4, 234 (1932) Ford, 0. W., J . Assoc. Oficial A g r . Chem., 23, 264 (1940) Kraybill, H. R . , and Thornton, S. F., Ibid.. 18, 263 (1935) Ibid., 18, 274 (1935). (6) St. John, J. L., and Midgley. M . C., IND.ENQ.CHEM.,ANAI En 14, 301-2 (1942).
(2) (3) (4) (5)
