Fertilizers Charles W . Gehrke and James P. Ussary, Department of Agricultural Chemistry, University o f Missouri, Columbia, Mo.
T
review covers the literature reported from September 1, 1962, to December 1, 1964, and includes procedures recorded in readily available journals, in Chemical Abstracts, and in .lnaLytical Abstracts. Some selectivity has been exercised to include only those procedures especially pertinent to, or which, in the author’s judgment, could be adapted easily to, fertilizer analytical problems. .1 number of papers presented a t the 148th annual meeting of the American Chemical Society a t Chicago (1964) related to fertilizer technology and analysis covering subjects of sampling; nitrates; water; determination of N, P, and K ; reactivity rates; dissolution rates: and various phases of manufacturing technology. Considerable information of importance to laboratories engaged in fertilizer analyses was obtained in a broad study by 12 state control and 11 industry laboratories. This study was supported by the Association of American Fertilizer Control Officials (AAFCO), the Sational Plant Food Institute ( S P F I ) , and industry members of the Chemical Control Committee of the N P F I . A summary report on this study was made by Quackenbush at the h.1FCO meetings in -1ugust 1964 at Poland Springs, Maine; and bo the llIagruder Committee, meeting in Washington, D. C., October 1964. A manuscript is in preparation for publication in the J o u r n a l of the dssociation of Oficial -1gricultural Chemists, The objective of this study was to determine the normal variations of analytical results within and among laboratories. The samples were sent to the respective laboratories with no efforts made to produce a uniform mix, and with particular emphasis that each laboratory did not know that this was a special study. The standard deviation within laboratories was about 0.3 and much larger than the usual Magruder within-laboratory standard deviations. The real amonglaboratory variation was about one half of the within-laboratory variation. I t was also concluded that the reduction and mixing of nonuniform samples by the 23 laboratories were performed in a satisfactory wag: however, there is a real need for standardization of reporting procedures. HIS
OFFICIAL
METHODS
The Association of Official Agricultural Cheniists (AOAC) gave “official” status to the gravimetric quinolinium phosphomolybdate method based on the “quimociac” reagent for total, watersoluble, citrate-insoluble, and available phosphorus. Also, a quinolinium phosphomolybdate method, without acetone, previously applicable only to total and citrate-insoluble phosphorus was given official status for available phosphorus. A method involving EDTA titration of acid- and watersoluble magnesium in fertilizers was adopted as official. The AOAC removed from official status a volumetric quinolinium phosphomolybdate method for phosphorus and an air-flow method for free water ( S , 4 ) . WATER
Studies were conducted on the moisture content of fertilizers by the vacuum oven method a t 60” C., the official 100’ C. oven method, and the Cenco balance method (53). No significant differences were noted for raw materials, with the exception of superphosphates, urea, and other similar products with low decomposition temperatures. Results on mixed fertilizers were 35% lower by the vacuum oven method and varied widely depending on the total amount of moisture present, amount and type of solution used as a source of nitrogen, and type of fertilizer. Results by the Cenco balance method fell between the two oven methods. Ruggedness tests revealed that the only experimental variable to which results in the vacuum oven method were sensitive was the oven temperature (14). The allowable temperature range within the oven chamber was less than 5” C. Collaborative results showed that the vacuum oven method was as precise as the AOAC official vacuum desiccator method. h method was proposed for free water in fertilizers, which involves extraction with p-dioxane and titration of the extract with Karl Fischer reagent ( I S ) . The values obtained compared favorably with the official vacuum desiccator method. The need for collaborative tests of the two AOAC official methods (100” C. oven and vacuum desiccator) for water
in fertilizers has been pointed out ( 2 5 ) . A collaborative study showed that the analyst must be very empirical in his manipulation of the methods and that the difference between total water and free water is much too great for urea-containing fertilizers (12). NITROGEN
The AO;IC continued studies on methods for determining nitrogen in fertilizers. A collaborative study (18) of the rlO.AC reduced iron method (2.039), the Gehrke (22) modified reduced iron method, a chromous solution reduction method, and a chromium powder method indicated that all of the methods gave equally reliable results. I n a critical evaluation of the reduced iron method for the reduction of nitrates it was concluded that hydrogen-reduced iron was a more efficient reductant than electrolytic reduced iron and that 250-mesh is the largest particle size that should be used (11). The AOAC official reduced iron m2thod has certain limitations mainly due to imprecision of results. The modified reduced iron method has corrected many of these. Further research is being conducted on these methods by the .10AC and various laboratories. I n a study of several Raney catalyst powders for the reduction of nitrates the most efficient reductant in acid medium was an h l and S i alloy; others, in decreasing order of reducing efficiency, were Cu-dl, Si-Cr-A, Co-hl, and Fe-hl (10). I n the reduction, nascent hydrogen, generated by the reaction of A1 with the acid, was adsorbed on the alloying metal and there reduced the nitrate. A relatively small amount of the alloy was required, and the resulting salts did not interfere with the subsequent Kjeldahl digestion and distillation. The method was applied successfully to mixed fertilizers containing nitrates and to mixtures of potassium nitrate with organic materials. The method is suitable for solid and liquid fertilizers that contain a considerable quantity of chloride. A method was proposed for separating the various forms of nitrogen in fertilizers by cation and anion exchange (46). The method was accurate, rapid, and direct, and gave results not now attainable by AOAC methods. VOL. 37, NO. 5, APRIL 1965
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A simple and rapid nitrogen method using Devarda’s alloy has been proposed (49). Reduction of nitrates and distillation of ammonia were completed in 45 seconds. The method is applicable to samples containing nonvolatile amides. I t was automated so that up to 50 samples per hour could be handled (61). This method could be a valuable tool to the laboratory analyzing a large number of inorganic fertilizers. I n a comparison of the Kjeldahl, Sessler, and formaldehyde titration methods it was found that the Kjeldahl method was more reliable than the other methods and the Nessler method more reliable than formaldehyde titration ($7). The formaldehyde titration yielded low results when the sample was not finely ground. Sitrogen was determined by the Kjeldahl method without distillation (16). An aliquot of the digestate was neutralized with NaOH, a known excess of standard XaOH added, the solution boiled for one hour to drive off the ammonia, and the excess NaOH backtitrated with standard HzS04. Ten per cent accuracy was achieved. A method using a modification of the Pregl-Dumas method was found to give accurate results ( 1 ) . The sample was placed in a small quartz test tube, which was then placed in a larger quartz tube packed with precalcined X i 0 or CuO and burned a t 850” to 900’ C. Carbon dioxide was used to flush the combustion products into the azotometer. Sauchelli (60) edited a comprehensive book providing a single source of reference to the latest research and technology of synthetic ammonia and its derivatives, especially as applied to the fertilizer industry. PHOSPHORUS
The need of a separate acid hydrolysis step to convert all phosphates to the ortho form was required in the determination of water-soluble phosphorus by the AOAC official volumetric method but not in the quinolinium phosphomolybdate method. I n the direct determination of “available” phosphorus complete precipitation of phosphorus as quinolinium phosphomolybdate was obtained by carefully controlling the phosphorus and ammonium citrate concentrations (30). The quinolinium phosphomolybdate method was simplified by the use of a single precipitating solution (31) which contains quinoline, sodium molybdate, citric acid, and acetone, and has been designated the quimociac reagent. Acetone eliminated interference from ammonium ions in the precipitation and made the method suitable for the determination of available phosphorus in fertilizers. A study was made to determine the 68 R
ANALYTICAL CHEMISTRY
reliability of the AOAC volumetric ammonium phosphomolybdate method for Pz05($6). I t was found that the method was reliable if more explicit directions were made for each step. End point detection and solution standardization required careful control. A study was made on the effect of extraction time on the citrate-soluble P205 content of fertilizer (24). The data show that for mixed fertilizers and straight materials, all water- and citratesoluble phosphorus was extracted in 60 minutes. Two exceptions were noted : Further solubilization of citrate-insoluble PzOs occurred in samples of tricalcium phosphate and ammoniated triple superphosphate for extraction times of 2 to 3 hours. An automated spectrophotometric determination of total phosphorus in fertilizer based on the formation of the yellow phosphomolybdovanadate was reported (20). The average automated results agreed closely with quimociac results, but precision with the automated method was not so good as with the quimociac method. Results of field tests of ammoniated superphosphate fertilizers correlated well with availabilities determined by an alkaline ammonium citrate extraction, but showed no relation to availabilities determined by the official AOAC neutral ammonium citrate extraction procedure. It was recommended that the alkaline ammonium citrate extraction method be considered for determination of the availability of phosphate fertilizers (9). Water-soluble phosphorus was determined by neutralizing the sample solution using bromocresol green as indicator, then titrating with 0.1N NaOH to a phenolphthalein end point. The PzOs content was calculated from the difference of the volumes used in titrating with the two indicators, bromocresol green and phenolphthalein (36)* POTASSIUM
During the past two years the flame photometric determination of potassium has undergone further study and refinement. An automated method which gave accurate and precise results with a significant saving in analytical time was reported. Anion exchange cleanup was found to be unnecessary for samples containing less than 16% K 2 0 (25). The halogen acids and their ammonium salts decreased the emission intensities of alkali metal lines in flame spectrophotometry (62). The effects of HC1, H2SO4,Na, Ca, Mg, and A1 on the determination of potassium by atomic absorption spectrophotometry were studied (68). All except H2SOtsuppressed the absorption by potassium. This influence could
be detected down to 10 p.p.m. of potassium. Potassium was determined in the presence of sodium and magnesium by converting the chlorides to hydroxides with Ag,O, and titrating the potassium conductometrically in ethanol with a n ethanol solution of chloroplatinic acid (43, 44). The method is claimed to be superior to that using titration with an ethanol solution of perchloric acid. A precision comparison study was made between the perchlorate method and the titrimetric sodium tetraphenylborate method (47). Standard deviations of 0.12 and 0.095, respectively, were obtained on analyses of 15 routine samples. Potassium was determined by precipitating with a measured excess of sodium cobaltinitrite, filtering, and titrating the excess cobalt with disodium EDTA (34). Dugger (19) recommended a substitution of lithium for sodium as added base in the Lindo Gladding method. Complete drying of the chloroplatinate precipitate was suggested as preferable to drying to a mush, provided enough sodium or lithium was present to ensure that upon drying all excess chloroplatinic acid was converted to sodium or lithium chloroplatinate. Small amounts of potassium have been determined by precipitating the potassium with sodium tetraphenylborate, dissolving the precipitate in acetone, and determining the boron in the precipitate with carminic acid ( 8 ) . The accuracy was + 1% relative. Potassium was determined by precipitating, in an ethanol solution, with a measured excess of tartaric acid, filtering, and titrating the excess tartaric acid with 0.1N sodium hydroxide (36). One standard deviation was less than 0.08. SECONDARY A N D MICRONUTRIENTS
Complexometric titration of calcium and magnesium with EDTA sontinues to attract fertilizer chemists. I n a collaborative study it was shown that calcium and magnesium can be determined by EDTA titration equally as accurately and more precisely than by the official AOAC gravimetric methods (66). The EDTA method for calcium and magnesium in limestone was modified to give more accurate results for samples containing 2 to 4’3 of magnesium (23). The effect of phosphate on the EDTA titration of calcium and magnesium was also investigated. Complete recovery of calcium and magnesium wa6 obtained a t all levels of added phosphate, but at the higher levels the time required to complete the titration increased. Interference of phosphate was completely
avoided by using an anion exchange resin column. ,iluniinon was used as an indicator for calcium, magnesium, or calcium and magnesium a t pH 8.5 to 9.9. When methylene blue was added to the aluminon, the color change was wine red to green (35). Interferences can be marked with S a C F in S a O H , and triethanolamine (60). Xtomic absorption has received some attention in the determination of calcium and magnesium. Silicon interference was obviated by removal as SiF4 ( 7 ) . Lanthanum was used to buffer the interference due to aluminum. Millet (40) and Barker (5) in studies of methods for determining sulfur in fertilizer recommended gravimetric precipitation of BaS04 as one of the best methods. These investigators also reported on the following methods. Sulfate can be titrated directly with Pb(XO& using dithizone indicator; BaC1, with sodium rhodizonate indicator; or Ba(C104)2 with thorin or sodium alizarin-sulfonate indicator. The last indicator was preferred, but it was found necessary to remove phosphate. Data were presented on the precipitation of sulfate with excess barium ion and titration of the excess with E D T A and Eriochrome Black T indicator. Reduction of H2S04 to H2S with HI and H3P04in a special apparatus followed by titration of the H2S with Hg(OAc)2 was found to be rapid. These procedures had to be modified for various fertilizers. An indirect spectrophotometric method for the determination of boron with carminic acid was reported (32). The method obeys Beer’s law u p to 8 p.p.m., and is sensitive and relatively free from chemical interferences. A simple and sensitive complexometric method was reported by Heyes and Metcalfe (29) using the boron-curcumin complex in a nonaqueous medium. Fluoride and nitrate interfere but can be overcome. Hiiro (33) used alizarin S for the spectrophotometric determination of boron in an aqueous solution instead of in concentrated sulfuric acid. Iron and aluminum interfere but can be tied up with EDTA. Protocatechuic acid (3,4-dihydroxybenzoic acid) forms a 1 to 1 complex with boron at p H 8 and can be read with an ultraviolet spectrophotometer a t 302 mM (33). Atomic absorption spectrophotometry has been applied by Morgan (41) in the determination of copper. Low levels were determined by extracting copper from a n HC1 solution into methyl isobutyl ketone with ammonium pyrrolidine dithiocarbamate by the method of Allan ( 2 ) . When iron and a mixed acid were added, a slightly smaller slope was observed ( 5 7 ) . The sensitivity of the determination was increased by analyzing the exhaust gases
of the flame (62) or using high pressure atomization (28). A gravimetric method for copper using resacetophenone phenylhydrazone as the precipitant was reported (59). I n the absence of cadmium, copper could be accurately determined up to 64 mg., whereas in the presence of cadmium the method was good to 32 mg. Copper and cobalt have been titrated with E D T A using bis(carboxymethylaminomethyl) dichlorofluorescein as a metallofluorochromic indicator (6).
The addition method in flame photometry has been applied to the determination of copper (42). Special attention was paid to the use of variable apparent blank readings for different concentration ranges on a calibration curve with a maximum of 200 p.p.m. of copper. Sirois (55) reported quantitative results for iron, copper, and zinc by plasma jet spectroscopy under noninterference conditions in multielement environments. Results were precise and accurate, and the method appeared to be widely applicable. Zinc has been determined by EDTA titration in the presence of nickel and cobalt, and was based on the selective decomposition of the cyano complex of zinc by formaldehyde (37). By using a photometric end point detection system and a weight buret, precision of 0.1% absolute or better for the E D T A determination of zinc was obtained (51). Mekada, Yamaguchi, and Ceno (39) used dimercaptosuccinic acid as a masking agent of cadmium, copper, and mercury in the determination of zinc, It was reported by Fuwa et al. (21) that an atomic absorption spectroscopy method for zinc was 10 to 100 times more sensitive than the dithizone method, required only one tenth as much time, and gave more precise results. The presence of HCl or HX03 influenced the results for zinc, but when the concentration of acid was changed from 0.3.V to 1.2N little further change in absorbance values was observed ( 4 8 ) . Cobalt was determined spectrophotometrically with 4-(2-pyridylazo)resorcinal, but iron and nickel interfered even on addition of E D T A or K C N (54).
Molybdate was reduced to Mo(V) by the stannous chloride-perchlorate reaction and determined spectrophotometrically as tricaprylmethylammonium oxytetrathiocyanatomolybdate(V) (38).
The Association of Official Agricultural Chemists conducted a collaborative study on colorimetric determinations of alurninum, iron, manganese, phosphorus, and titanium in liming materials (17 ) . Aluminum was determined with ammonium aurintricarboxylate (aluminon) ; iron with 2,4,6-tripy-
ridyl-s-triazine (TPTZ) ; manganese by oxidation to permanganate with KI04; phosphorus by a heteropoly blue method; and titanium with disodium1,2 - dihydroxybenzene - 3,5 - disulfonate (Tiron). Sample solutions were prepared by perchloric acid digestion or sodium hydroxide fusion. The results were precise and accurate, and no significant differences were observed for the two methods of sample solution preparation. Yields of Aspergillus niger mycelia were used to determine magnesium, zinc, manganese, copper, and molybdenum in fertilizers, plants, and soils, their ashed residues, or water extracts. The error was 4 to 8% and the time required was 2 to 5 days (45). PESTICIDES
Techniques that are well suited to handling an analytical problem such as that posed by residues of pesticides are gas-liquid (GLC) and thin-layer chromatography (TLC) supplemented with confirmation by infrared. These chromatographic methods offer the obvious advantages inherent in the refined separations that are possible: very small sample size, speed, accuracy, and simplicity. Gas-liquid chromatography with electron capture and microcoulometric detection systems have been used most widely and successfully for the determination of most pesticides. Thinlayer chromatography has been used for the separat>ionof pesticides (qualitative and semiquantitative) and identification of degradation products. This paper does not discuss methods for pesticide residues in fertilizers, since they are covered thoroughly in another review. LITERATURE CITED
( 1 ) Abramyan, A. A., Kocharyan, A. A., Karapetyan, A. G., Izv. A k a d . N a u k Arm. SSR, K h i m . AVauki 15, 225-30 (1962). ( 2 ) Allan, J. E., Spectrochim. Acta 17, 467 (1961). ( 3 ) Assoc. Offic. Agr. Chemists, J . Assoc. O f i c . Agr. Chemists 46 ( l ) , 127 (1963). ( 4 ) Ibid., 47 ( l ) , (1964). ( 5 ) Barker, J. E., Ibid., 47 (3), 436 (1964). (6) Bermejo-Martinez, F., Margalet, A,, Chemist-Analyst 53 (2)) 45 (1964). ( 7 ) Billings, G. K., Adams, J. A. S., Atomic Absorption Xewsletter (PerkinElmer Corp.), No. 23 (August 1964). (8) Bovalini, E., Casini, A., Ann. Chim. ( R o m e ) 52, 482 (1962). ( 9 ) Brabson, J. A., Brirch, W. G., Jr., J . Assoc. Ofic.A g r . Chemists 47 ( 3 ) , 439 (1964). 10) Ibid., p. 1035. 1 1 ) Brabson, J. A., Burch, W. G., Jr., Woodis, T. C., Jr., Ibid., 46 (4), 599 (1963). 12) Caro, J. H., Ibid., 47 (3), 443 (1964). 13) Ibid., p. 626. 14) Caro, J. H., Heinly, S . A,, Ibid., 47 ( 6 ) , 1040 (1964). 15) Caro, J. H., Rexrond, P. R., Ibid., 46 ( 4 ) , 582 (1963). VOL. 37, NO. 5, APRIL 1965
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(16) Chang, K., Hua Hsueh Tung Pao 6, 3 (1960). (17) Chichilo, P.,J . Assoc. Ojic. Agr. Chemists 47 (6),1019 (1964). (18)Davis, H. A,, Durgin, 0. B., Ibid., 46 (4),595 (1963). (19) Duaaer. J. F., Jr., Ibid., 46 (4),773 (20) Ferretti, R. J., Hoffman, W. M., Ibid., 45 (4),993 (1962). (21) Fuwa, K., Pulido, P., McKay, R., Vallee, B., ANAL.CHEM.36,2407(1964). (22) Gehrke, C. W., Beal, B. M., Johnson, F. J., J . Assoc. Ojic. Agr. Chemists 44, 239 (1961). (23)Gehrke, C. W.,Johnson, F. J., Ibid., 46 (4),611 (1963). (24) Gehrke, C. W., Kramer, G. H., Jr., Ibid., 47 (3),457 (1964). (25) Gehrke, C. W., Ussary, J. P., Kramer, G. H., Jr., Ibid., 47 (3), 459 (1964). (26)Harwell. A. 0.. Parks. K. L.. Harwood. wood, W.’M., Baker, J.’ E., Ibid., 47 428 (1964). ((3), 3 ) ; 42, Hecht, H., Fritz, A., Brauwissen(27) Hec schaft 15 (ll),347 (1962). (28) Herrmann, R., Lang, W., Collop. Svectros. Intern.. Sth, Lvons. 1961. 3; 291 (Pub. 1962). (29) -~ Heves. M. R.. Metcalfe. J.. C . K. I
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