Fert iIizers E. D. Schall, Department of Biochemistry, P urdue University, Lafayetfe, Ind.
T
HIS REVIEW covers the literature reported during the biennium ending September 1, 1960. Procedures recorded in readily available journals, in Chemical Abstracts, and in Andytacal Atistiacts are included, although some selectivity has been exercised to include only those procedures especially pertinent to or which, in the author’s judgement,, could be adapted easily to fertilizer analytical problems. The most recent review in this series appeared in April 1959 (6). A later review summarized recent developments and current problems in the analytical chemistry of fertilizers (39).
OFFICIAL METHODS
The Associabinn of Official $gricultural Chemists (AOAC) incorporated several changes into its methods, adopting as “first action” a procedure for determining the availability of nitrogen in urea-formaldehyde products, and a colorimetric procedure for biuret in urea. It also reduced; from 20 to 10, the number of cares constituting an official sample. -4 number of new methods were adopted as “official” during the biennium. These included a procedure incorporating constant agitation in the preparation of citrate-insoluble residues, a photometric method for the determination of total and of citrate-insoluble phosphorus, and volumetric and flame photometric methods for potassium. Also given official status was a volumetric method for copper, a colorimetric method for urea in mixed fertilizers, a procedure for sampling ammoniacal solutions and liquid fertilizers, and a method for determining watersoluble magnesium. SAMPLING
Samples from 10 bags, ;andomly selected, were considered adequate to represent a lot of fertilizer (68). Where less than 10 bags are present, 10 cores shoylld be taken with a t least one core from each bag. I n small package lots the sample consists of one entire package. Riffling is the preferred procedure of reducing the bulk sample to a laboratory sample. These techniques were adopted as first action by the AOAC (84). 46 R
ANALYTICAL CHEMISTRY
Statistical mrthods were employed also in devising sampling plans in which defined risks, from both the consumer’s and the manufacturer’s standpoint, were included (22). AMMONIACAL SOLUTIONS AND LIQUID FERTILIZERS
Fertilizer solutions containing free ammonia were sampled by dissolution in a known weight of water contained in a partially collapsed polyethylene bottle (76, 83). The vapor pressure was reduced to negligible proportions by this dilution and samples could be weighed for analysis in open containers R ithout significant loss of ammonia The techniques employed were slight modifications of procedures reviewed earlier (6). Apparatus for the analysis of anhydrous ammonia in the field and for sampling nonpressure liquid fertilizers was described (76). Stainless steel pressure containers were used in sampling and detcrmining the aaltingaut temperatures of nitrogen solutions containing free ammonia (105). Existing procedures were modified to ascertain the nitrogen distribution in fertilizer solutions. Urea, ammoniacal, nitrate, and total nitrogen wer’. determined (83). The conceiitration of ammonium sulfate in an aqueous solution of this salt can be determined rapidly by refractive index measurements (99). The temperature need not be known. WATER
The residual water content of fertilizer salts was determined from the dielectric properties of the sample in benzene saturated with water (62). The water content should be below 5% and water of crystallization is not determined. Superphosphate samples vary greatly in their intensity of and capacity for sorption of atmospheric moisture and should be handled to minimize water losses to, or gains from, this source (32). NITROGEN
The quality of water-insoluble nitrogen in mixed fertilizers containing
urea-formaldehyde products was determined by a modified activity index procedure (11, 15, 16). The correlation bet-xeen these values and the nitri5.ation index, obtained by incubating the sample in soil a t 30” C., was significant a t the 5% level (10) and the procedurcl was adopted as first action follo-xing collaborative studies (84). The time-honored Kjeldahl method continued to receive attention. A study of some of the variables in this procedure (71) indicahed HgO was thc best catalyst and that the additior? of oxidizing agents resulted in loss of nitrogen. The optimum quantities of K$04 and HgO were found to be 7.5 grams per 20 ml. of HaOaand 1.0 gram per flask, respectively. Nitrate nitrogen was determined through reduction to ammonia by chromous ion in an acid solution (60). Reduction with zinc dust in aqueous ammonia stopped a t the nitrite iori (81) which was measured by permanganate titration or colorimetrically by the Griess-Ilosvay reagent. Several cations interfere and must be complexcd or precipitatcd before the reduction, I n a gravimetric procedure, the nitrogen content of Ca(NO3)t was determined by precipitating the nitrate ion as the nit,ron salt (63). A number of ions interfere and the method is applicable only to relatively pure salts. ii procedure for determining urea in mixed fertilizers (16) was studied collaboratively and given official status by the AOAC (84). I n this method the ammonia produced by hydrolyzing urea with urease, following phosphate precipitation with barium hydroxide, was titrated in the presence of a mixed indicator. In a second hydrolytic procedure the carbon dioxide produced by treating urea with nitrous acid was absorbed in standard barium hydroxide (27). The method is applicable in the presence of NHa and NH4NOs and was adapted to nitrogen solutions. Partial hydrolysis of urea was found to introduce a positive error for ammoniacal nitrogen when determined by distillation from a magnesium oxide slurry (104) in the presence of this compound. Biuret was separated from urea by passing an aqueous solution of the sample over an anion exchange resin,
(AaY). Xfter \vasliing t,he column free of ur(':i the biuret was eluted with dilute I1( '1 :tnd dcttwnined by micro Kjeldahl. In fertilizers an xnhydrous CHIOH ii.\trsc,t of the sample mixiirniaed intercnri-s from other anions. The MsOH had no detrimentni eiyect on !lit. dvterminrttion. 3pcctrophotometric methods based !i1mn nietsl-biuret coinplexes in an dkaline tart.rate so!ution mere also rcported. A procedure employing the ~.oi)pcr-l,iriret,comple:: was studied col::ihor:itivdy (16,1 6 ) and found to be :tliplii.able directly to urea samples; ic.rtilizers containing urea were passed m'er rxchange resins to remove inter:cLring cations before color deirelopment. 'The method was aciopted as first action ,, iq+: , .\ -4 nickel-biuret complex gave cjmparable results (87). The nitrogen distribution in mixtures wntaining amnioniacal, cyanide, nitrite. :ind nitrate forme was determined 'SO) --Idirect potentiometric titration 7 4 ) of calcium ryanamide with AgNOs !\-:LS :ound to provide satisfactory re. d t s !'or industrial analyses. InterFerenw by free CaO was eliminated hy an osnlnte Precipitation. In a gasometric procedure the iimrnonin content of ammonium salts was determined from the volume of nitrogen produced by an alkaline hypo5roniite oxidat,ion of the salt (66). G:w chromatographic techniques showed that the primary products of NHdT\'O* i-leconi~~osition in mixed fert,ilizer were S.0 : i n d elemrntary nitrogen (4). PHOSPHORUS
C'ol1:ibor:itive studies of continuous intrrniittent agitation in the citrate idigx:stion for insolublc phosphorus (41) shorved lower results by the continuous method and it was given official status 184). Citric acid w:is more effective t h a n niiinion~umcitrate as an extractant for ~.~liosphorus from raw and steamed hone iiie:i!s (96). Siic,ctrophotometric methods based on the molybdovanadophosphate comp l m receiyed considerable attention tluring t h e past two years. Successful :ippliontions of the methods were reported for determining totai (18, 4 6 ) xiid ritrate-insoluble ( 7 ) phosphorus. 'I'he method was also applied to the rlirclct determination of available phosphate by combining the watersoluble and citrate-soluble extracts. Citrnte, nliich interferes with the color tiewlopment, was removed by a NaC103 oridstion (Sj or by digestion with a tern:iry acid (25) mixture (HN03, HCIO,, H2S04). Volumetric procedures were also applicable to the c.itrnte-free solutions (8) and the influence of a number of variables on accuracy aiid precision were studied (34, 40). vs.
Studies on the composition of the molybdovanadophosphate complex indicated the molnr ratio of molybdenum to phosphorus and of vanadium to phosphorus should be greater than 22 and 1.7, respectively (57). In the presence of citrate the molybdenumphosphorus ratio should be increased t o 27 or higher. Increasing the "Os concentration in the reagent reduced interference from silicic acid but resulted in increased color fading a t conrentrations greater than 1N (58). The influence of temperature, acidity, and interfering ions on the stability of the complex was also investigated (27). Interference from several cations was eliminated by extracting the colored phosphate complex with isobutyl methyl ketone and reading spectrophotometrically in this solvent (44). The Volumetric procedure based on quinolinium phosphomolybdate proposed earlier (101) was studied further from the standpoint of the effects of varied amounts of silica, citric acid, and other interferences (19). Initial attempts to adapt this salt to a gravimetric procedure were unsuccessful due to the tenacity with which it retained water (101). This difficulty R'&S overcome by drying a t 250" C. and a gravimetric method was proposed for total phosphorus (64) and for the direct determination of available phosphorus without prior destruction of citrate (65). Later thermogravimetric studies (98) showed that the salt began losing water of hydration a t 107" C., maintained a constant weight level corresponding to the anhydrous salt from 155" to 370" C., and mas converted to the anhydride above 500" C. Methods Lased on the precipitation of phosphorus as metal phosphates also received considerable attention. The solubility product of BiPO, was ( 4 9 ) and reported to be 7.24 X indirect methods were proposed in which an excess of standard Bi(N02)3 was added to a slightly acidic solution of the sample. After filtering, the excess bismuth was titrated with EDTA [(ethylenedinitrilo)tetraacetic acid] (4S, 49. 6 9 ) . Direct titration with this rea.gent was possible when dithizone was t'he indicator (91j , Direct titrations were also possible with standard higSO4 solutions in the presence of Eriochrome Black T ( I , 88), after complexing iron and aluminum with DCTA (diaminocyclohexanetetraacetic acid) and calcium and magnesium with EDTA. Buffering the solution to p H 10 to 10.5 with ",OHWH&1 accelerated the Precipitation of MgNH4POa. In a related procedure, phosphorus was precipitated as zinc ammonium phosphate at p H 7.6. After filtering, the precipitate was dissolved in a buffered ammoniacal solution a t p H 10
and the zinc was titrated with EDTA (36). Orthophosphate in superphosphate was determined by an alkalimetric resulting from titration of the "03 the precipitation with AgNOs (33). Calcium, aluminum, and iron were masked with EDTA but a number of other cations interfered. Lead nitrate was also used in the same manner following cation removal by exchange resins ( 0 1 ) . Resins were also used to convert phosphate and sulfate to their respective acids (2). Total acidity was determined by titration to p H 8.98 and Hap04 by back-titration to 4.63. Sulfate was then calculated by difference. I n a novel spectrophotometric method, solid lanthanum chloranilate was added to an acidic solution of the sample, precipitating LaPO4and liberating an equivalent amount of the colored chloranilate ion (29). Since many cations interfere, prior treatment with exchange resins is essential. Instrumental methods included an amperometric titration applicable in the presence of ferri,c iron (go), a potentiometric titration with H g ( W 0 h (ST), and a turbidimetric method for glassy phosphates (13). Pyrophosphates were precipitated from an ammoniacal solution by ammonium cobaltinitrate, forming a stable complex which served as the basis for a gravimetric procedure for these compounds
(86). In a colorimetric procedure, molybdophosphate was reduced to the stable molybdenum blue by ascorbic acid (23). The primary applications of this method are limited to samples of low phosphorus content. POTASSIUM
Factors affecting the precision of the volumetric tetraphenylborate method were studied collaboratively (24) and the procedure, after the incorporation of Clayton Yellow as the indicator (21), was given official status by the -40.4C (84). In a different approach, potassium was precipitated as the ' tetraphenylborate which was then converted to phenylmercuric acetate. Reaction of the latter with KBrO3 followed by the addition of K I liberated iodine which was titrated with Na2S203(14). Potassium was also titrated directly with sodium tetraphenylborate in a high frequency (14-megacycles) titrimeter (92). The accuracy was good with simple solutions but decreased in the presence of large amounts of other salts. Investigation of possible interference errors in the flame photometric method (56) showed that NaC1, MgC12, and "&I, and (NH4)&3O4had little effect; CaClz led to high results and ("I), VOL. 3 3 NO. 5, APRIL 1961
47 R
HI'OI lcd to low results. Adjusting the s:iniplc solution to pH. 1 eliminated the
interference from these ions (59). The AOAC gave official status to a method in which anion interference was circumvented by resin treatment (84). Radiometric methods based on the natural radioactivity of potassium (0.0119% K40) were applied to fertilizer analyses (28, 48,86). A Geiger-Muller count,er insensitive to yradiation was recommended (28). Analyses were performed on either the dry materials or solutions of the sample (48). In the latter method the counter was placed in the cell filled with distilled water for 10 minutes to measure backg! iund. The sample (25 grams in 50 nil. of water) was then read following instrument calibration with a standard 10% KCl solution. In counting dry samples no difference was found for granular or pulverized products (48)but the presence of iodide or bromide salts led to high values (28). Potassium was precipitated as the complex salt K3Bi(S203)3.K2NaBi(S203)3 (67); the bismuth was then determined complexometrically. The precipitating reagent was a mixture of Na&Os and Bi(NOs)*in solution. A comparative study of the cobaltinitrate and perchlorate methods showed results were consistently higher by the latter procedure (70). CALCIUM AND MAGNESIUM
Complexometric methods continue to attract considerable attention in the determination of calcium and magnesium, especially techniques for preventing interferences and for increasing specificity. A procedure was outlined for the selective titration of calcium in the presence of magnesium with EGTA [ethylene glycol bis (2-aminoethyl ether)-N,N,N',N '-tetraacetic acid] using Zincon as the indicator; magnesium was then determined in the same solution with EDTA and Eriochrome Black T (72). Methods proposed for the direct determination of calcium and magnesium in the presence of phosphates included titration with a mixed solution of the monozinc and disodium salts of EDTA (9) or DCTA (93). Calcium was determined also in rock phosphate by an indirect EDTA titration (103). Small amounts of magnesium and iron did not interfere. Conventional complexometric methode were applied to the analysis of slags after removal of iron and aluminum by a chloroform extraction of their or cupferronates acetylacetonates (12, 60). In another approach, aluminum, chromium, manganese, and magnesium were masked with triethanolamine, which forms complexes with
48 R
ANALYTICAL CHEMISTRY
these metals that are mor+; stable than those with EDTA ( 4 5 , . 1 procedure was also proposed for the determination of each of these metala in slags (20). A study of 26 azo dyes indicated that the structural features necessary for union with calcium and magnesium were two hydroxyls, or one hydroxyl and one carboxyl, ortho and orthoprinic to the azo group (17). As a result of this study a new indicator, 1 (1 hydroxy - 4 methyl - 2 - phenylazo)-2-naphthol-4sulfonic acid (Calmagite), was recommended for the titration of total calcium and magnesium (64). The color change is the same as Eriochrome Black T but IS clearer and sharper. Factors influencing the accuracy of complexometric titrations include pH and the metal-indicator stability (53, 96), the metal-EDTA stability, and other complexing ions present in the solution (G3). The ratio of magnesium to calcium is important also and the maximum value which can be tolerated depends upon the indicator selected (3) Calcium in phosphate, carbonate, and silicate rocks was determined by flame photometry (47). Interferences were overcome by adding the interfering elements to the sample solution and standards a t relatively high levels. Magnesium was determined in calcium phosphate and limestone following the removal of iron as the basic acetate and calcium as the oxalate. The 8quinolinol precipitated by magnesium was liberated and titrated amperometrically with a bromate-bromide reagent (42). Procedures were described for conducting complexometric titrations automatically with the Sargent-Malnistadt spectrophotometric Titrimeter (65) Details were given for the titration of total calcium and magnesium in lirnestone with EDTA. Other proposals for the analysis of limestone included a rapid system in which calcium and magnesium were titrated complexometrically and iron, aluminum, and silicon were determined by adaptations of conventional methods (75).
- -
-
BORON
The identical p H method, simplified so that removal of carbon dioxide and phosphate was unnecessary, was recommended over four other methods studied for boron analysis in fertilizers (56). It was applicable to colored fertilizers without prior extraction. The separation of heavy metal ions from mixtures with boric acid was accomplished by adsorption on an exchange resin (Zeo-Karb-225) (86). Boron was then determined by conventional methods-eg., the carminic acid procedure (61).
No sppreciabit: loss of boron was observed aftcw prolonged Jirs1ting of the sample with HCl, HXOa, H,SO,, or fuming H2S01 or with aqua regia (95). However, the boron tvas no longer reactive to carminit, acid. In an indirect mcthorl boron was precipitatcd ija barium borotartra te (6). Dissolving t,he precipitate in HZSO, Ilbrratec! the tartaric acid whicii was determined colorimetrically with Pesez reagent (resorcinol and KBr in dilute H2S0,). FLUORINE
The bleaching action of fluorine, following isolation from intprfering ions, on metal-indicator lakes continues to attract interest and was applied to the analysis of rock phosphate (82). Cations were removed by exchange resin and the bleaching of an aluminumAlizarin Red S lake was determined. A thorium-2-(2-arsonophen~lazo)-l,Sdihydroxynaphthalene-3,6-disulfonic acid lake provided a basis for the determination of fluorine in fertilizer materials, after distillation as Hr8iF6 (94). The highly colored chloranilate ion provided a new, direct approach to fluorine analyses. The addition of a metal chloranilate, such as the lanthanum (106) or thc thorium (SO) salt, to the sample solution precipitates the insoluble metal fluoride while releasing an equivalent amount of the chloranilate ion. The latter is determined colorimetrically. Since a number of ions interfere, a clean-up of the sample solution prior to color development is necessary. COPPER, COBALT, AND ZINC
The colored complex tormeti between cuprous ion and ?.9-diniethpI-l,10phenanthroline K ~ Y t.mployed to determine copper in fertiilaers (97). Extraction of the coupiex h n i :in aqueous solution with CHCln, prir 1, t'o reading spectrophotometrically: interference from ions four:;+ ommcnly in fertilizers. The analytical chemistry ui c8ob:ilt was surveyed in a review of !mthotls for its detection and rlpterrnin:itiu;i. The literature during the past &cade was covered i/OO). In a procedure fo: dctcmiiiiing ion. levels of zinc in fertilizers, :I preliminary extraction with dithizone served to concentrate the zinc and e1iniin::tc a number of interfering ions ( 7 9 ) . The color developed with indo-8-quinolinol W:LS read photometrically. MINOR ELEMENTS. GENERAL
Preliminary extractive procedures were also applird for the srpnration of
trace elcments from p &d~ salts. Once isolated, spectrogmpkae methods were utiiized io dekrmine the manganese, copper, zinc; boron, lead, and aluminum content, of the salt (51). Iron was determined colorimetrica!lg with, a,a’dipyridyl. Existing methods for seven trace elements were zdaptecl tu the analysis of fertilizers (78). A double extraction procedure was described for the determination of small amounts.of molybdenum in agricultural materia.s ( 7 7 ) . An initial extraction of the benzoin-a-oxiae cumplex concentrated the molybdenum; a butyl acetate extract of a &thiol complex was then read photometrica!ly. Aluminum was dete;mined in rock phosphates by colorirnt. after removal of iron as die cupfwonate (98). Samples in which t,he :3,tio of Fez& to -41203was > 7 :1 were handled successfully.
‘LITERATURE CITED
(1) Bakacs-Polgar, E., Przemysl C‘hem. 34, 641 (1955). ( 2 ) Behr, I., Bull. Research Council Israel SA, 295 (1958). (3) Belcher, R., Close, R. A., West, T. S., Talanta 1, 238 (1958). (4) Borland, C . C., Schall, E. D., J. Assoc. Ofic. Agr. Chemists 42,579 (1959). (5) Bovalini, E., Piazzi, M., Ann. chim. ( R o m e )48,305 (1958). (6) Brabson. J. A.. AKAL. CHEY. 31. 688 (1959). (7) Brabson, J. A,, Jacob, K. D., Wilhide, W. D.., Ho!Tman. W. M.. J . Assoc. 0 c. A q r . Chemist$ 43, 504 (1960). ( 8 ) rahson, J. -4., Wilhide, JV. D., Ibid., 43,57+ (1960). (9) Cimerman. C., A h , A , Marshall, J., ,41~aI.Chim. Acta 19, 461 (1958). (10) Ciark, K.. G., Ltmiont3, T. G., J . Assoc. Ofic. Ayr. Chemists 43, 504 (1960). (11) Clark, xi G., Yee, J. Y . , Lundstrom, F. 0 , hamont, T. G., Ibid., 42, 592
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