Ind. Eng. Chem. Res. 2002, 41, 139-144
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Pyro- and Tripolyphosphates and Citrates as the Complexing Agents for Micronutrients in Liquid Fertilizers Barbara U. Grzmil* and Bogumił Kic Institute of Chemical and Environment Engineering, Technical University of Szczecin, ul. Pułaskiego 10, 70-322 Szczecin, Poland
The conditional stability constants of tripolyphosphate, pyrophosphate, and citrate complexes of Zn2+, Mn2+, Co2+, and Fe2+ as function of pH have been determined. The polarographic method was employed in these studies, and the ionic strength of the solution used was 0.5 mol/dm3. P3O105- complexes with the respective micronutrients were found to show higher conditional stability constants more often than P2O74- and citrate complexes. In comparison with complexes containing other cations, conditional stability constants of complexes with zinc studied in this work showed higher values. Distinct from other micronutrients, Cu2+ ions were observed as significantly accelerating the hydrolysis process of P3O105- in a range of pH from 4.5 to 10.0. Hence, the degree of hydrolysis increased together with the pH value of their solutions. In the case of other micronutrients, the degree of hydrolysis of P3O105- was lower for higher pH values of their solutions. Additionally, it was stated that the structure of Fe2+ tripolyphosphate complexes differs from complexes with other central ions. The preparation conditions of liquid fertilizers stable over long periods of time, containing macroelements and micronutrients in the complexing form by polyphosphates, have been elaborated. Stability of the solutions depended on the content of individual components, pH, and kind of complexing agent. It remained closely related to stability constants of complexes. Introduction The additional supply of macro- and micronutrients for plants through the leaves belongs to the most effective and simultaneously ecologically safe methods of the applications of fertilizers.1-4 Micronutrients exhibit 10-20 times higher effectiveness in the supply through the leaves than that in the application through the soil. Nitrogen is utilized 1-2 times better, whereas magnesium is 2-3 times better. The discussed method of fertilizer applications prevents the pollution of the soil by heavy metals, which comprise a part of the micronutrients, because the amounts of supplied trace elements are insignificant and they are utilized effectively. The supply of plants through the leaves is the most advantageous for those components that rapidly undergo retardation in the soil, and they are required by the plant in minor amounts, as well as for components that are uptaken quickly. The rate of absorption of the nutrients from solution by the plants depends on the following factors: plant age, the form in which the element is supplied, and the solution concentration. The physicochemical parameters of the solution assigned for spraying play a significant role. The surface tension of the solution and the critical relative humidity of the salt after evaporation of water are of crucial importance. The advantage resulting from the application of liquid fertilizers is also the possibility of the incorporation of the growth regulators, crop protection products, wetting agents, dispersing agents, the substances increasing the adherence, and the rate of penetration of fertilizer components through the leaf tissue into solutions.1,3,4 The protective action of liquid fertilizer results also from * To whom correspondence should be addressed. Phone: (+48 91) 449 47 30. Fax: (+48 91) 4494686. E-mail: bg@ mailbox.ps.pl.
the presence of micronutrient at a toxic concentration for pathogens, being harmless for plants or for the alkalinity of the solution.1,3 The liquid fertilizers applied for additional feeding of plants should be characterized by defined properties such as an appropriate content and ratio of macro- to micronutrients (to match the nutrition requirements during the vegetative phase of the plant, the micronutrients contained in these fertilizers should occur in the complexed form, and the ligand of the sequestering agent should undergo a metabolic conversion or biodegradation), a high total content of nutrients, the adherence to the leaf, durability during storage and transportation, and a low temperature of the crystallization of salt from them.1,3,4 The foliar liquid fertilizers with micronutrients are produced from pure, readily water-soluble salts of the respective components or their solutions. The most frequently used raw materials are comprised of (NH2)2CO, NH4NO3, KNO3, KCl, K2SO4, MgSO4, and MgCl2. Ammonia and pure superphosphate acid with the P2O5 content ranging from 68 to 76% (including ca. 36% pyrophosphates and ca. 10% tripolyphosphates) as well as potassium phosphates were also utilized but to a lesser degree.4,5 The content of N, P, K, and Mg in the liquid fertilizers is not unlimited, because the equilibrium state of liquid-solid in the considered systems must be taken into account. Micronutrients are applied in the form of simple complexes or as the chelates. Several compounds are used as the complexing agents. They can be of both natural and synthetic origin such as acids citric, formic, ascorbic, propionic, tartaric, succinic, lactic, gluconic, and salicylic or their K, Na, and NH4+ salts, lignosulfonates, natural and synthetic amino acids (glycine, cysteine, and glutamine), ethylenediaminetetraacetic
10.1021/ie010223d CCC: $22.00 © 2002 American Chemical Society Published on Web 12/20/2001
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acid (EDTA) and its derivatives, and polyphosphates.2-4,6,7 The stability of complex compounds depends on several factors. Hence, apart from the nature and concentration of the cationic complexing agent, pH, and ionic strength of the solution, the stability is influenced by both the concentration and type of sequestering cation (its charge, ionic radius, and coordination number).6-9 Additionally, when polyphosphate is utilized as a ligand, the stability is influenced by both the chain length of polyphosphate and the location of the oxygen atom in tetrahedryl PO4 responsible for metal binding.7 Polyphosphates and other complexing agents often show comparable stability constants and costs related to an appliance. Despite that, in a production of liquid fertilizers containing micronutrients, polyphosphates are used to a lesser extent.9 In numerous cases, utilization of expensive chelating agents (e.g., EDTA demonstrates a very low biodegradation rate) is not justified. In the production process of liquid fertilizers containing both micro- and macronutriens, it is very important to choose a suitable complexing agent. Knowledge of complexes’ stability constants is the basis necessary for choosing the agent. The stability constants of complex compounds are most frequently determined in solutions with an ionic strength of 0.1 mol/dm3. However, the chemical composition of the solutions considerably differs from that of liquid fertilizers. The ionic strength is significantly higher for liquid fertilizers containing macro- and microelements, and often it achieves a value of ca. 10 mol/dm3. Additionally, data related to this scope and collected from literature differ from one paper to another. This may result from utilization of different analytical methods. The objective of the studies presented in this publication was the determination of (i) the conditional stability constants depending on the pH value for P2O74-, P3O105-, and citrate complexes with Zn2+, Mn2+, Co2+, and Fe2+ at an ionic strength of the solution of 0.5 mol/dm3, (ii) the degree of hydrolysis depending on both the pH of the solution and the kind of micronutrient (Zn, Mn, Co, Cu, and Fe), and (iii) and the possibilities of production of liquid fertilizers NPKMg stable over time with microelements (Zn, Cu, Co, Mo, B, Mn, and Fe) as a function of the pH value with the utilization of P2O7,4P3O105-, and citrate as the complexing ligands. Experimental Section The conditional stability constants of the complexes were determined by the polarographic method. Differences of the half-wave reduction potential of both complexed and noncomplexed metal ions at the same ionic strength and pH value of the solution are utilized in this method.8,9 In the calculations, a method developed by Ford and Hume was used.8 The following reduction wave was utilized in the polarographic studies: Zn2+ f Zn0, Mn2+ f Mn0, Co2+ f Co0, and Fe2+ f Fe0, the half-wave potentials of which amounted to ca. -1.0, -1.5, -1.3, and -1.4 V, respectively. They varied toward more negative values in the case when these ions were complexed. The determinations for the particular series of experiments were performed at the three values of pH of 4.5, 6.0, and 7.5. A solution of sodium or ammonium chloride was used for regulation of the ionic strength.
The liquid fertilizers were prepared using the reagentgrade compounds as a source of the respective macroand micronutrients. Mg, Cu, Zn, Mn, Fe, and Co were incorporated in the form of sulfates, B as boric acid, and Mo as ammonium molybdate. K4P2O7 and K5P3O10 were obtained as a result of dehydration of the appropriate potassium phosphates. Potassium was used in the form of KCl and K2SO4 and phosphorus as (NH4)2HPO4, whereas nitrogen was additionally introduced as NH4+NO3- and urea. In the subsequent series of experiments, the content of micronutrients and magnesium in fertilizers, the range of pH (4.5-7.5), and the molar ratio of the complexing ligand to the sum of Mg, B, Cu, Zn Mn, Fe, Mo, and Co varied, while the content of N, P, and K (12% N, 4% P2O5, and 6% K2O) remained unchanged. The analytical control included determination of the total contents of both phosphates and particular forms (i.e., ortho-, pyro-, and tripolyphosphates and higher condensed phosphates) of phosphates in the liquid fertilizers. Individual forms of phosphates were measured with the use of ion-exchange column chromatography.10 For the estimation of the phosphate contents, a colorimetric vanadium-molybdenum method was employed.11 The contents of both, ammonium and nitrate were determined using ion-selective electrodes.12 The urea content was measured by a colorimetric method with use of p-(dimethylamino)benzaldehyde.11 The potassium content was determined by utilizing flame photometry, while the contents of Mg, B, Cu, Zn, Mn, Fe, Mo, and Co were estimated with an atomic emission spectroscope that uses inductively coupled plasma.13,14 Results and Discussion Determination of the Conditional Stability Constants. The conditional stability constants were not determined in the case of Mn2+ pyrophosphate complexes depending on the pH value of the solution because of the precipitation of Mn2P2O7 at a low mole ratio of ligand to metal and the interference of a polarographic wave corresponding to the reduction of complexes Mn2+ with the wave of reduction of the hydrogen ions at a higher ratio of ligand to this ion. The reduction of Fe2+ that was complexed by P3O105- also proceeded at too negative values of the potential. This excluded the possibility of utilizing the polarographic method for the determination of the stability of such complexes with Fe2+. For the reason that Cu2+ ions underwent both two-stage reduction (in a NaCl or NH4Cl solution) and one-stage reduction (in the presence of studied complexing agents), conditional stability constants of complexes containing copper were not determined. In the earlier paper15 the authors stated how (i) the ionic strength, (ii) the simultaneous presence of two complexing agents, and (iii) the presence of several central ions influence the values of complexes’ conditional stability constants. On the basis of other experiments, it was found that the complexes of micronutrients in a solution at an ionic strength equal to 0.5 mol/dm3 have higher conditional stability constants for pH values of 6 and 7.5 than for a pH of 4.5. This property is retained independently on the type of used sequestering agent, which is presented in Table 1. These differences resulted from the side reaction of the protonation of ligand proceeding simultaneously with the main reaction.8,9 It was found that
Ind. Eng. Chem. Res., Vol. 41, No. 2, 2002 141 Table 1. Dependence of the Conditional Stability Constant of Complexes on the pH Value of the Solution
central ion Zn2+
complexing agent [L] tripolyphosphate pyrophosphate citrate
Mn2+
tripolyphosphate pyrophosphate citrate
Co2+
tripolyphosphate pyrophosphate citrate
Fe2+
pyrophosphate citrate
pH 4.5 6.0 7.5 4.5 6.0 7.5 4.5 6.0 7.5 4.5 6.0 7.5 4.5 4.5 6.0 7.5 4.5 6.0 7.5 4.5 6.0 7.5 4.5 6.0 7.5 4.5 6.0 7.5 4.5 6.0 7.5
regulator of ionic strength NH4Cl NH4Cl NH4Cl NH4Cl NH4Cl NH4Cl NH4Cl NaCl NaCl NH4Cl NH4Cl
type of complex
conditional stability constant [log β′]
Zn(H2L)3 Zn(HL)2 Zn(HL)3 Zn(H2L)2 Zn(H2L)4 Zn(HL)4 Zn(H2L)2 Zn(HL)3 Zn(HL)2 Mn(H2L)2 Mn(HL)2 Mn(HL)2 MnH2L MnH2L Mn(HL)2 MnHL Co(H2L)3 Co(HL)2 Co(HL)3 CoH2L Co(H2L)3 Co(HL)3 Co(H2L)3 Co(HL)2 Co(HL)3 FeH2L Fe(H2L)4 Fe(HL)4 Fe(H2L)2 Fe(HL)4 Fe(HL)4
12.54 15.90 18.00 3.25 18.71 18.98 8.39 13.80 6.27 8.05 8.39 10.95 3.17 2.54 6.24 4.71 12.17 13.56 21.90 2.67 11.83 16.95 11.63 12.14 17.46 7.73 16.00 18.44 8.72 14.92 14.71
Table 2. Influence of the Metal Concentration on the Stability of Complexes (pH 6)
complexing ligand [L]
concn of regulator central central of ionic type of ion ion [mol] strength complex
pyrophosphate
Zn2+
pyrophosphate
Co2+
citrate
Co2+
citrate
Fe2+
tripolyphosphate
Mn2+
5 × 10-4 1 × 10-4 5 × 10-4 1 × 10-4 1 × 10-3 1 × 10-4 5 × 10-4 2 × 10-4 5 × 10-4 1 × 10-4
NH4Cl NH4Cl NaCl NH4Cl NaCl
Zn(H2L)4 Zn(H2L)4 Co(H2L)4 Co(H2L)3 Co(HL)3 Co(HL)2 Fe(HL)4 Fe(HL)3 Mn(HL)3 Mn(HL)2
conditional stability constant [log β′] 23.59 18.71 14.10 11.12 15.59 12.14 14.92 10.20 12.47 8.34
P3O105- complexes with the respective microelements possess higher conditional stability constants more often than P2O74- and citrate complexes. In comparison with other cations, the conditional stability constants of the studied complexes with zinc are higher. Higher conditional stability constants were determined for the complexes formed in solutions, keeping the same pH value and ligand concentration but having a higher concentration of metal (Table 2). For liquid fertilizers, nitrogen is introduced in the form of either NH4+ or NO3-, or as urea. The influence of the presence of NH4+ on the value of the conditional stability constants was checked. As an ionic strength regulator, the solution of NH4Cl was used instead of NaCl (Table 3). It was found that higher values of the constants showed Zn complexes and lower values showed Mn complexes. The complexes containing Co revealed similar values of the constants. This results from an ability of above-mentioned metals to form complexes with ammonia. The ability is the strongest in the case of Zn, weaker for Co, and the weakest for Mn.
Determined conditional stability constants of the complexes relate to solutions with ionic strength lower than that in liquid fertilizers. Yet, knowledge of the constants made possible a comparison of their values (the same analytical method) and use of these drawn conclusions for further investigations. Determination of the Degree of Hydrolysis for Pyro- and Tripolyphosphate Complexes. Depending on the solution pH, temperature, presence of catalysts, and other factors, polyphosphates may undergo hydrolysis. The degree of hydrolysis increases for lower pH values and for higher temperatures. The influence of pH and the central ion on the stability of polyphosphates containing solutions was determined. This is essential in liquid fertilizer manufacturing processes. The solutions contained 2-4 wt % P2O5. This value corresponds to the content of this component in practically used liquid fertilizers. The stability of neutral pyro- and tripolyphosphate complexes (pH value ca. 10) with Cu2+, Zn2+, Mn2+, and Co2+ in solution was investigated by the determination of the lowest value of the molar ratio of the ligand to a given central ion which ensures the maintenance of the formed complex in a solution over sufficiently long periods (Table 4). It was found that the Cu2+ polyphosphate complexes were the most stable under independently studied conditions on the type of sequestering agent. However, in the case of Mn2+, the P3O105chelates exhibit a higher stability in comparison with P2O74- chelates, but their stability period was significantly shorter than that of the respective chelates with Cu2+ or Zn2+. This is a consequence of the reaction of the precipitation of insoluble pyro- and tripolyphosphates with these cations because the formation of a precipitate is competitive with the complexation reaction of metal ions. The process of hydrolysis of discussed micronutrient complexes in solution (pH value of ca. 10) with preservation of the optimal ratio of the ligand to the central ion was investigated. It was found that during the studied period (5 months) only the P3O105- complexes with Cu2+ undergo hydrolysis with the formation of P2O74- and PO43-; however, this did not influence the decrease of the solution stability. The degree of hydrolysis amounted to ca. 58%. This phenomenon was most probably due to the formation of the strong oxygenmetal bond in the PO4 tetrahedral of tripolyphosphate and to stabilization of the system through the splitting of the polyphosphate chain rather than the abstraction of the metal.7 The stability of analyzed chelates versus the excess of PO43- simulating the phenomenon of retardation was also determined. The studies were performed for a micronutrient content of ca. 0.03%, with the molar ratio of ligand to Me2+ given in Table 4 and the weight ratio of PO43- to Me2+ equal to 100, for a pH value of ca. 8.5. It was found that the pyrophosphate complexes with copper at the molar ratio of P2O74- anion to Cu2+ equal to 2 were stable over 1 month in comparison to unstable tripolyphosphates (Figures 1 and 2). Under the influence of an excess of phosphates proceeded the decomposition of complexes that were insufficiently stable under the measurement conditions and the precipitation of insoluble phosphates containing micronutrients. The P2O74- chelates with Zn2+ or Co2+ exhibit a slightly higher stability than that of P3O105-. However, in the case of Mn2+, the complexes of P3O105- exhibited a
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Table 3. Dependence of the Conditional Stability Constant of Complexes on a Kind of Ionic Strength Regulator (pH 6.0) central ion
concn of central ion [mol]
complexing agent [L]
Zn2+
1 × 10-4
tripolyphosphate
1 × 10-4
pyrophosphate
Mn2+
5 × 10-4
tripolyphosphate
Co2+
5 × 10-4
tripolyphosphate
1 × 10-4
pyrophosphate
5 × 10-4
citrate
regulator of ionic strength
type HT of complex HT
conditional stability constant [log β′]
NaCl NH4Cl NaCl NH4Cl NaCl NH4Cl NaCl NH4Cl NaCl NH4Cl NaCl NH4Cl
Zn(HL)3 Zn(HL)3 Zn(H2L)3 Zn(H2L)4 Mn(HL)3 Mn(HL)2 CoHL Co(HL)2 Co(H2L)3 Co(H2L)3 Co(HL)3 Co(HL)3
15.53 17.39 16.41 18.71 12.47 8.39 12.64 13.56 11.83 11.12 17.46 18.07
Table 4. Stability of Polyphosphate Complexes with Micronutrients complexing ligand [L] 4-
P3O105-
P2O7 micronutrient Me2+
molar ratio L/Me2+
concn of Me2+ [mol]
stability time [days]
molar ratio L/Me2+
concn of Me2+ [mol]
stability time [days]
Cu2+ Zn2+ Mn2+ Co2+
2.34 2.40 13.10 3.41
0.082 0.089 0.017 0.044
100 90 30 20
2.10 2.05 2.64 2.00
0.077 0.079 0.064 0.083
90 30 5 90
Figure 1. Influence of phosphates on the stability of pyrophosphate complexes with micronutrients: 1, Cu; 2, Zn; 3, Co; 4, Mn.
higher stability, in regards to the excess of PO43- than of pyrophosphates. The course of hydrolysis for P3O105- at pH 4.5 and 6.0 was also determined. The molar ratio of a ligand to micronutrient used for experiments was kept as in Table 4. At lower pH, measured after 2 months the degree of hydrolysis for polyphosphates was 16-29%. The degree for the solution containing Zn2+ cations was found as the lowest and that for the solution containing Co2+ as the highest. However, during the same period of time and at pH 6.0, the fastest hydrolysis of polyphosphates was observed in the presence of Cu2+ and the slowest in the presence of Mn2+. The degree of hydrolysis was ca. 39% and ca. 8% in the presence of Cu2+ and Mn2+, respectively (Figures 3 and 4). To determine the influence of pH on the degree of polyphosphates hydrolysis in the presence of Fe2+, solutions with the ratio of ligand/central ion ) 2.5 were prepared. Despite the lack of higher forms of phosphates in the starting P3O105- (Table 5), these were found during the determination of the contents of individual forms of phosphates in the obtained solutions. In this
Figure 2. Influence of phosphates on the stability of tripolyphosphate complexes with micronutrients: 1, Cu; 2, Zn; 3, Co; 4, Mn.
Figure 3. Influence of the time on the degree of hydrolysis of tripolyphosphates in a solution of pH 4.5.
case it indicates the formation of polyphosphate complexes of a structure different from those formed in the presence of Cu2+, Zn2+, Mn2+, and Co2+. Likely, complex compounds of a higher charge and of a high stability are formed, e.g., as a result of the combination of two (HnP3O10)5-n ions with the central ion (Fe2+). Determination of the Possibilities of Production of Liquid Fertilizers. Based on the experimental results obtained, a series of studies were carried out in which the objective was to achieve stable clear liquid
Ind. Eng. Chem. Res., Vol. 41, No. 2, 2002 143
Figure 4. Influence of the time on the degree of hydrolysis of tripolyphosphates in a solution of pH 6.0. Table 5. Influence of pH and Time on the Degree of Hydrolysis of FeII Polyphosphate Complexes fraction of individual forms of phosphates [%] higher time P2O5 condensed pH of solution [days] total [%] ortho- pyro- tripoly- phosphates initial K5P3O10 4.5 6.0 7.5 9.0
1.12 11.11 0 34 84 0 34 84 0 34 84 0 34 84
2.42 2.31 2.41 2.29 2.26 2.28 2.15 2.06 2.16 2.45 2.47 2.51
2.07 3.03 14.52 2.18 2.21 7.02 3.26 2.43 11.57 2.36 5.43 6.37
11.16 11.26 25.73 10.04 9.73 11.84 11.16 12.14 13.89 8.26 11.31 16.33
87.78
0.00
28.93 28.57 8.71 27.95 30.53 26.75 46.05 46.12 39.81 30.58 28.14 23.51
57.85 57.14 51.04 59.83 57.52 54.38 39.53 39.32 34.72 59.09 55.12 53.78
fertilizers containing macro- and micronutrients. The stability of NPKMg fertilizer solutions with micronutrients was determined at constant contents of the major nutrient elements (12% N, 4% P2P5, and 6% K2O) and variable contents of the remaining elements, i.e., Mg, B, Zn, Mn, Cu, Fe, Mo, and Co. The molar ratio of ligand to the sum of micronutrients and magnesium ranged from 4 to 8 depending on both the pH value of the solution and the nature of the complexing agent.
It was found that a significant influence on the stability of liquid fertilizers had the pH value of solution, independently of the complexing agent, utilized for chelating of microelements. Stable over long period (812 months) were solutions of P3O105- with pH values of 4.5-7.5 and solutions of citrates with pH values of 4.5 and 6.0, whereas from solutions in which the complexing ligand was P2O74-, precipitate formed over the studied range of pH and at pH 7.5 in the case of citrate. The kinds of micronutrients included in the solutions also decided their stability. It was affirmed that P2O74might be employed as a chelating agent in those liquid fertilizers not containing Mn2+ ions (Table 6). Apart from Mn2+, diffusion goes from the solutions containing P2O74- ions to the precipitating solid other ions. This phenomenon was observed to a large extent (even up to 100%) for ions of the following metals: Mg, Co, and Zn. From citric solutions (pH 7.5), Mn and Mg precipitated to a large extent. This resulted from a diverse stability of individual complexes that depended on the kind of ligand and on the pH value of a solution. No precipitation was observed from the solutions of fertilizers containing ions of both Mo and B. This remained independent of the pH and of the kind of utilized ligand. Additionally, it remained closely connected with stability constants of the complexes. Additionally, it was concluded that despite similar molar ratios of the ligand to the sum of micronutrients, precipitation from those fertilizers with the whole composition assumed (i.e., containing Mg2+, Zn2+, Cu2+, Mn2+, Co2+, Mo6+, and Fe2+) took place more frequently than that from those containing only some of the micronutrients. It was found that the P3O105- introduced into the liquid fertilizers underwent a partial hydrolysis with the formation of P2O74- and PO43-. The degree of hydrolysis depended on both the pH value and time and amounted to from ca. 10% to even 87%.
Table 6. Influence of Time on the Stability of Liquid Fertilizers Containing Pyrophosphate as a Complexing Agent (12% N and 6% K2O, pH 6.0)a time [days] 0
3.67
21
3.63
62
3.68
119
3.39
278
3.37
1/24 15 56 113 177 293 a
total
content of individual forms of phosphates [%] ortho pyro tripoly
Mg
Zn
degree of precipitation of magnesium and micronutrients [%] Cu Mn Co Mo
0.01% of B, Zn, Cu, and Fe, 0.005% Mo, 0.001% Co; Molar Ratio of H2P2O72-/∑M ) 5.0 2.72 0.83 0.12 0.0 0.0 0.0 74.11 22.62 3.27 2.71 0.85 0.07 0.0 0.0 0.0 74.66 23.42 1.93 2.78 0.90 0.00 0.0 0.0 0.0 75.54 24.46 0.00 2.71 0.63 0.04 0.0 0.0 0.0 80.18 18.64 1.18 2.52 0.85 0.00 0.0 0.0 0.0 74.78 25.22 0.00
Fe
B
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.17% MgO, 0.01% of B, Zn, Cu, and Fe, 0.005% Mo, 0.001% Co; Molar Ratio of H2P2O72-/∑M ) 4.0 3.45 1.17 2.19 0.09 51.2 49.5 5.8 91.7 80.0 0.0 33.92 63.47 2.61 3.06 1.21 1.78 0.07 92.6 67.7 11.6 96.4 80.0 0.0 39.54 58.17 2.29 3.08 1.40 1.68 0.00 98.8 67.7 11.6 96.4 80.0 0.0 45.45 54.55 0.00 3.11 1.27 1.72 0.12 98.8 67.7 11.6 96.4 80.0 0.0 40.84 55.31 3.86 2.92 1.37 1.55 0.00 98.8 67.7 11.6 96.4 80.0 0.0 46.92 53.08 0.00 2.94 1.38 1.56 0.00 98.8 67.7 11.6 96.4 80.0 0.0 46.94 53.06 0.00
Bold type indicates fraction (%).
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Pyrophosphates, in comparison with tripolyphosphates, are weaker complexing agents, and an increased content of PO43- favors the precipitation of hardly soluble precipitates of mixed phosphates. Therefore, this phenomenon unfavorably affected the stability of fertilizer solutions. Together with the progressive hydrolysis process, microelements diffused from initially clear solutions to a solid phase. In some cases, it was noticed that urea (as a partial source of nitrogen) utilized in the process of fertilizer preparation also undergoes hydrolysis, yet to a slight extent (4-13%). Moreover, in the produced liquid fertilizers proceeds the oxidation of metabolically active Fe2+ ions [incorporated in the form of (NH4)2Fe(SO4)2‚6H2O] with the formation of Fe3+ ions. This could result from the presence in solution of (i) other ions (showing variable valence), (ii) dissolved oxygen and oxidizing nitrate ions (source of nitrogen), and in some cases (iii) citrate ligands. Additionally, the cementation process of copper ions was noticed when larger amounts of Fe2+ ions (ca. 0.7%) were introduced to a fertilizer. Conclusion During the determination of the stability of complex compounds with micronutrients, the difficulties resulting from the limitations involved with the measuring techniques used have been faced. It was found that the studied complexes at pH values of 6 and 7.5 had higher conditional stability constants than those at a pH value of 4.5; the same tendency was established for P3O105complexes when they were compared with P2O74- and citrate complexes. On average, the complex compounds with zinc exhibit the highest stability in comparison with complexes with the other bivalent ions studied. It was concluded that hydrolysis of polyphosphates (in the range of pH 4.5-10.0) to the highest extent was catalyzed by Cu2+ ions, and the degree of their hydrolysis rose together with the pH value of their solution. After 2 months, the degree amounted to ca. 20% and to ca. 42% in a solution showing pH 4.5 and 10, respectively. In the case of other micronutrients, the degree of hydrolysis decreased along with an increase of the pH value and varied from 29% to 6%. It was stated that the structure of Fe2+ tripolyphosphates complexes differs from complexes with other central ions. Probably chain connections consisting of at least two (HnP3O10)5-n ligands are formed.
On the base of the performed studies, it was found that it is possible to attain liquid fertilizers stable over prolonged periods of time (at pH values of 4.5-6.0), with a diverse content of micronutrients and utilizing P3O105or P3O105- and citrates as complexing ligands. Results obtained during numerous experiments have been used to draw up a patent application concerning the preparation of liquid fertilizers containing micronutrients.16 Literature Cited (1) Czuba, R. Mineral Fertilization of Cultivated Plants; Chemical Plant “Police”: Police, Poland, 1996. (2) Go´recki, H. Novel Technologies and New Techniques of Fertilizers Application in the World. Chemik 1994, 47, 48. (3) Kotuła, E.; Faber, A.; Winiarski, A. Liquid Multicomponent Fertilizers with Microelements in Foliar Fertilization. Przem. Chem. 1988, 67, 157. (4) Nielsson, F. T. Manual of Fertilizer Processing; Marcel Dekker: New York, 1987. (5) Machej, J.; Cichy, B. Sequestering of some Bivalent Metals in the Agrochemical Solutions of Ammonium Polyphosphate. Przem. Chem. 1999, 78, 142. (6) Kvarackhelia, M. Z.; Kudejarova, A. U. The Complexation Tendency of Polyphosphate and its Significance in Conversions Under the Soil Conditions. Agrokhimia 1989, 1, 119. (7) Prodan, E. A.; Prodan, L. J. Tripolyphosphates and their Application; Science and Technique: Min´sk, Belorussia, 1969. (8) Inczedy, J. Complexation Equilibria in Analytical Chemistry; Scientific Publishers: Warsaw, Poland, 1979. (9) Ringbom, A. Complexation in Analytical Chemistry; Interscience Publisher: New York, 1963. (10) Maten´ko, H. Analytical Methods for Quality Control of Superphosphoric Acid. Personal Communication, Institute of Inorganic Chemistry, Gliwice, Poland, 1975. (11) Marczenko, Z. Spectrophotometric Determination of Elements; Scientific Publishers: Warsaw, Poland, 1979. (12) Orion Research, Instruction Manual Ammonia and Nitrate Electrode, Orion Research: Cambridge, U.K., 1979. (13) Połuektov, N. S. Analysis by Flame Photometry; ScientificTechnique Publisher: Warsaw, Poland, 1969. (14) Szczepaniak, W. Instrumental Methods in Chemical Analysis; Scientific Publishers: Warsaw, Poland, 1997. (15) Grzmil, B.; Kic, B. Stability of Pyro- and Tripolyphosphate Complexes with Micronutrients and Liquid Fertilizers. Polish J. Technol. 2000, 2, 5. (16) Grzmil, B.; Kic, B.; Kałucki, K. Liquid Fertilizer Concentrate for Onto-Leaf and Onto-Soil Fertilization. Patent Application P 340609, 2000.
Received for review March 9, 2001 Revised manuscript received October 16, 2001 Accepted October 30, 2001 IE010223D
