stability of urea solutions in presence of mineral …storing neutral urea solutions at 278.15 k, or...

Mladen E. Lilov, Plamen P. Kirilov

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STABILITY OF UREA SOLUTIONS IN PRESENCE OF MINERAL ACIDS AND THEIR AMMONIUM SALTS


ABSTRACT

The stability of urea solutions (20 % N) in presence of mineral acids (HNO3, H2SO4, H3PO4) and their ammonium salts (NH4NO3, (NH4)2SO4, NH4H2PO4) is studied in the course of 150 days storage at 298.15 K. The study is carried out by periodical measurement of the electric conductivity and pH values of the solutions.

Keywords: urea solution decomposition, stabilized urea solution, stabilizing additives, pH, conductivity.

Received 23 October 2018Accepted 21 January 2019

Journal of Chemical Technology and Metallurgy, 54, 2, 2019, 319-325

University of Chemical Technology and Metallurgy8 Kliment Ohridski, Sofia 1756, BulgariaE-mail: [email protected]

INTRODUCTION

Urea, CO(NH2)2, is a nitrogen-containing (46 % N) chemical product which is used in many industries, including agricultural, automotive, medical, biochemi-cal, cosmetic, and pharmaceutical one [1]. The most important application of this product is as a nitrogenous fertilizer. According to Glibert et al. [2] the usage of urea as a fertilizer accounts for more than 80 % of the total production.

Urea is commonly used for both soil and foliar application [3]. Several potential benefits of provid-ing nitrogen to cereals via the foliage as urea solution are suggested [4]. These include: reduced nitrogen losses through denitrification and leaching compared to nitrogen fertilizer applications to the soil; an ability to provide nitrogen when the root activity is impaired, e.g. in saline or dry conditions, as well as an uptake late in the season to increase grain nitrogen concentra-tion. Urea is considered as the most suitable form of foliar nitrogen because of its unique physicochemical properties, including non-polarity, rapid absorption, low phyto-toxicity, high solubility and increased penetration of the accompanying nutrients [5].

When urea is applied to the leaves of cereal plants, visual symptoms, described as leaf „scorching”, „burn-ing”, or „tipping” are often noted [4]. The causes of scorch are still not clearly understood but accumula-tion of toxic amounts of urea or NH3, formed through

hydrolysis by plant urease, are commonly cited as a cause. The scorch is not a major problem in case urea application rates are relatively low.

Another problem refers to the fact that the urea solutions tend to be unstable during prolonged storage, with urea decomposition to ammonium cyanate, and hydrolyzation of CNO- to ammonium carbonate [6-11]. Although the extent of urea degradation under general conditions is generally small, the untoward effects, such as an ammonia odor and an alkaline shift of pH, can be very significant [12]. The high pH value and the presence of non-dissociated ammonia are the reason for appearance of plants leafs scorching after spraying such solution, as well as for formation of insoluble salts if other nutrients as calcium and magnesium are present in the urea solutions. The micronutrient iron chelated with EDTA is unstable at pH > 7.5 [13].

Numerous attempts are carried out to stabilize the urea solutions through the use of different additives. Walker and Kay [6] and Burrows and Fawsitt [7] state that the addition of ethyl alcohol diminishes the rate of urea decomposition. According to Nicloux and Welter [14], and Montgomery [15] the cyanate can be converted into urea by incubation with a suitable ammonium salt solution (NH4Cl, (NH4)2SO4). The change is not quan-titative as some of the cyanate undergoes hydrolysis to ammonia and carbon dioxide depending on the time and the temperature of the incubation and the concentration of the added ammonium salt. According to Marrier and

Journal of Chemical Technology and Metallurgy, 54, 2, 2019

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Rose [11] the cyanate accumulation can be prevented by storing neutral urea solutions at 278.15 K, or by buffer-ing the urea solutions at pH of 4.7.

The urea stability in a solution is analyzed as a func-tion of the temperature (298.15 K - 333,15 K), pH (3.11 - 9.67) and the initial urea concentration (2.5 % - 20 %) [15]. The analysis shows that the urea is more stable in the pH range of 4 - 8, while urea stability decreases with temperature increase at all pH values.

EP 0 463 075 B1 [17] discloses stabilized urea based fertilizers for foliar application as a solution, containing urea and a divalent cationic and monovalent anionic salt selected among calcium, magnesium and zinc chlorides/nitrates. US Patent 4 591 375 [18] describes a process for the preparation of neutralized, clear and stable solutions through acid catalyzed reaction of urea and acetaldehyde or mixtures of propion-aldehyde and acetaldehyde. The products of the process are useful as spray sources of nitrogen which are non-phytotoxic and have a prolonged fertilizer activity.

The literature review shows that there are different methods to prevent urea decomposition. The use of low pH and temperature values and minimized storage time combined with a choice of effective buffers and addi-tives appear a possibility for urea solutions stabilization.

The aim of the present study is to investigate the possibility to prevent or delay the decomposition pro-cess of urea solutions (20 % N) by additives such as mineral acids and/or their ammonia salts. The foliar applied nutrient solutions could be phytotoxic due to their high pH and conductivity by affecting important physiological processes [19]. These factors are very important for consideration when spraying liquid ferti-lizers to the foliage. The study reported is carried out by periodical measurement of the aforesaid factors (electric conductivity and pH values) of urea solutions (20 % N) containing different additives during prolonged storage at T = 298.15 K.

EXPERIMENTALMaterials

Urea of a technical grade containing 46.6 % of N, 0.02 % of free NH3 and 0.75 % of biuret was used. The other reactants referred to HNO3 (Merck, 65 %, ρ = 1.40 g.ml-1), H2SO4 (Merck, 97 %, ρ = 1.84 g.ml-1), H3PO4 (Merck, 85 %, ρ = 1.71 g.ml-1), NH4NO3 (Merck), (NH4)2SO4 (Merck), NH4H2PO4 (Merck).

MethodsThe urea and the additives were dissolved in distilled

water to prepare solutions containing 20 % of N (nitro-gen). 100-ml samples of the solutions were transferred by a pipet to 100-ml plastic bottles which were sealed and maintained at a constant temperature of 298.15 ± 0.5 K for 150 days. The bottles were opened at regu-lar time intervals (days) for measurement of pH and specific conductivity values. The solution density was determined at 298.15 K with a 25-ml pycnometer and Mettler Toledo balance with an accuracy of 0.0001 g. The conductivity was measured with a BOECO model conductivity meter. pH was measured with a calibrated, pH meter Mettler Toledo, model FE 20. The urea content was determined using the standard procedure described in ISO/DIS 22241-2, Annex B. The ammount of free ammonia (alkalinity) in the urea solutions was measured according to the standard procedure described in ISO/DIS 22241-2, Annex D. The determination of the biuret content was done using the standard procedure described in ISO/DIS 22241-2, Annex E.

RESULTS AND DISCUSSION

Urea solutions containing mineral acids as additivesThe solutions studied refer to:20 % N + 0.18 % HNO3, ρ = 1.1203 g.cm-3;20 % N + 0.19 % H2SO4, ρ = 1.1213 g.cm-3;20 % N + 0.41 % H3PO4, ρ = 1.1235 g.cm-3.

The changes of the pH and conductivity values of the aqueous urea solutions in presence of HNO3, H2SO4 and H3PO4 as additives are presented in Table 1 and Figs.1.1 and 1.2. The acid amount in the solution provides the same starting pH value for the three solutions studied. The dependencies pH = f(t) presented in Fig.1.1 are of an identical mode for all acids investigated. At the end of the studied period the highest pH value is reached in the solution containing H2SO4 (pH120 = 8.61), while the lowest one - in the solution with H3PO4 additive (pH120 = 6.94).

The dependencies σ = f(t) presented in Fig.1.2 for solutions containing HNO3 and H2SO4 have the same mode, while that of the third solution (with H3PO4 ad-ditive) differs. In this case, the conductivity increases exponentially with time. Nevertheless, this solution is more suitable as a foliar fertilizer compared to the other two solutions because of the considerably lower pH value during the whole studied period.


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Urea solutions containing ammonium salts as additivesThe solutions studied refer to:20 % N + 3 % NH4NO3, ρ = 1.1320 g.cm-3;20 % N + 3 % (NH4)2SO4, ρ = 1.1365 g.cm-3;20 % N + 3 % NH4H2PO4, ρ = 1.1355 g.cm-3.

Table 2, Figs. 2.1 and 2.2 illustrate the time changes of pH and conductivity values in case of aqueous urea solutions containing ammonium salts of the acids studied above - NH4NO3, (NH4)2SO4 and NH4H2PO4.

Fig. 2.1 shows that the time changes of the pH values

Table 1. Effect of the time on the pH and the conductivity (σ) of the urea solutions in presence of additives of HNO3, H2SO4 and H3PO4

Day Urea + HNO3 Urea + H2SO4 Urea + H3PO4 pH σ, mS.cm-1 pH σ, mS.cm-1 pH σ, mS.cm-1

0 2.87 2.06 2.87 2.28 2.87 1.78 5 3.00 2.08 2.96 2.30 2.92 1.82 10 3.11 2.10 3.05 2.32 2.99 1.87 15 3.29 2.12 3.13 2.35 3.07 1.93 20 3.77 2.15 3.44 2.40 3.17 1.98 30 5.50 2.32 5.58 2.55 3.46 2.09 40 6.43 2.63 6.60 3.19 3.82 2.20 50 6.62 2.85 7.04 3.48 4.38 2.33 60 6.75 3.00 7.28 3.61 5.56 2.52 70 6.96 3.10 7.50 3.69 5.86 2.75 80 7.18 3.18 7.75 3.75 6.07 3.01 90 7.39 3.23 8.00 3.82 6.30 3.32

100 7.66 3.27 8.24 3.87 6.57 3.64 110 7.87 3.30 8.42 3.92 6.75 3.99 120 8.09 3.33 8.61 3.95 6.94 4.34

ΔpH=5.22 Δσ=1.27 ΔpH=5.74 Δσ=1.67 ΔpH=4.07 Δσ=2.56

Fig. 1.1. pH of 20 % N solutions with mineral acids as a function of time at 298.15 K.

Fig. 1.2. Conductivity of 20 % N solutions with mineral acids as a function of time at 298.15 K.


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for the urea solutions with additives of NH4NO3 and (NH4)2SO4 are almost the same. A very small decrease of the pH followed by a slight increase is observed on the ten-th day. In contrast to these solutions, the dependence pH = f(t) of the solution urea+NH4H2PO4 is quite differ-ent. It starts from the relatively low pH value and shows a significant increase with time. Nevertheless, the final pH value (pH150 = 7.13) is the lowest for this group of solu-tions. To a certain extent the situation is the same in case of the conductivity change with time shown in Fig.2.2.

Here again, the curves σ = f(t) for the solutions of urea with NH4NO3 and (NH4)2SO4 are completely identical, but the conductivity values of the solution containing (NH4)2SO4 are higher than those referring to the presence of NH4NO3 at the same time. The conductivity values of the urea solution with NH4H2PO4 are appreciably lower than those of the other two solutions. This is another indicator which provides to conclude that the solution of urea containing NH4H2PO4 is the most convenient as a foliar fertilizer compared to the rest of the group studied.

Fig. 2.1. pH of 20 % N solution of urea with ammonium salts as a function of time at 298.15 K.

Fig. 2.2. Conductivity of 20 % N solution of urea with ammonium salts as a function of time at 298.15 K.

Table 2. Effect of time on pH and conductivity of the urea solutions with additives of NH4NO3, (NH4)2SO4 and NH4H2PO4.

Day Urea + NH4NO3 Urea + (NH4)2SO4 Urea + NH4H2PO4 - pH σ, mS.cm-1 pH σ, mS.cm-1 pH σ, mS.cm-1

0 7.41 23.76 7.43 27.09 5.34 11.20 10 7.31 25.20 7.33 28.55 5.75 12.20 20 7.36 24.85 7.39 28.17 6.00 12.49 30 7.42 25.00 7.45 28.32 6.18 13.13 40 7.46 25.05 7.49 28.39 6.31 13.69 50 7.50 25.11 7.52 28.44 6.36 14.24 70 7.54 25.16 7.56 28.48 6.40 15.06 90 7.56 25.22 7.59 28.51 6.44 15.52

110 7.59 25.27 7.61 28.53 6.64 15.88 130 7.60 25.32 7.62 28.55 7.01 16.20 150 7.61 25.37 7.63 28.57 7.13 16.49



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Urea solutions containing mineral acids and their ammonium salts as additivesThe solutions studied refer to:20 % N + 3 % NH4NO3 + 0.56 % HNO3, ρ = 1.1358 g.cm-3;20 % N + 3 % (NH4)2SO4 + 1.18 % H2SO4, ρ = 1.1458 g.cm-3;20 % N + 3 % NH4H2PO4, + 5.72 % H3PO4, ρ = 1.1693 g.cm-3.

Table 3 and Figs. 3.1 and 3.2 illustrate the changes of the pH and conductivity values of the aqueous urea

solutions containing the three acids studied and their ammonium compounds (salts). The salt content (3% w/w) in the solution is the same as in the previous part, while the acid content is determined to provide low and identical pH start values for the three solutions studied. The pH value increase of the solutions urea + (NH4)2SO4

+ H2SO4 and urea + NH4H2PO4 + H3PO4 during the whole studied period is insignificant - ΔpH150 = 0.73 for the first,

Table 3. Effect of the time on pH and conductivity of urea solutions with presence of acids and their ammonium salts.

Day U + NH4NO3 + HNO3 U + (NH4)2SO4 + H2SO4 U + NH4H2PO4 + H3PO4 - pH σ, mS.cm-1 pH σ, mS.cm-1 pH σ, mS.cm-1

0 2.15 32.16 2.22 32.20 2.32 17.06 10 2.30 33.82 2.31 34.46 2.32 18.14 20 2.34 32.24 2.27 32.55 2.33 17.27 30 2.36 33.07 2.33 33.44 2.30 17.95 40 2.44 33.58 2.35 33.92 2.35 18.30 50 2.51 34.28 2.38 34.54 2.35 18.90 60 2.64 34.02 2.47 34.30 2.40 18.88 70 2.82 33.50 2.55 34.25 2.50 18.78 90 3.04 33.82 2.63 34.77 2.48 19.02

110 3.72 33.60 2.77 35.58 2.58 19.55 130 5.83 34.81 2.83 36.22 2.59 19.57 150 6.11 34.68 3.05 36.18 2.61 20.44


Fig. 3.1. pH of 20 % N solution of urea with mineral acids and their ammonium salts as a function of time at 298.15 K.

Fig. 3.2. Conductivity of 20 % N solution with mineral acids and their ammonium salts as a function of time at 298.15 K.


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while ΔpH150 = 0.30 for the second solution (Fig.3.1). A sharp pH value increase is seen in the solution urea + NH4NO3 + HNO3 after the 110-th day. Regardless of this sharp increase, the final pH value (pH150 = 6.11) is also acceptable for practical purposes.

The dependencies σ = f(t) presented in Fig.3.2 are very similar for the three solutions studied. They show a slight conductivity increase. The lowest conductivity value refers to the solution of urea + NH4H2PO4 + H3PO4.

Urea solution (20 % N) with no additivesTable 4 and Figs. 4.1 and 4.2 show the changes of

the pH and conductivity values of the aqueous urea solution with no additives. The time dependence of pH goes through a minimum on the 20-th day, then it starts to increase, while at the end of the period studied it reaches a value of 9.26. Unlike pH, the conductivity increases sharply until the 20-th day, then continues to increase but now at a much lower rate.

The conductivity of the urea solution is much lower than that of the urea solutions in the presence of additives. This is perfectly logical considering that the urea, in contrast to the tested additives, is not an ionic compound. The time increase of the conductivity of the urea solution with no additives is due to the formation of ammonium cyanate and ammonium carbonates. The use of additives suppresses the occurrence of these side processes.

CONCLUSIONS

The investigations of the stability of aqueous urea solutions (20 % N) in presence of additives of inorganic acids (HNO3, H2SO4 and H3PO4) and their ammonium salts (NH4NO3, (NH4)2SO4 and NH4H2PO4) provide to conclude that:

• only the phosphoric acid among the three acids studied can be used as a stabilizing additive. The solution 20% N + 0.39% H3PO4 can be preserved within 5 - 6 months without exceeding the critical pH value of 7.5;

Table 4. Effect of the time on pH and conductivity of urea solution (20 % N).

Fig. 4.1. pH of 20 % N solution of urea as a function of time at 298.15 K.

Fig. 4.2. Conductivity of 20 % N solution of urea as a function of time at 298.15 K.

Day pH σ, µS.cm-1

0 9,18 241 10 8,72 686 20 8,58 1000 30 8,71 1086 40 8,82 1123 50 8,91 1139 70 9,02 1181 90 9,1 1228

110 9,17 1274 130 9,22 1340 150 9,26 1408

ΔpH = 0.08 Δσ = 1167


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• the three ammonium salts NH4NO3, (NH4)2SO4 and NH4H2PO4 studied are very suitable for the urea solution stabilizing even in presence of minimal amounts (3% w/w). The solutions have stable pH values and show an insignificant time change of the conductivity;

• the third variant studied – addition of a mixture of an acid and its ammonium salt – is also suitable for preparing a stable foliar liquid fertilizer. No substantial changes of the initial pH and the conductivity values of the solutions urea + (NH4)2SO4 + H2SO4 and urea + NH4H2PO4 + H3PO4 are found within 5 months storage. Only the solution urea + NH4NO3 + HNO3 shows a sharp pH increase after the 110-th day but the final value of 6.11 is acceptable.

• it is possible to follow the stability of aqueous urea solutions during their storage for a long period of time by reading two easily and precisely registered parameters – pH and conductivity.

• it is possible to stabilize concentrated urea solu-tions (20 % N) with minimal amounts of the additives studied. The use of these additives in greater amounts will give a double effect - stabilization of the solution with a simultaneous introduction of supplemental nutri-ent elements, i.e. producing of stable NPK urea-based liquid fertilizers.

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stability of urea solutions in presence of mineral …storing neutral urea solutions at 278.15 k, or...

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