Ammonium Di-Hydrogenocitrate and Mono-Hydrogenocitrate Synthesis by Citric Acid Neutralization with Ammonia Using Ethanol as Co-Solvent for the Crystallization – Swelling Test to Confirm Gases Emissions Capacity

Citric acid is a α-hydroxylated tricarboxylic acid present in abundance in lemon. More than one million tons of citric acid are industrially produced throughout the year. Our objective in this manuscript was to increase the value of the citric acid to ammonium citric acid salts by crystallization such as ammonium Di-hydrogenocitrate and ammonium mono-hydrogenocitrate. Studies and tests were carried out in this direction but the characteristic of our last process was the use of a co-solvent ethanol which proved more effective and more economical. At the end, we tested the capacity of the ammonium Di-hydrogenocitrate and the ammonium mono-hydrogenocitrate to swell a mixture and compared theirs capacities to the sodium bicarbonate. Results showed an excellent swelling capacity of the ammonium Di-hydrogenocitrate and ammonium mono-hydrogenocitrate to produce a uniformly very not much porous product’s texture.


Introduction
Citric acid which is a tricarboxylic acid was neutralized by ammonia. According to the solution's pH, we obtained either ammonium Di-hydrogenocitrate either mono-hydrogenocitrate which are citric acid salts and have their future in agricultural, food and medicinal fields like calcium citrate and sodium citrate [1]. Studies on the effectiveness of the use of ethanol as co-solvent during the crystallization were made [2] and we have used this method.
Immediately, ammonium salts were formed. They were treated then we tested their capacity to swell a mixture by following a cooking procedure. Uniformly very not much porous texture was obtained which confirmed not only ammonia and carbon dioxide gases emissions but also water and molecules formations by esterification between citric acids and starches molecules. Citric acid C 6 H 8 0 7 is a tricarboxylic acid α-hydrolyzed. It contains three acids with pKa such as pKa 1 = 3.14, pKa 2 = 4.77 and pKa 3 = 6.39 and an α-alcohol function with pKa = 14.4 [3,4,5,6] " Figure 1". By its reactivity, the citric acid was the object of several studies and was used in several fields like the cosmetics, the food one, the chemistry and others [7].

Citric Acid Generalities
We noticed that the acid form is AH with pKa(AH). It was shown that if the pH ≤ [pKa(AH) -2], the quantity of basic Aassociated to the acid/base couple AH/Ais negligible in comparison with the AH quantity. And if the pH ≥ [pKa(AH) + 2], the quantity of acid AH associated to the acid/base couple AH/Ais negligible in comparison with the Aquantity [8]. For [pKa(AH) -2] ≤ pH ≤ [pKa(AH) + 2], the basic Aand the acid AH forms coexist but if [pKa(AH) -2] ≤ pH ≤ pKa(AH) the acid form AH dominate and if pKa(AH) ≤ pH ≤ [pKa(AH) + 2] the basic form Adominate [8]. Consequently, for the citric acid we noted in the following Table 1 the acids and basics forms according to the pKa and pH:

Characteristics of Citric Acid
Citric acid is solid with monoclinic as crystal structure, white, odorless and excessively sour flavor (

Ammonium Hydroxide Generalities
Ammonium hydroxide is a colorless aqueous solution obtained by the solubilization of ammonia in water [4,11,12] according to the reaction NH 3 + H 2 0↔NH 4 + + OHwith pKa (Dissociation constant of the couple Acid/Base NH 4 + / NH 3 ) = 9.2 [11]. We noticed again that the acid form is AH with pKa(AH). It was shown that if the pH ≤ [pKa(AH) -2], the quantity of basic Aassociated to the acid/base couple AH/Ais negligible in comparison with the AH quantity. And if the pH ≥ [pKa(AH) + 2], the quantity of acid AH associated to the acid/base couple AH/Ais negligible in comparison with the Aquantity [8]. For [pKa(AH) -2] ≤ pH ≤ [pKa(AH) + 2], the basic Aand the acid AH forms coexist but if [pKa(AH) -2] ≤ pH ≤ pKa(AH) the acid form AH dominate and if pKa(AH) ≤ pH ≤ [pKa(AH) + 2] the basic form Adominate [8]. Consequently, for the ammonia we noted in the following table 4 the acids and basics forms according to the pKa and pH:

Characteristics of Ammonia (Ammonium Hydroxide)
The concentration of ammonia ranges up to approximately 30%. Its solution irritates eyes and suffocating odor confirms the presence of ammonium hydroxide which is relatively very volatile. The boiling point of ammonia 25% is 311.15°K [10].

Ammonium Citrate (Formula and Characteristics)
The Ammonium citrate or Triammonium citrate ( Figure 2) was salt obtained by acid/base reaction between citric acid (C 6 H 8 0 7 ) as acid and ammonia (Ammonium Hydroxide -NH 4 OH) as base. It was shown that the respect of the couple Citric acid (acid)/Ammonia (base) solution's pH (cf. §2.1 - §3.1) ( Table 1) and crystallization temperature controls led to the tri-ammonium citrate salt formation [13,15]. We putted 150 [g] of citric acid with 234. 23 [ml] of ammonium hydroxide (25%) in a baker, the solution's pH is on 9.45. According to the table 1 and the table 4, the dominant form of ions are A 3and NH 4 + . Then, baker was carried in a 373.15 °K water bath during 60 mn to evaporate the water and to decrease the solubility of the tri-ammonium citrate formed. Then, the solution became tri-ammonium saturated and to increase the amount of this salt we brought this saturated solution to labile region which is over the metastable limit by cooling the baker on magnetic stirrer [14]. After filtration on filter paper, we obtained odorless white crystals of slightly acid tri-ammonium citrate (C 6 H 17 N 3 O 7 ) [13].

Ammonium Di-hydrogenocitrate and Ammonium Mono-hydrogenocitrate
We synthesized the Ammonium Di-hydrogenocitrate (AH 2 -/ NH 4 + ) and mono-hydrogenocitrate (AH 2-/2NH 4 + ). We putted 250 [g] of citric acid with 39 [ml] of ammonium hydroxide (25%) in a baker, the solution's pH is on 3.55. According to the table 1 and the table 4, the dominant form of ions is AH 2 and NH 4+ . Then, baker was carried in a 373.15 °K water bath during 60 mn to evaporate the water and to decrease the solubility of the Ammonium Di-hydrogenocitrate formed. Then, the solution became Ammonium Di-hydrogenocitrate saturated and to increase the amount of this salt we brought this saturated solution to labile region which is over the metastable limit by cooling the baker on magnetic stirrer [14]. After filtration on filter paper, we obtained odorless white crystals of Ammonium Di-hydrogenocitrate [ Figure 3] salts which taste acid [13]. To obtain the mono-hydrogenocitrate [ Figure 4] we brought the pH of solution between 4.77 and 6.39 by putting 9.6 [g] of citric acid with 20 [ml] of ammonium hydroxide (25%) in a baker. According to the table 1 and the table 4, the dominant form of ions are AH 2and NH 4 + . Then, baker was carried in a 373.15 °K water bath during 60 mn to evaporate the water and to decrease the solubility of the mono-hydrogenocitrate formed. Then, the solution became mono-hydrogenocitrate saturated and to increase the amount of this salt we brought this saturated solution to labile region which is over the metastable limit by cooling the baker on magnetic stirrer [14]. After filtration on filter paper, we obtained odorless white crystals of mono-hydrogenocitrate (AH 2-/2 NH 4 + ) salts which taste non-acid [13]. To summarize we put in the following figure ( Figure 5) the Ammonium citrate, Ammonium Di-hydrogenocitrate and Ammonium mono-hydrogenocitrate crystallization procedure [13]. The output of ammonium salts was all the time over 100% because of water presence (Hydrated salts - Figure 2). To limit the water presence and to improve the quality of the ammonium salts we elaborated a new procedure using a co-solvent [14] miscible with water, ammonium hydroxide, soluble in citric acid but practically insoluble in Ammonium Di-hydrogenocitrate and Ammonium mono-hydrogenocitrate: ethanol which is not only healthy but also an environmentally responsible solvent than methanol.

Procedure Used for Ammonium
Di-hydrogenocitrate and Mono-hydrogenocitrate Synthesis: Salts Precipitation Using the Co-solvent Ethanol

Procedure
We putted 62 [g] of citric acid with 9.8 [ml] of ammonium hydroxide (25%) in a baker, the solution's pH is on 3.5. According to the table 1 and the table 4, the dominant form of ions are AH 2 and NH 4 + . Then, baker was carried in a 313.15 °K water bath during 15 mn. Critical nucleus of the new solid phase "Ammonium Di-hydrogenocitrate salt" is formed. We are on region between undersatureted regions and metastable limits. To accelerate the formation of the solid phase and to reduce its solubility in water, we cooled the baker and we used the co-solvent ethanol 97% which is not only miscible in water but also naturally cool [14,15,16]. It was shown that c*, the equilibrium saturation concentration value, is empirically correlated with the concentration of the co-solvent x like ln[C*] = A+Bx+Cx2. In this case, we used a quantity of ethanol which didn't bring the pH of solution more than 4.77 and stirred the solution baker. Instantaneously, the Ammonium Di-hydrogenocitrate precipitation occurs, we are on the labile region [14]. After a few minutes, we filtered the Ammonium Di-hydrogenocitrate with a filter paper and dried it in a drying oven. Knowing that the ethanol boiling point is 351.39 [°K] [17] and it's completely miscible in water with the possibility of having hydrogen bond connections with oxygens and hydrogens molecules of water and ammonium citric acid salts [18], drying with temperature higher than 358. 15 [°K] is used to eliminate not only the rest of water molecules but also ethanol molecules. Then, we obtained odorless white crystals of Ammonium Di-hydrogenocitrate salts which taste acid.
To obtain the mono-hydrogenocitrate we brought the pH of solution at 5.5 (between 4.77 and 6.39 -cf. . Then, baker was carried in a 313.15 °K water bath during 15 mn. Critical nucleus of the new solid phase "Ammonium mono-hydrogenocitrate salt" is formed. We are on region between undersatureted regions and metastable limits. To accelerate the formation of the solid phase and to reduce its solubility in water, we cooled the baker and we used the co-solvent ethanol 97% which is not only miscible in water but also naturally cool [14,15,16]. In this case, we used a quantity of ethanol which didn't bring the pH of solution more than 6.5 and stirred the solution baker. Instantaneously, the Ammonium Di-hydrogenocitrate precipitation occurs, we are on the labile region [14]. After a few minutes, we filtered the Ammonium mono-hydrogenocitrate with a filter paper and dried it in a drying oven. Knowing that the ethanol boiling point is 351.39 [°K] [17] and it's completely miscible in water with the possibility of having hydrogen bond connections with oxygens and hydrogens molecules of water and ammonium citric acid salts [18], drying with temperature higher than 358. 15 [°K] is used to eliminate not only the rest of water Ammonia Using Ethanol as Co-Solvent for the Crystallization -Swelling Test to Confirm Gases Emissions Capacity molecules but also ethanol molecules. Then we obtained odorless white crystals of Ammonium mono-hydrogenocitrate salts which taste non-acid. To summarize we put in the following figure ( Figure 6) the Ammonium Di-hydrogenocitrate and Ammonium mono-hydrogenocitrate precipitation procedure using a co-solvent ethanol.

Ammonium Di-hydrogenocitrate and Ammonium Mono-hydrogenocitrate Yields Using Co-solvent Ethanol Precipitation Procedure
Compared with the crystallization procedure (figure 2), the precipitation procedure using a co-solvent ethanol was not only rapid but also energetically profitable. In the following table, we show the yields of Ammonium Di-hydrogenocitrate and ammonium mono-hydrogenocitrate obtained by the precipitation procedure.

Ethanol Solubility of the Ammonium Di-hydrogenocitrate and the Ammonium Mono-hydrogenocitrate Obtained by Co-solvent Ethanol Precipitation Procedure
Bibliography informs us that the ammonium mono-hydrogenocitrate is very slightly soluble in ethanol [19]. To confirm this solubility to our ammonium mono-hydrogenocitrate (Table 6), we used the oversaturated method. In this case, solid in excess of the amount required for saturation is added to the solvent and agitated until apparent equilibrium is reached [14]. At ambient temperature, we took 2 [g] of ammonium mono-hydrogenocitrate into 2 [ml] of ethanol (97%) using a test tube. After prolonged agitated contact, the weight of the rest of ammonium mono-hydrogenocitrate was 1.950 [g]. That is to say, only 0.05 [g] of ammonium mono-hydrogenocitrate was soluble in ethanol (97%) and considering that the solubility of the ammonium mono-hydrogenocitrate in water is 1mg/1mg [19], this solubility is exactly 0.0485 [g] in ethanol (100%); it correspond to 2.425% of the initial weight and confirm that the ammonium mono-hydrogenocitrate is very slightly soluble in ethanol.
We used the same method to determine the solubility of the ammonium Di-hydrogenocitrate in ethanol. At ambient temperature, we took 2 [g] of ammonium Di-hydrogenocitrate into 2 [ml] of ethanol (97%) using a test tube. After prolonged agitated contact, the weight of the rest of ammonium mono-hydrogenocitrate was 1.935 [g]. That is to say, only 0.065 [g] of ammonium mono-hydrogenocitrate was soluble in ethanol (97%); it correspond to 3.25% of the initial weight and confirm that also the ammonium Di-hydrogenocitrate is very slightly soluble in ethanol. But, after comparison we saw that, the ammonium Di-hydrogenocitrate was more soluble in ethanol (97%) than the ammonium mono-hydrogenocitrate at ambient temperature.

Swelling Tests of the Ammonium Di-hydrogenocitrate and Mono-hydrogenocitrate Salts
The objective of this part was to test the capacity of the Ammonium Di-hydrogenocitrate and the Ammonium mono-hydrogenocitrate to swell a mixture. As we see, these salts contains ammonium function (from -ONH 4 ) (Figure 3 -Figure 4) which can generate the ammonia (NH 3 ) ( Table 4 -Table 5) and acids functions (from the citric acid function) which catalyzed the ammonia reactions formation (Figure 7 -Figure 8) [20]. In addition, we noticed that according to the Van't Hoff equation the pKa value was influenced by the temperature [21]. In the end, we compared these salts and the sodium bicarbonate baking powder (rising powder) capacity to swell a mixture.  With water, we noticed that the ammonia can formed NH 4 OH according to the pH and the temperature (Table 4 -Table 5) [21].

Sodium Bicarbonate and Ammonium Bicarbonate Baking Powder Swelling Characteristics
Baking powders are white powders used to swell biscuits and pastry during cooking. According to their natures, they produced carbon dioxide (Sodium bicarbonate - Figure 9) and or ammonia (Ammonium bicarbonate - Figure 10) with water. The paste swelled and developed. Then, cavities and pores was been left by these gases and steams emission (Table 7) - [22].   According to the figure 7, figure 8 and figure 9 we showed in the following table 8 the theoretical swelling capacity of the ammonium Di-hydrogenocitrate, the ammonium mono-hydrogenocitrate and the sodium bicarbonate by respectively NH 3 and CO 2 gas emission with water. We noticed that at the same quantity 1 [g], the NaHCO 3 salt should have 2.5 times and 1.4 times capacity to swell than respectively the ammonium Di-hydrogenocitrate and the ammonium mono-hydrogenocitrate salts considering only the NH 3 and CO 2 gas emission. However, it was shown that water was not only the solvent which is the responsible of the ingredients repartition but its evaporation was also responsible of the porous formation [22]. Consequently, the ammonium Di-hydrogenocitrate and the ammonium mono-hydrogenocitrate should have respectively less 7.5 times and 4.1 times capacity to swell than the sodium carbonate. Discussions and explanations about the swelling salts capacities will be broached at the paragraph 7.3.

Cooking Procedure
During the swelling capacity tests of ammonium Di-hydrogenocitrate and ammonium mono-hydrogenocitrate in comparison with sodium bicarbonate [23]. We adopted the following cooking procedure: we preheated the oven at 423. 15 [°K] and prepared the mixture in the cake pan. When the 423.15°C was stable we putted the mixture in the oven. We progressively increased the temperature at 473.15°C. The cooking at 473.15°C lasted 30 [mn] after which we took out the product obtained.

Tests Results and Comparisons, Discussions
About the swelling tests, we prepared cake pastries using respectively 3 [g] of ammonium Di-hydrogenocitrate, ammonium mono-hydrogenocitrate and sodium carbonate. Then, we adopted the cooking procedure previously described [23]. We noticed that flour is the pastry principal ingredient which is compound with starch (70%), water (16%), Gluten (11%), Sugar (2%) and fatty substance (1%) [4]. We noticed also that the pastry height in the cake pan before cooking was all the time equal to 15 [mm] and for each salts we read not only the pastry height after the cooking procedure but also its taste and texture to assess each salts swelling capacity. We showed in the following table (Table 9) the tests results and comparisons for all salts [23]. According to the results in the table 9, the ammonium Di-hydrogenocitrate had 4.25 times less capacity to swell than the sodium carbonate but theoretically we was in a hurry for 7.5 times (cf. § 7.1). It was the same for the ammonium mono-hydrogenocitrate which had 3.4 times less capacity to swell than the sodium carbonate but theoretically we was in a hurry for 4.1 times (cf. § 7.1). These results was due to the water molecules and carbon dioxide CO 2 formations from the ammonium Di-hydrogenocitrate and the ammonium mono-hydrogenocitrate. Water molecules formed are solvent and responsible of the ingredients repartition until the ammonium salts evaporation to form porous particularly uniform (Table 9). For these ammonium Di-hydrogenocitrate and the ammonium mono-hydrogenocitrate, water and carbon dioxide molecules were formed either by their dehydration and decarboxylation like a citric acid [13-24-26], either by the dehydration and decarboxylation of citric acid molecules formed (figure 7) and either by the esterification reaction [24] between the acid of the citric acid formed and the starch's alcohol functions [27][28] according to the figure   11. That explained the uniformly very not much porous of the ammonium Di-hydrogenocitrate and mono-hydrogenocitrate salts samples texture (table 9).

Conclusion
The crystallization of the ammonium Di-hydrogenocitrate and the ammonium mono-hydrogenocitrate obtained by the ammonia and the citric acid reaction using an ethanol as co-solvent while respecting the pH rule was not only easier but also energetically economical. The swelling test capacity of these salts by following a cooking procedure informed us the gases emissions like ammonia, carbon dioxide, water vapor and consequently the real formation of water molecules by esterification between citric acids and starches molecules. The molecules formed by this esterification reaction like the one showed by the figure 11 was interesting because in certain conditions like the cooking procedure that we adopted on the paragraph §7.2, they are potential source of water molecules and carbon dioxide by dehydration and decarboxylation reactions ( figure 12). Being given that the ammonium Di-hydrogenocitrate and the ammonium mono-hydrogenocitrate contains azote and citric acid which plays a significant role in biochemistry as metabolite of the Krebs' cycle, a major metabolic way at all the aerobic organism, it's possible in certain conditions to use these salts as fertilizer supplements.