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Abstract:This study compares the pH and buffering capacity differences of various types and sources of feed-grade calcium phosphate and analyzes the correlation between pH and buffering capacity. Different manufacturers’ calcium dihydrogen phosphate (MCP), tricalcium phosphate type III (DCP type III), and tricalcium phosphate type I (DCP type I) were selected for testing their pH, buffering capacity, and free acid content. The results show that the pH and buffering capacity of MCP are 3.62 and -279 mmol/100g, respectively, while those of DCP type III are 4.48 and -194 mmol/100g, and those of DCP type I are 7.92 and 33 mmol/100g, respectively. There is a significant correlation (P<0.01) between the pH and buffering capacity of MCP and DCP type I, while there is no correlation between the pH and buffering capacity of DCP type I. These results indicate that the pH and buffering capacity of MCP and DCP type III are much lower than those of DCP type I, making them suitable as inorganic phosphorus sources for weaned piglets.
Keywords:Calcium dihydrogen phosphate; Tricalcium phosphate type III; Tricalcium phosphate type I; pH; Buffering capacity; Free acid
The digestive tract development of nursing piglets is poor, and insufficient gastric acid secretion necessitates consideration of the buffering capacity in their diet. By controlling the dietary buffering capacity, a normal acidic environment in the piglet’s stomach can be maintained to ensure normal digestion and absorption functions. Research shows that various raw materials can affect the dietary buffering capacity of nursing piglets, and calcium hydrogen phosphate, as a high buffering capacity feed ingredient, significantly impacts the dietary buffering capacity of nursing piglets (Ding Hongtao et al., 2005). With the development of feed-grade phosphate production technology, there are now over 20 types of feed phosphates, with three commonly used feed calcium phosphates: tricalcium phosphate type I (DCP type I), tricalcium phosphate type III (DCP type III), also known as mono-dicalcium phosphate (MDCP), and calcium dihydrogen phosphate (MCP). The nutritional value of different feed phosphates varies for animals. Current research focuses more on the comparative differences in phosphorus bioavailability, with results in pigs, poultry, and aquaculture indicating that MCP>DCP type III>DCP type I. However, there are differences in the physical and chemical properties between different calcium phosphates, and whether there are differences in pH and buffering capacity can help identify superior feed phosphates for nursing piglets. Therefore, this study aims to compare the pH and buffering capacity of three feed-grade calcium phosphates, providing experimental evidence for selecting better feed phosphate products for nursing piglet diets.
Different types and sources of calcium phosphate products were selected: MCP (P content 22%), DCP type III (P content 21%), and DCP type I (P content 17%).
1.2 Test Measurement Indicators
pH, buffering capacity, and free acid content.
1.3 Test Instruments and Reagents
Analytical balance; pH meter (PHS-25C); beakers; conical flasks; acid burette; base burette; water bath; grinder; glass rod (magnetic stirrer); filter paper; funnel.
Hydrochloric acid (analytical grade); sodium hydroxide (analytical grade); deionized water; 1 mol/L HCl standard solution; 1 mol/L NaOH standard solution; 0.1 mol/L NaOH standard solution; bromocresol green indicator solution.
1.4 Test Analysis Methods
1.4.1 Determination of pH
Refer to GB/T 22548-2008 method for determining the pH of feed-grade calcium dihydrogen phosphate.
Weigh 0.24g±0.01g of the sample, place it in a 150mL beaker, and add 100mL of water to dissolve. Measure the test solution using a calibrated pH meter.
1.4.2 Determination of Buffering Capacity
Concept of feed buffering capacity: The total milliliters of hydrochloric acid consumed to titrate 100g of feed added to 200mL of deionized water until the pH of the solution reaches 4.0.
Weigh 50g of the sample in a 250mL cup, add 100mL of deionized water, and heat in a constant temperature water bath to 37°C. Stir thoroughly using a glass rod (or magnetic stirrer), insert the pH meter electrode into the solution, and titrate with 1 mol/L hydrochloric acid or 1 mol/L NaOH solution to pH=4.0, recording the volume of hydrochloric acid or NaOH used, which is then calculated as millimoles, indicating the buffering capacity of the sample (Ding Hongtao et al., 2005).
1.4.3 Determination of Free Acid Content
Refer to the fertilizer inspection method SN/T 0736.10-1999 for determining free acid. Weigh 5g (accurate to 0.001g) of the sample into a conical flask, add 100mL of distilled water, and shake for 30 minutes. Filter using dry filter paper and funnel, discard the initial portion of the filtrate, add 2-3 drops of bromocresol green indicator solution, and titrate with 0.1 mol/L NaOH standard solution, recording the milliliters of NaOH consumed as the solution color changes from yellow to blue-green.
The results are expressed as mean ± standard deviation, with one-way ANOVA performed using SPSS 15.0 statistical software and multiple comparisons conducted using the LSD method, along with correlation analysis between pH and buffering capacity.
2.1 pH and Buffering Capacity of Different Types and Sources of Calcium Phosphate
The pH and buffering capacity results for MCP, DCP type III, and DCP type I are shown in Table 1. As seen in Table 1, the pH of MCP and DCP type III are 3.62 and 4.48, respectively, both significantly (P<0.01) lower than the measured value of DCP type I at 7.92; the pH of DCP type III is between that of MCP and DCP type I. The buffering capacities of MCP and DCP type III are -279 and -194mmol/100g, respectively, both significantly lower than the measured value of DCP type I at 33mmol/100g, with a significant difference between MCP and DCP type III.
Comparing the pH and buffering capacity of MCP from different manufacturers also shows differences. Manufacturer C’s MCP has the lowest pH and buffering capacity, significantly (P<0.01) lower than manufacturer D, and significantly (P<0.05) lower than manufacturer A; the buffering capacity of manufacturer B’s MCP is also significantly lower than that of manufacturer D; other groups show no significant differences.
There are significant differences in the pH of DCP type I from different manufacturers, but no significant differences in buffering capacity. Manufacturer F’s DCP type I has the highest pH, significantly higher than manufacturers G and H, and significantly higher than manufacturers D, E, and J; manufacturer I’s DCP type I is next, significantly higher than manufacturer G; other groups show no significant differences.
2.2 Correlation Analysis of pH and Buffering Capacity of Calcium Phosphate
The correlation analysis of the pH and buffering capacity of MCP and DCP type III indicates a significant correlation (P<0.01), with a correlation coefficient of 0.998 (Figure 1). However, there is no significant correlation (P=0.987>0.05) between the pH and buffering capacity of DCP type I.
2.3 Free Acid Content of MCP and DCP Type III
Table 2 shows that the free acid content of MCP is significantly higher than that of DCP type III, approximately 10 times; there are also differences between MCPs from different manufacturers. Manufacturer C’s MCP free acid content is significantly (P<0.01) higher than the other three manufacturers; manufacturer D’s MCP free acid content is next, significantly (P<0.05) higher than manufacturer A.
3.1 pH of Feed Calcium Phosphate
This experiment determined the average pH of MCP (2.4 g/L) to be 3.62, meeting the national standard requirement of not less than 3.0 for MCP. This result is close to the pH (2.4 g/L) measured by Wang Leilei (2013) for MCP from nine different manufacturers (average value of 3.87, range 3.58-4.15). The average pH of DCP type I (2.4 g/L) determined in this experiment is 7.92. There are no national standards for the pH of DCP, and compared to other experimental results, this result is higher than the 7.39 measured by Tu Yan et al. (2010) and the 7.23 measured by Ding Hongtao et al. (2005); it is consistent with the result of 7.92 measured by Jiao Xiaoli et al. (2010). DCP type III is a complex of MCP and DCP type I, with MCP accounting for more than 60%, and the water-soluble phosphorus content being over 10%, thus its pH is between that of MCP and DCP type I and is slightly acidic. The result of 4.48 measured in this experiment is consistent with the 4.4 measured by Peadar et al. (2005). This indicates that the results of this experiment are consistent with most measurement results, but there are still differences between different experiments, which may be attributed to various factors such as the sample concentration used for measurement, the different sources of samples, and measurement errors.
Comparing the pH between MCP, DCP type III, and DCP type I shows differences primarily due to the differences in chemical composition of the various calcium phosphates, leading to differences in their physical properties. The molecular formulas (compositions) of MCP, DCP type III, and DCP type I are Ca(H2PO4)2·H2O, Ca(H2PO4)2·H2O + CaHPO4·2H2O, and CaHPO4·H2O, respectively. From the molecular formulas, it can be seen that Ca(H2PO4)2 and CaHPO4 produce H2PO4– and HPO42- ions when dissolved in water, while the former has a higher degree of ionization in aqueous solution than hydrolysis, producing hydrogen ions, thus being acidic; the latter has a higher degree of hydrolysis than ionization, producing hydroxide ions, thus being basic. DCP type III contains both H2PO4– and HPO42- ions, with the former being dominant, thereby making its aqueous solution acidic but lower than that of MCP. Additionally, since the generation of calcium phosphates currently primarily involves the reaction of phosphoric acid with limestone/calcium oxide, the pH of MCP or DCP type III being slightly acidic may be due to the excess phosphoric acid leading to a high free phosphoric acid content, thus resulting in a low pH.
Therefore, this experiment measured the free acid content of MCP and DCP type III. As seen in Table 2, the free acid content of DCP type I is very low, indicating that its acidity is not caused by unreacted free phosphoric acid, while the free acid content of MCP is higher than that of DCP type III. This is due to the large amount of H2PO4– ions in the aqueous solution ionizing to produce hydrogen ions, which are detected as free acid. Only the food-grade calcium dihydrogen phosphate (GB25559-2010) standard specifies a free acid standard, without a specified limit, indicating that the calculated free acid content should be within 7%. Therefore, the measured free acid content of MCP in this experiment meets the standards.
Moreover, for the same type of phosphate product, there are also differences in pH from different manufacturers. Wang Leilei (2013) reported that the pH of MCP is affected by different processing techniques, with steam method > calcium salt + sodium salt precipitation method > calcium salt precipitation method. In this experiment, the lowest pH among different sources of MCP was found in manufacturer C, possibly due to the influence of different production processes. For DCP type I, there are also significant differences in pH among products from different manufacturers, ranging from 7.83 to 8.21, indicating a significant variation. A high pH suggests that more Ca(OH)2 or CaCO3 was added during the neutralization reaction, with less phosphoric acid, hence the calcium content in the product would also be correspondingly higher. Some studies indicate that as the calcium content in phosphates increases, the biological utilization rate of phosphorus decreases.
3.2 Buffering Capacity of Feed Calcium Phosphate
For calcium dihydrogen phosphate, there have been no relevant experimental results reported for buffering capacity, with an estimated value of -60mmol/100g; however, this experiment measured -279mmol/100g, which is far below the estimated value. Therefore, the impact of calcium dihydrogen phosphate on feed buffering capacity should be given greater consideration. For DCP type III, the measured result in this experiment is -194mmol/100g, while Peadar et al. (2005) measured a buffering capacity of 29mmol/100g. The difference is due to the different concentrations used for measurement; this experiment used a 1:2 ratio (calcium phosphate salt: water) for dissolution, while Peadar et al. (2005) used a 1:100 ratio, resulting in a significant difference, whereas the 1:2 ratio is commonly used for measurement domestically. Using this method, the buffering capacity of DCP type I measured by domestic researchers Tu Yan et al. (2010) was 42.35mmol/100g, which is close to the measured result of this experiment but lower than the 69 and 77mmol/100g measured by Ding Hongtao et al. (2005) and Jiao Xiaoli et al. (2010), respectively. This indicates that the measured buffering capacity is influenced by the measurement method, dissolution concentration, and differences between samples, but overall, there are significant differences in buffering capacity between different types of calcium phosphates, while differences between sources of similar products are smaller. Research shows that in the diets of weaned piglets, calcium hydrogen phosphate, as a feed ingredient with high addition and buffering capacity, significantly affects the feed’s buffering capacity (Ding Hongtao et al., 2005). Therefore, substituting low buffering capacity MCP or DCP type III for conventional DCP type I should significantly reduce the buffering capacity of the piglet diet.
3.3 Relationship Between pH and Buffering Capacity of Feed Calcium Phosphate
The results of this experiment indicate that there is no significant correlation between the pH and buffering capacity of DCP type I from different sources, while there is a significant correlation (P<0.01) between the pH and buffering capacity of MCP and DCP type III. Comparing the results of pH and buffering capacity for feed ingredients measured by Ding Hongtao et al. (2005), Jiao Xiaoli et al. (2010), and Tu Yan et al. (2010) shows that there is also no correlation; the measured buffering capacity results do not align consistently at the same pH. The reason for this discrepancy may be that although both pH and buffering capacity are indicators of acidity, pH is the negative logarithm of the H+ concentration in the feed aqueous solution, while buffering capacity indicates the ability of the diet to neutralize acids, reflecting the amount of bound acids in the feed. Therefore, for buffer systems and salts, the pH and buffering capacity do not necessarily align perfectly. Since MCP and DCP type III contain a large amount of H2PO4–, which can release hydrogen ions, their pH and buffering capacity are consistent, showing a positive correlation between the two.
(1) Among different types of feed calcium phosphates, the order of pH and buffering capacity is MCP<DCP type III<DCP type I, with MCP and DCP type III having negative buffering capacities, indicating acidity; DCP type I has a positive buffering capacity, indicating weak alkalinity.
(2) There are differences in the pH of MCP and DCP type I from different manufacturers, but the differences in buffering capacity are small.
(3) There is a significant correlation between the pH and buffering capacity of MCP and DCP type III, while there is no correlation between the pH and buffering capacity of DCP type I.
(4) The acidity of MCP and DCP type III is not entirely influenced by the free acid content.
(Li Xia, Wan Rong, Wu Bo, Wang Xuejuan, China Chemical Yunlong Co., Ltd.)
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