UNDERSTANDING PHOSPHATE SOURCES “REMOVING A MAJOR MISCONCEPTION ABOUT FEED-GRADE PHOSPHATES”
The animal feed industry throughout the world uses millions of tons of high quality commercial sources of inorganic feed-grade phosphate each year. The relative bioavailability of the phosphorus (P) as well as the calcium (Ca) in these high-quality products is usually high. Even though the guaranteed amount (%) of P and Ca in these commercial phosphate sources is known, the purchaser is normally not aware of the exact chemical form in which the P exists. However, using X-ray diffraction techniques and other quality control methods, companies manufacturing feed-grade phosphates are able to determine the actual form and quantity of P and other minerals present in their products. In this issue of Milwhite’s Journal a major misconception about Ca phosphate sources is clarified and especially the mistaken belief about dicalcium phosphate that exists among most people throughout the world. Elemental structures are provided for a visual representation of the molecules even though the natural atomic bonding within the molecular structures is different than those presented here for simplification purposes.
Three major sources of inorganic P are used in the worldwide animal feed industry.These three sources are commonly referred to as monocalcium phosphate, dicalcium phosphate and tricalcium phosphate. It is very common for nutritionists and feed manufacturers to also refer to these three sources of P as MONOCAL, DICAL and TRICAL, respectively. However, almost no one within the animal feed industry as well as in academia seems to remember that from a chemical standpoint there is a more correct way to refer to these three phosphate sources. These “chemically correct” names are monobasic calcium phosphate, dibasic calcium phosphate and tribasic calcium phosphate. It is because the word “basic” has been forgotten and eliminated when referring to these sources that a misconception exists with regards to these high quality phosphate sources.
THE MISCONCEPTION: Most everyone assumes when referring to mono, di and tricalcium phosphate that the prefixes “mono”, “di” and “tri” are indicating the number of Ca atoms in the phosphate source. So, using this logic, it is easy to understand why people that have been asked the question: “How many calcium atoms are in MONOCAL, DICAL and TRICAL? “, answer by saying, “they contain one, two and three atoms of Ca, respectively.” However, this is not true. DICAL (dicalcium phosphate) only contains one Ca atom even though MONOCAL and TRICAL, do indeed, contain one and three Ca atoms, respectively. In fact, “mono”, “di” and “tri” are not referring to the number of Ca atoms nor to their relative bioavailability, but instead are referring to the particular properties of the phosphoric acid molecule.
At this point, in order to clarify the misconception, it is essential to understand to what the word “basic” is referring with regards to the properties of the phosphoric acid molecule. First, we must consider a molecule of phosphoric acid which has the molecular formula, H₃PO₄ , and remember in chemistry we learned that a molecule that donates hydrogen to a solution is referred to as an “acid” and the donated hydrogen is referred to as a “proton” which is simply a hydrogen atom without an orbital electron. On the other hand, a “base” accepts protons. So, chemically speaking, the term “basic” is used here to denote how many hydrogens have been displaced (removed) from a molecule of phosphoric acid. For instance, if one hydrogen is removed the phosphoric acid molecule is referred to as “monobasic phosphate” with the formula of H₂PO₄– or .
If two hydrogen’s are removed the molecule is referred to as “dibasic phosphate” with the formula HPO₄= or and when all three hydrogens are removed the phosphate is in the “tribasic” form (i.e.,“tribasic phosphate”, or .
Being in the mono, di or tri basic form now allows for the formation of a specific mineral salt. Salts of P can easily be formed by reacting with monovalent cations such as Na+ and K+ and divalent cations such as Mg++, Zn++, Ca++, etc. As an example, if sodium phosphate is formed the formula would be NaH₂PO₄, , keeping in mind that only one hydrogen has been displaced from the phosphoric acid molecule, which gives rise to the more appropriate chemical name of “monobasic sodium phosphate”.
Another example, using K+ to form mono, di, and tribasic potassium phosphate, respectfully, is as follows: , , . So, basic atoms or groups have the ability to combine with the phosphate molecule depending on how many of its hydrogens have been displaced.When Ca++ reacts with phosphoric acid to form MONOCAL, DICAL and TRICAL the formula and structure of each is as follows: Monobasic calcium phosphate (MONOCAL), Ca(H₂PO₄)₂ . Notice that only one Ca atom is involved and that it has combined with only one of the negative charges of each phosphate in order to satisfy the two positive charges in its valence. Often, monobasic calcium phosphate is also referred to as “calcium biphosphate”. However, when dibasic calcium phosphate (DICAL), CaHPO₄) is formed the single Ca atom attaches to two negative charges of only one phosphate as seen here and this is why it is sometimes referred to as “calcium monohydrogen phosphate” . In order to form tribasic calcium phosphate (TRICAL), Ca₃(PO₄)₂, three Ca atoms and two phosphate molecules are involved in order to balance out their positive and negative valences, .
Through the years, the word “basic” has been eliminated when referring to mono, di and tricalcium phosphate and this has led to erroneously interpreting the true chemistry of these phosphate sources, especially dicalcium phosphate. Even though in chemistry “di” indicates two of something, it does not mean that there are two atoms of Ca in a molecule of DICAL. In this case the “di” is simply referring to the basic nature of the phosphate molecule because it has lost two of its hydrogens and in-so-doing is capable of forming mineral salts with monovalent and divalent cations. Hopefully, the above information has assisted in clarifying the misconception about these phosphate sources and in the future when you hear someone ask the question, “How many Ca atoms are in dicalcium phosphate?” it will be easier to understand why most people answer the question by saying, “two”, which you now know is the wrong answer.
The information presented in this issue of Milwhite’s Journal was compiled by Dr. Orlando Osuna, Director of Health Science at Milwhite, Inc. and Dr. Richard Miles, Professor Emeritus, University of Florida, Gainesville, FL, USA.