Calcium Oxalate Dihydrate vs. Calcium Oxalate Monohydrate

What's the Difference?

Calcium Oxalate Dihydrate and Calcium Oxalate Monohydrate are two different crystal forms of calcium oxalate, a compound commonly found in kidney stones. The main difference between the two lies in their water content. Calcium Oxalate Dihydrate contains two water molecules per formula unit, while Calcium Oxalate Monohydrate contains only one water molecule. This disparity in water content affects their physical properties, such as solubility and crystal structure. Calcium Oxalate Dihydrate is more soluble in water and has a more flexible crystal lattice, making it more prone to dissolution and less likely to form kidney stones. On the other hand, Calcium Oxalate Monohydrate is less soluble and has a more rigid crystal structure, making it more likely to precipitate and contribute to the formation of kidney stones.


AttributeCalcium Oxalate DihydrateCalcium Oxalate Monohydrate
Chemical FormulaCaC2O4 · 2H2OCaC2O4 · H2O
Crystal StructureMonoclinicMonoclinic
SolubilityLess soluble in waterMore soluble in water
AppearanceColorless or white crystalsColorless or white crystals
FormationForms in the presence of excess waterForms in the absence of excess water
Common OccurrenceKidney stones, urinary tract stonesKidney stones, urinary tract stones

Further Detail


Calcium oxalate is a common compound found in nature, particularly in plants and human kidney stones. It exists in two main forms: calcium oxalate dihydrate (COD) and calcium oxalate monohydrate (COM). While both forms share similarities, they also possess distinct attributes that set them apart. In this article, we will explore and compare the characteristics of COD and COM.

Crystal Structure

COD and COM differ in their crystal structures. COD has a monoclinic crystal structure, meaning its atoms are arranged in a specific pattern with unequal sides and angles. On the other hand, COM has a rhombic crystal structure, characterized by equal sides and angles. This difference in crystal structure affects their physical properties and behavior.


When it comes to appearance, COD and COM can be distinguished by their visual characteristics. COD crystals are typically colorless and have a prismatic shape with pointed ends. They often appear as clusters or aggregates. On the other hand, COM crystals are also colorless but have a more elongated and needle-like shape. They tend to form in a radiating pattern, resembling bundles of needles.


One of the significant differences between COD and COM lies in their solubility properties. COD is more soluble in water compared to COM. This means that COD crystals dissolve more readily in water, leading to higher concentrations of calcium and oxalate ions in solution. COM, on the other hand, is less soluble, requiring more time and effort to dissolve. The solubility difference between the two forms can have implications in various fields, including medicine and geology.


Both COD and COM can form under different conditions and environments. COD is commonly found in urinary stones, where it precipitates from urine rich in calcium and oxalate. It can also be found in plants, such as rhubarb and spinach, where it serves as a defense mechanism against herbivores. COM, on the other hand, is the most prevalent form of calcium oxalate in kidney stones. It forms when urine is supersaturated with calcium and oxalate, leading to the crystallization of COM in the kidneys.

Medical Significance

Understanding the differences between COD and COM is crucial in the field of medicine, particularly in the diagnosis and treatment of kidney stones. Calcium oxalate stones are the most common type of kidney stones, and they can be composed of either COD or COM. Identifying the specific form of calcium oxalate present in a patient's kidney stones can help determine the underlying causes and guide treatment strategies. For instance, COM stones are more likely to recur compared to COD stones, necessitating different preventive measures.

Impact on Health

While both COD and COM can contribute to the formation of kidney stones, they may have varying impacts on an individual's health. COM stones are generally considered more problematic as they tend to be larger and more jagged, causing more pain and discomfort during passage. On the other hand, COD stones are often smaller and smoother, making them less likely to cause severe symptoms. However, it is important to note that the impact on health can vary depending on the size, location, and number of stones present.

Environmental Significance

Calcium oxalate crystals, including both COD and COM, play a significant role in the environment. They are involved in various biological processes, such as calcium regulation in plants and the formation of kidney stones in animals. Additionally, calcium oxalate crystals can act as a defense mechanism against herbivores, deterring them from consuming certain plants. The presence of calcium oxalate crystals in fossils can also provide valuable information about ancient plant species and their evolutionary history.

Industrial Applications

Both COD and COM have practical applications in various industries. For example, calcium oxalate crystals can be used as a polishing agent in the manufacturing of optical lenses and mirrors. The abrasive nature of the crystals helps to achieve a smooth and reflective surface. Additionally, calcium oxalate crystals have been investigated for their potential use in drug delivery systems due to their biocompatibility and ability to encapsulate pharmaceutical compounds.


In conclusion, calcium oxalate dihydrate (COD) and calcium oxalate monohydrate (COM) are two distinct forms of calcium oxalate with different crystal structures, appearances, solubility properties, and formation processes. Understanding the differences between these two forms is essential in various fields, including medicine, geology, and industry. Further research and exploration of COD and COM will continue to shed light on their unique attributes and potential applications.

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