What Is Calcium Oxalate?
Calcium oxalate is the oxalate of calcium.
It is found in plants and also occurs as an intermediate in the production of oxalic acid. At room temperature, it is a colorless needle-like crystal that is virtually insoluble in water.
It exists not only in anhydrous form but also in monohydrate and dihydrate (weddellite) forms. In plants, it is found in rhubarb leaves, yams, taros, capsicum, untreated konjac, and unripe pineapple.
In high concentrations, it causes skin irritation and a strong gustatory taste, and is toxic to the human body.
Uses of Calcium Oxalate
Calcium oxalate alone is used as a standard solution for qualitative and quantitative analysis of calcium and oxalate ions, as a glaze for ceramics, and in other applications. Calcium oxalate is generated as an intermediate in the production of oxalic acid.
When sulfuric acid is added to calcium oxalate, calcium sulfate precipitates and the oxalate ion is released. Calcium oxalate crystals precipitate when calcium sulfate is removed by filtration and the filtrate is evaporated.
Characteristics of Calcium Oxalate
Calcium oxalate is the oxalate of calcium and has the chemical formula CaC2O4. It can form monohydrate or dihydrate by coordination bonding with water.
Some laboratory reagents are sold in the monohydrate state, so it is necessary to check the labeling when using calcium oxalate in experiments. When calcium oxalate hydrate is heated to 200°C, it loses its crystalline species and becomes anhydrous.
The basic properties of calcium oxalate (molecular weight, specific gravity, and solubility) are as follows:
- Molecular weight: 146.11
- Density: 2.2 g/cm3
- Solubility: Almost insoluble in water and ethanol. Soluble in dilute hydrochloric acid and dilute sulfuric acid.
Other Information on Calcium Oxalate
1. Toxicity to Humans
Calcium oxalate is highly irritating and corrosive to skin and mucous membranes. Wear protective equipment, such as nitrile gloves and protective glasses, to prevent contact with hands and eyes.
If calcium oxalate adheres to the skin, wash thoroughly with soap while rinsing under running water. Soap is used because the solubility of calcium oxalate is increased by raising the pH level, making it easier to wash off.
If it gets into the eye, wash the eye with running water with the face on the side so that the crystals of calcium oxalate can easily flow out of the eye. At this time, spread the eyelids with your fingers and move the eyeballs slowly while washing well behind the eyelids. If irritation persists in the eyes, medical attention is required.
2. Regulation as a Deleterious Substance
Calcium oxalate is a skin irritant and is therefore designated as a “deleterious substance.”
3. Calcium Oxalate in Food
Calcium oxalate is also found in foods because it accumulates as a secondary metabolite in plants. Spinach, yams, and black tea are particularly rich in calcium oxalate.
Eating yam may cause itchy lips, because the calcium oxalate in the yam irritates the mucous membranes of the lips. Untreated konjak yam has even higher calcium oxalate content, so it is treated with strong alkali before being crushed, mixed, and coagulated into the food “konjac.”
Touching a raw konnyaku potato with bare hands can cause irritation due to the calcium oxalate and may require hospital treatment. Some wild plants that grow in the mountains and fields also contain large amounts of calcium oxalate, such as the tenkansho plant, which grows wild.
It is important for risk management to know the forms of these plants from an illustrated book so that you do not accidentally touch them with your bare hands when camping or climbing mountains.
4. Urinary Tract Stones
Calcium oxalate ingested from food is hardly excreted in urine because it is almost insoluble in water, and gradually accumulates in the body. Calcium oxalate in the body is deposited as crystals in the kidneys and urinary tract.
5. Decomposition by Heating
Calcium oxalate itself is nonflammable, but decomposition occurs when heated or exposed to strong oxidizing agents. Since toxic carbon monoxide is produced as a decomposition product, care must be taken to avoid inadvertent contact of calcium oxalate with heating elements or oxidizing agents.