As a high-quality source of calcium and phosphorus for livestock and poultry, the demand for dicalcium phosphate has surged in recent years, leading to increasingly serious issues of adulteration and the substitution of inferior products. How can one select high-quality dicalcium phosphate from the numerous options available?
Common adulterants in dicalcium phosphate include: stone powder, sand, tricalcium phosphate, agricultural superphosphate, phosphate rock powder, and light calcium carbonate. The identification methods for each are described below.
1. Identification of Stone Powder or Light Calcium Carbonate:
Stone powder, when ground to 80 mesh or finer, resembles dicalcium phosphate in appearance and morphology, but its relative density is greater. Light calcium carbonate is similar to dicalcium phosphate in both sensory characteristics and relative density, making it difficult to distinguish visually. However, it can be identified using dilute hydrochloric acid. Stone powder and light calcium carbonate react violently with dilute hydrochloric acid, producing a large number of bubbles. After the reaction, the solution is relatively clear.
2. Identification of Tricalcium Phosphate:
Tricalcium phosphate, also known as calcium phosphate, is white or grayish-white in appearance and has a high phosphorus and calcium content (but it is not easily absorbed by animals), making it an "ideal raw material" for adulteration by unscrupulous merchants. However, it can be identified. Take a small sample and place it in a small beaker, dissolve it with a small amount of acetic acid, then add tartaric acid solution and ammonium molybdate solution to immerse the sample. Place it in a constant temperature oven at 60-70°C. After several minutes, if a yellow precipitate appears, it indicates the presence of tricalcium phosphate.
3. Identification of Bone Meal:
The purpose of adding dicalcium phosphate to bone meal is to reduce its fluorine content. However, its color is grayish or yellowish-brown, and if more than half is added, it will have a bone meal odor. Alternatively, take a small amount (about 1-2g) of dicalcium phosphate, add excess dilute hydrochloric acid, and after the reaction, a large amount of turbid foam will be produced. After the reaction, the solution will be cloudy yellow, and insoluble matter will be present at the bottom, indicating the presence of bone meal.
4. Identification of Phosphate Rock Powder:
Phosphate rock powder is a fine powder ground from phosphate rock. It is grayish-white, yellowish-brown, or white, with a fluorine content of approximately 2% and a calcium content of approximately 32%. It is insoluble in dilute hydrochloric acid, which can be used for identification.
5. Identification of Agricultural Superphosphate:
Agricultural superphosphate is grayish-white to dark gray. When dilute hydrochloric acid is added, the solution turns grayish-brown, with some insoluble matter at the bottom, which can be used as a basis for identification.
6. Identification of Talc:
Talc has similar sensory characteristics to high-quality dicalcium phosphate, but it does not dissolve in dilute hydrochloric acid, and a translucent film floats on the surface, which can be used for identification.
7. High-quality dicalcium phosphate should be a white or grayish-white powder with uniform fineness, a soft feel, and good flowability. It is insoluble in water but soluble in 2% citric acid solution and 0.4% hydrochloric acid (no bubbles are produced during dissolution). Alternatively, a small amount can be placed in a watch glass, and a few drops of 5% silver nitrate solution can be added; the sample will turn into a yellow precipitate.
Furthermore, the sample quality standard (HG2636-2000) stipulates that the calcium content should be ≥21.0%, the phosphorus content ≥16.5%, and the fluorine content ≤0.18%. There is no upper limit specified for the content, which provides a legal guise for adulterated batches. However, the normal calcium and phosphorus content of high-quality dicalcium phosphate should be 21%~23.2% calcium and 16.5%~18.5% phosphorus, with theoretical values of 23.2% calcium and 18.0% phosphorus. The calcium-to-phosphorus ratio in unadulterated dicalcium phosphate should be close to 31:40. Generally speaking, even if an adulterated sample can achieve a suitable phosphorus content, it is difficult to achieve a suitable calcium content simultaneously.
EN
