Calcium hydrogen phosphate is a mineral raw material that provides effective phosphorus and calcium for animals. With the rapid development of the feed industry, the market demand for calcium hydrogen phosphate is increasing, but various fake and shoddy products also appear.
This article discusses how to distinguish between calcium hydrogen phosphate and tricalcium phosphate and the problems and matters needing attention in the determination of fluorine, phosphorus and calcium in feed grade calcium hydrogen phosphate standard (HG 2636-94) and puts forward Suggest accordingly.
1 Correctly distinguish between calcium hydrogen phosphate and tricalcium phosphate
1.1 Physical identification
Generally speaking, hydrogen calcium is white or off-white powder or particles, fine and uniform in texture, bulky, and light in specific gravity, while tricalcium is made by calcination and other processes, with more impurities, dark color, and inconsistent particle size. Secondly, by burning (800~900°C), the weight of hydrogen calcium can be reduced by about 30% due to decomposition, while the weight of tricalcium remains basically unchanged.
1.2 Chemical identification
Calcium hydrogen is soluble in dilute hydrochloric acid and slightly soluble in water, while tricalcium is insoluble in dilute hydrochloric acid and water. Take a small amount of sample and add dilute acid, and the one that can dissolve quickly, clear and without precipitation is calcium hydrogen; it is insoluble, turbid, and has precipitation. is tricalcium.
Through the silver nitrate test, hydrogen calcium should have obvious yellow precipitation, and tricalcium should not be obvious, showing light yellow precipitation. Another sample needs to be dissolved in acetic acid, and put in an oven at 60~70°C for several minutes under tartaric acid and ammonium molybdate solution immersion. A yellow precipitate appeared.
Judging by the determination of calcium and phosphorus content, the phosphorus content is 16%~17%, and the calcium content is 21%~23%; if the phosphorus content is 17%~18%, the calcium content is 25%~30%, or even More than 30%, it can be broken into tricalcium.
2 Determination of fluorine content
The standard (HG2636-94) uses a fluoride ion selective electrode to directly measure the electrode potential under the condition of citric acid-sodium citrate buffer solution, and uses the standard curve to determine the fluorine content, which is fast and accurate under certain conditions. The factors determining the accuracy are: 1. temperature; 2. the sensitivity of the potentiometer (1 mV error in the potential measurement will produce a 3.9% relative concentration error); 3. the stability of the electromotive force of the working battery itself; 4. interfering ions; 5. the pH of the solution; ⑥ ion concentration to be measured; ⑦ potential equilibrium time. Among them, the errors caused by factors such as ①②③⑤⑦ can be reduced as much as possible through strict operation. For example, keep the measurement temperature and pH constant, choose a potentiometer and electrode with high sensitivity, keep the stirring speed consistent for each measurement, fully balance and correct the marking line, etc. The influence of factors such as the concentration of interfering ions and the concentration of ions to be measured will be discussed below.
2.1 Interfering ions
The interfering ions can react with the measured ions to generate a substance that does not respond on the electrode. Its existence not only brings errors to the measurement, but also increases the electrode response time. As far as calcium hydrogen is concerned, iron, aluminum, magnesium, etc. affect the determination of fluorine. The buffer conditions specified in the standard (HG 2636-94) are practical for low-fluorine and low-impurity samples, but not for high-fluorine and high-impurity samples. Because the conditions stipulated in the standard can only eliminate the influence of a small amount of aluminum oxide, but the influence of interfering ions such as iron cannot be eliminated.
In order to improve the accuracy of the determination, the standard addition method can be used for the determination of samples with complex components. Since the fluoride ion selective electrode measures the fluoride ion activity rather than the concentration, the activity coefficient is close to 1 only in a very dilute solution without interference, and the activity and concentration can replace each other. To determine the total concentration of fluoride ions (including free and complexed), to eliminate the influence of interfering ions, use the Nernst equation to quickly and accurately determine the concentration of fluoride ions in samples with complex compositions.
2.2 Fluoride ion concentration
The measurement range is not only related to the electrode system and properties, but also related to the concentration of interfering ions. The higher the concentration of interfering ions, the higher the lower limit of the ion concentration that can be determined. In order to measure high-fluorine and high-impurity samples under standard (HG 2636-94) conditions, it is possible to reduce the sample weight (0.25~0.5 g), set the volume to 250 ml and increase the dilution factor (5~10ml). Control the amount of interfering ions and the concentration of fluorine ions so that they are within the optimum measurement range of the electrode and improve the accuracy of the measurement.
3 Determination of phosphorus
The determination of phosphorus in the standard (HG 2636-94) is by gravimetric analysis. The precipitating agent quinomolybdenone reacts with phosphate to generate quinoline phosphomolybdate precipitation.
The precipitate is filtered, dried to remove water and weighed to obtain the phosphorus content. The role of citric acid in the precipitant is to complex with molybdic acid in the solution to reduce the concentration of molybdic acid, avoid the precipitation of quinoline silicomolybdate (which interferes with the determination), and prevent the hydrolysis of sodium molybdate from MoO3, acetone The function of this method is to make the precipitated particles larger and looser, which is convenient for washing, and at the same time, it can increase the solubility of quinoline, so as to prevent its precipitation from interfering with the determination.
Gravimetric analysis is an accurate and precise analysis method, but the operation is cumbersome, time-consuming, and there are many factors affecting the measurement error. Skilled laboratory personnel are required to operate to improve the accuracy of the measurement.
3.1 Precipitation
In the precipitation operation, it is necessary to make the precipitation as complete as possible to obtain a pure and large-particle crystalline precipitate that is easy to separate and wash, and to avoid the formation of an amorphous precipitate or even a colloid. This requires slowly adding a precipitating agent dropwise into the hot solution. After the precipitation is formed, the precipitation should be left to age to eliminate tiny particles and obtain coarse precipitation.
3.2 Filtration
In order to prevent the glass sand core from being quickly blocked by precipitation, it should be filtered by pouring filtration method, that is, the precipitation supernatant is carefully poured into the filter along the glass rod, so as to keep the precipitation in the cup as much as possible. After starting to filter, observe whether the filtrate is clear. If it is turbid or contains precipitated particles, it means that the operation is not good or the filter is not selected properly, and the reason should be found out and the experiment should be repeated.
3.3 Washing, drying, constant weight
The purpose of washing the precipitate is to wash away the impurities adsorbed on the surface of the precipitate and the mother liquor mixed in the precipitate. When washing, try to minimize the dissolution loss of the precipitate and avoid the formation of colloids. At the beginning of washing, the pouring method is generally still used, that is, add an appropriate amount of washing liquid into the beaker containing the sediment, stir it fully and place it for clarification, then filter the clear liquid by pouring, and wash it several times in this way, and submerge the clear liquid as much as possible each time. After washing several times, the precipitate can be transferred to the filter, and after all the precipitate is transferred, it can be washed several times until the precipitate is washed. Washing must be carried out continuously and completed at one time. It cannot be left halfway, let alone dry for too long. It is not easy to wash after being placed for condensation.
To wash the precipitate without increasing the dissolution loss of the precipitate, an appropriate amount of washing liquid can be used to wash it several times. Before adding the washing liquid each time, the previous washing liquid should be drained as much as possible. After washing, dry the precipitate to constant weight, the difference between the two weights shall not be greater than 0.2 mg, and calculate the phosphorus content from the weight of the precipitate.
4 Determination of Calcium
In the standard (HG 2636-94), the titration of calcium adopts the back titration method, that is, adding excess EDTA to complex with calcium ions, and then back titrating with zinc standard solution to indirectly calculate the calcium content. However, hydrogen calcium products generally contain iron, aluminum, calcium, magnesium, fluorine, phosphate and other ions. Under the condition of pH 10, phosphate does not affect the determination, but fluoride ion interferes seriously (especially for samples with high fluorine content).
Because fluoride ions can form calcium fluoride precipitates with calcium ions, and the order of EDTA complexation under this condition is iron, calcium, magnesium, aluminum and other ions, it can be seen that iron interferes most seriously, followed by magnesium and aluminum. Therefore, the calcium content measured under standard conditions is actually the sum of the above-mentioned ion contents, and the error of the measurement results is relatively large. In order to improve the accuracy of calcium determination, firstly, the interference of fluoride ions should be removed. It can be heated and boiled under acidic conditions to make the fluoride ions generate hydrogen fluoride gas to be volatilized and removed. Secondly, to avoid the interference of iron and aluminum ions, triethanolamine should be added for masking. Through the above treatment, direct titration with EDTA can be used to quickly and accurately measure the calcium content (when the magnesium content is not large and does not affect the analysis results).