Products that need to be freeze-dried are generally pre-prepared into water solutions or suspensions, so their freezing point is different from that of water. Water freezes at 0 °C, while seawater only needs to be at a temperature lower than 0 °C. Freeze because seawater is also an aqueous solution of many substances. Experiments indicate that the freezing point of the solution will be lower than that of the solvent.
In addition, the freezing process of a solution is different from that of a pure liquid. When a pure liquid such as water freezes at 0°C, the temperature of the water does not drop until all the water freezes, which means that the pure liquid has a fixed freezing point. freezing point. The solution is different. It does not completely condense into a solid at a fixed temperature, but at a certain temperature, crystals begin to precipitate. As the temperature decreases, the number of crystals increases continuously until the solution is completely condensed. In this way, the solution does not condense at a fixed temperature. Rather, it condenses within a certain temperature range, and the temperature at which crystals begin to form when cooled is called the freezing point of the solution. The temperature at which all the solution condenses is called the freezing point of the solution. Because the freezing point is the point at which melting begins (ie the melting point), in the case of a solution, it is the point where the solute and solvent melt together. So, it is also called the eutectic point. The freezing point of the solution is not the same as the eutectic point. The eutectic point is the temperature at which the solution actually condenses into a solid.
Obviously, the concept of eutectic point is important for freeze-drying, because the freeze-dried product may contain substances such as salts, sugars, gelatin, proteins, blood cells, tissues, viruses, bacteria, etc. Therefore, it is a complex liquid, and its freezing process must also be a complex process. Similar to a solution, it also has a temperature at which it all condenses into a solid. the eutectic point. Since freeze drying is carried out under vacuum. Only after the product is completely frozen can it be sublimated under vacuum. Otherwise, when some liquid exists, it will not only evaporate rapidly under vacuum, causing the concentration of the liquid to reduce the volume of the freeze-dried product; and the gas dissolved in water will quickly evaporate under vacuum. Bubbling out, creating the appearance of a boiling liquid, bubbling the lyophilized product, and even out of the bottle. This is not what we want. Therefore, the freeze-dried product must be cooled to a temperature below the eutectic point at the beginning of sublimation, so that the freeze-dried product is truly frozen.
During the freezing process, it is impossible to determine whether the product is completely frozen into a solid from the external observation; it is also impossible to determine the internal structural state of the product by measuring the temperature. And the change of electrical properties as the structure of the product changes is extremely useful, especially when freezing is the measurement of resistivity, which allows us to know whether the freezing is in progress or has been completed. After all freezing, the resistivity will be very large, so the solution is ionically conductive. Freezing means that the ions are immobilized and cannot move, so the resistivity increases significantly. In the presence of a small amount of liquid, the resistivity will drop significantly. Therefore, measuring the resistivity of the product will determine the eutectic point of the product.
The formal eutectic measurement method is to immerse a pair of platinum electrodes in a liquid product, insert a thermometer into the product, cool them to a low temperature below -40°C, and then slowly heat the frozen product. The resistance is measured with a Wheatstone bridge. When the resistance suddenly decreases, the temperature at the time is the eutectic point of the product. The bridge is powered by alternating current, because direct current will electrolyze, and the whole process is recorded by the meter.
It can also be measured in a simple way. Two copper wires of appropriate thickness and insulated from each other are inserted into the container containing the product as electrodes. Insert a thermometer near the copper electrode, the insertion depth is like that of the electrode, put them together near the observation window hole in the freeze-drying box, and fix them in an appropriate way, and then pre-freeze them together with other products. At this time, we use the multimeter continuously measures the resistance value during the cooling process and determines the eutectic point according to the change of the resistance value.
Connect the electrode leads to the multimeter through a switch, regardless of the positive and negative poles. If there is no wire lead-out connector in the freeze-drying box, you can use two thin wires to lead out from the seam of the box door, and apply some vacuum sealing wax near the wires, so as not to affect the vacuum degree.
After the thermometer dropped to 0°C, the measurement was started and recorded. Put the switch of the multimeter on the highest gear (×1K or ×10K) for measuring the resistance. Since the multimeter uses direct current, in order to prevent electrolysis, the switch should be turned off immediately after each measurement, and the temperature and resistance value of each measurement should be recorded one by one. The resistance value is small at the beginning, and then gradually increases. At a certain temperature, the resistance suddenly increases, almost infinite, and the temperature value at the time is the eutectic value.
The eutectic point measured by this method has a certain error because of some electrolysis at the copper electrode. The multimeter is not sensitive to the high resistance value of the bridge; in addition, the changes of the resistance during the freezing process and the melting process are not the same, but the measured value still has practical reference value.
The value of the eutectic point ranges from 0°C to -40°C, which is related to the type of product, type, and concentration of protective agent. The eutectic points of some substances are listed twenty-two for reference, as the actual freeze-dried product has other ingredients. So, it's not the same.
