Products that require freeze-drying are usually pre prepared as water solutions or suspensions, so their freezing point is different from that of water. Water freezes at 0 ℃, while seawater freezes at temperatures below 0 ℃ because seawater is also an aqueous solution of various substances. The experiment indicates 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. Pure liquids, such as water, freeze at 0 ℃, but the temperature of the water does not decrease until all the water freezes, indicating that pure liquids have a fixed freezing point. But the solution is different. It does not solidify into a solid at a fixed temperature, but at a certain temperature, crystals begin to precipitate. As the temperature decreases, the number of crystals continues to increase until the solution finally solidifies. In this way, the solution does not condense at a fixed temperature. But it condenses within a certain temperature range, and the temperature at which crystals begin to precipitate when cooled is called the freezing point of the solution. The temperature at which the solution completely solidifies is called the freezing point of the solution. Because the freezing point is the starting point of melting (i.e. melting point), and for a solution, it is also the point where the solute and solvent melt together. So it is also called eutectic point. It can be seen that the freezing point and eutectic point of a solution are not the same. The eutectic point is the temperature at which a solution truly solidifies into a solid.
Obviously, the concept of eutectic point is important for freeze-drying, as freeze-dried products may contain substances such as salts, sugars, gelatin, proteins, blood cells, tissues, viruses, bacteria, and so on. Therefore, it is a complex liquid, and its freezing process is certainly a complex process, similar to a solution, with a temperature at which it truly solidifies into a solid. The eutectic point. Due to the fact that freeze-drying is carried out in a vacuum state. Only when all products are frozen can sublimation be carried out under vacuum. Otherwise, when some liquid is present, it will not only evaporate rapidly under vacuum, but also cause concentration of the liquid, resulting in a reduction in the volume of the freeze-dried product; And the gas dissolved in water will quickly emerge under vacuum, causing it to boil like a liquid, causing the freeze-dried product to bubble and even protrude from the bottle. This is what we do not want. Therefore, the freeze-dried product must be cooled to a temperature below the melting point at the beginning of sublimation, so that the freeze-dried product is truly completely frozen.
It is impossible to determine whether a product has frozen into a solid by observing its appearance during the freezing process; Measuring temperature alone cannot determine the internal structural state of the product. The change in electrical performance as the product structure changes is extremely useful, especially when measuring the resistivity during freezing, which can help us know whether freezing is in progress or has already been completed. After complete freezing, the resistivity will be very high, so the solution is ion conductive. Freezing means that ions will be fixed and unable to move, resulting in a significant increase in resistivity. When there is a small amount of liquid present, the resistivity will significantly decrease. Therefore, measuring the electrical resistivity of the product will be able to determine its eutectic point.
The formal eutectic point 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 ℃, and then slowly heat up the frozen product. Use a Wheatstone bridge to measure its resistance. When the resistance suddenly decreases, the temperature at this point is the eutectic point of the product. The bridge needs to be powered by alternating current because direct current undergoes electrolysis, and the entire process is recorded by instruments.
It can also be measured using a simple method, by inserting two appropriately thick and insulated copper wires into a container containing the product as electrodes. Insert a thermometer near the copper electrode at a depth similar to that of the electrode, place them together near the observation window in the freeze-drying box, fix them in place using appropriate methods, and then pre freeze them together with other products. At this point, we continuously measure the resistance value during the cooling process using a multimeter, and determine the melting point based on the change in resistance value.
Connect the electrode leads to a multimeter through a switch, without distinguishing between positive and negative poles. If the freeze-drying box does not have a wire lead out connector, two thin wires can be used to lead out from the box door gap, and some vacuum sealing wax can be applied near the wires to avoid affecting the vacuum degree.
Start measuring and recording after the thermometer drops to 0 ℃. Place the conversion switch of the multimeter at the maximum position (x 1K or x 10K) for measuring resistance. Due to the use of direct current in the multimeter, in order to prevent electrolysis, the switch should be turned off immediately after each measurement, and the temperature and resistance values measured each time should be recorded one by one. At the beginning, the resistance value is very small, and it gradually increases afterwards. At a certain temperature, the resistance suddenly increases, almost infinitely, and the temperature value at this point is the melting point value.
There is a certain degree of error in measuring the eutectic point using this method, as there is some electrolytic effect at the copper electrode. A multimeter is not as sensitive to high resistance as a bridge; In addition, the changes in resistance during the freezing and melting processes are not the same, but the measured values still have practical reference value.