The supercooling phenomenon is a key factor affecting the measurement accuracy in the determination of osmotic pressure using the freezing point descent method. When the temperature of the solution drops below the theoretical solidification point, if there is a lack of crystal cores (such as impurities, crystal seeds) or the cooling rate is too fast, solute particles cannot be arranged in an orderly manner to form crystals in a timely manner, resulting in the solution being in a supersaturated state without precipitating crystals. This phenomenon is called undercooling. For example, high-purity water can be cooled to -40 ℃ under specific conditions and still remain liquid. Its essence is that liquid molecules fail to form stable structures through crystal nuclei, and thermodynamic energy is retained in the solution in the form of latent heat.
The impact of supercooling on measurement results is mainly reflected in two aspects: firstly, the temperature of the solution cannot rise to the true solidification point after supercooling, resulting in a lower measured solidification point. According to the formula for calculating solute molar mass
M2=ΔTf⋅W1Kf⋅W2， When the decrease in freezing point Δ Tf is too large, the calculated solute molar mass M2 is too small, resulting in a lower measured osmotic pressure; Secondly, when the supercooling process is too large, excessive solvent precipitation will change the concentration of the solution, further exacerbating measurement errors.
The strategy for controlling supercooling phenomenon needs to start from two aspects: cooling rate and crystallization induction. Firstly, a graded cooling method is adopted, which first lowers the temperature to near the solidification point at a rate of 2 ℃/min, and then switches to a slow cooling rate of 0.5 ℃/min to give the molecules sufficient rearrangement time. For example, when measuring plasma osmotic pressure, this method can control the degree of supercooling within 0.2 ℃. Secondly, crystallization is induced by mechanical disturbance or the addition of seed crystals. When the temperature drops near the solidification point, a glass rod is used to rub the inner wall of the container or a small amount of air bubbles are injected to provide heterogeneous nucleation sites, or 0.01% of the same solute is added as the seed crystal to reduce the nucleation energy barrier. In addition, optimizing the design of sample containers by using glass tubes with rough inner walls or pre-set metal mesh can increase the probability of crystal core formation and reduce dependence on external disturbances.