In chemical analysis, the transfer operation of high viscosity digestion solution is far from simple "suction and discharge", and it involves complex thermodynamic and fluid mechanics principles behind it. From the perspective of fluid mechanics, the intermolecular attraction of high viscosity liquids (such as glycerol and oils) is much greater than that of aqueous solutions, resulting in a significant increase in internal friction during their flow. According to Newton's law of internal friction, shear stress is proportional to velocity gradient. High viscosity liquids have small velocity gradients and high flow resistance, which makes traditional forward pipetting (first suction to the first stage and then discharge) prone to volume errors due to residual material. Reverse pipetting technology involves first suctioning to the second level (excessive suction), releasing only the set volume during drainage, and compensating for the adhesion of the suction head inner wall with excess liquid, effectively improving pipetting accuracy.

On a thermodynamic level, temperature has a significant impact on the viscosity of high viscosity liquids. The viscosity of a liquid decreases with increasing temperature, as the increase in intermolecular distance weakens the intermolecular attraction. For example, at 20 ℃, the viscosity of water is 1cP, while glycerol can have a viscosity of over 1000cP at the same temperature. Moderately preheating the liquid before pipetting can reduce its viscosity and flow resistance, but attention should be paid to avoiding excessive temperature that may cause decomposition or volatilization of the digestion liquid components. In addition, the heat transfer coefficient of high viscosity liquids is relatively low. If the ambient temperature fluctuates greatly during the transfer process, it may cause density changes due to local temperature gradients, further affecting the volume accuracy.

In practical operation, it is necessary to optimize the process based on the type of pipette. Gas piston pipettes rely on air cushions to transmit pressure and are suitable for low viscosity liquids; The external piston type pipette comes into direct contact with the liquid through the piston, without interference from the air column, making it more suitable for high viscosity and volatile liquids. For example, in the transfer of soil heavy metal digestion solution, the use of reverse pipetting combined with an external piston pipette can significantly reduce residual errors and ensure the accuracy of analysis results.