When using a volumetric pipet calibrated for "to deliver" (TD), the intent is to ensure accurate delivery of a specific volume of liquid. After the pipet is drained, a small amount of liquid remains in the tip due to surface tension and the characteristics of the liquid.
This residual liquid is not removed after draining, as it is considered an inherent part of the measurement process. The design of TD pipets anticipates this remaining volume, and the pipet is calibrated accordingly. This means that when you fill a TD pipet and then drain it into a receiving vessel, the volume delivered accounts for the residual liquid left in the tip. It is understood that this liquid is part of the total volume intended to be delivered, and thus it is not physically removed or measured separately.
In practical laboratory applications, technicians operating with TD pipets should keep this in mind to ensure accurate delivery of solutions, as disregarding the residual liquid could lead to errors in measurements or concentrations.