The pharmacist is concerned with liquid measurement from two perspectives.
First, he/she should be concerned with the ability to accurately measure
the components of a prescription preparation. Secondly, he/she must be
concerned with how the patient will measure and deliver an accurate dose
of a liquid medication.
The techniques used to measure liquids are probably the simplest of
the operations related to prescription compounding. At the same time, they
are also the most susceptible to errant selection and unprofessional execution
leading to inaccuracies. In this section, we will consider factors which
influence selection of liquid measurement devices.
Precision volumetric glassware is used to measure and/or deliver exact
volumetric quantities of liquid substances. The capacity of the vessel
(1 ml, 50 ml, 1000 ml, etc.) is inscribed on the vessel, and some types
of devices will have calibration marks for measuring multiple volumes.
The inscriptions TD or TC mean, respectively, "to delivery" and
"to contain". A moment's reflection should indicate the significance
of these designations as they apply to the function of the glassware. Calibrated
pipets, burets, syringes and droppers are T.D. glassware; volumetric flasks
and cylindrical or conical graduates are T.C. glassware although in
practice, graduates are used as T.D. vessels for volumes of 1 ml or more.
Erlenmeyer flasks, beakers, and prescription bottles, regardless of
markings, are NOT volumetric glassware, but are simply containers
for storing and mixing liquids. The designated volume(s) express the approximate
capacity of the vessel.