The Pharmaceutics and Compounding Laboratory
Accuracy in Measurements

Nonvolumetric Glassware and Devices


Erlenmeyer Flasks, Beakers, and Prescription Bottles

Erlenmeyer flasks, beakers, and prescription bottles are not volumetric devices and should NOT be used to measure liquids. "Graduation" marks on such vessels are only approximations of liquid capacity. These devices should only be used to mix or store solutions or other liquid preparations, unless you first calibrate them to a known volume.

Teaspoon

The Teaspoon and the problem concerning the actual volume of liquid contained in a "teaspoonful" has existed for many years. Despite various recommendations, the problem persists. Patients continue to self-administer liquid doses from spoons which, in all probability, are far from standard.

The U.S. Pharmacopoeia/National Formulary (USP/NF) recognizes the problem and provides the following statement: "Agreement has not been reached on a standard official teaspoonful, in spite of the need for such a standard measure in connection with compounding and labeling liquid medicines. For household purposes, an American Standard Teaspoon has been established by the American Standards Association as containing 4.95 ± 0.24 ml. In view of the almost universal practice of employing teaspoons ordinarily available in the household for the administration of medicine, the teaspoon may be regarded as representing 5 ml." Household measures are, however, far from standard. Teaspoons may be found with capacities ranging from 3 to 7 ml and the volume of liquid which may be held in a given teaspoon varies with the viscosity and surface tension of the liquid. Thus, the dosage becomes an almost unpredictable variable. Errors in dosage can be minimized by:

  • Writing and formulating all prescriptions using the metric system of measure.
  • Using the symbol for one fluid dram in the dosage instructions to represent one teaspoonful.
  • Requesting physicians to designate on the prescription the number of ml or mg desired in each dose.
  • Dispensing a standard calibrated teaspoon, oral syringe, or other calibrated measuring device with each prescription for an oral liquid where such a dosage is used.