Drop (unit)


The drop is an approximated unit of measure of volume, the amount dispensed as one drop from a dropper or drip chamber. It is often used in giving quantities of liquid drugs to patients, and occasionally in cooking and in organic synthesis. The abbreviations gt or gtt come from the Latin noun gutta.
The volume of a drop is not well defined: it depends on the device and technique used to produce the drop, on the strength of the gravitational field, and on the viscosity, density, and the surface tension of the liquid.
There are several exact definitions of a "drop":
In organic synthesis, a synthetic procedure will often call for the addition of a reagent "dropwise" with the aid of a syringe or a dropping funnel. The rate of addition for such a procedure is taken to be slow, but is otherwise vague, since one chemist might consider dropwise to be one drop per second, while another might consider five to ten drops per second to be dropwise. Furthermore, the volume of a drop will depend on needle gauge or the dimensions of the glassware. Thus, to improve reproducibility, noting the total amount of time required to add the liquid or another measure of addition rate in the experimental procedure is now considered prudent. In a related usage, the amount of a reagent, whose precise quantity is unimportant, will sometimes be given in terms of the number of drops, often from a glass pipette. In this usage, a drop is typically considered to be approximately 0.05 mL. Giving quantities this way was more common in the past but this practice is now generally considered to be sloppy or unrigorous.

History

In the first decade of the 19th century, the minim, the smallest unit of Apothecary Measure, was promoted by the pharmaceutical and medical establishments as an alternative to the drop. It was noted that the size of a drop can vary considerably depending on the viscosity and specific gravity of the fluid, as well as the size and shape of the vessel from which it is poured. The minim came with a set of procedures for ensuring accurate measurement, specifically, diluting powerful medicines that had previously been measured by the drop, then using a "minimometer" or "minim glass" with minim marks at regular intervals. The minim was defined as one 60th of a fluid dram or one 480th of a fluid ounce. This is equal to about 61.6 μL or 59.2 μL.
Pharmacists have since moved to metric measurements, with a drop being rounded to exactly 0.05 mL. In hospitals, intravenous tubing is used to deliver medication in drops of various sizes ranging from 10 drops/mL to 60 drops/mL. A drop is abbreviated gtt, with gtts used for the plural, often seen on prescriptions. Other sources abbreviate gt for singular, and gtt for plural. These abbreviations come from gutta, the Latin word for drop.