The drop is an approximated unit of measure of

volume Volume is a measure of occupied three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch). ...

, the amount dispensed as one drop from a

dropper An eye dropper, also called Pasteur pipette or simply dropper, is a device used to transfer small quantities of liquids. They are used in the laboratory and also to dispense small amounts of liquid medicines. A very common use was to dispense e ...

or drip chamber. It is often used in giving quantities of liquid

drug A drug is any chemical substance that causes a change in an organism's physiology or psychology when consumed. Drugs are typically distinguished from food and substances that provide nutritional support. Consumption of drugs can be via inhala ...

s to patients, and occasionally in cooking and in organic synthesis. The abbreviations gt or gtt come from the Latin noun ''gutta'' ("drop"). 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 The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the int ...

, density, and the

surface tension Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension is what allows objects with a higher density than water such as razor blades and insects (e.g. water striders) t ...

of the liquid. Several exact definitions exist: * In medicine, IV drips deliver 10, 15, 20, or 60 drops per ml. Micro-drip sets deliver 60 drops per ml and 10, 15, or 20 drops per ml for a macro-drip set. * Prior to the adoption of the minim in the early 19th century, the smallest unit of fluid measure in the

Apothecaries' system The apothecaries' system, or apothecaries' weights and measures, is a historical system of mass and volume units that were used by physicians and apothecaries for medical prescriptions and also sometimes by scientists."Medicinal-Gewicht, Apothek ...

s of the

United States customary units United States customary units form a system of measurement units commonly used in the United States and U.S. territories since being standardized and adopted in 1832. The United States customary system (USCS or USC) developed from English uni ...

and pre-1824

English units English units are the units of measurement used in England up to 1826 (when they were replaced by Imperial units), which evolved as a combination of the Anglo-Saxon and Roman systems of units. Various standards have applied to English units at ...

was, while inexact, presumed to be equal to 1/60 of a fluid dram or 1/480 of a fluid ounce. In

organic synthesis Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...

, a synthetic procedure will often call for the addition of a reagent "dropwise" with the aid of a syringe or a

dropping funnel A dropping funnel is a type of laboratory glassware used to transfer fluids. They are fitted with a stopcock which allows the flow to be controlled. Dropping funnels are useful for adding reagents slowly, i.e. drop-wise. This may be desirable when ...

. The rate of addition for such a procedure is taken to be slow but is otherwise vague: one chemist might consider dropwise to be one drop per second, another five to ten drops per second (almost a stream). Furthermore, needle gauge or the dimensions of the glassware also affect drop volume. To improve

reproducibility Reproducibility, also known as replicability and repeatability, is a major principle underpinning the scientific method. For the findings of a study to be reproducible means that results obtained by an experiment or an observational study or in ...

, experimental procedures also note the total amount of time required to add the liquid or another measure of addition rate. 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. The practice of giving quantities this way has declined in usage.

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. (At the time,

was not well understood.) 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" (graduated pipette) with minim marks at regular intervals. The minim was defined as one 60th of a

fluid dram The dram (alternative British spelling drachm; apothecary symbol ʒ or ℨ; abbreviated dr) Earlier version first published in '' New English Dictionary'', 1897.National Institute of Standards and Technology (October 2011). Butcher, Tina; Cook, ...

or one 480th of a

fluid ounce A fluid ounce (abbreviated fl oz, fl. oz. or oz. fl., old forms ℥, fl ℥, f℥, ƒ ℥) is a unit of volume (also called ''capacity'') typically used for measuring liquids. The British Imperial, the United States customary, and the United ...

. This is equal to about 61.6 μL (U.S.) or 59.2 μL (Britain). Pharmacists have since moved to metric measurements, with a drop being rounded to exactly 0.05 mL (50 μL, that is, 20 drops per milliliter). 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'' (plural ''guttae''), the Latin word for drop.

References

{{Reflist Units of volume Cooking weights and measures Metricated units

History

See also

References