Editing Drop (unit) (section)

{{Short description|Unit of measure of volume}}

[[File:Surface tension, Penny dropper experiment.jpg|thumb|One drop hanging from the dropper.]]
The '''drop''' is an [[approximate measures|approximated unit of measure]] of [[volume]], the amount dispensed as one drop from a [[Pasteur pipette|dropper]] or [[drip chamber]]. It is often used in giving quantities of liquid [[drug]]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]], density, and the [[surface tension]] of the liquid.<ref name="phisics">{{cite web 
| title = Drop - size
| publisher = Physics and Astronomy Online
| url = http://www.physlink.com/education/askexperts/ae681.cfm
| access-date = 2010-03-29
}}</ref>

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.<ref>{{Cite web|url=http://www.cwladis.com/math104/lecture7.php|title = Lecture 7: IV Flow Rate and Infusion/Completion Times}}</ref>
* Prior to the adoption of the unit of the [[Minim (unit)|minim]] in the early 19th century, the smallest unit of fluid measure in the [[Apothecaries' system]]s of the [[United States customary units]] and pre-1824 [[English units]] was, while inexact, presumed to be equal to {{sfrac|1|60}} of a fluid dram or {{sfrac|1|480}} of a fluid ounce.
*Under the modern US customary measurement system, 1 drop is {{sfrac|1|72}} of a [[Dram (unit)#Unit of volume|US customary fluid dram]].

{|
|-
|1 US customary drop&nbsp;
||=
|align=right|{{sfrac|5|6}}||US customary minim
|-
|||=
|align=right|{{sfrac|1|72}}||[[Dram (unit)#Unit of volume|US customary fluid dram]]
|-
|||=
|align=right|{{sfrac|1|288}}||[[Tablespoon#United States|US customary tablespoon]]
|-
|||=
|align=right|{{sfrac|1|192}}||[[Dessert spoon#Culinary measure|US customary dessert spoon]]
|-
|||=
|align=right|{{sfrac|1|96}}||[[Teaspoon#United States customary unit|US customary teaspoon]]
|-
|||=
|align=right|{{sfrac|1|48}}||US customary coffee spoon
|-
|||=
|align=right|{{sfrac|1|24}}||[[Salt spoon|US customary salt spoon]]
|-
|||≈
|align=right|0·87||[[Minim (unit)|British imperial minim]]
|-
|||≈
|align=right|0·0036||[[Tablespoon#United Kingdom|UK tablespoon]]
|-
|||≈
|align=right|0·0072||[[Dessert spoon#Culinary measure|UK dessert spoon]]
|-
|||≈
|align=right|0·014||[[Teaspoon#British culinary measurement unit|UK teaspoon]]
|-
|||≈
|align=right|0·029||[[Salt spoon|UK salt spoon]]
|-
|||≈
|align=right|0·87||UK drop
|-
|||≈
|align=right|0·051||[[Litre#SI prefixes applied to the litre|millilitre]]
|-
|||≈
|align=right|0·0034||[[Tablespoon#International metric|international metric tablespoon]]
|-
|||≈
|align=right|0·0026||[[Tablespoon#Australian metric|Australian metric tablespoon]]
|-
|||≈
|align=right|0·0051||[[Dessert spoon#Culinary measure|metric dessert spoon]]
|-
|||≈
|align=right|0·01||[[Teaspoon#Metric teaspoon|metric teaspoon]]
|}

* In the [[United Kingdom]], subsequent to the adoption of the minim and the creation of the [[Imperial units|British imperial system of units]] in the 1820s, a drop is defined as 1 [[Minim (unit)|British imperial minim]],<ref>[https://archive.org/details/bookofhouseholdm0000mrsi/page/40/mode/2up Page 40], ''The Book of Household Management'' (1861)</ref> the equivalence of {{sfrac|1|60}} of a [[Dram (unit)#Unit of volume|British imperial fluid drachm]] or {{sfrac|1|480}} of a [[Fluid ounce|British imperial fluid ounce]].

{|
|-
|1 UK drop&nbsp;
||=
|align=right|1||British imperial minim
|-
|||=
|align=right|{{sfrac|1|60}}||British imperial fluid drachm
|-
|||=
|align=right|{{sfrac|1|240}}||[[Tablespoon#United Kingdom|UK tablespoon]]
|-
|||=
|align=right|{{sfrac|1|120}}||[[Dessert spoon#Culinary measure|UK dessert spoon]]
|-
|||=
|align=right|{{sfrac|1|60}}||[[Teaspoon#British culinary measurement unit|UK teaspoon]]
|-
|||=
|align=right|{{sfrac|1|30}}||[[Salt spoon|UK salt spoon]]
|-
|||=
|align=right|{{sfrac|1|480}}||British imperial fluid ounce
|-
|||≈
|align=right|0·96||[[Minim (unit)|US customary minims]]
|-
|||≈
|align=right|0·016||[[Dram (unit)#Unit of volume|US customary fluid dram]]
|-
|||≈
|align=right|0·004||[[Tablespoon#United States|US customary tablespoon]]
|-
|||≈
|align=right|0·006||[[Dessert spoon#Culinary measure|US customary dessert spoon]]
|-
|||≈
|align=right|0·012||[[Teaspoon#United States customary unit|US customary teaspoon]]
|-
|||≈
|align=right|0·024||US customary coffee spoon
|-
|||≈
|align=right|0·048||[[Salt spoon|US customary salt spoon]]
|-
|||≈
|align=right|1·15||US customary drops
|-
|||≈
|align=right|0·059||[[Litre#SI prefixes applied to the litre|millilitre]]
|-
|||≈
|align=right|0·0039||[[Tablespoon#International metric|international metric tablespoon]]
|-
|||≈
|align=right|0·003||[[Tablespoon#Australian metric|Australian metric tablespoon]]
|-
|||≈
|align=right|0·006||[[Dessert spoon#Culinary measure|metric dessert spoon]]
|-
|||≈
|align=right|0·012||[[Teaspoon#Metric teaspoon|metric teaspoon]]
|}

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: 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]], 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.