Drop (unit)
{{Short description|Unit of measure of volume}}
File:Surface tension, Penny dropper experiment.jpg
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 ("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.{{cite web
| title = Drop - size
| publisher = Physics and Astronomy Online
| url = http://www.physlink.com/education/askexperts/ae681.cfm
| access-date = 2010-03-29
}}
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.{{Cite web|url=http://www.cwladis.com/math104/lecture7.php|title = Lecture 7: IV Flow Rate and Infusion/Completion Times}}
- Prior to the adoption of the unit of the minim in the early 19th century, the smallest unit of fluid measure in the Apothecaries' systems 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 US customary fluid dram.
| 1 US customary drop | =
|align=right|{{sfrac|5|6}} | US customary minim |
| =
|align=right|{{sfrac|1|72}} | US customary fluid dram | |
| =
|align=right|{{sfrac|1|288}} | US customary tablespoon | |
| =
|align=right|{{sfrac|1|192}} | US customary dessert spoon | |
| =
|align=right|{{sfrac|1|96}} | US customary teaspoon | |
| =
|align=right|{{sfrac|1|48}} | US customary coffee spoon | |
| =
|align=right|{{sfrac|1|24}} | US customary salt spoon | |
| ≈
|align=right|0·87 | British imperial minim | |
| ≈
|align=right|0·0036 | UK tablespoon | |
| ≈
|align=right|0·0072 | UK dessert spoon | |
| ≈
|align=right|0·014 | UK teaspoon | |
| ≈
|align=right|0·029 | UK salt spoon | |
| ≈
|align=right|0·87 | UK drop | |
| ≈
|align=right|0·051 | millilitre | |
| ≈
|align=right|0·0034 | international metric tablespoon | |
| ≈
|align=right|0·0026 | Australian metric tablespoon | |
| ≈
|align=right|0·0051 | metric dessert spoon | |
| ≈
|align=right|0·01 | metric teaspoon |
- In the United Kingdom, subsequent to the adoption of the minim and the creation of the British imperial system of units in the 1820s, a drop is defined as 1 British imperial minim,[https://archive.org/details/bookofhouseholdm0000mrsi/page/40/mode/2up Page 40], The Book of Household Management (1861) the equivalence of {{sfrac|1|60}} of a British imperial fluid drachm or {{sfrac|1|480}} of a British imperial fluid ounce.
| 1 UK drop | =
|align=right|1 | British imperial minim |
| =
|align=right|{{sfrac|1|60}} | British imperial fluid drachm | |
| =
|align=right|{{sfrac|1|240}} | UK tablespoon | |
| =
|align=right|{{sfrac|1|120}} | UK dessert spoon | |
| =
|align=right|{{sfrac|1|60}} | UK teaspoon | |
| =
|align=right|{{sfrac|1|30}} | UK salt spoon | |
| =
|align=right|{{sfrac|1|480}} | British imperial fluid ounce | |
| ≈
|align=right|0·96 | US customary minims | |
| ≈
|align=right|0·016 | US customary fluid dram | |
| ≈
|align=right|0·004 | US customary tablespoon | |
| ≈
|align=right|0·006 | US customary dessert spoon | |
| ≈
|align=right|0·012 | US customary teaspoon | |
| ≈
|align=right|0·024 | US customary coffee spoon | |
| ≈
|align=right|0·048 | US customary salt spoon | |
| ≈
|align=right|1·15 | US customary drops | |
| ≈
|align=right|0·059 | millilitre | |
| ≈
|align=right|0·0039 | international metric tablespoon | |
| ≈
|align=right|0·003 | Australian metric tablespoon | |
| ≈
|align=right|0·006 | metric dessert spoon | |
| ≈
|align=right|0·012 | 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.
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.{{cite book|last=Nicholson|first=William|author-link=William Nicholson (chemist)|title=The British encyclopedia, or Dictionary of arts and sciences comprising an accurate and popular view of the present improved state of human knowledge|url=https://books.google.com/books?id=jYAPAAAAQAAJ&pg=PT264|access-date=18 December 2011|year=1809|publisher=Whittingham|page=264}} 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, surface tension 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 or one 480th of a fluid ounce.{{cite book|author1=Royal College of Physicians of London|author2=Richard Powell|title=The pharmacopoeia of the Royal College of Physicians of London, M. DCCC. IX|url=https://books.google.com/books?id=pCPrAAAAMAAJ&pg=PA6|access-date=18 December 2011|year=1809|publisher=Longman, Hurst, Rees, and Orme|pages=6–7}} 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.{{cite web|url=http://www.dictionary.com/browse/gutta|title=Gutta - Define Gutta at Dictionary.com}}{{cite web|url=http://medical-dictionary.thefreedictionary.com/gutta|title=gutta|via=The Free Dictionary}} These abbreviations come from gutta (plural guttae), the Latin word for drop.{{cite book|author=Hugh Cornelius Muldoon|title=Lessons in pharmaceutical Latin and prescription writing and interpretation|url=https://archive.org/details/lessonsinpharma00muldgoog|access-date=8 March 2012|year=1916|publisher=John Wiley & sons, inc.|page=[https://archive.org/details/lessonsinpharma00muldgoog/page/n160 147]}}{{cite journal |author=McGarry RC, McGarry P |title=Please pass the strychnine: the art of Victorian pharmacy |journal=Canadian Medical Association Journal |volume=161 |issue=12 |pages=1556–8 |year=1999 |pmid=10624415 |pmc=1230877}}