Policeman (laboratory)
A policeman is a hand-held flexible natural-rubber or plastic scraper. The common type of it is attached to a glass rod and used in chemical laboratories to transfer residues of precipitate or solid on glass surfaces when performing gravimetric analysis. This equipment works well under gentle, delicate and precise requirement. A policeman also comes in various sizes, shapes, and types. Some of them come in one-piece flexible plastic version and some in stainless. The origin of the policeman and its name cannot be identified for sure but some clues led back to the 19th century from the chemist named Carl Remigius Fresenius.
Origin
In chemistry, gravimetric analysis is essential. After precipitating the chemical element of interest, successfully transfer all of the precipitate to the filtration funnel for separation from the supernatant liquid is required. Normally, it used to be done by using a stream of distilled water from a wash bottle. However, such a way is ineffective because dense precipitates may become compacted at the bottom of the beaker or, in the case of light precipitates, may become dispersed on the walls of the beaker. Therefore, by using glass rod to take them off may result in poking a hole in the bottom of the beaker or scratching of the beaker wall. In the 19th-century, German chemist, Carl Remigius Fresenius suggest the solution to overcome this problem.
Then rubber policeman also recorded in 1910 edition of J. C. Olsen’s textbook of quantitative analysis that states "...particles adhering to the glass must be removed by means of a so-called policeman, which is made by inserting the end of a rather thick large-sized glass stirring-rod into a short piece of rubber tubing. The rubber tube should be left protruding slightly beyond the end of the glass tube and sealed together with a little bicycle [i.e. rubber] cement."
However, it seemed that Olsen has nothing to do with the production and sales of this invention. Instead, Oesper Collections catalog indicated that policemen sold by the Henry Heil Company of St. Louis as early as 1904.
There is no answer on where the name "policeman" comes from. However, there are two assumptions:
- It likes police that protected the beaker from scratching.
- It likes the police that gathering up any stray or escaped particles of precipitate on the beaker wall.
The second speculation is the most likely the one since in the 1937 edition of Hackh’s Chemical Dictionary “platinum policeman,” defined as “a platinum-iridium claw that fits over a glass rod and is used to hold a quantitative filter during ignition,” which the purpose of the policeman was to prevent the escape of stray filter paper from the crucible during the ignition process that causes from thermal updrafts from the burner. Therefore, for policeman, it likely means to prevent the escape of stray precipitate.[1]
Structure
A policeman is generally a flexible natural-rubber attached to a 5mm and 6mm diameter of 15 cm long glass rod. However, it also comes in various size and shapes depending on its uses. This rubber also provides a chemical resistant. In some of the designs, there is no need for a glass rod, instead, the whole piece makes of plastic or stainless and at the end, it has either spatula or scarpers shape.
Uses
A policeman can be used for cleaning the inside of glassware, or for getting the last bit of precipitate out of a vessel. Especially in chemical laboratories it often uses to transfer residues of precipitate or solid on glass surfaces when performing the gravimetric analysis.
It also used in biological laboratories, to transfer tissue culture cells from a plate to a suspension. Their feature is to prevent the glass rod from scratching or breaking glassware.
Applications
These are some examples of the experiment that use the policeman.
Gravimetric Analysis of a Soluble Chloride
The experiment intends to introduce the student to the technique called Gravimetric Analysis by using the Chloride as an example. This technique helps determine amount of an analyte by the measurement of mass.[2]The policeman uses here to scrape down any of the precipitates that have dried on the sides of the beaker or the glass stirring rod, so that 100% of the precipitate will be used for the Gravimetric Analysis[3]
Isolation of Casein from Milk
The policeman can be used for stirring the mixture in the fragile containers and for draining the solid substance. As in the experiment called Isolation of Casein from milk which has the objective to isolate chemical substances found in milk. A policeman uses to stir the mixture of milk and acetic acid. Then it uses to push the casein up onto the side of the beaker so that most liquid drains from the solid.[4]
Cell Lysates Preparation
In the cell lysates preparation, it requires a very gentle procedure for preparing them after transfection of cells growing in well plates. The policeman which is soft and flexible uses for a scrape or detach those cells for transferring or observe. [5]
See also
References
- ^ Jensen, William B (2008). "The Origin of the Rubber Policeman" (PDF). Journal of Chemical Education. 85 (776). doi:10.1021/ed085p776. Retrieved 13 October 2017.
- ^ "Gravimetric Analysis". WeirdChemist. Retrieved 13 October 2017.
- ^ "Gravimetric Analysis of a Soluble Chloride" (PDF). University of Kentucky. Retrieved 13 October 2017.
- ^ Pavis, Donald L.; Lampman, Gary M.; Kris, George S. (2005). Introduction to Organic Laboratory Techniques: A Small Scale Approach (2 ed.). Belmount, CA, USA: Davis Haris. p. 464. ISBN 0534408338.
{{cite book}}:|access-date=requires|url=(help) - ^ Deb, Sumitra; Deb, Swati Palit (2003). Methods in Molecular Biology. Totowa, New Jersey: Humana Press Inc. p. 98. ISBN 1588291065.
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