ANALYTICAL & LABORATORY EQUIPMENT
ADVANTAGEOUS ACCESSORIES
Matthias Stein introduces some useful accessories for bottle-top dispensers in special applications
B
ottle-top dispensers and bottletop burettes enable liquid handling directly from bottles, canisters or other containers. Te bottle-
top concept saves time in the laboratory and ensures greater safety since no intermediate steps are required. For many laboratory applications, bottle- top instruments can be used universally. Tere are several innovative solutions for applications that are particularly challenging due to the media or container being used.
DRYING TUBE
Bottle-top dispensers and bottle-top burettes come with a ventilation channel, which ensures that the pressure inside the media bottle is the same as the ambient pressure. If the inside of the bottle were to be hermetically sealed from the ambient
atmosphere, a vacuum would be created in the bottle as removal of the medium increases. Depending on the vapour pressure of the medium, it would begin to evaporate as a result. At the very least, the dispensing process would be impaired by the vacuum in the bottle. Te drying tube is used when the medium must be protected from the penetration of moisture or carbon dioxide from the air. Te drying tube is a plastic tube with a screwed-on elbow piece and removable closure cap with hose nozzle. Te tube is screwed onto the valve block of the Dispensette or Titrette where the ventilation channel is sealed with a perforated plug on delivery. Te drying tube must be filled with
a solid reagent that is suitable for the application. Adding a bit of mineral wool into the tube is recommended as a filter material so that the solid material or its dust cannot enter the bottle.
A drying tube can be screwed onto the valve block of the Dispensette
18
www.scientistlive.com
THE FUNCTIONS OF THE DRYING TUBE Binding water is a primary function of the drying tube. Water content in organic solvents that increases over time can have a disturbing effect on the chemical reaction or the analytical procedure. In hygroscopic aqueous media, such as concentrated sulphuric acid, the concentration changes. Silica gel (made from silicone dioxide) and a molecular sieve with pore size 4Å (made of zeolites), used individually or in combination, are a good drying agent for air since they do not dissolve when the water content
Dispensette S Digital bottletop dispenser
increases. Silica gel in combination with a humidity indicator is also referred to as blue gel or orange gel. As the water content changes, the changing colour makes it easy for the user to tell when the drying tube filling has expired. Binding CO2
is also a key function. When CO2 from the air dissolves in water,
carbonic acid forms. If the solution is a base, for example an aqueous sodium or potassium hydroxide solution, the CO2
is
bound as a carbonate, which changes the content of the base via a neutralisation
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60
