Advantageous accessories

Matthias Stein introduces some useful accessories for bottle-top dispensers in special applications

Bottle-top dispensers and bottletop burettes enable liquid handling directly from bottles, canisters or other containers. The 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. There 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. The drying tube is used when the medium must be protected from the penetration of moisture or carbon dioxide from the air. The drying tube is a plastic tube with a screwed-on elbow piece and removable closure cap with hose nozzle. The tube is screwed onto the valve block of the Dispensette or Titrette where the ventilation channel is sealed with a perforated plug on delivery.

The 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.

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 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 reaction. This is particularly a problem when the base solution is a titration medium, since titrating a solution with an unknown content leads to erroneous results. In addition, carbonates that are by comparison poorly soluble can break down in the solution – for example in an ethanolic potassium hydroxide solution. In turn, the precipitates then form salt crystals which can wear out the piston of the instruments faster and lead to leaky ball valves. To bind the CO2 from the air, granulated absorbers such as soda lime can be used. Soda lime consists primarily of calcium hydroxide and sodium hydroxide, with an indicator also mixed in.

A third function of the drying tube is binding odours. The Dispensette is a laboratory instrument and should therefore also be operated in a laboratory environment that includes technical ventilation with sufficient air exchange. In practice, however, local and temporary exposure to strong smelling compounds may occur. A drying tube filled with activated carbon can provide relief in these situations.

Venting stopper for microfilters

If scientists use the Dispensette bottle-top dispenser to dispense sterile media, for example nutrient solutions for cultivating microorganisms, the instrument should first be autoclaved. Instructions on preparing instruments can be found in the operating manual. The venting stopper
for microfilters with Luer cone is used to attach a microfilter over the Luer cone.

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