The graduated burette is an instrument used for accurate dosing of measured volumes of a chemical solution. It is most commonly used in experimental analytical chemistry tests, where the dosing of a solution is required. It differs from pipettes in that the amount of solution inside the glass tube is variable.
Correct handling of liquids is one of the main problems in a chemical laboratory. This is due to the fact that many experiments are based on reactions taking place in aqueous solutions, between a sample of a certain substance and reagents. Accurate dosing therefore becomes essential before carrying out experimental tests. For this reason, the burette, which is used for dosing and dispensing liquids, is among the most important instruments in a chemical laboratory. They are crucial when working with samples.
Graduated burette, composition and use
Thetraditional graduated burette consists of a graduated glass tube. This is open at the upper end and fitted with a tap at the lower end. The gradation of the burette's glass tube is usually measured in tenths of a millimetre. In addition, it is fitted with a blue longitudinal strip, known as a 'Schellbach band', which facilitates measurement. There are, however, different gradations in measuring the burette, which distinguish different accuracies. For example, an accuracy of up to 1/20 millilitre distinguishes a class A burette. An accuracy of up to 1/10 of a millilitre distinguishes a class B. The tap attached to the bottom of the glass tube, on the other hand, has the task of keeping the solution inside the glass tube, and allowing accurate dosing. Mohr burettes with a PTFE tap.
Burette with 50 ml capacity
They are produced in numerous sizes, and are used according to their function. For example, a 50 ml capacity is an average size for burettes that are used in the laboratory, or even at home. A 50 ml model is ideal for the titration and precise dispensing of liquid substances. Other models, such as the 100 ml model, are preferred by, for example, motor racing and motorbike enthusiasts, who need to accurately measure cylinder head displacement.
Typologies of burettes
The volumetric burettes do not only consist of traditional models. They are divided into two different types of burettes: analogue, or traditional, and digital, or piston burettes.
Analogue burettes. These are the traditional model of burette, consisting of a graduated glass tube and a tap at its lower end to contain the solution. After zeroing the instrument, and placing it on the stand, the tap is operated to dispense the measured amount of solution.
Digital volumetric meters. They are based on the model of a syringe, and are characterised by the fixed position of the solution container. Suction of the solution is automatic, while its dispensing can be manual or motorised. The volume of the solution is shown in the display, and unlike analogue volumetric burettes, they do not require resetting. Consequently, digital volumetric burettes allow a more accurate estimation than analogue ones.
The Schilling model consists of a particular type of instrument. It consists of a bottle, usually 1 L and made of PE, a filling tube, a burette calibrated to zero, a discharge nozzle and a quick discharge button. The adjustment of the discharge nozzle allows the discharge rate to be adjusted from a large quantity down to the drop. The burette is filled simply by applying pressure to the bottle in the reservoir. As soon as the bottle is released, liquid is drawn in until the burette is set to zero. The excess liquid returns through the filling tube to the bottle.
The automatic is a further model of burette, which is used for series testing. It is connected to a bottle containing the titration solution. Air is pumped into the bottle by a small rubber pump, which lifts the solution to the top of the burette. When it is full, the valve is released, the pressure in the bottle drops and the burette automatically sets itself to zero. Working with automatic ones is much faster and the consumption of standard solution is lower.
Pellet is a special model of automatic burette. It retains the general characteristics of an automatic model, such as the presence of an air pump. However, it is distinguished by the fact that it is applied on the neck of the bottle, and not remotely applied via a hose to the reservoir.