The original DURAN® burettes are made of highly chemically resistant borosilicate glass 3.3. Their precise scaling makes them particularly suitable for titrimetric purposes. The amount of liquid required for titration, but previously unknown, can be read off very accurately after the reaction has been completed. Adjustment is made to drain ("Ex") for + 20 °C reference temperature. This means that the quantities can be taken exactly according to the scale, since the adhesion of the liquid to the glass was taken into account during calibration. The tolerances for the volume correspond to the error limits of ISO and DIN. DURAN® burettes are available with glass or PTFE keys. Unlike glass keys, PTFE keys do not have to be greased, which simplifies work in the laboratory. Burettes with a batch number and accuracy class AS are supplied with a batch certificate. This certificate documents the mean value obtained from measuring the batch in question, the standard deviation and the day of issue. The batch certificates can also be retrieved online. The batch number consists of four digits. The first two numbers specify the production year, and the following two numbers specify the batch, e.g.: 18.01.