FAQ Laboratory Glassware
- Generally, taper joint sizes are designated by two numbers separated by a slash. The first number is the diameter of the ground part of the joint at the top (largest diameter), expressed in millimeters, while the second number is the length of the ground part. For example, a 24/40 joint is 24 mm at the top (widest diameter), and 40 mm long. This means that if the first numbers of a male and female set are not the same, those joints will not fit together. Conversly if the numbers of the two joints are the same, then they will fit together. For example, a 24/25 male joint will fit into a 24/40 female joint, despite the lengths being different.
Class A glassware provides the highest accuracy in Volumetric Glassware. It is permanently marked "A" and is guaranteed to comply with Class A volumetric tolerances defined in DIN EN ISO. It is also supplied with a serialized certificate of precision.
Volumetric glassware that is permanently marked "B" has volumetric tolerances twice those of Class A (with the exception of Graduated Cylinders). These tolerances comply with Class B tolerances as defined in DIN EN ISO.
For glass-to-glass joints, immerse the joints in a glass container of freshly poured carbonated liquid. Let them soak in the solution for about 15 minutes to allow the liquid to penetrate. Remove the joints, rinse with water and then wipe. Wearing heat-resistant gloves gently heat the two joints by rotating them over a Bunsen burner flame for about 20 seconds. Make sure that at least 50% of the joints are moist prior to heating. After removing from the flame, gently twist the joints apart. If this does not work, then repeat the procedure. Never force the two joints apart when using this method.
For PTFE-to-glass connections, place the piece of glassware containing the PTFE part (stopcock, stopper, etc.) into an ice bath for about 15-20 minutes. This causes the PTFE to contract allowing the parts to be separated.
Fritted filters and fritted glassware should be washed by suction with hot hydrochloric acid and then rinsed with water before use. This treatment will remove loose particles of foreign matter such as dust.
It is advisable to clean fritted glassware as soon as possible after use.
Remove all residues from the frit and dispose of these in appropriate waste containers.
Scrub off any remaining residue with a brush, taking care not to damage the frit. Clean the frit using appropriate solvents (water, acetone etc.) and vacuum filtration.
Precipitates can also be removed from the fritted glassware by rinsing from the reverse side with water under pressure not exceeding 1 BAR (15 PSI). Soak the frit overnight in an appropriate cleaning solution. Always make sure that your cleaning solution is compatible with the chemical residues on your frit! Use vacuum filtration to wash the frit thoroughly with distilled water, dilute ammonium hydroxide (if you used acid as the cleaning solution) and distilled water.
Volumetric instruments of class A and AS have identical error limits as established by DIN EN ISO. For class AS volumetric instruments, calibrated to deliver (TD, Ex), the additional ‘S’ means swift delivery.
The delivery time is defined as the period of time required for the free fall of the meniscus (discharge of water due to gravity) from the upper volume mark to reach the lower volume mark or the tip. This is related to the defined waiting time for class AS volumetric instruments. In class AS pipettes the delivery time is much faster than in class A pipettes (see example below).
The waiting time is the time taken from when the meniscus comes to rest at the lower volume mark or in the tip, until the residual liquid has finished flowing down from the glass wall of the pipette into the vessel.
Waiting time for class AS: The waiting time for class AS bulb and graduated pipettes is 5s which is indicated on the pipette by the manufacturer. The pipette should not be removed from the receiving vessel until the time for the meniscus to come to rest in the tip/lower volume mark. has elapsed.
Class A pipettes have no waiting time and thus compensate for the slower delivery time. The following are delivery and waiting times for different classes of 25 ml bulb pipettes:
Class A Class AS Delivery time: 25-50 s Delivery time: 15-20 s Waiting time: 0 Waiting time: 5 s