Plastic Volumetric Flasks

Choose polypropylene when breakage is the real risk or when the chemistry rules glass out. A school class, a field kit, a busy quality-control bench and anything involving hydrofluoric acid or hot strong alkali all point to plastic. Stay with glass when you need Class A accuracy with a certificate, when you want to see the meniscus at its sharpest, or when the solution has to be warmed at any stage.

The same rule as glass: a volumetric flask measures one volume only, so buy the size you actually intend to make up. For school work 100 mL, 250 mL and 500 mL cover almost everything. Because plastic flasks cost less and do not break, it is worth buying a full class set of one or two sizes rather than a scattered handful across many volumes.

Polypropylene flasks are Class B. At 250 mL that is a tolerance of roughly ±0,30 mL, about double a Class A glass flask and still far better than any measuring cylinder. For teaching, routine dilutions and field sampling that is comfortably enough. If a method or an accreditation demands Class A with a batch certificate, that only comes in glass.

Do not heat them and do not autoclave them with the stopper fitted. Polypropylene distorts well below the temperature borosilicate tolerates, and a flask that has been warmed has quietly lost its calibration whether or not it looks changed. Wash with warm water and mild detergent, never an abrasive pad, because scratches cloud the neck and make the ring hard to read.

The same job as a glass one: making a solution up to a single exact volume. Dissolve the solid, transfer it in, rinse the beaker into the flask, then add solvent until the bottom of the meniscus rests on the ring. Cap it and invert about ten times. The reason to reach for plastic rather than glass is that it will survive a school laboratory, a field trip or a reagent that attacks glass.

Yes, to Class B. At 250 mL that is a tolerance of about ±0,30 mL. That is roughly twice the uncertainty of a Class A glass flask and many times better than a measuring cylinder, which at the same volume is around ±2 mL. For teaching, routine dilutions and sample preparation it is more than adequate. Class A with a batch certificate is only available in glass.

Treat them as not heatable. Polypropylene softens and distorts at temperatures borosilicate handles without trouble, and any distortion changes the volume the flask contains, permanently. Never put one on a hotplate or in a drying oven. If a solution warmed up while dissolving, let it cool to room temperature before making up to the mark.

Hydrofluoric acid is the obvious one: it dissolves silica, so it cannot be stored or measured in glass at all. Hot concentrated sodium and potassium hydroxide also etch borosilicate over time, frosting the surface and eventually blurring the calibration ring. Polypropylene is unaffected by both. It is less suited to strong oxidisers and to some organic solvents, so check compatibility before use.

Rinse straight after use, wash in warm water with a mild laboratory detergent, rinse several times with distilled water and stand it inverted to drain. Never use an abrasive pad or a stiff brush: polypropylene scratches far more readily than glass, and a scratched neck makes the calibration ring genuinely hard to read. Air dry only, never in an oven.