If you still want the concentration as %v/v you can still weigh the Tween and compensate the weight with the density of the Tween 20 or Tween 80.

Tween 20 has a density of 1.095g/ml and Tween 80 has a density of 1.064g/ml so for 1litre of a 1%v/v solution you would weigh 10.95g and 10.64g respectively.

Take the mass of tween 80 corresponding to that volume. Use density of tween 80 to calculate it.

The answer to the question of how to dose the required volumes of Tween 20 or Tween 80 depends on what measurement accuracy is sufficient and whether these are small or large volumes.

1. If these are large volumes and high measurement accuracy is required, weighing is the most accurate and easiest, but you need to have an appropriate scale.

2. If these are small volumes, up to several mL and high measurement accuracy is not required, you can use a regular medical syringe with an appropriate tip (needle or tip of a manual automatic pipette).

3. If these are small volumes, up to several mL, but greater measurement accuracy is required, you can use a manual automatic pipette. This means that we take the appropriate volume of Tween 20 with such a pipette, to the appropriate, preferably slightly larger tip. Tween 80 is already too thick, but you can first quantitatively dilute it many times. And this volume, sucked into the pipette tip, is dosed into the prepared intermediate or target vessel, and the remaining Tween in the tip is washed with e.g. water, and the rinsings are added to this vessel.

4. If we can use Tween not in its original, initial concentration, but diluted many times, it is best to do it this way. And then use the usual methods of dosing the volume, possibly with the modifications described above.

Greetings

ZJ

Prepare 5%, then use serial dilution (C1V1 = C2V2)to prepare lower concentrations

I agree with Patrick Mccabe , Zbigniew Jońca and Ngueliatou Morain that weighing can be the most appropriate approach if the densities are known and this fits current routine practice in the particular laboratory (time can be an important factor).

Otherwise you could use volumetric apparatus that (by design or application) can be used 'to contain' as opposed to 'to deliver'. Classical glass pipettes (at least) are marked accordingly; a pipette 'to deliver' has to be flushed out with diluent. There exist many websites on the subject.