[masspecs]

I'd like to take a quick survey if I could. For all who routinely create diluted solutions using volumetric flasks, I'm sure you've observed volumes changing sometimes due to chemical interactions (with the meniscus going well above or below). In instances like these, do you prefer to do weight-to-weight dilutions instead of weight-to-volume and volume-to-volume? For example, I created a 20% solution (w/v) of sodium bicarbonate in water. After mixing, the meniscus dwindled well below the line. Should the solution be diluted to the line again in situations like this? Should a weight-to-weight solution be used instead?

[Babcock_Hall]

While we wait for the true analytical chemists to respond, I can say that I was under the impression that the final volume was the critical one.  In other words it is the volume of the solution.  My own opinion is that it is very important to specify exactly what one means when using percentages.  If I mix 70 mL of acetonitrile with 30 mL of water, the final volume is noticeably less than 100 mL.  The way that I understand the definition of a 70% volume to volume aqueous solution of acetonitrile is to take 70 mL of acetonitrile to a final volume of 100 mL with water, regardless of the exact volume of water.

[AWK]

I created a 20% solution (w/v) of sodium bicarbonate in water

Really?

[jeffmoonchop]

Maybe your solution is undergoing an endothermic or exothermic reaction. With dissolution usually, it's endothermic, as with the mixture of certain solvents, which can cause contraction or expansion of liquids. Perhaps you need to wait until the solution has returned to equilibrium before considering adding more.

[Borek]

Add enough solvent to make something like 95-99% of the flask volume, mix thoroughly, let it equilibrate the temperature with the surroundings, fill up to mark, mix again. This way final change of the volume will be negligible.

[Corribus]

We do everything gravimetrically.

[masspecs]

I created a 20% solution (w/v) of sodium bicarbonate in water

Really?

I misspoke. Sodium carbonate--not bicarbonate.

[shchavel]

I agree with Borek. It's the right way. In our lab we do the same.