Troubleshooting: Weighing Reactants and Reagents

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Problem: I forget to tare my flask.

Solution: For all of your flasks, write the weight twice on the inside of the joint in pencil. The pencil will eventually rub off, but since the tare is written twice, having one blur is no problem. Then you can erase the other and re-tare, so the weight is always accurate.

Problem: I need to weigh 0.5 mg of catalyst. The balance is not accurate for such small weights.

Solution: Make a solution of (say) 5 mg of the solid in (say) 1 mL of solvent, and then syringe out 100 microliters.

Problem: I do not know the density of a liquid- how do I know how much to add via syringe?

Solutions:

Problem: When I try to use a syringe to measure a certain liquid, nothing happens because it is too viscous to be drawn into the syringe.

Solutions:

Problem: I have a small amount of an oil (say, 40 mg) in a flask. I only want to use some of it (say 10 mg), but I do not know how to weigh it accurately.

Solution: For the example given, dissolve the material in 4 mL of solvent, and remove 1 mL. You can remove the solvent to check the weight. This method is more accurate than you might think.

Problem: I want to weigh a hygroscopic solid, but it "melts" as I weigh it, and then I can not easily tranfer it to my reaction.

Solutions:

Problem: I want to weigh a reagent with a melting point near room temperature. It keeps changing states on me- it clogs a syringe, but melts on weighing paper! (t-BuOH is one example).

Solution: Warm it up slightly (if it is heat- stable) and syringe quickly, before it cools enough to solidify.

Problem: I want to syringe a liquid that evaporates at room temperature, and I never get an accurate volume.

Solution: Cool it to a low temperature before syringing it.

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