I am a undergraduate student in chemistry, and my university is very weak in teaching solution preparation. I need to prepare a 10 mg / L methylene blue solution and now the way I do is, using an analytical balance of 5 decimal places, I tare a 50 mL beaker and simply add the 10 mg of methylene blue, trying be as accurate as possible (the amount is very small due to the large molecular weight). Afterwards, I add deionized water to a certain volume and transfer to a 1 L volumetric flask using a funnel (I wash the small beaker several times to guarantee that I transfer as much as possible of the compound) and complete with deionized water until the meniscus line. And so, I shake it many times.

Is this procedure correct? If not, how can I prepare this solution avoiding errors?

  • $\begingroup$ I like the final part about shake many times, years ago when I was a young academic I had a student who put an iron solution in a large volumetric flask. The student then added distilled water and topped it up to the mark. The student then used a small short pipette to take a sample. The student could not understand why there was no iron in the sample they had taken. I then told the student to give the volumetric flask a mighty shake. Then they could find iron in the liquid. $\endgroup$ Jul 5 '18 at 10:32
  • $\begingroup$ @Macro G. If you are to make a solution of 10 mg/liter, then what does the molecular weight have to do with the mass? Why would you say that the amount is very small due to the large molecular weight? The amount is 10 mg which is not "very small". $\endgroup$
    – Dr. J.
    Jul 5 '18 at 10:50
  • $\begingroup$ @Dr.J. I think I didn't make it clear enough, my mistake. What I was trying to say is that due to the fact that methylene blue has a large molecular weight, the amount of actual solid particles I have to take from the reagent bottle and transfer it to the beaker is so little that it is hard to nail the mark of 10 mg in the balance scale. English is not my native language, sometimes is hard to express what you really is thinking. Thanks. $\endgroup$
    – Macro G.
    Jul 5 '18 at 14:35

Methylene blue isn't a primary standard so I'm not sure how much accuracy/precision you need.

  • Typically when wanting the best accuracy the solid would be dried in a hot oven, then cooled in a desiccator.

  • Do not to handle the beaker with your hands. Even if it has been "left out" in humid air that amount of water would be stable. So use tongs to pull in and out of balance.

  • Another option is to weigh more and do a serial dilution. So 1 gram in liter, then 10 ml of that solution into 1 liter.

  • $\begingroup$ Not that much of precision since it's going to be used for plasma degradation tests and the kinectics will be measuring its rate of degradation by time, (Co-Ct)/Co vs. Time. (Co being its initial concentration, Ct the concentration in a given time) $\endgroup$
    – Macro G.
    Jul 5 '18 at 4:01

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