If this happens in the course of preparing a volumetric solution, what should be done?
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14$\begingroup$ Best solution : start again ! $\endgroup$– MauriceCommented Aug 30, 2020 at 8:26
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3$\begingroup$ Secondbest soloution: Pour into a flask of twice the volume, and get it right there. $\endgroup$– KarlCommented Aug 30, 2020 at 8:37
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5$\begingroup$ Volumetric flasks are TC(to contain) not TD (to deliver) type glassware, one cannot pour the contents into another flask, until and unless the first one is washed quantitatively. $\endgroup$– ACRCommented Aug 30, 2020 at 14:53
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$\begingroup$ You can wash the first flask into the second though $\endgroup$– Tristan MaxsonCommented Aug 30, 2020 at 20:06
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1 Answer
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If you have plenty of solution and reagents, the solution should be prepared again, because the volume is not the nominal volume written on the flask. However, the proper trick to fill the volumetric flask is to use a dropper when you are about to reach the calibration line. Experienced hands can use a squeeze bottle until the end but the chances of this mistake are high.
However, if the solution was was just a single sample solution (provided by somebody else, or the compound is too expensive or rare), slightly overfilling the volumetric flask is not the end of the world. You can mark the liquid level after proceeding normally (dissolve and invert several times). With a help of a high quality volumetric syringe or gas tight syringe (e.g., 2.5 mL Hamilton used in gas chromatography injections) draw the excess liquid until the liquid level is back to the calibration line. Measure that volume in the syringe and add it to the nominal volume of the volumetric flask.
If the volumetric flask had a nominal volume = 100.0 mL and you had to withdraw 1.80 mL then the total volume would be 101.8 mL for molarity or other concentration calculation.
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5$\begingroup$ (+1) Excellent answer! One semester I was teaching quant or instrumental analysis and I was in the lab with the students. One student went missing. Turns out they overshot the mark, so they went to the attached lab next door (the analytical balances were there), attached a hose to a water aspirator, put the other end into the neck of the volumetric flask and turned on the aspirator! When found, they said they were trying to get rid of the extra water by pulling a vacuum on it. Nice teachable moment! ;) $\endgroup$– Ed VCommented Aug 30, 2020 at 20:57
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$\begingroup$ Evaporating the extra water is perfectly legitimate approach - as long as you don't induce boiling in the tiny flask neck. $\endgroup$– fraxinusCommented Aug 31, 2020 at 14:49
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3$\begingroup$ Evaporation is a bad idea if your solute is also volatile as is the case in gas chromatography or sometimes in HPLC. $\endgroup$– ACRCommented Aug 31, 2020 at 15:22