Discussion on the proper selection of graduated pipettes
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This post was last edited by Yang Qing on 2020-12-24 at 13:34. Today, a technician said that if one wants to use a graduated pipette to draw up 5 mL of liquid, he would usually choose a pipette with a 10 mL scale. The reason is that it’s difficult to control the residue left in the tube; a 10mL graduated pipette is more reliable than a 5mL one. I don’t agree with this. When there is no pipette with a single marking, or when high precision isn’t required, if one wants to transfer 5 mL, it’s still appropriate to use a pipette with a 5 mL marking under the same conditions. As long as care is taken to follow the operating procedures for pipettes when transferring liquids – blowing into the pipette only when necessary and not when it’s not required – everything will be fine. As for volume errors, your reading error is likely greater than the error caused by residues; residual amounts are also taken into account when calibrating the pipette, so just follow the instructions. That technician said that he’s used to it anyway, and he’s sure he can control the liquid level so that it stops exactly at the 5mL mark during pipetting; this way, he feels more confident while performing the procedure. Similarly, he said that when drawing 1 mL, a 2 mL pipette should be used. Anyway, I personally think this claim isn’t valid; I wonder what others think? It seems I’ve seen a post on some forum before saying that transferring 1 mL 10 times is more accurate than transferring 10 mL once: lol. I also think that makes no sense; regardless of whether the hassle of handling the sample is taken into account or not, the best way to transfer 10 mL is to do it in one go using a 10 mL tube. Let’s see what everyone does. The instructor said that if a determination is to be made, 5 mL of solution is required for titration analysis; it’s better to draw 4.8 mL for the titration and use 4.8 mL as the volume value in the calculations, as this yields more accurate results than using 5 mL directly. I think it’s some form of psychological comfort for him? Let’s hear everyone’s opinions
Weigh the amount of distilled water released by the two types of straws using an analytical balance, repeat the process several times to check the accuracy and precision, and that will make it clear. Additionally, individual *habits may result in higher precision.
What you mentioned about giving it a try is fine, but I’m afraid the readings of the balance won’t stay stable (due to liquid evaporation). When we need to obtain a more accurate measurement sometimes, we use the method of first determining the density of the liquid, then carrying out a weighing, and finally converting it into the solution concentration.
Using a weighing flask, along with washed, degreased, and dry cotton gloves, is perfectly fine.
That makes sense; if one is going to give it a try, it indeed should be done carefully in accordance with the relevant requirements. Thank you~~
Actually, a 10ml graduated cylinder can also be used. When transferring the solution, the 5ml mark should not be taken as the \"0 point\"; instead, the 10ml mark should be used as the \"0 point\", and pouring should stop when the solution reaches the 5ml mark. For the analysis of sulfite content in reagent hydrochloric acid, 8.5 ml of sample needs to be taken; that’s how the procedure is carried out.