# How to identify a blow-out pipette?

Is my pipette a blow-out pipette?

I suspect that it is a blow-out pipette, though the double lines aren't rings. My guess is it is a TC blow-out pipette. It's a big 50 mL pipette, longer than my arm.

I wonder, how am I supposed to know? I also have another one with a single dark band on the top.

EDIT: This video explains how blowing out a pipette is actually done.

Good to see classical analytical chemistry queries. These graduated pipettes or Mohr pipettes they are called, confuse even the relatively advanced users. First you have to check where the graduations end. If the pipette was meant to deliver 10 mL or 50 mL you should not see 10 or 50 mL mark near the tip of the pipette. That is one hint. Add a picture of the tip as well. Secondly, a double ring indicates that all liquid has to be expelled.

Notice that when you are doing very high precision work, one has to know the full drainage time as well after the liquid has been delivered. For example, after delivering 10 mL you need to wait for 5 s before blowing out the last drop. Very high quality glassware use to mention this time on the glassware.

In your blurred picture one can se Ex 5s, which means wait for 5 s before blowing out the last drop.

Volumetric pipettes should never ever be blown out, but they also have a drainage time, you have to wait for a certain time before dislodging the last drop purely from surface tension effects.

• Can't I just "help" blowing it out with the bulb? I could blow air into the pipette with the bulb. – user1095108 Dec 23 '20 at 17:42
• If you require high precision, shouldn't you avoid graduated pipettes all together? – Martin - マーチン Dec 23 '20 at 18:06
• Yes, this pipette is a blow out pipette because the 50 mL mark is missing and it has double rings. – M. Farooq Dec 23 '20 at 18:13
• @M.Farooq: I guess you need to wait so all liquid, even from the top, drains into a container. You're not supposed to "help" it go down with the bulb? – user1095108 Dec 23 '20 at 18:20
• @user1095108 I was actually asking more general. If I needed a volume of 50 mL; shouldn't I use a more precise approach? I also did not claim that volumetric pipettes have infinite accuracy, just more than what you'll be able to do with a graduated one. – Martin - マーチン Dec 23 '20 at 18:25

Elsewhere here, M. Farooq has given excellent answer for this analytical chemistry question. I like to explore this answer by explaining what are these lines means and some confusions arose from M. Farooq's answer.

The pipette is an important tool in many fields of science. There are few types of them:

• Volumetric pipettes: This type of pipettes is used to deliver a single specific volume of liquid (e.g., $$\pu{1, 10, 25, 50 mL}$$, etc.). This type of pipettes are reminiscent of big rolling pin with a belly. When emptying its content, the liquid is allowed to drain-out by gravity, should never be blown-out. After emptied the content, a small amount of liquid should remain in the tip of the pipettes.
• Measuring pipettes: This type of pipettes is used to measure various amounts of liquids using the same pipette. This type of pipettes are straight glass or plastic tubes with one tapering end. The straight tube is calibrated into small sections so that different amount of volumes can be measured (e.g., see images attached to the question). Measuring pipettes can be divided into two other types: Mohr pipettes and Serological pipettes.
• Mohr pipettes: The graduation of this type of pipettes is end before the tip (maximum volume does not include the volume in the tip).
• Serological pipettes: The graduation of this type of pipettes is continued to the tip (maximum volume includes the volume in the tip).

There are a few specifications on Measuring pipettes. Closed to the tapered end, some pipettes contain two circles, which indicate the pipette is a blow-out one (see below):

After that, there are calibration information, which contains the maximum volume and the temperature where the measurement is most accurate. For example, if it reads "$$\pu{10 mL}$$ in $$1/10$$ TD at $$\pu{20 ^\circ C}$$" that means maximum volume is$$\pu{10 mL}$$ and minimum volume is $$\pu{0.1 mL}$$ at $$\pu{20 ^\circ C}$$, and pipette is to deliver (TD). If it is TC, then it for to contain (see above image). Some manufacturers use a color coded ring to indicate the maximum volume. For example, Eppendorf uses yellow, green, blue, orange, red, and violet colors to indicate $$1, 2, 5, 10, 25,$$ and $$\pu{50 mL}$$ Serological pipettes.

Reference:

Karen Guzman, "Pipetting: A Practical Guide," The American Biology Teacher 2001, 63(2), 128-131.

• +1 Sadly, all these basic skills and classical calibration concepts are no longer taught except in universities with strong analytical chemistry programs. Even if you ask a PhD student, they will not be aware of these things, let alone undergraduates. – M. Farooq Dec 24 '20 at 5:49
• The reason for me asking this question is this video and the Wikipedia article, which says: "Less commonly, some TD pipettes are made "to contain" as per manufacturer and made to be blown out". As you can see, my pipette has EX specified, but is a TC pipette, so saying this is "less common" is wrong IMO. It seems to be the norm for serological pipettes. – user1095108 Dec 24 '20 at 7:13
• In the Wiki link you provided, I this is wrong " The delivery time is described as the duration of time that the meniscus reaches the end of the tip starting from the topmost volume, which is specified as 5 seconds for Class AS bulb and graduated-pipette volumetric equipment.". I think Ex+5s means to deliver plus 5s wait before you blow out the last last drop. I don't think it will take 5 s seconfs to empty the full 50 mL pipette as claimed here. – M. Farooq Dec 24 '20 at 14:43
• Where does your pipette mention TC? – M. Farooq Dec 24 '20 at 14:45
• I think M. Farooq is correct here. The article I mentioned didn't explain 'Ex' but it make sense it to be for extra. – Mathew Mahindaratne Dec 24 '20 at 16:47