Yttrium2
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Can Someone Explain This? -
The volume metric flask, when and why is it used as opposed to a graduated cylinder or similar volumetric measuring device?
Also, how off can it throw off calculations wherein; one is using a syringe 100 times to measure 100mL vs a 100mL graduated cylinder used 1 time,
which would be more accurate? ( I could probably figure this one out in my head ) - figured it would be a good question to ask for all of us newbies
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Sulaiman
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A volumetric flask is mainly to contain a given volume,
for making up 'standard' solutions,
a known weight of a soluble substance is diluted to a precise volume,
the narrowness of the neck improves resolution.
A measuring cylinder is cylindrical, giving low precision readings,
and is used to deliver a known volume of liquid,
like a low-resolution burette.
There may in theory be some clever statistics involved,
but the accuracy will depend mainly upon;
. calibration accuracy of syringe and measuring cylinder,
. ratio of width to length of measured volume
. most significantly, filling/emptying a syringe 100 times is so boring that I am likely to become careless 
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Tsjerk
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With a 100ml graduated cylinder you can for example measure 50, 70 or 100 ml. With a 100 ml volume metric flask you can only measure 100 ml but very
precise.
[Edited on 11-10-2018 by Tsjerk]
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Ubya
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a 100ml graduated cylinder has a resolution of 1ml most of the times, so the error let's say is 0.5ml
a 100ml volumetric flask has a precision of 0.05ml, so for analytical work this is important.
which would be better, measure 100ml in 1ml increments with a syringe or just 100ml measured with a graduated cylinder?
error propagation theory, if the syringe as an error of +-0.05ml for example, you need to add all those errors together, so if you measure 100 times
1ml+-0.05ml at the end you would have 100ml+-5ml, but using a graduated cylinder with an error of 0.5ml you would have 100ml+-0.5ml
when i was doing lab for my quantitative analitical chemistry class i had to titrate with a burette (error of +-0.05ml) usually 200mg+-0.1mg of a
compound dissolved in 250ml using a volumetric flask, adding just 1 or 2 drops in excess during the titration means an error of around 1% already, so
precision is a must. normally in an amateur lab this kind of precision is not necessary so a graduated cylinder is fine for 99% of things
[Edited on 11-10-2018 by Ubya]
[Edited on 12-10-2018 by Ubya]
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feel free to correct my grammar, or any mistakes i make
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XeonTheMGPony
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temperature will effect measure, you need to know the stc that it was calibrated at and then ensure the lab is maintained at this.
Normal stc's seem to be between 20c to 25c for the most part which sucks as too damned hot, I keep my place between 14 to 17c much above that it gets
uncomfortable.
STC = Standard test conditions.
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Yttrium2
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Quote: Originally posted by Ubya  | a 100ml graduated cylinder has a resolution of 1ml most of the times, so the error let's say is 0.5ml
a 100ml volumetric flask has a precision of 0.05ml, so for analytical work this is important.
which would be better, measure 100ml in 1ml increments with a syringe or just 100ml measured with a graduated cylinder?
error propagation theory, if the syringe as an error of +-0.05ml for example, you need to add all those errors together, so if you measure 100 times
1ml+-0.05ml at the end you would have 100ml+-5ml, but using a graduated cylinder with an error of 0.5ml you would have 100ml+-0.5ml
when i was doing lab for my quantitative analitical chemistry class i had to titrate with a burette (error of +-0.05ml) usually 200mg+-0.1mg of a
compound dissolved in 250ml using a volumetric flask, adding just 1 or 2 drops in excess during the titolation means an error of around 1% already, so
precision is a must. normally in an amateur lab this kind of precision is not necessary so a graduated cylinder is fine for 99% of things
[Edited on 11-10-2018 by Ubya] |
I appreciate it.
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TheMrbunGee
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| Quote: Originally posted by XeonTheMGPony | temperature will effect measure, you need to know the stc that it was calibrated at and then ensure the lab is maintained at this.
Normal stc's seem to be between 20c to 25c for the most part which sucks as too damned hot, I keep my place between 14 to 17c much above that it gets
uncomfortable.
STC = Standard test conditions. |
Measure and then chill. Making standard flasks for every temperature would get confusing very fast.
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Sulaiman
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@TheMrbunGee :Because water has predictable thermal expansion
and the thermal expansion of borosilicate glass is usually negligible
it is common to measure at whatever ambient temperature
then make a mathematical correction to compensate for non-standard conditions.
A lot easier than having everything at standard temperature. 
@Yttrium2 : "... adding just 1 or 2 drops in excess during the titolation means an error of around 1% already, ..."
If 1 or 2 drops = 1% then 100% = 100 or 200 drops = 5 or 10 ml
so using a more dilute solution in a 50 ml burette would give more precision.
If you have ever bought a cheap made-in-China plastic volumetric flask
or graduated glass syringe
you will understand why my top priority is
"calibration accuracy of syringe and measuring cylinder,"
Because I already have a range of volumetric flasks (10, 25, 50, 250 and 500 ml)
I tend to use them,
but it is more accurate to use weights than volumes
as no temperature compensation is required AND precision is increased..
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JJay
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There's generally nothing wrong with using a class A graduated pipette or cylinder for measuring chemicals for reactions. (I don't own any class B
glassware, and if I did, I'd probably crush it for column packing.)
For analytical work, I try to use volumetric glassware as much as possible for anything that has to be measured by volume.
Oh and if you use a 1 mL pipette to measure out 100 mL, any bias in measurement will be multipled by 100x. Normal measurement errors won't multiply in
the same way; if there is no bias, the average error will be zero with a standard deviation is nearly fixed no matter how large a quantity you measure
with your pipette.
Generally, though, there is some bias, so you can't reliably measure entire cubic meters of liquids with a 1 mL pipette.
[Edited on 12-10-2018 by JJay]
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Ubya
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| Quote: Originally posted by Sulaiman |
@Yttrium2 : "... adding just 1 or 2 drops in excess during the titolation means an error of around 1% already, ..."
If 1 or 2 drops = 1% then 100% = 100 or 200 drops = 5 or 10 ml
so using a more dilute solution in a 50 ml burette would give more precision.
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i was already using a 50ml burette
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Deathunter88
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| Quote: Originally posted by JJay | There's generally nothing wrong with using a class A graduated pipette or cylinder for measuring chemicals for reactions. (I don't own any class B
glassware, and if I did, I'd probably crush it for column packing.)
[Edited on 12-10-2018 by JJay] |
??? That doesn't make any sense to me. Class B glassware just means it has twice the tolerance of Class A (so a class A 50ml volumetric pipette would
be +/- 0.06ml, whilst a class B is +/- 0.12). This hardly makes a different in 99.9% of amature chemistry applications, and certainly doesn't warrant
destroying glassware.
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Deathunter88
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| Quote: Originally posted by Sulaiman | A volumetric flask is mainly to contain a given volume,
for making up 'standard' solutions,
a known weight of a soluble substance is diluted to a precise volume,
the narrowness of the neck improves resolution.
A measuring cylinder is cylindrical, giving low precision readings,
and is used to deliver a known volume of liquid,
like a low-resolution burette.
|
To add on, there are two types of glassware.
The first is marked TC, which stands for "to contain". This includes volumetric flasks and volumetric pipettes. This means that they are designed to
contain exactly the stated volume. AKA if you fill a 500ml volumetric flask with water and pour it out into another measuring device, it will read
less than 500ml, since some liquid is stuck to the wall of the flask. For the same reason,volumetric pipettes are also called blow-out pipettes, since
to deliver the correct amount you have to blow out the last drop that is stuck due to capillary action.
The second is marked TD, which stands for "to deliver". This usually includes graduated cylinders and pipettes with graduations. They are designed to
deliver the stated volume, taking into account the liquid that will remain. This means that a graduated cylinder filled to the 50ml mark will have
slightly more liquid than 50ml. This becomes apparent when you weigh the liquid, say for calculated density. Similarly, these pipettes don't need to
be blown, and if the last drop is included then you are actually using more than you intend.
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unionised
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| Quote: Originally posted by JJay | (I don't own any class B glassware, and if I did, I'd probably crush it for column packing.)
[Edited on 12-10-2018 by JJay] |
What's the density of your standard masses?
If you can't answer that straight away then you can't be allowing for the density of the air. Depending what you are measuring, you may well be
failing to take account of a bigger source of error than class B glassware.
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macckone
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Let's assume you need to measure 100ml of water at 20C.
That has a density of .9982 g/cm3
The 100ml class A volumetric flask is accurate to .1ml.
Class B is .2ml.
Density of air is .0012041 g/cm3
So 100ml of air will be .12g
While if you don't account for air, you will be off more than class A spec you will exceed class B spec.
So weighing is more accurate than class B but not class A unless you account for air.
If you don't know the density of a liquid and you are trying to measure 100ml obviously you can't use weight since you don't know the density.
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nimgoldman
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Also when you make larger amount of solution, say 1L or 2L, then it is more practical to use volumetric flask as cylinders of such volume would be
very tall.
The volumetric flask can also be capped and allow thorough mixing by inversion.
[Edited on 13-10-2018 by nimgoldman]
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morganbw
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| Quote: Originally posted by JJay | (I don't own any class B glassware, and if I did, I'd probably crush it for column packing.)
[Edited on 12-10-2018 by JJay] |
My class B glassware surpasses my class C lab skills.
Of course, I am also fine using a jelly glass for a beaker.
To the original poster, volumetric flask will make sense to you if you have a need to work with a lot of standard solutions.
[Edited on 10/15/2018 by morganbw]
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