By Anonymous on Friday, July 12, 2002 - 08:15 am:
I have a simple question.
how to prepare 20% methanol in water?
Should I put 20ml methanol and 80 ml water together or put 20ml methanol in a cylinder and add water to 100ml?
Thanks!
By Anonymous on Friday, July 12, 2002 - 10:39 am:
Test instructions should be specified exactly to prevent such possible misinterpretations. I personally would do the first option, because that's how a multi-channel HPLC would do such, automatically. But specify such so that all do the same in the future.
By B.Buglio on Friday, July 12, 2002 - 02:56 pm:
Definitley the first option. If you add 20ml MeOH
to 80 ml of water you'll see that the total volume
is less than 100ml.
By Anonymous on Saturday, July 13, 2002 - 06:42 am:
I would put 20mL meoh and i´d go to 100mL in a flask.
By A.Nonymous on Sunday, July 14, 2002 - 03:49 am:
Last Anonymous, what is the reason for this?
By Anonymous on Monday, July 15, 2002 - 01:18 am:
Could it be done if you only had a 1L and a 3L
flask, not graduated?
By juddc on Monday, July 15, 2002 - 08:41 am:
Yes, if you wanted to make 5L of mobile phase at a time.
3L + 1L Water, then add 1L methanol.
Use the 20 mL Methanol + 80 mL water method and don't bother with a 20 mL MeOH QS'd to 100 mL. This is just standard practice for HPLC and is done for the reasons noted above.
By H W Mueller on Monday, July 15, 2002 - 11:40 pm:
This has been discussed before, anyway, there are more reasons for rejecting this "20mL filling to 100mL" method. Mixing water with methanol is an exothermic process, one would have to refill several times after the mix cools. Also, with the method 20+80... you know how much of each solvent you have, which is important for comparisons (Example: The rule of three), and possibly for theoretical calculations.
By B.Buglio on Tuesday, July 16, 2002 - 05:35 pm:
For the record, when MeOH and water are mixed
there is a decrease in total volume. The
expression for the change in volume is:
(Delta)Vm =b(Delta)Em
where b is the compressibility and Em is the
energy of mixing.
Only when mixing is athermal is there no change
in volume.
By accepted definition the mobile phase
composition in chromatographic systems is given in
% v/v. By so defining we don’t have to worry about
the above equation: a 20/80 solution is 20mL +80
mL. If you dilute a methanolic solution to volume
you do have to take (Delta)Vm into consideration
when expressing the % composition. The extent of
volume change of course depends on the solvents
being blended however as a bottom line - follow
the accepted practice and avoid trouble.
By Anonymous on Wednesday, July 17, 2002 - 03:08 am:
OK, what about a 2L bucket and a standard
cup?
By Anonymous on Wednesday, July 17, 2002 - 06:48 am:
Just buy a 2 liter and a 500 mL graduate cylinder. You are making mobile phase, not doing baking.