problem on preparing 90% methanol for gas chromatography???

[bobey]

i have problem to prepare 90% methanol. i used 100 ml volumetric flask to prepare it. by using burette( 50 ML 1X AND 20 ML 2X) , i pour 90 ml of methanol SupraSolv (R) by MERCK into 100ml volumetric flask. then i top up with distilled water up to calibration mark. my problem is, when i left the solution for 15 mins preparing for the gas chromatograph, the calibration mark seems to drop... my question is, is the concentration of the methanol is still 90% or it has been dropped??? do i need to to up the distilled water again up to calibration mark???? or do i need to prepare a new solution???

since my experiment lasting for 5 hours, i need to use the solution again for the different temperature the next day, is the solution contains 90% of methanol of % has been dropped??? do i need to prepare the fresh solution again??

i do understand this is due to the hydrogen bonding btw methanol and water... is the concentration inside the solution is 90% methanol even after the solution is left for 1 week???

your help is highly appreciated.

[Don_Hilton]

If you just added the water to the volumetric flask and did not mix it, then on standing the water and methanol begain to mix. If you take a 10 mL water and 90 mL methanol and mix them, you will not come up with 100 mL solution.

If you mixed the solution well before taking any out of the volumetric flask and if you make sure that the flask has been well stoppered since it was mixed, it should be consistant over a week.

The percentage of methanol in the solution would not have changed significantly. If you left the flask standing with water in the neck of the volumetric flask (not yet mixed), I doubt any significant quantity of water or methanol evaporated. But , as far as the solution being 90%, this is tricky because the quantity of water you added by topping up the flask will be more than 10 mL, because as the first bit of water into the flask reached the methanol it did mix and the volume decreased.

There are two ways people will prepare a 90% methanol/water solution - based on volume. One is to measure 90 mL methanol and 10 mL water and mix them. The other would be to add 90 ml methanol to the volumetric flask and then add water and mix, then add more water and mix until there is no significant change in the level of the solution in the flask. And these two ways of making solutions make two different solutions. The difference is not large, and in many applications is not sufficiently significant to matter. You can also make solutions based on weight pecent. (You can find discussions of percent solutions on this site.)

(And in most cases, volume measurements with graduated cylinders are adequate for making percent solutions.)

[Johnny Rod]

There is a temperature change (increase) when you mix methanol and water, this could be making the volume change as it cools back to room temperature. As said you need to decide how you want your mixture, for example if it were an HPLC phase you would mix 10ml and 90ml together and not care about the final volume. Maybe you need to mix them first then allow to cool, top up again then you are good for several days. Whatever the answer, stick to doing it one way.

[tom jupille]

For what purpose will it be used? The statement "90% methanol" implies 1 significant figure. Even "90.% methanol" would only imply two significant figures; i.e., the actual value is between 89.5% and 90.5%. Which means the volume change on mixing or the temperature change effect really won't matter.

Whatever you do, *document it* for future reference.

[michaelbarnes42]

100 mL of 90% methanol v/v is made by taking 90 mL of methanol and diluting to 100 mL total volume. You need to mix while performing the dilution due to the miscibility of Alcohol and water there is volume lost when mixing, if there is volume lost then you will not have 90 mL MeOH in 100 mL total volume you will have 90 mL MeOH in ~99 mL total volume or 91% MeOH.

[Consumer Products Guy]

For stuff like this, you must:
(1) be consistent in how you make it, like always dilute to a volume, or mix volume A to volume B
(2) DOCUMENT exactly how you did this, so others will not similarly be confused.

That's most likely way more important than if the methanol is exactly 90.0000%