[xALmoN]

There are some things about the procedure which i do not quite understand. I need to understand this for a school project. I thank you all in advance..

i'll type out the procedure first. questions later...

A. Reagents

(a) Salicylic acid standard solution. -- 20micrograms/mL. Dissolve 0.2500g reagent grade salicyclic acid in 95mL CHCl3 in 250mL volumetric flask and dilute to volume with alcohol. Dilute 2.00mL to 100mL with alcohol.

(b) Sodium bicarbonate solution. -- Dissolve 5g NaHCO3 in 100mL of H2O to which 1 drop HCl has been added.

B. Determination

Prepare sample dilution, if necessary to contain about 5% methyl salicylate. Pipet 5mL sample or dilution into 50mL ether-petroleum ether mixture(1+1) in separator and wash with 2 5mL portions, cold, freshly prepared NaHCO3 solution. Discard unemulsified aqueous phases. Extract organic layer with 2 portions 5% NaOH solution followed by 2 5mL posrtions H2O. Let phases separate 5min and drain unemulsified aqueous layers into another separator. Wash combined extracts with 10mL petroleum ether and drain aqueous phase into another separator. Acidify cautiously with HCl ( litmus paper ) and extract with four 20mL and one 15mL portions CHCl3. Filter each extract through CHCl3 moistened plug of cotton into 250 volumetric flask. Dilute to volume with alcohol and transfer 2.00mL aliquot to 100mL volumetric flask.

Dilute to volume with alcohol and determine A at maximum ( ca 305nm). Calculate as salicylic acid by comparison with A of standard solution. Salicylic acid x 1.1016 = methyl salicylate.

Ah. and its time for the questions..

1. regarding the preparation of reagent b.. why must 1 drop of HCl be added?
2. why must cold NaHCO3 solution be used.?
3. why will the solution emulsify when NaHCO3 is added?
4. when washed with NaOH and H2O. which is discarded? the emulsified or unemulsified layer?
5. after washing with petroleum ether, what is the acidification of HCl for?

thats about all i have to ask.... once again. thanks in advance.

[movies]

1) I would guess this is just to make sure that the NaHCO₃ dissolves.  It should be soluble in water anyway, but if your water is a little basic then the solubility might go down a bit.  This step does seem a little odd though.

2) Not sure about this one either, but usually you use cold solutions to either slow down a reaction (in this case an acid/base neutralization) or to lower solubility.  Either of these may be the case here.  I think it's more likely the latter, since you want to minimize the amount of material that goes into the NaHCO₃ solution so you can get all of it out in the NaOH extractions.

3) Emulsification is a funny thing.  It tends to happen when you have a solution of some material that has a functional group that is highly soluble in aqueous media and another that is highly soluble in organic media.  In this case, an aromatic ring (organic soluble, aqueous insoluble) and a deprotonated carboxylic acid (aqueous soluble, not very organic soluble).  So these two functional groups are competing to find the most stable way for them to dissolve.  The carboxylate end sticks to the water, the aromatic end sticks to the pet. ether.  These attractions make a crude mixture of the two solvents: an emulsion.  (BTW, this is a pretty bad explaination.  I'm sure you can find a better one)

4) Be aware of the subtle difference between "washing" and "extracting."  Washing is treating something that is in one phase (say it's the organic phase) with a solution of some reagent in another phase (e.g. aqueous ammonium chloride soln.) to effect some change on the molecules in the phase that is being washed.  In this case the "washing" layer is discarded as the desired material never moves from one phase to the other (the aqueous phase in my example would be discarded).  Extracting is adding a solvent (say, ether) in a different phase from solution you are going to extract from (say, some aqueous solution of an organic molecule).  The extracting fractions take advantage of the fact that the molecule you want is more soluble in the phase you are extracting with than in the phase you are extracting from.  You save the extracting phase (ether) is saved and usually combined after a few extractions.  Once the extractions are complete, the phase that was extraced from is discarded (aqueous phase, in my example).  Don't dispose of anything until you are convinced that you have all of your material in your extracts!

5) The acidification is to change the solubility of the salicylic acid.  Recall that it becomes soluble in water because it is deprotonated.  The HCl re-protonates the salicylic acid so that it is again more soluble in organic media than in aqueous media.  Then you can extract it into chloroform.

[xALmoN]

A few more questions.

1. Why is litmus paper used during acidification? Can universal indicator strips be used as well?

i've thought it through and i feel that because litmus paper gives a change that is easy to see, like from blue to red, but in the case of universal indicator strips, the change is a little hard to detect. And from what i've learnt. pH does not equal acidity, but i wonder if blue to red litmus reading really indicates an acid solution.

2. Why do we need to wash with petroleum ether after extraction with NaOH and H2O? I'm not sure about this because petroluem ether would not wash out any of the Methyl salicylate since they're all deprotonated.

last question for now, i promise...

3.How do you do subscript? like the 3 in NaHCO3...

once again thanks in advance...

[movies]

I don't know the ins and outs of litmus paper vs. universal indicator paper.  Maybe someone else can tackle that question.

The washes with pet. ether are probably to remove any organic stuff that is in the aqueous layer that isn't supposed to be.  Some organics may have been pulled in by the emulsification.  The deprotonated acids should stay put in the water layer.

for subscript you use the tags "[ sub]" and "[ /sub]" (without the spaces or quotes) around whatever you want subscripted.  Or you can just use the "sub" button and type what you want between the tags.

[xALmoN]

ah i was thinking more on the lines of alt + something so i can use it my project report as well. but. heh.. this is good enough. i'll just.. copy and paste!

[movies]

Oh you wanted to use subscript and superscript in word or something?  If you highlight the text you want to sup/sub and then go to the font dialog box you should be able to change it.  I just have a button on my toolbar to do it, I don't know the shortcut command.

The tag thing is for adding sup/sub to posts here on the forums.

[xALmoN]

thats okay. dont have to worry about it.