[khanhduy0388]

I have a solution with 2 phases: phase at top is diethyl ether contain Stearic acid, phase at bottom is HCl solution. How can i reduce the concentration of HCl solution at the botom by using a chemical agent?
I had used water to dilute HCl solution but it so lost of time because i had to do that repeat many time.
Note: i wanna the concentration of Stearic acid in diethyl ether don't change.
please help me 

[Arkcon]

Please explain why you want to reduce the concentration of HCl in the aqueous phase.  If you dilute with water, it doesn't affect the organic phase.  If you separate the two liquids in your separatory funnel, they're not in contact anymore.  If you put the organic phase back into the funnel, you can add an aqueous phase of any HCl concentration you want.  What application you have that requires any of this work, however, remains a mystery to me.

I had used water to dilute HCl solution but it so lost of time because i had to do that repeat many time.

I realize we're facing a language barrier, but this statement doesn't make much sense.  You lost time, because you diluted?  You had to do it many times, because you diluted?  That doesn't follow logically for me.

[khanhduy0388]

thank you very much for your reply.
My product is Hydrotalcite (HT) which is used as a additive in plastic industry. HT was coated to its surface by Stearic acid. my jod is calculate accurately the amount of Stearic in HT.
I had used HCl acid to dissolve HT (because HT contain Mg(OH)2 and Al(OH)3), and used diethyl ether to solute Stearic acid. (that why i had a solution with 2 phases above)
As i said above, my goal is: take the organic phase by using separatory funnel, and then use acid-base titration to calculate the amount of Stearic soluted in diethyl ether. But, when i use separatory funnel to take the organic phase, it always has a amount of HCl remain in organic phase, and this thing effect to acid-base titration.
In order to resovle this problem, i did as following:
  - put water into solution and then use separation funnel to take the organic phase, but leave a little amount of HCl phase (because i dont want the organic phase go out).
  - put water into separation funnel again to dilute the HCl remain in separation funnel.
repeat two procedure above 4 times, (that why i said it lost time).

[Arkcon]

I'm following you a little bit better.  I recognize you're extracting the stearic acid to quantify it.  ANd yes the organic solvent is a good way.  And yes, there will be traces of water in the organic phase, carrying with it some HCl, and that impurity will be bad for your titration.

You have to separate out the aqueous acid.  You have to wash with plain water repeatedly, and each time you do, you reduce the HCl impurity in your organic phase.  Doing it four times may be enough. Then again it may not be enough.  And you mention you run the risk of losing the organic phase.

Of course, as careful as you try to be, a drop of aqueous phase will remain above the closed stopcock.  Now what do you do.  Do you risk losing organic phase to get that last drop out?  No, not for a quantitative test.  That drop of water has to be left with your same.  That water and the water dissolved have to be clean of HCl.

Maybe you need some more practice with the use of the separatory funnel.  You can try shaking organic with HCl without having any stearic acid.  Then titrate your organic.  That way, when you can't see any acid at all, you'll know your doing it correct.  Maybe after 4 or 5 washes, there is still a trace of HCl titratable.  You can use this as a blank -- you know this trace will always be there, so you subtract the titration volume of this blank from your sample titration.  USP NF titrations are often written in just this way.

Are you having a problem with separation, is an emulsion forming, or maybe the organic and aqueius just don't separate easily enough?  Maybe you'd like to add a neutral salt to help separate the aqueous and organic.

[khanhduy0388]

yes, you have some point there.
 But, we weight 2g of HT first (this weight may be change belong to the inspector), the precision of the scale  is 1/10,000 and then we alway put 10ml HCl (37-38%) in it, i do this to ensure that HT always be dissolved completely. thus, the remain of HCl acid is unstable, this thing leads to the change of concentration of HCl in the trace  That's why we can't use a blank as your way. I wanna reduce the time of washing (the aim of the washing is reduce the concentration of HCl, and increase pH to nearly 6,5-7), I can use a chemical agent like a base to neutralize HCl acid, but it will effect to Stearic acid concentration. Whether having a other agent which can react with HCl but don't react with Stearic acid or not?
Could you tell me more about using a neutral salt to help separate the aqueous and organic, can this thing meet my aim?
thanks!

[Arkcon]

Hmmm...tricky.  Here's what I'm thinking: you try to include a dilute solution of silver nitrate in one of the washes.  That should help remove any traces of HCl as insoluble silver chloride.  But now you have traces of nitric acid.  You want something to react with the "H" in HCl, and not react with the "H" in stearic acid.  That can't really be done.

Maybe you can switch quantification methods.  Instead of analyzing stearic acid by acid, you analyze stearate, by total carbon, or HPLC or GC.  If you can switch methods.

I still don't really see your dilemma.  With through washing, you should get accurate results.  With practice, you should become as efficient with the process as possible.  You don't seem to know how accurate you need to be and how accurate you already are.  And you really aren't clear as to how fast the process has to be.  Just because you want more done in a day?  Just because the boss says, "Go faster?"

[khanhduy0388]

Thanks for your oponion.
Stearic acid content is aproximately 1.5%, acceptable range is 1.3%-1.8%. That means in 2g HT has about 0.03 g of Stearic acid , so if a little organic phase be separated out, it will efftect to the final result. The current method is ok, but as i said it really lost time.
yes, i wanna reduce the time of this method in order to more sample be done a day.

[Arkcon]

If you have dozens a day, you may want to switch to an instrumental method -- HPLC or GC.  If you have a few a day, your technique will improve with practice.  If this were a pharmacopeia method, you wouldn't be allowed to deviate without validating.  If this is an ASTM method, likewise, the accreditation of your results depends on you not changing methods.  If you've set this method up yourself, well, you can do anything you want, but your accuracy depends on your ability to develop and validate the method.

Simply put, I can see no chemical way to remove hydrochloric acid, and not effect stearic acid.  No reagent is selective for one and not the other.  There are acid absorbing resins, but they will likewise affect the stearic acid.

And I still have no idea of the limitations of your technique.  You have to tell me something about your validation process.   If you dissolve 0.03 g of chemically pure stearic acid in solvent, add 10 ml conc. HCl, and wash four times with water, and then titrate to measure the stearic acid,, you should calculate a number, and it should match 0.03 g stearic acid.  If you do this 10 times, your relative standard deviation should be less than 2%.  If another worker in your facility does it, they should also get less than 2% deviation, and your 20 results should also have less than 2% deviation. If you deliberately make the stearic acid 0.036 and 0.024g, then you should get those results 5 or more trials with 2% or less standard deviation, otherwise your method isn't a good one.  Your manager should look at all of these results and say, "This method can be relied on for good results."  This is a basic method validation.  Has this process been done before?  If not then what you're doing isn't analytical chemistry.