(Hive Bee)
09-29-03 15:23
No 461708
(Rated as: excellent)
It seems that many peracid oxidations of isosafrole have been failing due to "old" H2O2. The label may say "30% w/w" but how do you know if that is really what is in your container? Would you consider a 10minute titration in order to save yourself from wasting precious isosafrole? Due to my recent failure, I will no longer use H2O2 without first making sure it is within acceptable limits.
This post is geared towards newbees, as most chem-bees would probably think this is second nature, or common knowledge perhaps. I would hope everyone would have access to these items, as they are not watched, and may even be "borrowed" from your University, or workplace.
What you will need:
Suspect H2O2: More than likely 30%. ~4 grams will be needed for the titration.
Glassware: Accurate volume measurement device: Volumetric flask or volumetric pipette ideally; however you should be able to use a non volumetric pipette with not too much variability in endpoints. A 50mL burette, beakers, pipettes, etc.
Analytical digital balance ideally, triple beam should be sufficient however.
Reagents:
Conc. Sulfuric Acid. (H2SO4) ~98% ~36N.
Potassium Permanganate (KMnO4) (0.3N)
Procedure:
Weigh ~4g of H2O2 in a tared container, beaker what have you. Record your weight, and wash with distilled water into a 250mL volumetric flask. Fill the remainder of the flask with distilled water up to the fill line. Stopper, and mix thoroughly.
Add to a seperate 400mL beaker a volume of 250mL of distilled water, and 10mL of conc. Sulfuric Acid (H2SO4). Remembering always to add the acid to the water.
Add stir bar, and place on stirring plate.Add 25mL of H2O2 solution from your volumetric flask into the 400mL beaker. Start stirring.
Fill your burette with .3N Potassium Permanganate (KMnO4), place the burette directly over the 400mL beaker, record starting point, and add dropwise the (KMnO4) to the beaker. Titrate until a permanant "pink" color is achieved.
Record your end point, then subract your endpoint from your starting point, which will give you your total amount of .3N (KMnO4), which will be used in the equation below.
Equation
% H2O2 (by weight) = [(mL's of KMnO4) x (N) x (0.01701) x (1000)] / (grams of H2O2 sample used)
where: N = Normality of the standardized potassium permanganate
Example
Let us say that it took 24.0mL KMnO4 to achieve a permanent pink solution. We multiply this by the normality of the KMnO4 which in this case is .3 Multiply the product by 0.0701 and the product of this by 1000 then divide by the inital weight of H2O2, in this case 4g.
[(24) x (.3) x (.01701) x (1000)] / 4 = 30.618% assay. Not bad, let's make some peracid,
I have found one discrepancy in the search engine that stumps me. Post 249881 (LaBTop: "Re: Chromic's DRAFT peracetic (BIG MISTAKE)", Newbee Forum) states While acquiring your H2O2, be especially prepared to AVOID any product which has additives and/or stabilizers in it, they will totally ruine your reaction outcome. Now I would think it would be hard to find a commercial supplier of 30%H2O2 that does not add any kind of stabilizers, I never really payed much attention to this however, this post concerns me. I happen to have the bottle that I used in the failed synth and it says "30% H2O2 w/w stabilized" Sigma is the supplier, and I would imagine it was the same part number as I have always used in performics. Hmmmm. I wonder if there are some stabilizers that may not harm the reaction, I understand that there are quite a few different options when stabilizing H2O2 solutions. Something to look into further I imagine......
I wanted to post the USP/NF procedure for the assay, however I could not access it from my residence. If I find any discrepencies in the USP compared to the above post I will edit the post accordingly, with the USP procedure.
I hope this post helps some newbees out, as it seems quite a few bees including myself have been failing peracid oxidation due to decomposed H2O2
Further newbee reading.
H2O2 FAQ
http://www.h2o2.com/intro/faq.html
Discussing topics such as, What factors contribute to the decomposition of H2O2?, What can I do to minimize H2O2 decomposition? , What are H2O2 stabilizers and will they affect my application? What is the pH of H2O2 solutions?
Unipolar Mania, It's good for life...
(Stoni's sexual toy)
09-29-03 16:08
No 461719
The usual stabilisers will not hurt the reaction. Buy stabilised only (it's unlikely you will get anything else anyway).
Keep in mind though that the KMnO4 solution cannot be kept and must be titrated before each use too if you want true results. If you only want to estimate the approx. concentration then you can probably avoid this.
And another important fact: 0.3N KMnO4 which is to be used in acidic titrations has a different concentration than 0.3N KMnO4 used in basic environment! To avoid confusion the molar concentration should be given too.
I'm not fat just horizontally disproportionate.
http://www.whatreallyhappened.com
(Stranger)
09-30-03 11:24
No 461928
How to make sure your H2O2 is still 35%?Just check density,no need to mess with titration.If it's 1.135 or something like that,it's ok,anyways there were some charts posted some time ago...TFSE should help