[Steenrod]

Sometime back, I heard the phrase "active hydrogen".Can someone please tell me what this is all about?

They were talking of active hydrogen determination or something. I shall be obliged if someone could please tell me what this is all about.(Is there a method to quantitatively determine the number of active hydrogens)

Thanks !

[Babcock_Hall]

At first I thought you might be referring to acidic hydrogen atoms in an organic molecule, but I am beginning to think I was wrong.  When I googled the phrase "active hydrogen," I got a bunch of hits that look like nonsense at best:  "An abundance of active hydrogen is generated when tap water passes through the Sosei Water Generator. "Active hydrogen" (atomic symbol "H") is hydrogen that exists alone. H2 is molecular hydrogen, not active hydrogen. Molecular hydrogen is highly acidic and bonds with active oxygen, thought to be the cause of many ailments. Atomic active hydrogen can convert active oxygen to water (H2O). It is known that the world-famous water found in caves in Nordenau, Germany is rich in active hydrogen. According to test results, the amount of active hydrogen contained in Sosei Water is from 2 to 10 times this amount."
http://www.soseiworld.co.jp/know/esir111.html

[Steenrod]

Actually I looked up the internet later and instead found this:

http://en.wikipedia.org/wiki/Zerewitinoff_determination

I am sure this is what I wanted but the link does not tell me how to determine  the no. of active hydrogens.

Can anyone please explain what this is all about?I mean what are the quantitative aspects of this ?(how to really calculate?)

Secondly, the link or the one it refers to does not define the term "active hydrogen".So,I am also ignorant of the definition too!

[Babcock_Hall]

From your link, "The Zerewitinoff determination or Zerevitinov determination is a quantitative chemical test for the determination of active hydrogens in a chemical substance. A sample is treated with the Grignard reagent, methylmagnesium iodide, which reacts with any acidic hydrogen atom to form methane. This gas can be determined quantitatively."  The Grignard reagent is a strong base, and it forms methane when it reacts with an acidic hydrogen (so my first supposition was correct).  Here is a link to an article from 2010 in the Wiley library:  http://onlinelibrary.wiley.com/doi/10.1002/9780470638859.conrr692/abstract

[Steenrod]

But is an active hydrogen?(I do not have access to the article either.But first I need to know what an active hydrogen is.

I don't see proper definitions on the internet.To be precise, I just need a definition of active hydrogen which can be understand  by a high school student whose chemistry skills  are pathetic.

[Arkcon]

You may need to find the original place you found the term 'active hydrogen' to be sure that its not just some pseudoscience screed, and what exactly it is meant to mean.  The Wikipedia article give the example of the alpha H of an alkyne, but Wikipedia defines alpha hydrogen by jumping to Bronsted-Lowery acid.  Meanwhile, the abstract of Babcock_Hall: link implies that the Zerevitinov determination was amended to include other proton donors, like amines and alcohols.  It may simply be an archaic term to mean proton donor, under a variety of circumstances.  Why they don't just say 'proton donor' I don't know.

[Babcock_Hall]

I would say that active hydrogens are (at least) slightly acidic, but I don't have a particular pKa cutoff.  Here is one more reference:
http://pubs.rsc.org/en/content/articlelanding/1963/an/an9638800782/unauth

[fledarmus]

Can you give more context to your "active hydrogen"? What were you reading about when you found it?

I don't know of any specific definition of active hydrogen, but I have heard it used loosely in at least two different contexts. One is the number of acidic hydrogens in a compound, typically in small peptides. -OH hydrogens, -NH hydrogens, -COOH hydrogens, -SO₃H hydrogens, any hydrogens that can be pulled by a reasonable base in water (for example NaOH). Compounds were describes as having "molecular weight X with Y active hydrogens".

There is also a use I've seen in polymer chemistry, where prepolymers are prepared with molecular weights in the 100-1000 range, then reacted with other prepolymers or initiators to form the full length polymer. Most recently, a polyurethane preparation starting with isocyanates. In that case, "active hydrogens" on the prepolymer meant sites that could react with the isocyanate. This gives you a measure of how much branching the polymer will have, and how solid a three dimensional matrix will form. If there are only two active hydrogens, the chains can only be linked end to end, and any branching would have to come from the isocyanate piece. If there are more active hydrogens, each will reacti with isocyanate and form a more heavily branched and therefore more rigid final product.

Then there are a number of pseudoscience applications which have already been alluded to, where "active hydrogen" means as little as "phlogiston".

[Steenrod]

Ok,thanks!