Since the information of OH# was already provided (i.e. "OHV,mgKOH/g = 47.5"), then assuming that OH# is accurate, one can use this following equation:

   OH-equiv. weight (in g/mol) = 56100 / OH# = 56100 / 47.5

or 1180 g per mole of OH end group.

The reciprocal of that is the mole of OH end group per gram (8.47 e-4 moles OH/g, or 0.847 millimoles OH/g).

Check any standard method. Polyol is usually refluxed with acetic anhydride in presence of pyridine. After requisite time, it is cooled to R.T. and the amount of residual carboxyl group present in the sample is titrated. There will be control with similar reflux without sample. By difference you come to know how much anhydride is used for esterfication giving the estimate of hydroxyl group.

Since the information of OH# was already provided (i.e. "OHV,mgKOH/g = 47.5"), then assuming that OH# is accurate, one can use this following equation:

   OH-equiv. weight (in g/mol) = 56100 / OH# = 56100 / 47.5

or 1180 g per mole of OH end group.

The reciprocal of that is the mole of OH end group per gram (8.47 e-4 moles OH/g, or 0.847 millimoles OH/g).

Dear Kaoru Aou

Thanking you for your time and consideration.

I am in field of mechanic and I have a little information about chemical field so I have confused from your procedure ( i.e. what is the 56100 or 1180?)

would you tell me exactly about your procedure or calculate the number of OH group for me based on following information?

thank you in advance and I look forward to hearing from you in your earliest convenient time.

Of the titration-based methods, there are two ASTM methods on hydroxyl number, and one ISO method.  They all work based on reacting the polyols with a reagent, so that you can count the number of hydroxyl groups that were on the polyol, via titration.  Two of them are actually acid-base titration.

56100 just comes from 56.1 g/mol (molar mass of KOH) and 1000 mg (in a gram). Please see the Dow Answer Center page (link attached) for more detailed explanation.

1180 is the equivalent weight in grams per mole of hydroxyl group.  This is the reciprocal of what you asked ("number of OH group in this Polyol").

http://www.astm.org/Standards/E222.htm

http://www.astm.org/Standards/E1899.htm

http://www.iso.org/iso/catalogue_detail.htm?csnumber=13294

http://dowac.custhelp.com/app/answers/detail/a_id/13359/~/dow-polyurethanes---functionality%2C-oh-number%2C-and-equivalent-weight-definitions

You can also determine this value by 1H NMR. Usualy methylene or methine end groups atached to the hydroxyl of the polyol appear at different chemical shift from the rest of methylenes. Then, by integration of this signal and the signals corresponding to the repeating unit you can determine easily the hydroxyl number and the number average molecular weight of the polyol

You can determine this value by IR-spectroscopy. of your samples

In my experience, the 1H NMR method is especially useful if you have a very small amount of sample (this was especially the case for exploratory studies on new polyols).  The ASTM methods that use titration are selected as standard methods, in part because of their accuracy even at very high equivalent weights (i.e. very low OH#, so you're searching for small numbers of OH), and probably mostly because titration devices are much easier to maintain and is cheaper, than an NMR device.  But accessibility to NMR is much better than before, and universities frequently allow outside users access, so it's an important optiion.

The Near-IR (NIR) method is both an ASTM and an ISO method (link attached) - the method requires that you measure several polyols of known OH# (determined most likely via titration....) to build a calibration plot between the known OH# and a NIR signal that happens to be sensitive to OH groups.  So the advantage is that you may be able to use the NIR probe as a rapid way to determine OH# - this can be of interest for example as a probe placed in the polyol synthesis reactor in situ.  Since it is a method reliant on a calibration plot, you need to be willing to accept a higher statistical uncertainty level than the OH# from titration on which the calibration is built - the only time you would accept that would be cases where the speed of acquiring data is valuable (such as the in-situ monitoring case as above).

http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=52878

http://www.astm.org/Standards/D6342.htm

Dear Mr Izadi,

While all the above answer are accurate and useful for someone who has some background in chemistry. I feel that, the fellow chemists who have answered have not quite addressed your difficulties.

As a mechanical engineer, you may have trouble understanding the concepts of a molecule and molecular weight. Further, you may have to know the safe handling procedures of chemicals. Some of those involved may be quite corrosive.

In my view, the best approach would be to team up with some other students in the department of chemistry from your university.

Best Regards

Guy Havet, PhD

Immediately available for a R&D Manager position in France

Kaoru's answer is right. Worth mentioning that if you are using this for a system that contains solvent such as solventborne polyester, don't forget to consider the factor of the solvent

All the answers above are correct I suppose. However, you need to know that pyridine is highly toxic and you requires a high standard of safety. DO NOT WORK WITH IT before having a look at its MSDS document.

You can IR spectrum

A little late but useful

Kaoru showed

1) Ew=56100/OH#

- get the OH# for your lot from the manufacturer. Keep the container closed and store under dry nitrogen as it can absorb water.

and we know that

2) Ew = Mn/fn

- where Mw is the number average molecular weight of the material and fn is the number of functional groups (-OH) on the molecule.

Combining 1) & 2): 56100/OH#=Mn/fn

Rearranging: Mn=56100 x fn/OH#

- this is how you can calculate the Mn

You can obtain the # OH in polyol from GPC data and structure of monomer.

tale a look at this website, it is pretty clear.

https://dowservice.custhelp.com/app/answers/detail/a_id/15855

Equation 1: OH#= 56100/Ew

Ew (equivalent weight): this is the molecule weight (size) containing only 1 OH. Keep in mind, it does not tell you the total size/weight of a polyol, which can have 2 or more OH per molecule.

If the functionality is given, for example fn=3 (3 OH per molecule), then you can calculate the molecule weight by rearranging and adjusting the equation.

Equation 2: fn x Ew = Mn (number average molecular weight)

Equation 3: Ew = Mn/fn (rearrangement of Eq. 2)

replace Ew in eq. 1 with Mn/fn

Equation 4: OH# = 56100/(Mn/fn)

Equation 5: Mn = 56100 x fn / OH#

you can obtain the number of OH in polyol from the molecular weight of the polyol divided by the 17

ASTM D 1957 and ASTM E222-10 describe several versions of this method of determining hydroxyl value.