Hydroxyl value in surfactant samples

[bdhedhy19]

Hi!
Can anybody help me with the hydroxyl value testing. Vendor method is as follow:

Digestion of samples with 25 ml *acetylating reagent. Sample digested with the condensor on and cold water flow for two hours at 95-100C, 25-50 ml H2O was added and 25 ml of reagent alcohol was added through the condensor. cooled to room temperature and titrated with 1N KOH methanolic and blank was digested at the same time with just 25 ml acetylating reagent. * Acetylating reagent was made with 500ml Pyridine+1.75ml DI H2O
+ 70ml Acetic anhydride.
My question is when i did two more samples 3 hours later and use the same blank value, results were higher than expected value for those two samples.

Hydroxyl value calculation: (B-V)x1.003Nx56.108/weight of sample
where B is amount of titrant (KOH) used to titrate blank
S is amount of titrant used to titrate sample.
I wonder if it is necessary to do the blank and sample side by side? or may be this acetylating reagent is not very stable because of pyridine? I dont get it first sample results came out within the specification and other two with
three four hours later with the same reagent gives Out of specification results. Please let me know if you have any suggestions? Thanks

[krickos]

Hi

Well, pyridine is a catalyst in this reaction so itself is not the cause, but will speed up degradation (reaction with H2O) of the anhydrine if the pyridine is high in water content (use dried or commercial "seccosolv"). Also if exposed to high moisture air anhydride will degrade overtime to acetic acid.

So I would spontenously use only freshly made reagent mixture just prior to start of acetylation. Consequently I would recommend to always do blanks side by side with samples with sensitive reagents.

[bdhedhy19]

Thanks Krickos!
you are expert in chemistry! i will try again today to do blank side by side with the samples.

thanks

[Consumer Products Guy]

I've used phthalic anhydride as hydroxyl value reagent. But nowadays I'd typically let a contract lab do it.

[bdhedhy19]

Thanks Krickos!
i did the blank parallel to the sample and added the amount of acetylating agent at the same time, exactly same amount in both plus i sealed the joints with the grease to avoid moisture, and it works great.
results came out within the vendor specification

thanks alot:)

[krickos]

Thanks for the feedback, always appriciated.

[saxon211]

We've had trouble getting our hydroxyl value test to yield reproducible results.

The test is as follows:

5 mL of acetyling agent (15 mL pyridine and 5 mL acetic anhydride) is added to a sample flask and a blank.

The sample is refluxed for 1 hour with a condensor, and 10 mL DI water is added. The reflux continues for 10 mins and the sample is titrated with 0.7 N KOH in Ethanol (standardized). After the 10 minutes, the flasks are cooled and 40 mL of neutralized ethanol is added to each.

The formula is: (mL blank - mL titrant)(N titrant)(56.1)/sample size = _____

Sometimes the test yields normal numbers and other times it yields numbers that are OOS for the same material.

Any suggestions on what we could be doing wrong?

[krickos]

Hi Saxon

When you say OOS do you mean lower or higher than expected? Or is it both? This would indicate what the cause could be.

Too low values may indicate that your glassware like condensors are not dry or contaminated. When you kill off excess reagent with water each acetic anhydride molecule gives 2 molecules of acetic acid, but when acetic anhydride reacts with an alcohol it gives one acetate and one acetic acid molecule.So excessive moist would lower your result.

A too high value could mean that you have a contamination (residues in glassware condensros), for instance if you grease your glass joints too freely or with the "wrong" grease that contains for example stearyl alkohol.

[bdhedhy19]

Hi saxon!

Mr. Krickos helped me with the Hydroxyl testing and plus i got the procedure of making the acetylating reagent from the vendor and together with krickos techniques and vendor procedure works great for me. I got some tips from online vendor procedures too.
Let me know if you need the procedure of making the acetylating reagent???