Arsenic testing by USP <211> in soapy samples

[bdhedhy19]

Hi!
I was wondering if anyone can help me in recommending the right anti-foaming agent/other thing to get rid off interference during Arsenic testing (color development) step. I would really appreciate. My samples are Bio-soft N91-6.

thanks

[krickos]

Hi

Have tried method II for sample work up? it includes charring of sample and should help reduce foaming.

You can also consider increasing the isopropanol content during the colour development step.

[bdhedhy19]

Thanks! I will try 5 ml IPA, i am using 1.0 ml IPA during color development. It still produce lots of foam and slows the color development in the monitor/spike sample.
thank you!

[Consumer Products Guy]

Do you really mean soap, or anionic or nonionic surfactants?

[bdhedhy19]

It is kind of surfactant and it is called Bio-Soft N91-6.

[AICMM]

Gentlemen,

Just playing around with the subject here, so please humor me. Would conversion to Arsine followed by headspace GC get you the answer you are interested in?

Best regards,

AICMM

[krickos]

Gentlemen,

Just playing around with the subject here, so please humor me. Would conversion to Arsine followed by headspace GC get you the answer you are interested in?

Best regards,

AICMM

Intresting, with what detector? AED?

In my recollection, the Gutzeit based test generally is not to bad but of course a bit messy to work with. Depending on colour reagents you can move from visual comparision to UV-VIS measurement for better quantification, how good I do not recall.
The level in USP etc typically is around 1ppm levels or so if memory serves.

[bdhedhy19]

Sorry Guys! i am not gentlemen! Well i am using USP <211> method II(Arsine conversion) for my non ionic surfactant sample and the digestion procedure with H2SO4 (1:1) and H2O2 goes very well, until I starts the color development
with the addition of 7N H2SO4+2.0 ml of Potassium Iodide+0.50ml of stannous chloride and 1.0ml of IPA and it starts foaming in the flask. i just need to know? what anti foaming agent i should use to stop this interference. that's all.
Please help me if you can.
Thanks

[krickos]

Hi

Well given the additional information, I am more inclined to that your charring is incomplete, perhaps it goes to fast and you get sample residues on the walls? if thats the case:

If the test substance reacts too rapidly and begins charring with 5 mL of sulfuric acid before heating, use instead 10 mL of cooled dilute sulfuric acid (1 in 2), and add a few drops of the hydrogen peroxide before heating.
We use this for a few substances when charring starts too fast at roomtemperature.

Excessive foamin is nothing we typically see during the colour development.

[AICMM]

bdhedhy19,

I am not trying to hijack the thread... so I hope that Krickos' suggestions will help you out. You might also look at:

http://www.restek.com/catalog/view/5502/31822

from another thread on anti-foaming in VOA samples. Might be worth looking at.

Krickos,

First it appears that USP 211 already converts to arsine (I will look to see if I can get my hands on the method in the next few days.) Which raises another question in my mind about loss to the headspace. Second, arsine has an IP of 9.9 so it should be easily do-able with a PID and has already been done by argon ionization detector. Relatively inexpensive compared to AED or ICP but one has to be aware of interferences of course.

Best regardss,

AICMM

[bdhedhy19]

Thanks Krickos and AICMM!
I will start working on my samples next week. And will let you know how it goes.

thanks a bunch!:)

[bdhedhy19]

Hi krickos!
arsenic digestion by USP <211>
I did the sample digestion for 8 hours with 1:1 H2SO4 and 20 drops of H2O2 and finally after 8 hours
of digestion when the sample looks like pale straw color, cooled the samples and added 10 ml DI H2O and increased the temperature to get rid off traces of per oxide. I started the color development with the addition of 4 ml of Isopropyl alcohol + 20ml 7N H2SO4+2ml KI+ 0.5 ml Stannous chloride and still it was fine. When I added 3 grams of Zinc granules
and closed it with the absorber contained 3ml of Silverdiethyldicarbamate reagent. it started forming foams in 30 minutes i saw almost like an inch of foam in it. My Spike/Monitor messed up otherwise i would see if the foam is causing the interference. But next week i will try again. Any trick or clue to digest these samples to get rid off or to destroy the organics in it. let me know.
Thanks

[krickos]

Hi

OK, well an inch of foam might be doeble (non excessive), lets see how the spiked sample becomes next week.

There are some more aggressive sample preparations like the one below from the Chinese pharmacopiea, note though that sample should initially not be exposed to temperatures above ~540°C, depeding of arsenic salt formed melting may occur just below 600°C.

weigh 1g of product accurately, mixed with 1g of CaOH, add water with small amount, mix up, warm lightly (avoid boling off liquid) then burn to ashing totally and to off white, cool.

Adding water to dissolve would not be enough, a few ml 7N H2SO4 and water totaling 10ml as in USP would be recommended, of course running a spiked sample and pure standard side by side.

[bdhedhy19]

Thanks Krickos!
I will try this USP <211> next week with better temperature conditions and spike too. if it doesn't work than i will try Chinese Pharmacopoeia.
I will add more glass beads in the digestion to avoid the bumping of sample plus i could increase the temperature up to
130C during digestion and later after digestion i can go higher than 130C to get rid off trace of H2O2. Will let you know
Thanks
Have a nice weekend:)

[bdhedhy19]

Hi krickos!

Arsenic works really great this time:) I added 20ml of 1:1 H2SO4 and 20 drops of H2O2 and around 50 glass beads and digested at 99C for 8 hours, Get rid off traces of H2O2 at 110C for almost 1 hour and during color development< I added 3 ml IPA to avoid the foams and it was perfect, only 1 cm of foam, sample and spike works great too after addition of 20 ml 7N H2SO4+2ml KI+0.5 ml stannous chloride etc etc...

thanks a bunch!