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Old 08-17-2011, 11:24 AM   #21
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Originally Posted by dwarven_stout View Post
I've tried them, and was not impressed. OTOH, I have access to those $200 quartz cuvettes so it doesn't matter how not impressed I am. Great writeup, btw.
I guess I should have said I haven't tried them at UV or in particular for this MOA. I use them all the time for color determination and they are fine for that. The concerns with this MOA would be differences in absorption between the blank and sample cuvets and the fact that these are open to the air but I suppose one could put parafilm over the tops. For color determination one eliminates the differential problem by using the same cuvet for blank (DI water) and sample.
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Old 08-17-2011, 11:28 AM   #22
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Right. I've got a 20-yr old boy taking chem in college starting next week. He has his instructions. The plan is coming together.
I'll be interested to hear about how that works out. Regulations (anti terror, environmental, drug precursor...) have beer proposed and in some cases implemented that impose accounting requirements on users of chemicals that would practically prevent a university from teaching lab courses (I would think). I know this is just gasoline (very expensive gasoline but still gasoline) but the world is now just a crazy place.
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Old 08-17-2011, 12:03 PM   #23
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Due to the anti-terror environmental and blah blah blah regulations most of my chem labs are built round household consumer products for cleaning and cooking. They just happen to use all the fancy glass and electronic whizbang gizmos.

I was actually looking at doing this testing for a local brewery as a school project. It was tough to get anyone to agree to purchasing the necessary reagents for the reaction. The brewery isn't tiny but they're not big and $300 in alcohols was not entirely interesting to them. The school wanted to allow me to have the experience but didn't want to buy the $300 in alcohols either. I'd still like to get some experience doing the testing I guess I'll just have to wait.

I want to come hang out at your house AJ. I'd love to geek it up beer science wise.

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Old 08-17-2011, 02:14 PM   #24
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I guess I should have said I haven't tried them at UV or in particular for this MOA. I use them all the time for color determination and they are fine for that.
Yeah, same. I use the plastic ones all the time for photocatalysis studies, but whenever I go into UV I use the quartz.
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Old 03-25-2013, 02:03 AM   #25
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Didn't think the book was that expensive!

What you need:

[snip]
6. 3 N hydrochloric acid
[snip]

What you do:

[snip]
5. Pipet in 1 mL 3 N hydrochloric acid
I've measured the pH of three of my beers. They were all 6.0 +/- .1
For this IBU test what is the target pH?

Thanks,
Paul
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Old 03-25-2013, 04:27 AM   #26
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Something funny there. Most beers have pH's in the 4's. But that's a separate question. There is no 'target pH' for the test. It just needs to be low enough that none of the bittering acid molecules are dissociated i.e. they are not charged and therefore more soluble in the gasoline phase than in the water (beer) phase. One cc of 3 N acid would deliver (3/1000) moles to 11/1000 liter of non gasoline phase i.e. 3/11 moles/L which is enough to set the pH to something like 0.6. That's obviously plenty low enough.

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Old 03-25-2013, 09:49 AM   #27
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Something funny there. Most beers have pH's in the 4's. But that's a separate question. There is no 'target pH' for the test. It just needs to be low enough that none of the bittering acid molecules are dissociated i.e. they are not charged and therefore more soluble in the gasoline phase than in the water (beer) phase. One cc of 3 N acid would deliver (3/1000) moles to 11/1000 liter of non gasoline phase i.e. 3/11 moles/L which is enough to set the pH to something like 0.6. That's obviously plenty low enough.
ajdelange,
Thanks for your reply. I have muriatic acid (31.45 HCl) which should be about 10.2 N. I can go from there with your answer.

I measured the pH of the beers right out of the tap without degassing them. I'll pull another sample of one and let it go flat and then measure the pH again. Don't know if that is the reason that my pH doesn't match your experience (i.e., ~4 pH).

Paul
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Old 03-25-2013, 12:57 PM   #28
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Carbonic acid lowers the pH so a degassed sample will read slightly higher than one that still has CO2 dissolved.

Suggest you review http://www.homebrewtalk.com/f128/ph-...ration-302256/ to make sure there isn't something you are overlooking there. Your mash should read 5.4 or so and the pH should be a bit lower than that in each successive step of the brewing process finishing well under 5 (how much depending on the beer).

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Old 03-26-2013, 01:16 AM   #29
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For reference, I've used the disposable plastic (both PS and PMMA) in my QC chem lab for all sorts of routine analyses without issue, so long as the detection wavelength is in the Vis range - turbidity, color matching, etc. For true UV applications (anything below 300 nm) the quartz cuvettes are unfortunately a necessity. I don't have the comparative absorbances in front of me, but the PMMA will pretty much dampen any uv light to useless levels below 300 nm. Pity, because they're so cheap.

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Old 03-26-2013, 01:37 AM   #30
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Carbonic acid lowers the pH so a degassed sample will read slightly higher than one that still has CO2 dissolved.

Suggest you review http://www.homebrewtalk.com/f128/ph-...ration-302256/ to make sure there isn't something you are overlooking there. Your mash should read 5.4 or so and the pH should be a bit lower than that in each successive step of the brewing process finishing well under 5 (how much depending on the beer).
I let a small sample of beer sit overnight to go flat. The next morning it had dropped from 6.0 to 5.5 pH.

I checked the calibration of my pH meter when I first got it a few weeks ago. It read the 4.0 buffer as 4.0.; the 7.0 as 7.0; and the 10.0 buffer as 10.1. Close enough.
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