Author Topic: A simple model for pH buffers  (Read 1813 times)

Offline Kaiser

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A simple model for pH buffers
« on: January 24, 2011, 06:45:01 PM »
This is something I started working on last June before I took a break from writing about brewing science. I finally completed this short article which demonstrates pH buffers using a simple model of vertical pipes that are filled with water:

A simple Model for pH Buffers

Kai

Offline hopfenundmalz

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Re: A simple model for pH buffers
« Reply #1 on: January 24, 2011, 07:23:39 PM »
Kai,I will study that tomorrow after a cup of coffee.   It looks to be something that I can learn from, as ususal.  Right now it makes my head hurt.
Jeff Rankert
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Offline majorvices

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Re: A simple model for pH buffers
« Reply #2 on: January 25, 2011, 05:31:42 AM »
Hey Kai, when you check pH are you rinsing the probe with tap water between calibrations? Thats what I do, then I shake off excess water and gently dry the tip - but I always wonder if this is correct procedure.
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Offline wingnut

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Re: A simple model for pH buffers
« Reply #3 on: January 25, 2011, 06:00:56 AM »
Hmph, I never thought of it like that before... but that is really a great way visualize.  A lot of the things I have been reading just went "click" after reading through the article and viewing the graphic representations.

Thanks Kai!
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Offline Kaiser

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Re: A simple model for pH buffers
« Reply #4 on: January 25, 2011, 07:15:00 AM »
Hey Kai, when you check pH are you rinsing the probe with tap water between calibrations? Thats what I do, then I shake off excess water and gently dry the tip - but I always wonder if this is correct procedure.
Ideally you want to rinse the probe with distilled water in order to remove as much buffer substance as possible. Tap water, especially when it is highly alkaline, is also a buffer.

Personally I use R/O water from the tap and I actually reuse the buffer solution over a few months. I found that those calibration buffers are strong enough that they will not drift when the probe is properly rinsed. Every 6-8 month I’ll refresh the buffers from a larger bottle and never noted a drift of more than 0.02 pH.

I even go so far that I don’t calibrate the pH meter before every use. In the past I haven’t seen much change in the calibration which is why I only calibrate when I really care about precise pH measurements. For experiments for example.

Hmph, I never thought of it like that before... but that is really a great way visualize.  A lot of the things I have been reading just went "click" after reading through the article and viewing the graphic representations.
 
You’re welcome. This is what I was going for.

The commonly used model of a scale with alkalinity on one side and malt and acids on the other is somewhat flawed since it does not take into account that more malt means more buffering potential. I can also not account for the fact that a pH buffer can be broken when the pH shifts far enough away from the buffer point (pKa). But that doesn’t really matter too much for pH adjustments in the mash.

Kai



Offline Mark G

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Re: A simple model for pH buffers
« Reply #5 on: January 25, 2011, 07:26:57 AM »
Very well done. Thank you for putting that together.
Mark Gres

Offline roger

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Re: A simple model for pH buffers
« Reply #6 on: January 25, 2011, 07:32:57 AM »
Being a beginner, I always have questions about water chemistry. I've been using that 5.2 buffer product, because it seems the "safe" way to keep my mash pH in the correct area. If I could attempt to correlate that product to this example - would that be like adding a very thick vessel right at the 5.2 pH level, thereby requiring a great amount of either an acidic solution or alkaline solution to change the pH?

Oh, BTW this was a "two" coffee cup article. Thanks for sharing it.
Cheers,

Roger
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Offline Kaiser

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Re: A simple model for pH buffers
« Reply #7 on: January 25, 2011, 08:08:52 AM »
Being a beginner, I always have questions about water chemistry. I've been using that 5.2 buffer product, because it seems the "safe" way to keep my mash pH in the correct area. If I could attempt to correlate that product to this example - would that be like adding a very thick vessel right at the 5.2 pH level, thereby requiring a great amount of either an acidic solution or alkaline solution to change the pH?
 

Yeah, the 5.2 stuff. Don’t get me started on that ;)

Well, you did. 5.2 is not a buffer in the correct sense of a pH buffer. And if it is, it is a very weak buffer. I have done a number of experiments with it and it doesn’t work as advertised. First off, it is made with phosphate salts (namely sodium phosphate) and using the recommended amount boosts your beer sodium content by 100 mg/l. Not really a problem when your water is low in sodium.

Secondly, phosphate salts are not good for forming a good buffer at 5.2. This is because there is no “bulge” at 5.2 for phosphate. The “bulges”, i.e. pH ranges for good buffering for phosphate are at 3.1-4.1, 6.2-8.2 and 11.3-13.3.

Since it is not a good buffer it does little from keeping the pH from falling too far. Although it is not a common problem in brewing it is an indication that this product doesn’t work as advertised. With respect to keeping pH from rising it doesn’t seem to buffer the pH at 5.2 but it seems to “cap” the pH at 5.8. This means using this product prevents your pH from rising too far which might be why this product seems to work when brewers use it. This effect may come from calcium and phosphate reactions and not the buffering action of the phosphate. I haven’t confirmed that yet.

If this product makes a difference for you I recommend spending the money on a Ward Labs water test or buying a GH&KH test kit to do your own simple water test. With that information you can figure out simple water and/or grist modifications that allow you to get your pH into the appropriate range w/o having to add a lot of sodium.

Kai

Offline tomsawyer

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Re: A simple model for pH buffers
« Reply #8 on: January 25, 2011, 08:17:20 AM »
As a practical matter, what do you think are the primary compounds that buffer in the correct mash pH range?  Its a little unsatisfying to say that the "malt" is the buffer.

I think we're talking about phosphate at the low end of its useful buffer range, phytin which is a more complicated buffer, bicarbonate/carbonic/CO2 which is functional down to pH 5.1, and free amino acids that can act as pseudo-buffers.
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Offline mabrungard

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Re: A simple model for pH buffers
« Reply #9 on: January 25, 2011, 08:25:28 AM »
I'm with Kai on this issue of brewers using the 5.2 stuff.  With just a little bit of education, a brewer can better accomplish what this product tries to do.  Note that I said 'try to do'.  It is not always successful and some waters cannot be corrected with this product.  

This stuff adds sodium to the mash and that carries over to the finished beer.  Sodium is only desirable in wort at modest concentrations.  Using this product to correct mash pH can lead to some unwelcome taste consequences.  

And as Kai pointed out, this product does not really help a mash stay at a truely desirable pH, it keeps it from moving to a too high range.  Unfortunately, the mash is already compromised at that upper end.   Stay away from snake oil.  Learn how to test your water alkalinity and how to adjust it.  Your beer will be better for it.

With regard to Lennie's quest, read one of AJ's articles on Malt phosphates here:
http://ajdel.wetnewf.org:81/Brewing_articles/Cerevesia/Final_galley
« Last Edit: January 25, 2011, 08:29:32 AM by mabrungard »
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Offline Kaiser

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Re: A simple model for pH buffers
« Reply #10 on: January 25, 2011, 08:33:06 AM »
As a practical matter, what do you think are the primary compounds that buffer in the correct mash pH range?  Its a little unsatisfying to say that the "malt" is the buffer.

I should refine this. The primary buffers in malt are phosphates and amino acids, I think Lewis and Bamford mention this in their book “Essays in Brewing Science”. The latter are mostly bound in proteins. I think there will be a few more compounds in the mash that will not be in wort, but sugars and starch do not buffer pH.

Quote
I think we're talking about phosphate at the low end of its useful buffer range, phytin which is a more complicated buffer, bicarbonate/carbonic/CO2 which is functional down to pH 5.1, and free amino acids that can act as pseudo-buffers.

Lennie, you may have seen this already, but here is a titration curve for a mash:



There are no distinct pKas that one could identify since there are a multitude of buffers at work in the mash. What matters to us brewers is how well the mash is buffered between pH 5.0 and 6.0.

Kai



Offline tomsawyer

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Re: A simple model for pH buffers
« Reply #11 on: January 25, 2011, 08:38:31 AM »
Proteins probably aren't a good buffering moelcule since their amines and carboxylics are connected so only the ends of the molecule and any charges side groups are going to be available to bind ions.  The smaller proteins will have some more effect, I think the free amino acids are where its at.

Martin the link didn't work for me.
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Offline mabrungard

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Re: A simple model for pH buffers
« Reply #12 on: January 25, 2011, 08:43:21 AM »
The link works, it just takes time.  Give it several minutes.  It must be running from a home computer.
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Offline Kaiser

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Re: A simple model for pH buffers
« Reply #13 on: January 25, 2011, 08:52:38 AM »
Proteins probably aren't a good buffering moelcule since their amines and carboxylics are connected so only the ends of the molecule and any charges side groups are going to be available to bind ions.  The smaller proteins will have some more effect; I think the free amino acids are where it’s at.
 

Thanks. I didn’t think about the amines and carboxylics being occupied. I was more thinking about the charged side chains.

A.J.’s server has always been a bit slow and unavailable from time to time. When I asked he allowed me to host his alkalinity papers but I haven’t mirrored anything else.

Phosphate is a prime suspect for major mash/malt buffer since the titration curve I showed does have a distinct buffer around pH 7 where one of the pKas for phosphoric acid (7.21) lies.

Kai

Offline roger

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Re: A simple model for pH buffers
« Reply #14 on: January 25, 2011, 10:25:00 AM »
Wow, thanks for all the super advise and information. I have so much to learn. Instead of really understanding the water chemistry, I was taking the lazy approach and trusting the product did what was claimed. BTW, the link did work for me, and it did take several minutes. But I will have to make another pot of coffee.

One question, are the test kits necessary if I have the report from my municipal supplier? I do realize the reports are averages, and can vary quite a bit by season. Whoops, did I just answer my own question? The test kit would also be useful when using or blending different water.
Cheers,

Roger
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