Post boil Ph in Gose

[Morrey]

Kettle souring went nicely in the Gose with 3.22 going into the boil. 7 gallons was my pre-boil volume and 5.75G was my post boil volume going into the fermenter.

A small amount of hops were added into the boil as well as sea salt and cracked coriander. Since I boiled off 1.25 gallons, I naturally assumed my ph would lower slightly since my acidic wort was more concentrated at 5.75G than when it started at 7G.

To my surprise, the post boil ph actually rose and is now at 3.25. Was the ph change due to the additions I made?

 

[ajdelange]

A pH change for 3.22 to 3.25 is only worthy of comment if you were scrupulous with respect to the measurements e.g. calibration checked or repeated before each measurement, sample and buffer temperatures the same (or within a couple of degrees), meter known to be stable.

If indeed the pH did rise by 0.03 pH it could be from any number of causes. Note, first, that concentrating an acid moderately does not necessarily lower the pH of the solution. When you get into the lower end of the pH range you have to remember that, despite what you may have been told in highschool or college chemistry, pH isn't a measure of hydrogen ion concentration but rather of activity. If the concentration increases but the activity coefficient decreases (unlikely) the pH could go up. Beyond that at pH 3.3 the concentration of H+ ions is about 1/2 mmol per liter. If you remove a couple of liters of water you haven't returned that much H+ to the solution and if there is enough of the anion of the beer acids present (and there certainly will be more than 1/2 mmol/L) their pickup of those H+ won't change the pH much.

Hops and coriander are very probably basic relative to pH 3.2 so their addition would probably cause a pH rise. I'd put my money on that or measurement error.

 

[RPh_Guy]

Hops and coriander are very probably basic relative to pH 3.2 so their addition would probably cause a pH rise. I'd put my money on that or measurement error.

Yeah, what he said
Home pH meters have a margin of error.

 

[Morrey]

A pH change for 3.22 to 3.25 is only worthy of comment if you were scrupulous with respect to the measurements e.g. calibration checked or repeated before each measurement, sample and buffer temperatures the same (or within a couple of degrees), meter known to be stable.

If indeed the pH did rise by 0.03 pH it could be from any number of causes. Note, first, that concentrating an acid moderately does not necessarily lower the pH of the solution. When you get into the lower end of the pH range you have to remember that, despite what you may have been told in highschool or college chemistry, pH isn't a measure of hydrogen ion concentration but rather of activity. If the concentration increases but the activity coefficient decreases (unlikely) the pH could go up. Beyond that at pH 3.3 the concentration of H+ ions is about 1/2 mmol per liter. If you remove a couple of liters of water you haven't returned that much H+ to the solution and if there is enough of the anion of the beer acids present (and there certainly will be more than 1/2 mmol/L) their pickup of those H+ won't change the pH much.

Hops and coriander are very probably basic relative to pH 3.2 so their addition would probably cause a pH rise. I'd put my money on that or measurement error.

Thanks, AJ, I appreciate the feedback. Always learning and benefitting from your knowledge.

Small ph changes such as this are typically non-detectable on the tongue, but it was the theory that confused me...and you explained it nicely debunking the concentration myth.

I always calibrate my Hach Pocket Pro meter between uses, and I think this may be a good habit. You may remember you told me a product called "Zymit" may be helpful for cleaning, so I called the company and they sent me a pint sample bottle. When diluted 50 to 1, a pint will last me a longggg time. Thanks, AJ.

 

[ajdelange]

That's how I got on to it. A free sample from Cole Parmer. That's the first and last time I got anything free from those guys but it did last a long time!

 

[RPh_Guy]

despite what you may have been told in highschool or college chemistry, pH isn't a measure of hydrogen ion concentration but rather of activity. If the concentration increases but the activity coefficient decreases (unlikely) the pH could go up.

I'm trying to wrap my head around why the textbook authors might have been wrong, or rather perhaps oversimplifying.
If it's possible to decrease acidity by boiling wort with no boil additions, under what circumstances might this occur? Could you design an experiment to test this? (Something plausible that might occur during brewing)
I've always heard pH decreases during the boil because of phosphate precipitation.
What could change the lactic acid's activity coefficient?

@Morrey
One other possibility occurred to me:
If your Lacto (or contaminant) had trace acetic acid production it would be expelled during the boil. Seems like that would increase pH, from my limited understanding.

 

[ajdelange]

I'm trying to wrap my head around why the textbook authors might have been wrong, or rather perhaps oversimplifying.

I think you have and simplification is the answer. Were you taught about non-ideal behavior in your high school and college freshman courses? If I was I don't remember it. We hated equilibrium problems enough without added complexities of having to deal with activities.

If it's possible to decrease acidity by boiling wort with no boil additions, under what circumstances might this occur?

Could you design an experiment to test this? (Something plausible that might occur during brewing)[/QUOTE]

When I wrote

If the concentration increases but the activity coefficient decreases (unlikely) the pH could go up.

I meant to imply that I couldn't think of any conditions under which the activity coefficient would be reduced but supposed that there might be such conditions unknown to me. Thus no, I can't think of any demonstration of this in brewing. Also, to reiterate, I suspect OP's problem most likely relates to measurement variation or the probable alkalinity of hops and coriander relative to the low pH he observed.

I've always heard pH decreases during the boil because of phosphate precipitation.
What could change the lactic acid's activity coefficient?

What would change the activity of lactic acid would be the addition or removal of ions to or from the solution. The Debye-Hückle theory lets us calculate a (geometric) mean activity correction term from the sum of the concentrations of all the ions weighted by the squares of their charges. With the p operator applied this results in a term in the Hendersson Hasselbalch equation often symbolized by pfm. The form of Hendersson Hasselbalch of interest to us here is the expression for the ratio of the number of lactic acid ions that dissociate to the number that don't. This is r = 10^(pH - pK + pfm) = 10^(pH -(pK - pfm)). Thus the lactic acid's effective pK is pfm less than its ideally dilute pK if other ions are present (and their behaviour matches what Debye - Hückle predicts). Thus adding neutral salts will decrease the pH of a solution and removing them will increase the pH of a solution. So then we might consider a wort in which there is, for whatever reason, fair numbers of calcium and bicarbonate ions present. Boiling such a wort would concentrate it but the reaction 2HCO3- + Ca++ ---> CO2 + CaCO3 + H2O would also occur. Note that no H+ ions are produced in this reaction (as they are in the phosphate reaction). Removal of HCO3- and Ca++ ions will result in a decrease in pfm and an effective increase in lactic acids pK and the wort pH will rise. So now I have thought of conditions in which boiling would result in a pH increase but they are pretty far fetched. Further to that, as I mentioned in No. 2, some protons are returned to the mash when neutral water is boiled off and those might well offset the pH increase produced by diminished pfm which increase would be tiny.

@Morrey
One other possibility occurred to me:
If your Lacto (or contaminant) had trace acetic acid production it would be expelled during the boil. Seems like that would increase pH, from my limited understanding.

Yes, that sounds very reasonable and would apply to any other volatile acid as well.

 

[RPh_Guy]

That makes sense. Fewer ions results in less dissociation of a weak acid. Just a small part of the reactions happening during the boil I suppose. The explanation is much appreciated!

I don't recall learning about non-ideal behavior either, but my inorganic chemistry studies simply provided a foundation for organic chemistry, biochemistry, and medicinal chemistry. I wish I could remember half of it.

Thanks again!

P.S. If you highlight part of a post and then hit reply on the pop-up, it inserts that as a quote in your draft.

 

[Morrey]

I had a feeling that "concentrating" acidified wort wasn't the answer to my ph thoughts, and thanks to both of you for resolving my question. Water chemistry is a pretty in-depth subject, and it's no wonder so many brewers gloss over it and just use RO water w/o really digging in deeper. Based on the threads we see from less experienced brewers, some (or many) may feel water make-up is rather intimidating.