Reef Chemistry Question of the Day # 174 pH Buffering

[Randy Holmes-Farley]

Reef Chemistry Question of the Day # 174

Because of the various buffers in seawater, especially the bicarbonate/carbonate system, the pH of seawater is buffered against pH changes.

The buffering capacity of seawater depends on a variety of factors, and this question explores two of those factors.

If I add 5 mL of vinegar to a 100 gallons of seawater, the pH will drop. It drops a lot less than 5 ml of vinegar in 100 gallons of pure fresh water due to the buffering.

In which scenario below will the pH drop the least, immediately after adding the vinegar?

For purposes of this question, think about the actual change in pH. So a drop of 8.5 to 8.4 (0.1 pH unit) is less than a drop from 8.2 to 8.0 (0.2 pH units) and a drop from pH 7.8 to 7.7 is less than a drop from 8.5 to 8.3.

A. pH 8.5; alkalinity = 10 dKH
B. pH 8.5; alkalinity 7 dKH
C. pH 8.2; alkalinity 11 dKH
D. pH 8.2; alkalinity 8 dKH
E. pH 7.8; alkalinity 12 dKH
F. pH 7.8; alkalinity 6 dKH

There is only one correct answer, but partial credit for good reasoning on half of the answer.

Good luck!






















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[Myka]

Ok, so I know there is a formula for this, but I don't know it without Googling, and I'm not cheater, so this will be a guess...

First, let's write off B, D, and F because A, C, and E are the correlating pH with higher alkalinity, and high alkalinity will provide more buffering.

The alkalinity of A, C, and E differs by 1 dKH in each answer. The pH differs between A and C by 0.3 units and C and E by 0.4 units. ASSUMING the pH and buffering capacity of the alkalinity level rise in a graph with a straight line rather than curved line, I can remove answer A. This leaves me with answer C or answer E. Since RHF stresses the buffering capacity and the pH have a strong correlation, my GUESS is answer C.

So now I'm going to go Google it...

 

[beaslbob]

e

more alk and less ph to drop

 

[JimWelsh]

I agree with Myka about eliminating B, D, and F, and why. But I know that pH is a base 10 logarithmic scale. The number of moles of acidity (H+) being contributed by a given amount of vinegar is constant, but in the case of A, it will shift the pH down less than C by a factor of 2, and less than E by a factor of 5, because there is twice as much OH- at a pH of 8.5 than at a pH of 8.2, since 10^(8.5-8.2) = 10^0.3 = 2, and 5 times as much than a pH of 7.8, since 10^0.7 = 5. Answer is A.

 

[Cory]

I was also going to say A like Jim but dont have chemistry logic lol. My logic is A because the ph is highest and the alkalinity is highest so both would be a factor in "buffering strength".

 

[Myka]

I know that pH is a base 10 logarithmic scale.

This I didn't know - until I Googled the answer after I posted.

 

[JimWelsh]

This I didn't know - until I Googled the answer after I posted.

But that's not the "why" that Randy is going to describe in his answer. I'm right about the logarithmic thing, but wrong about the point Randy is trying to make (now that I've Googled it, too).

 

[Randy Holmes-Farley]

But that's not the "why" that Randy is going to describe in his answer. I'm right about the logarithmic thing, but wrong about the point Randy is trying to make (now that I've Googled it, too).

 

[Myka]

But that's not the "why" that Randy is going to describe in his answer. I'm right about the logarithmic thing, but wrong about the point Randy is trying to make (now that I've Googled it, too).

Yeah, but "the logarithmic thing" messes up my logic.

 

[nervousmonkey]

E. Less pH drop and a lot higher carbonate/bicarbonate buffer due to higher alk

 

[Myka]

But that's not the "why" that Randy is going to describe in his answer. I'm right about the logarithmic thing, but wrong about the point Randy is trying to make (now that I've Googled it, too).

Oh, and also I read the question wrong too, so I was doomed from the word "GO". I read the question as "Which would result in the highest pH after the vinegar addition?" If I would have read the dang question right, my answer would be different, but I think maybe I lead Jim down the wrong path too with my elimination of A, C, and E. Sorry Jim, I squirreled.

For purposes of this question, think about the actual change in pH.

Uhm, ya, that.

 

[Randy Holmes-Farley]

And the answer is...A. pH 8.5; alkalinity = 10 dKH

In seawater, it is the bicarbonate/carbonate and boric acid/borate buffer systems that help reduce the pH change when an acid or base are added.

Higher alkalinity means higher bicarbonate/carbonate (or, less likely, higher boric acid/borate), so there is inherently more buffering with more alkalinity, regardless of pH.

As to the pH effect, buffers are most effective near their pKa where there is half of the buffer couple in each form. For bicarbonate that is about pH 8.9 and for boric acid it is about pH 8.55. So the effect is highest above pH 8.5 and lower as the pH declines from there (the buffering picks up again down at the carbonic acid/bicarbonate pKa, which is around 5.8)

The more detailed explanation, including calculations at these different pH values, is here:

Chemistry And The Aquarium: Boron In A Reef Tank ? Advanced Aquarist | Aquarist Magazine and Blog
http://www.advancedaquarist.com/issues/dec2002/chem.htm

from it:

Buffering of Normal Seawater
The exact percentage of boric acid and borate in any aqueous system is dependent on pH. The pKa of boric acid in seawater is about 8.55, depending on the temperature.1 That is, the pH where both forms are equally represented is about 8.55 in a normal tropical reef tank. At lower pH values, such as those in most reef tanks, there is more boric acid than borate.

The fact that the two forms are related by equation 1 explains why boric acid and borate together form a buffer system:

1) B(OH)3 + H2O → B(OH)4- + H+ (pKa ~8.55)

If the pH in a system containing both forms were to begin to rise for any reason, some of the B(OH)3 would be converted into B(OH)4-, releasing a proton, H+. The pH would then not rise as much as it otherwise would. Likewise, if the pH in that system were to begin to drop for any reason, some of the B(OH)4- would be converted into B(OH)3, taking up a proton. The pH would then not drop as much as it otherwise would.

This effect is exactly how a standard buffer works.

The other system that significantly buffers the pH in normal seawater is the bicarbonate/carbonate system:

2) HCO3- → CO32- + H+ (pKa ~ 8.92)

The relative buffering of these two systems (equations 1 and 2) depends on the amounts of each system present, and also on the pH. For any given buffer system, it turns out that the pH at which it gives optimal buffering corresponds to the pKa, where there are equal amounts of each of the two forms of the buffer present. Why exactly this is true is beyond this article, but it relates to the fact that for a given incremental change in pH, more of the buffer will change form at that pH than would change form at any higher or lower pH, and moreover, the farther that the pH is from the pKa, the smaller the effective capacity of that buffer system to resist pH changes.

Buffering capacity can be quantified using the buffer intensity, b, defined mathematically in a way that is easy to calculate, but that isn’t worth detailing here.2 The units of the buffering intensity can be expressed as meq/L or meq/L/pH unit (these are equivalent since pH is really a dimensionless parameter). Thinking about it as meq/L/pH unit makes it easier to understand that it is a measure of the amount of alkalinity (or acidity; either one measured as meq/L) that needs to be added to impact the pH up or down by one unit (though that is a substantial simplification).

In the case of normal seawater at pH 8.2, b = 0.19 meq/L/pH unit for the boric acid/borate system, and 0.63 meq/L/pH unit for the bicarbonate/carbonate system. These values are additive, and result in a total buffering of b = 0.82 meq/L/pH unit. Under these conditions, the boric acid/borate system provides about 23% of the total buffering, while the bicarbonate/carbonate system provides about 77%.

If the pH of normal seawater is raised to 8.5, the total buffering is b = 1.2 meq/L/pH unit, or about 40% greater than at pH 8.2 (because both systems are closer to the pKa). At this pH, the relative contribution of the two systems to the total capacity is only slightly different than at pH 8.2, with 20% from borate and 80% from carbonate.

If the pH of normal seawater is lowered to 7.8, the total buffering is b = 0.42 meq/L/pH unit, or about half that at pH 8.2 (because both systems are farther from the pKa). At this pH, the relative contribution of the two systems to the total capacity is also only slightly different than at pH 8.2, with 29% from borate and 71% from carbonate.

 

[JonasRoman]

very interesting
question: how do you calculate b, total buffering intensity, b?
and the relative contribution, the distance from the pka, how do you calculate that?

 

[GARYSODERLUND]

I HAVE A PEN PH TESTER, IT SAYS TO CALIBRATE. GOT THAT, BUT THERE IS ONLY 1 PACKET OF EACH TEST RANGES, QUESTION: CAN I USE VINEGAR=2 AND BAKING SODA =9 TO CALIBRATE. THESE SEEM TO BE CONSTANT NUMBERS. AND =

 

[Randy Holmes-Farley]

very interesting
question: how do you calculate b, total buffering intensity, b?
and the relative contribution, the distance from the pka, how do you calculate that?

Buffering intensity is formally described as

B = d(Cb)/dpH or -d(Ca)/dpH

it gets hard to write these equations in this sort of forum, but here's a link to a book with it, plus they have some graphs:

https://books.google.com/books?id=Y...ved=0ahUKEwiUwsS415jLAhXFKh4KHT_vCjwQ6AEIOjAG

 

[Randy Holmes-Farley]

I HAVE A PEN PH TESTER, IT SAYS TO CALIBRATE. GOT THAT, BUT THERE IS ONLY 1 PACKET OF EACH TEST RANGES, QUESTION: CAN I USE VINEGAR=2 AND BAKING SODA =9 TO CALIBRATE. THESE SEEM TO BE CONSTANT NUMBERS. AND =

I don't think those are optimal choices, but most importantly, most pH pens require a specific pH for calibration, like 4, 7 or 10.

Does yours allow for input of the pH of the buffers? Some pH meters do (like one I have), but many do not.

 

[JonasRoman]

Buffering intensity is formally described as

B = d(Cb)/dpH or -d(Ca)/dpH

it gets hard to write these equations in this sort of forum, but here's a link to a book with it, plus they have some graphs:

https://books.google.com/books?id=YDRxgf-ZeOkC&pg=PA20&lpg=PA20&dq=calculation+of++"buffering+intensity"&source=bl&ots=I0ZW0oKT7E&sig=kuDir-kXlqTgxCEOqej242DUjqI&hl=en&sa=X&ved=0ahUKEwiUwsS415jLAhXFKh4KHT_vCjwQ6AEIOjAG

thanks Randy i will study the weblink