Reef Chemistry Question of the Day #19

Randy Holmes-Farley · Oct 1, 2014

Reef Chemistry Question of the Day #19

We'll go with a little bit harder one today!

The pH buffering capacity is an indication of the ability of a water sample to resist changes in pH when small amounts of acid or base are added.

In which of these solutions is the buffering capacity the highest?

A. Natural seawater at pH 7.7
B. Natural seawater at pH 8.0
C. Natural seawater at pH 8.2
D. Natural seawater at pH 8.4

Good luck!

.

cope413 · Oct 1, 2014

Hmm... I'll fancy a guess.
(A) Has the highest buffer capacity for bases.
(D) Has the highest for acids

leptang · Oct 1, 2014

I dont know but ill guess D, and wait for the answer.

beaslbob · Oct 1, 2014

NOt enough information?

Novastar808 · Oct 1, 2014

A! cuz 7 is neutral

Eric B · Oct 1, 2014

I will take a stab and say "D".

leptang · Oct 1, 2014

I think its D because you would need more acid to drop the alkalinity???

Reefing Madness · Oct 1, 2014

To buffer acid? Id say none of the above. They will all drop and will all return to normal ranges if properly aerated.

reggaedrummin · Oct 2, 2014

I'm curious to learn more on this

Randy Holmes-Farley · Oct 2, 2014

reggaedrummin said:
I'm curious to learn more on this

Stay tuned!

I usually post the answer in about 36 hours after posting the question, and this answer will have a detailed explanation of what buffering capacity is, what causes it, and how that is impacted by pH in seawater.

Happy Reefing.

cginter · Oct 2, 2014

guessing
"A"

Randy Holmes-Farley · Oct 2, 2014

And the answer is..................D. Natural seawater at pH 8.4

The buffering capacity of seawater and reef aquarium water is lowest at the lowest pH values normally obtained in the tank (in the 7's) and highest at the higheest pH vlaues obtained (mid 8's).

Here's an article section that explains why:

[h=2]What is "Buffering"[/h]Buffer and buffering are terms that are thrown around indiscriminately in the world of reefkeeping, and the actual meaning of these terms is often lost. Many aquarists refer to any alkalinity supplement as a buffer, but this isn't the case. For example, neither sodium bicarbonate nor sodium carbonate, taken alone, is a true buffer.
A buffer is something that helps minimize pH changes in the presence of added acid or base. No buffer can completely stop the pH from changing when acid or base is added. The change in pH, however, is made smaller when an appropriate buffer is used. A buffer is almost always comprised of two different chemical entities. Bicarbonate and carbonate together, for example, form a buffer in the pH range from about 8 to 11 in seawater, though the buffering is best between about 8.5 and 10.0 (ignoring that at high pH some calcium carbonate may precipitate).
Here's what is happening on a chemical level. When a base (such as OH^-) is added to the system (in an effort to raise pH), some of the bicarbonate is converted to carbonate. This process effectively "uses up" some of the OH^-that was added, and the pH does not rise as much as it would without the "buffer".
4. HCO₃^- + OH^- → CO₃^--
So overall, we have:
5. HCO₃^- and CO₃^-- + OH^- → less HCO₃^- and more CO₃^--
When an acid (H⁺) is added to the system (in an effort to lower pH), some of the carbonate is converted to bicarbonate. This process effectively "uses up" some of the H⁺ that was added, and the pH does not drop as much as it would without the "buffer".
6. CO₃^-- + H⁺ → HCO₃^-
So overall we have:
7. HCO₃^- and CO₃^-- + H⁺ → more HCO₃^- and less CO₃^--
Of course, in order for this process to buffer against both pH rises and pH drops, there must be a significant amount of both HCO₃^- and CO₃^-- present. At about pH 8.9 in seawater at 25 Â°C there are equal concentrations of HCO₃^- and CO₃^--. At lower pH, there is less CO₃^--, and at pH 8.0 there is really quite little carbonate in seawater (only about 4% of the bicarbonate level). Consequently, seawater is not especially well buffered against substantial pH drops when the pH is already less than 8.0. It is, however, well buffered against substantial pH rises.
Here's an actual experiment. Take artificial seawater (Instant Ocean made to S=35; alkalinity measured to be 2.26 meq/L by titration) and add 0.5 meq/L of either acid or base. The results of an immediate pH measurement (before atmospheric carbon dioxide has a chance to equilibrate) are:
Starting Solution pH = 8.10

0.5 meq/L OH^- added pH = 8.76
0.5 meq/L H⁺ added pH = 6.91

As we can see, the added base (OH^-) drives the pH up by only 0.66 units, while the added acid dropped the pH by 1.19 units. This result shows that the water is better buffered against a pH rise than a pH drop, and the reason for this difference is simply that there is more bicarbonate than carbonate at pH 8.1. The only reason that the drop stops at pH 6.9 is that at that point, bicarbonate is really doing the buffering as it is converted into carbonic acid:
8. HCO₃^- + H⁺ → H₂CO₃
The difference in the buffering against substantial pH rises and drops is obvious from such an experiment. Nevertheless, there is much more to fully understanding how a buffer works. Chemists have chosen the term "buffer intensity" (symbolized by b, which supposed to be a beta, but the software can't handle it) to reflect the buffering capacity of a solution at any given pH. While it has an exact mathematical definition, it is beyond the scope of this article to describe beta in detail. There are, however, a few details worth mentioning in the context of reefkeeping (with additional details are provided in "Aquatic Chemistry Concepts" by James Pankow).
The most important fact to reefkeepers is that the buffering due to bicarbonate and carbonate, at a given pH, is directly related to the carbonate alkalinity. If you double the alkalinity, you double b, and hence have twice as much buffering due to the carbonate and bicarbonate system. In normal seawater, the carbonate/bicarbonate system provides a substantial portion of the total buffering (which is quantified below). Consequently, marine aquaria with higher alkalinity tend to have greater buffering against pH swings.
Other, more esoteric tidbits arise from this system as well. For example, while the buffering against substantial changes can be different in the two different directions (as shown experimentally above), the buffering against very small changes is necessarily exactly the same. That's what b represents: the buffering against infinitesimally small changes in pH in either direction. b changes as a function of pH, and is maximal when the concentrations of the two forms of the buffer (e.g., bicarbonate and carbonate) are equal. In seawater, b is locally maximized around pH 5.8 where the buffer is H₂CO₃/HCO₃^- and at pH 8.9 where the buffer is HCO₃^-/CO₃^-- (ignoring the fact that other things happen at high pH, like precipitation of magnesium hydroxide and calcium carbonate). These are the points where seawater is most resistant to changes in pH as acid or base is added. Unfortunately, reef tanks are not usually kept at those pH values, and so the buffering effect of the carbonate system is not as effective at holding pH steady as it might otherwise be.
This lack of effective buffering at a normal tank pH is one of the reasons that some salt manufacturers (Seachem, as told to me by the late Leo Morin, and possibly to a lesser extent, Coralife) boost the borate concentrations in their salt mixes (or at least did so in the past). Since the pKa of borate in seawater is about 8.6, its maximal b (pH 8.6) is not far from the range experienced by reef tanks. In normal seawater at pH 8, borate provides about half of the buffering against a small downward pH change, despite the fact that it provides less than 3% of the total alkalinity. In Seachem salt, where borate is (was) about 10x natural levels, borate totally dominates the buffering in the pH range experienced by reef tanks.