Understanding Reef Chemistry...

[Randy Holmes-Farley]

Here it is...

I just completed reading "Reef Aquarium Chemistry for Beginners". While I have clarification on some questions, I think I have more now than when I started. For example...

I understood that dosing with baking soda would raise pH. I took 1000ml of RO/DI water and added 15g of baking soda. Stirred well and then added two 200ml doses, 15 minutes apart. The baking soda solution initially drove the pH down and then came back up to where it started prior to the dose.

Isn't the opposite supposed to happen?

Nope. Baking soda (sodium bicarbonate) slightly lowers pH when first added. If you bake it first, or use washing soda (sodium carbonate), that will raise pH when first added.

 

[Randy Holmes-Farley]

I'm just starting to read this. It's like Russian alphabet soup; lots of letters, I can't read any of them.

This is the problem; there isn't any "plain English" explanations on the subject.

lol

Sure there are. But it is science, and dumbed down explanations of many reef chemistry ideas tend to be factually incorrect and misleading. They really frustrate me when folks come here with ridiculous chemistry simplifications they have read elsewhere. :(

This one should fit your needs (as someone posted above):

The “How To” Guide to Reef Aquarium Chemistry for Beginners Part 3: pH by Randy Holmes-Farley - Reefkeeping.com
http://reefkeeping.com/issues/2007-05/rhf/index.php

 

[Dom]

Did you bake the baking soda in the oven first?

No.... this is something new to me.

So to use baking soda as a means to drop pH, I use it as is out of the box. But to bring up pH, I bake it first? How is that done?

(FYI: My pH is 8.15 with the lights out as I type this post).

 

[saltyfilmfolks]

No.... this is something new to me.

So to use baking soda as a means to drop pH, I use it as is out of the box. But to bring up pH, I bake it first? How is that done?

(FYI: My pH is 8.15 with the lights out as I type this post).

Nope. Baking soda (sodium bicarbonate) slightly lowers pH when first added. If you bake it first, or use washing soda (sodium carbonate), that will raise pH when first added.

 

[Randy Holmes-Farley]

Whats your alkalinity at? To understand it better I dont use the term alkaline. I prefer base. From what I under stand alkalinity and ph (even though they are related) are 2 different things. Ph is the potential of hydrogen which makes things either acidic or basic depending how many ions of hydrogen and OH are in a solution. If there are more h ions then its acidic and if there are more oh ions its basic. Alkalinity is the buffering capacity of a solution as in the ability to resist changes to ph. If you take 2 glasses of water with a ph of 7, one with a alkalinity of 0 and the other with a high alkalinity and pour some acid in them both, the one with 0 alkalinity will drop in ph and stay there and the one with high alkalinity will probably initially drop but then raise back up to 7. If your ph is swinging, and your calcium is off, it is probably an alkalinity problem

FWIW, some of these ideas are too simplified to be accurate, and swinging pH in a reef tank does not imply low alkalinity. Alk can be high, and if CO2 rises and falls due to photosynthesis, pH will swing. Swinging pH reflects aeration effectiveness as much as alkalinity.

Also, alkalinity and buffering are actually different concepts and do not necessarily track with each other. In a reef tank I can change them in the same or opposite directions depending on what i add or remove from the water.

 

[Randy Holmes-Farley]

No.... this is something new to me.

So to use baking soda as a means to drop pH, I use it as is out of the box. But to bring up pH, I bake it first? How is that done?

(FYI: My pH is 8.15 with the lights out as I type this post).

One would NEVER (or almost never) use baking soda to lower pH. While it will happen for a brief period, the alk will get too high.

 

[Conor Vaughn]

Hi Dom,

pH fluctuations in an aquarium arise from concentration ( ie hydronium per gallon ). In the ocean, the concentration of hydronium ions remain constant between night and day because the scale or sheer size of the ocean cannot be so easily affected over a period of hours and is more or less 'rebooted' diurnally. pH is a mathematical representation where 'p' stands for '-log'. it is the inverse measurement of the amount of hydronium ions in a certain amount of water. A hydronium ion is a water molecule with an extra hydrogen added to it, so instead of H2O, you now have H3O. This extra free Hydrogen comes from a natural reaction between H2O (water), CO2 (carbon dioxide) and CO3 (carbonate ion). All happening simultaneously and with countless amounts of each compound involved, the two hydrogen (H) break from their bond in water and move to the less stable carbonate ion (CO3) in an effort to electrically stabilize its' charge. This new compound comprised of water and carbonate is called bicarbonate (H2CO3)- this is essentially what you are adding to your tank through the addition of baking soda (sodium bicarbonate or NaH2CO3). The sodium (Na) is what is called a 'spectator ion' and irrelevant in this reaction and in your tank. So, from our original ingredients we now have one Oxygen from the water compound (O) floating around, a CO2 and our new H2CO3 compound. The free (O) will bind with the CO2 to form a new carbonate (CO3), thus replacing the one that H2CO3 took and CO2 is replenished from the atmosphere. Now we've established how 'baking soda' (minus the Na) is naturally formed in your aquarium. From this H2CO3, which is floating around in a bunch of H2O's, a H can sometimes become separated and bind with a water (H2O) to form a hydronium (H3O). This hydronium is what pH is. Now let's look at this in terms of day and night. During the day, there is more free oxygen in the water (mostly from photosynthesis). The amount of oxygen offsets the amount of CO2. This slows the cycle at the point which CO3 can be made- less CO3 equals less H2CO3. if there is less H2Co3, then less H3O can be made which leads to a higher pH. Remember, pH is inversely proportional to the amount of hydronium. Higher pH equals less hydronium (I always have to pause and think that through.. math). So, if the problem is a low pH at night (more hydronium) the answer is to boost the oxygen levels at night to offset synthesis of carbonate ions. Oxygen and CO2 levels don't really change according to night and day in the ocean due to its' magnitude. This is often done through the addition of a refugium with a photoperiod opposite that of your main tank. You'll be getting oxygen through photosynthesis of macroalgae in the refugium. Adding baking soda is only ever a temporary fix (like, a few hour fix-maybe a few days depending on the weather) because it is in greater concentration than CO2. It can't effectively degrade down to its' useful form without CO2 present. you're sort of adding a bunch of baking soda and only a small amount of it can really do anything unless CO2 is simultaneously added in the same proportion. Anyway, lights on in tank during the day and lights on in refugium at night. Your tank will eventually find a good equilibrium with this system- unless you're growing a lot of SPS, but that's an equally as long explanation. Also, if you understand all this you pretty much understand the process of ocean acidification and global warmings affect on the sea! This is a simple process when explained through pictures and diagrams. as with most things, it's the jargon that makes it difficult.

Good luck to you,
Conor

 

[Randy Holmes-Farley]

Baking soda (sodium bicarbonate) being added to seawater is a perfect example of a very counterintuitive thing, and is why chemists make the big bucks. :D

Baking soda in solution has a pH of about 8.3 at 0.1 M concentration.

If you add it to seawater at pH 8.1, the pH will decline.

Lesson: when two solutions mix, the final pH need not be in between the two starting pH value, but can be higher or lower (or in between). :0

 

[Randy Holmes-Farley]

pH fluctuations in an aquarium arise from concentration ( ie hydronium per gallon ). In the ocean, the concentration of hydronium ions remain constant between night and day because the scale or sheer size of the ocean cannot be so easily affected over a period of hours and is more or less 'rebooted' diurnally.
r

Actually, the pH in the ocean does fluctuate day to night due to photosynthesis and consumption of CO2.

Oxygen and CO2 levels don't really change according to night and day in the ocean due to its' magnitude.

Yes they do.

 

[Randy Holmes-Farley]

So, from our original ingredients we now have one Oxygen from the water compound (O) floating around, a CO2 and our new H2CO3 compound. The free (O) will bind with the CO2 to form a new carbonate (CO3), thus replacing the one that H2CO3 took and CO2 is replenished from the atmosphere.

The free O combines with CO2? No, free O (or more correctly, O--) is never formed in seawater. All of the processes involve H+ transferring around.

 

[Randy Holmes-Farley]

The amount of oxygen offsets the amount of CO2. This slows the cycle at the point which CO3 can be made- less CO3 equals less H2CO3. if there is less H2Co3, then less H3O can be made which leads to a higher pH.

No, O2 has no impact on pH. Photosynthesis consumes CO2 which raises pH by pushing this equation (the hydration of CO2 to carbonic and and its dissociation) to the left (by LeChatliers Principle) reducing H+ and thereby raising pH:

CO2 + H2O <---> H2CO3 <---> H+ + HCO3-

 

[Randy Holmes-Farley]

Adding baking soda is only ever a temporary fix (like, a few hour fix-maybe a few days depending on the weather) because it is in greater concentration than CO2. It can't effectively degrade down to its' useful form without CO2 present. you're sort of adding a bunch of baking soda and only a small amount of it can really do anything unless CO2 is simultaneously added in the same proportion. r

That is not correct. Adding baking soda actually adds more CO2 than is needed for calcification using the alkalinity in that baking soda, and even more CO2 than the bicarbonate holds at equilibrium in seawater. CO2 blows off the water after adding baking soda. That's why it initially lowers pH, then it rises back as CO2 leaves.

 

[TaylorPilot]

Ph between 7.8-8.5 is acceptable. Don't chase ph if within this range.

Check this video they put out this week. They said by scrubbing CO2 and jumping their PH from 8.0 to 8.3, their Alk and Ca usage doubled instantly. I wonder if it was some temporary mechanism I am unfamiliar with that equalized at the new PH.

 

[Randy Holmes-Farley]

Check this video they put out this week. They said by scrubbing CO2 and jumping their PH from 8.0 to 8.3, their Alk and Ca usage doubled instantly. I wonder if it was some temporary mechanism I am unfamiliar with that equalized at the new PH.

No, there's no magic reset. It takes continuous removal of CO2 to maintain pH in the presence of elevated room CO2 levels. There are several ways to do this, and a scrubber is a fine one.

Both coral calcification and abiotic precipitation of calcium carbonate rise as pH rises, which increase demand. :)

 

[TaylorPilot]

So really, all things being equal, we do want to be at the higher end of the range. Is that a fair assessment?

 

[Randy Holmes-Farley]

So really, all things being equal, we do want to be at the higher end of the range. Is that a fair assessment?

If you have control (we rarely do), I'd pick a pH of about 8.3.