Post about BIG PH drop from baking soda dosing???

[duganderson]

I'm currently using about 23 ml daily of baking soda solution mixed at the BRS ratio for Sodium Bicarbonate (1 cup and 4 T. baking soda per gallon) to maintain my Alk at about 10.0

I just read this post that makes me nervous about using sodium bicarbonate due to the potential of a big PH drop. Please see this post below and give me your thoughts if I should change to sodium carbonate instead for my daily dosing. By the way, I dose in the morning after my lights have been off for about 6-8 hours. I'm guessing I should switch my dosing time to evening when my lights have been on for several hours and the PH is highest....right?

Here is the post.......(please see the text in red that I am referring to)

"Several things to consider here.

Sodium bicarbonate is highly soluble. 1 gm of Sodium Bicarbonate dissolves in 10 ml (2 tsp) of room temperature water. The resulting solution of sodium bicarbonate predominately dissociates into sodium and a bicarbonate ions. The bicarbonate only slightly changes (a small amount) via hydrolysis by water to form carbonic acid and a hydroxyl ion. The resulting solution is only slightly alkaline, a pH of around 8.2.
The resulting hydrolysis and resulting pH can be affected by other ions in solution. One of the problems of using sodium bicarbonate over time is that it changes the composition of the conservative elements in seawater. Its use demands that we do regular water changes to avoid depletion and ratio changes of the essential conservative elements in seawater mixes on a frequent regular basis. Craig Bingman has a study using a computer model where many of the conservative elements change their quantities in seawater due to dilutional effect and consumption and elimination. If borate is one, then the buffer capacity of seawater is further diminished, leaving mostly bicarbonate and carbonate to handle the acid/base reactions.
the baking soda was doing what it was supposed to do. At best, this solution will dissociate further to form the carbonate ion, which in association with sodium will form a very basic solution (Na CARBONATE dissociates in water to a pH of 11.6, very basic). As the dissociation of bicarbonate to carbonate is very small, this is of little significance if there are a large amount of organic acids in the water column, as the bicarbonate (predominant specie in this reaction) will be consumed by the acids, driving carbonate back to the reactant bicarbonate and further dropping pH.

So what do these things mean to us? When adding sodium BICARBONATE to the system, if there were no other ions, the pH would only be 8.2, not a lot to affect a pH of 8.1 much to start with. If there are significant amounts of organic acids in the water column, they will quickly react with the bicarbonate ion to form the neutral form of that acid and release carbon dioxide gas into the water column. This results in a rapid drop of the pH of the water column due to the formation of carbonic acid and a depletion of the buffer capacity of the solution, not what we might expect when adding baking soda. However, in the presence of adequate amounts of calcium, and the presence of carbonate ions, the pH would be maintained, and bicarbonate is conserved rather than consumed to maintain alkalinity/buffer of the water column. This is why most reef buffer builders contain 8 parts sodium bicarbonate and 1 part sodium carbonate The high pH of the carbonate helps offset the resulting release of carbon dioxide which leads to the formation of carbonic acid in seawater reactions. Sodium Bicarbonate by itself is so slightly alkaline that it fails to turn phenolphthalein indicator red. Sodium Carbonate, on the other hand, turns phenolphthalein dark red due to it's extensive ionization in water. In addition, the carbonate is now available to replenish the bicarbonate ion in the water column, further conserving alkalinity of our seawater."

 

[blanden.adam]

This information is not correct.

When you add bicarbonate to your tank, you simply add bicarbonate to your tank. The pH doesn't change that much, maybe drops a little toward 8 ish, but this is only very mild and temporary as the equilibrium reestablishes -- an equilibrium which is driven primarily by CO2 concentration in the room.

Where the post starts to go wrong is it says that the organic acids quickly react with the bicarbonate upon it's addition to form carbon dioxide, dropping the pH and diminishing the buffering capacity. What the author doesn't discuss is that the organic acid concentration is relatively low, and the acids that are there (if they do indeed lower the pH) are already ionized. The H+ ions that drop pH would already be in solution. When they react with bicarbonate, they do indeed drive it back to carbonic acid, which then splits into H2O and CO2, but this doesn't drop pH much -- this is precisely how bicarbonate buffers the solution and why it works. The CO2 is then free to off-gas and equilibrate with the surrounding air.

Furthermore, the assertion that when you add bicarbonate you deplete the buffering capacity of the solution is kinda silly. The opposite happens -- you add bicarbonate which increases the buffering capacity of the solution. It's the organic acids that decrease the buffering capacity. So I'm not quite sure where the author was coming from.

Would you mind posting a link to the original thread?

At any rate, what you are doing is fine. Not to worry.

 

[Lostinthedark]

I've been using baking soda from the get go. Never had a ph problem. During the winter months when the windows are closed I bake it to maintain a ph above 8. It offsets the higher amounts of CO2 in a closed house.

 

[Allmost]

you are not adding carbonate to acid,

you are adding it to salt water, that contains other ions such as calcium, pottasium, magnesium and so on.

 

[Allmost]

This information is not correct.

When you add bicarbonate to your tank, you simply add bicarbonate to your tank. The pH doesn't change that much, maybe drops a little toward 8 ish, but this is only very mild and temporary as the equilibrium reestablishes -- an equilibrium which is driven primarily by CO2 concentration in the room.

Where the post starts to go wrong is it says that the organic acids quickly react with the bicarbonate upon it's addition to form carbon dioxide, dropping the pH and diminishing the buffering capacity. What the author doesn't discuss is that the organic acid concentration is relatively low, and the acids that are there (if they do indeed lower the pH) are already ionized. The H+ ions that drop pH would already be in solution. When they react with bicarbonate, they do indeed drive it back to carbonic acid, which then splits into H2O and CO2, but this doesn't drop pH much -- this is precisely how bicarbonate buffers the solution and why it works. The CO2 is then free to off-gas and equilibrate with the surrounding air.

Furthermore, the assertion that when you add bicarbonate you deplete the buffering capacity of the solution is kinda silly. The opposite happens -- you add bicarbonate which increases the buffering capacity of the solution. It's the organic acids that decrease the buffering capacity. So I'm not quite sure where the author was coming from.

Would you mind posting a link to the original thread?

At any rate, what you are doing is fine. Not to worry.

you are correct.

but so is the quote I think. it meant if you add it to straight acid, or salt water full of acids .... in that case, our rocks and sand would melt first, so a reef tank would never even get to that level seems like he just didnt make its point clear.

 

[blanden.adam]

you are correct.

but so is the quote I think. it meant if you add it to straight acid, or salt water full of acids .... in that case, our rocks and sand would melt first, so a reef tank would never even get to that level seems like he just didnt make its point clear.

I mean, he seems to be describing what would happen if you added a titratable acid to a buffered solution of bicarbonate, but even then it's the acid that's dropping the pH, not the bicarbonate.

Perhaps there is a larger context to what he's saying that is causing me to miss something in the explanation, like maybe someone asked him what would happen if you added HCl to their tank? If that were the case I'd give the explanation the benefit of the doubt and say there were just somethings mistyped. IDK.

Nonetheless, no need to worry about baking soda crashing your pH

 

[bertoni]

The post seems to have a lot of gibberish, at least as far as reefkeeping is concerned. Sodium bicarbonate can't drop the pH of your tank much, only a very small amount. I agree that comments about "conservative elements" don't make much sense. When he talks about "extensive ionization", for example, I have no idea what he means.

We can go through this post in more detail, if you're interested, but I agree that we might be missing some context that'd make it more coherent.

 

[duganderson]

We can go through this post in more detail, if you're interested, but I agree that we might be missing some context that'd make it more coherent.

Thank you for your thoughts everyone. No....I don't need to go through the post in more detail. Thank you everyone for clarifying the relevant chemistry content of this post.

 

[tmz]

The post is gibberish and misleading unless it's way out of context. Someone trying to sound smart,name dropping too but badly confused most likely.
Where did the post originate? It's difficult to comment further without context.
Sodium bicarbonate will only drop pH a little if it's 8 plus or higher to start with as ADam noted.. If it's low it will raise it.
Natural seawater at pH8.2 and a salinity of 1.0264 contains 110 ppm bicrabonate and 20 ppm carbonate,fyi. At that ph, the carbonae will even out to approximately those levels with athmospheric CO2 governing the pH for the most part.

 

[duganderson]

Where did the post originate? It's difficult to comment further without context.

The post was in response to a question about why PH drops with sodium bicarbonate. It was located in another forum. I will not give the name of the other forum since we are not supposed to direct RC users to other forums.

As I read the initial post, I'm wondering if the quote meant to suggest it is a quick drop in PH NOT a large drop. It sounds, however, like there are other errors in the understanding of reef chemistry according to the quote.