Mixing Sodium Carbonate with Sodium Bicarbonate

[Dr. Jim]

Is it true that if Sodium Carbonate is mixed with Sodium Bicarbonate at a 1:14 ratio, the temporary pH swing by either one used alone should be eliminated? (I got the 1:14 ratio from another thread and am wondering if this is the correct ratio for mixing the two).

 

[JimWelsh]

According to the CO2_Sys spreadsheet (https://www.nodc.noaa.gov/ocads/oceans/CO2SYS/co2rprt.html), seawater with a salinity of 35, temperature of 25C, an alkalinity of 8.0 dKH, and a pH (NBS scale) of 8.2 has a molar bicarbonate:carbonate ratio of around 8.2:1. Converting that into a dry weight ratio of the two compounds by multiplying by the MW of sodium bicarbonate (84) divided by the MW of sodium carbonate (106), we get 8.2 * 84 / 106 = 6.5. So a 6.5:1 ratio of grams of sodium bicarbonate to sodium carbonate should give a pH of around 8.2 when dissolved in seawater.

 

[Randy Holmes-Farley]

I think this is more of an intellectual curiosity than something of practical importance or utility.

Bear in mind that the effect of sodium bicarbonate alone is very small (about 0.06 pH unit drop for 1.4 dKH added) and the rise from sodium carbonate is small enough that very few people get pH too high when using it.

 

[Dr. Jim]

According to the CO2_Sys spreadsheet (https://www.nodc.noaa.gov/ocads/oceans/CO2SYS/co2rprt.html), seawater with a salinity of 35, temperature of 25C, an alkalinity of 8.0 dKH, and a pH (NBS scale) of 8.2 has a molar bicarbonate:carbonate ratio of around 8.2:1. Converting that into a dry weight ratio of the two compounds by multiplying by the MW of sodium bicarbonate (84) divided by the MW of sodium carbonate (106), we get 8.2 * 84 / 106 = 6.5. So a 6.5:1 ratio of grams of sodium bicarbonate to sodium carbonate should give a pH of around 8.2 when dissolved in seawater.

Thank you for the info, Jim.

This may be too much math to ask anyone to help me with so I won't be offended if I don't get a reply.... ;Happy …. but what I need to figure out is how to mix the two BRS solutions and use their online calculator. Each pack of bicarbonate and carbonate is mixed in 1 gallon of water where the final carbonate solution is twice the strength of the bicarbonate. So if 1ml of carbonate raises Alk by a certain amount of dKH (X), then 2ml of bicarbonate would be needed to raise the level the same amount.

I'm thinking this would be the math:
If (Y) ml of bicarbonate or 2(Y) ml of bicarbonate are needed to raise Alk by (X)dKH and you want to make a 1:6.5 dilution of the two:
1 + (6.5 x 2) = 14 So, a 1/14 stock solution can be made (e.g. 28.6ml carbonate solution + 400 ml bicarbonate).
Dosage using the bicarbonate calculator: (Y is the ml the calculator says to use if you were dosing only bicarbonate). So, when considering that carbonate was mixed with the bicarbonate, then: Y x 13/14. In other words, the volume that the bicarbonate calculator says to use would be multiplied by 13/14. Is this right? (I'm not sure if multiplying the value by 13/14 is correct but I think it is).

Anybody up for some "brain exercise"?

 

[Randy Holmes-Farley]

Why do you want to do this?

 

[Dr. Jim]

I think this is more of an intellectual curiosity than something of practical importance or utility.

Bear in mind that the effect of sodium bicarbonate alone is very small (about 0.06 pH unit drop for 1.4 dKH added) and the rise from sodium carbonate is small enough that very few people get pH too high when using it.

I guess this is good to know that it probably won't matter if I screw up my calculations
…..but now that I've got the two solutions and have gone this far, I might as well try to figure out the right mixture and dosage. (I'm referring to my previous response to @JimWelsh)

Thank you both for helping!

 

[Randy Holmes-Farley]

The pH neutral ratio varies with the pH, so it is different at pH 7.9 than at 8.2.

if you want such a solution, I’d mix roughy 5-10% of the carbonate solution into 90-95% of the bicarbonate solution (which is more dilute). If that gives a small pH change that you do not like, add more of the appropriate one.

 

[Dr. Jim]

Why do you want to do this?

I have a young tank (4 mo; 40 gal) loaded with SPS frags. I've been struggling to get the alkalinity down (from 10) and have had a lot of alk swings in the process (partly due to ATO-with-Kalk mishaps while trying to learn my new GHL Controller system). I'm not sure how much more stress these tiny frags can handle so I want to stabilize parameters as quickly as I can.
I recently managed to get Alk to 8.0-8.4 but the pH fell a little lower than I would like (7.9 at night, which I know is OK, but I would like it higher). Basically, I thought the mixture of bicarbonate and carbonate to minimize pH swings might be one less stress factor for the frags, and, I know some companies make their buffering solutions this way, (and may add some borate).
Also, since I already have the BRS carbonate and bicarbonate and would like to use them, I'm thinking that mixing them would be beneficial.
(pH issues are corrected as of yesterday after running a 1" PVC pipe from a window to my skimmer. The pH increased by 0.14 within a few hours, so I'm happy about that! Now, 8.11-8.25).

 

[Dr. Jim]

The pH neutral ratio varies with the pH, so it is different at pH 7.9 than at 8.2.

if you want such a solution, I’d mix roughy 5-10% of the carbonate solution into 90-95% of the bicarbonate solution (which is more dilute). If that gives a small pH change that you do not like, add more of the appropriate one.

Thanks Randy, I will try this.

 

[JimWelsh]

Thank you for the info, Jim.

This may be too much math to ask anyone to help me with so I won't be offended if I don't get a reply.... ;Happy …. but what I need to figure out is how to mix the two BRS solutions and use their online calculator. Each pack of bicarbonate and carbonate is mixed in 1 gallon of water where the final carbonate solution is twice the strength of the bicarbonate. So if 1ml of carbonate raises Alk by a certain amount of dKH (X), then 2ml of bicarbonate would be needed to raise the level the same amount.

I'm thinking this would be the math:
If (Y) ml of bicarbonate or 2(Y) ml of bicarbonate are needed to raise Alk by (X)dKH and you want to make a 1:6.5 dilution of the two:
1 + (6.5 x 2) = 14 So, a 1/14 stock solution can be made (e.g. 28.6ml carbonate solution + 400 ml bicarbonate).
Dosage using the bicarbonate calculator: (Y is the ml the calculator says to use if you were dosing only bicarbonate). So, when considering that carbonate was mixed with the bicarbonate, then: Y x 13/14. In other words, the volume that the bicarbonate calculator says to use would be multiplied by 13/14. Is this right? (I'm not sure if multiplying the value by 13/14 is correct but I think it is).

Anybody up for some "brain exercise"?

A couple of things here. First, while I agree with the "1 + (6.5 x 2) = 14" part, that's not a 1:14 ratio; it is a 1:13 ratio. So, it would be 30.8 mL of carbonate + 400 mL bicarbonate. Second, to dose this solution using the bicarbonate calculator, we must understand that each 1+13 = 14 parts of our mixed solution will contain (2*1)+13=15 "parts" of alkalinity, since the carbonate solution is twice as concentrated. So, the correction factor is 14/15, not 13/14. A minor difference, but since you asked.....

 

[Dr. Jim]

A couple of things here. First, while I agree with the "1 + (6.5 x 2) = 14" part, that's not a 1:14 ratio; it is a 1:13 ratio. So, it would be 30.8 mL of carbonate + 400 mL bicarbonate. Second, to dose this solution using the bicarbonate calculator, we must understand that each 1+13 = 14 parts of our mixed solution will contain (2*1)+13=15 "parts" of alkalinity, since the carbonate solution is twice as concentrated. So, the correction factor is 14/15, not 13/14. A minor difference, but since you asked.....

Thank you, again, Jim, for your time and for correcting my math. I feel much more confident with going ahead with my plan to mix the carbonates and bicarbonates, and how to adjust the BRS calculator!
Much appreciated!

 

[spicymikey]

I suppose everyone has different motives. For me my goal was not to get a ph neutral solution but rather to get a solution that would minimize the chance of calcium carbonate crystallization and precipitation. Straight sodium carbonate has a very high ph of 11.8ish and I find it just too high to easily dose without problems. My recipe turned out to be a simple 1:1 mix. It produces an alk mix with a ph of around 9.6-8. Still gives me a desirable ph boost yet its low enough that I can dose it into a low flow area of the sump with no precipitation. I still dose in a high flow area as a general practice.