Making a carbonate/bicarbonate buffer
- Thread starter Luis A M
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Luis A M
Premium Member
But check this: http://cshprotocols.cshlp.org/content/2011/2/pdb.rec12398
Buffer solutions are mostly carbonate,and little bicarbonate?
Buffer solutions are mostly carbonate,and little bicarbonate?
Luis A M
Premium Member
But with a more in depth analysis,the % of carb.vs bicarb depends of the desired pH of the buffer solution.
So,working with 0.1M solutions of both,4 parts bicarb/ 1 carb gives a pH of 9.4.While 4 carb/1 bicarb gives 10.5.
So the question is now what pH should we want for our aquarium buffer?
http://www.promega.com/~/media/files/resources/paguide/a4/chap15a4.pdf?la=en
So,working with 0.1M solutions of both,4 parts bicarb/ 1 carb gives a pH of 9.4.While 4 carb/1 bicarb gives 10.5.
So the question is now what pH should we want for our aquarium buffer?
http://www.promega.com/~/media/files/resources/paguide/a4/chap15a4.pdf?la=en
disc1
-RT * ln(k)
The ratio depends on the pH you want. Going strictly from the buffer tables you'd use 99% bicarb and 1% carbonate to hit pH 8.4. But in seawater at equilibrium with the atmosphere you have to account for the CO2 which is the third member of the carbonate buffer system. The math gets a lot more complicated when you add that in.
Really, any way you go you'll come to equilibrium pretty quick once it is added to the seawater. So except for the first few minutes after addition the exact ratio doesn't really matter much.
If you want more on how this math works, Google up the Henderson / Hasselbalch equation. Hope I spelled that right.
Really, any way you go you'll come to equilibrium pretty quick once it is added to the seawater. So except for the first few minutes after addition the exact ratio doesn't really matter much.
If you want more on how this math works, Google up the Henderson / Hasselbalch equation. Hope I spelled that right.
Carbonate(CO3) by volume gives you twice the alkainity of bicarbonate(HCO3). The bicarbonate has one H+ and one open spot to neutralize one more H proton. The carbonate has two open sites since it has no H+. The carbonate species shift in relation to the H+ in the water (ie the pH). The pH moves in relation to the CO2 in the water as CO2 adds H+ from the water . When you add carbonate ,much of it it takes up H+ balancing with bicarboante. This temporarily increases the pH and reduces the CO2 in the water. The air equilibrates with the tank water in a short time dropping th Ph back down. sing buffers for ph control is a poor strategy and leaves low pH and high alk.
Luis A M
Premium Member
But from a practical aquarium keeping point of view,acid production slowly but steadily reduces the original buffer capacity of SW and the pH falls.This is particularly seen in fish only tanks heavily populated.
Partial water changes at some point restores the buffer content,but not enough.So it is a common practice to add buffer salts (Sodium carbonate and bicorbonate) to sustain pH over 8.
Partial water changes at some point restores the buffer content,but not enough.So it is a common practice to add buffer salts (Sodium carbonate and bicorbonate) to sustain pH over 8.
The pH effects from the buffer solution are due to the effects of the supplement on the amount of carbon dioxide in the system. In systems with a high aeration rate, the choice of buffers tends to make only a small difference, because the effects are very temporary in nature.
I'm not sure that fish are going to be affected much by a lower pH level. Corals tend to have more issues with very low pH levels.
I'm not sure that fish are going to be affected much by a lower pH level. Corals tend to have more issues with very low pH levels.
From a practical aquarium keeping point of view adding buffers will increase alkalinity and does not maintain high pH.
It leaves you with higher alkalinity and the same pH you started with in a brief period of time as the water in the tank exchanges gases with the air around it and draws in CO2 to make up for the H+ neutralized by the extra alkalinity (buffer).
Bouncing the alkalinty around is harmful to many organisms in the tank. Adding buffers to sustain higher pH is a poor strategy; it doesn't drive pHin our tanks ;CO2 content does. It does raise alkalinity.
Maintaining alkalinity in the 7 to 11 dkh range for a reef tank is desirable because calcifying organisms consume carbonate along with calcium as they create calcium carbonate for skeletal mass.
Alkalinity also moderates precipitous daily swings in pH , up or down, by taking up some H+ or letting some go , thus allowing time for equilibriation with CO2 in the surounding the air. Thus the term buffer.
The carbonate alkalinity mix in a simplified version looks like this :
CO3(carbonate)<---> HCO3(bicarbonate) <---> H2CO3 (carbonic acid).
The carbonate has room to neutralize two H protons ;it is two units of alkainity. The bicarbonate ion is only one unit of alkalinity since it has room to neutralize only one H proton. The amount of carbonate vs bicarbonate at any time is dependent on the CO2 content of the water via the H+ it takes from the water;i.e., more H+ equals lower pH and vice versa.
As CO2 enters the water it hydrolizes to carbonic acid( H2O plus CO2 then H2CO3). Two H+ protons are added .
The carbonate ,bicarbonate and carbonic acid equilibrium shifts in response to the amount of H+ ( ie , it is pH dependent).More H+ moves the speciation to the right; less to the left. This happens instantaneously.
If CO2 enters the water from biological activity it will shift it to the right temporarily until the CO2 equilibriates with the air.
If CO2 is reduced below the levels in the surrounding air by photosynthesis it shifts to the left until CO2 in the water equilibrates with the air . This works this way not only in tanks but in the oceans as the seawater equilibriates with the atmosphere and athmospheric CO2 levels drive the oceans' pH.
Here's an example of what happens to a carbonate alkalinity ion dosed to a tank as CO2 interacts with the water( H2O).
The CO3 ion(buffer) dosed adds 2 units of alkalinity; it can neutralize 2 H protons. The carbonate equilibrium shifts to the left.
Then CO2 enters the water; CO2 plus H2O = H2CO3 (carbonic acid) ; the carbonate equilibrium shifts back to the right.
H2CO3(carbonic acid) plus the CO3(carbonate ion) = 2 HCO3(bicarbonate).
The end result contains two more H protons than before the CO2 entered the water and the same pH as before the carbonate was dosed.The carbonate's temporary effect on pH has been negated.
The alkalinity has been raised, however, by 2 units as each new bicarbonate ion adds one unit of alkalinity along with the two new H protons . So the ph is the same as before dosing the carbonate and the alkalinity is higher.
It leaves you with higher alkalinity and the same pH you started with in a brief period of time as the water in the tank exchanges gases with the air around it and draws in CO2 to make up for the H+ neutralized by the extra alkalinity (buffer).
Bouncing the alkalinty around is harmful to many organisms in the tank. Adding buffers to sustain higher pH is a poor strategy; it doesn't drive pHin our tanks ;CO2 content does. It does raise alkalinity.
Maintaining alkalinity in the 7 to 11 dkh range for a reef tank is desirable because calcifying organisms consume carbonate along with calcium as they create calcium carbonate for skeletal mass.
Alkalinity also moderates precipitous daily swings in pH , up or down, by taking up some H+ or letting some go , thus allowing time for equilibriation with CO2 in the surounding the air. Thus the term buffer.
The carbonate alkalinity mix in a simplified version looks like this :
CO3(carbonate)<---> HCO3(bicarbonate) <---> H2CO3 (carbonic acid).
The carbonate has room to neutralize two H protons ;it is two units of alkainity. The bicarbonate ion is only one unit of alkalinity since it has room to neutralize only one H proton. The amount of carbonate vs bicarbonate at any time is dependent on the CO2 content of the water via the H+ it takes from the water;i.e., more H+ equals lower pH and vice versa.
As CO2 enters the water it hydrolizes to carbonic acid( H2O plus CO2 then H2CO3). Two H+ protons are added .
The carbonate ,bicarbonate and carbonic acid equilibrium shifts in response to the amount of H+ ( ie , it is pH dependent).More H+ moves the speciation to the right; less to the left. This happens instantaneously.
If CO2 enters the water from biological activity it will shift it to the right temporarily until the CO2 equilibriates with the air.
If CO2 is reduced below the levels in the surrounding air by photosynthesis it shifts to the left until CO2 in the water equilibrates with the air . This works this way not only in tanks but in the oceans as the seawater equilibriates with the atmosphere and athmospheric CO2 levels drive the oceans' pH.
Here's an example of what happens to a carbonate alkalinity ion dosed to a tank as CO2 interacts with the water( H2O).
The CO3 ion(buffer) dosed adds 2 units of alkalinity; it can neutralize 2 H protons. The carbonate equilibrium shifts to the left.
Then CO2 enters the water; CO2 plus H2O = H2CO3 (carbonic acid) ; the carbonate equilibrium shifts back to the right.
H2CO3(carbonic acid) plus the CO3(carbonate ion) = 2 HCO3(bicarbonate).
The end result contains two more H protons than before the CO2 entered the water and the same pH as before the carbonate was dosed.The carbonate's temporary effect on pH has been negated.
The alkalinity has been raised, however, by 2 units as each new bicarbonate ion adds one unit of alkalinity along with the two new H protons . So the ph is the same as before dosing the carbonate and the alkalinity is higher.
Luis A M
Premium Member
Thank you for your comprehensive explanation about the pH/ buffer interaction.
My question was more modest really,i.e.what were the proportions of Sodium carb.vs Sodium bicarb.of the buffer mix I should add to a system to restore falling pH (and alkalinity).Or how to make a DIY buffer mix similar to the commercial offered products(which might contain other ingredients such as Borum).This proportion was given by M.Moe as 6 parts bicarb/1 part carb.
Addition of these chemicals do restore low pH and alkalinity to normal values.
But there are two main mechanisms producing a low pH (acidosis).
One is respiratory.In a tank where aeration is not enough for the given bioload,CO2 accumulates faster than it can diffuse into the atmosphere,and pH will fall,due to the reaction shown above by Tom.It only takes to optimize the aeration to reverse this process.
But if aeration is adequate,still another process lowers pH.The animals kept and bacteria produce nitrate (nitric acid) and other organic acids.This is metabolic acidosis.For some time the buffers (alkalinity) neutralize them,but then they start to be lost and as a result,pH falls.This is exactly what we do when we measure alk.with a test;we add acid drop to drop until,all the alk.is lost and pH falls,as evidenced by a shift in reagent´s colour.
It is in this case that the addition of carb/bicarb restores original alkalinity and pH in their original values.
But Jonathan rises a question which is almost philosophical;should we struggle to keep water in the "right" parameters,if deviation from them does not produce any obvious harm?.
Certainly,fish (though not corals and other inverts) seem to live well under conditions very different from the natural water:low salinity,very high No3/Po4 and low alkalinity/pH.
So,why bother to keep chemical/physical parameters within range if fish seem ok?.
I don´t have an answer,but feel that there might be negative effects of these deviations that we still didn´t find.
Frequent water changes or an open system will keep original water conditions.
This not being practical,restoring alkalinity/pH with carb./bicarb.seems advisable.
My question was more modest really,i.e.what were the proportions of Sodium carb.vs Sodium bicarb.of the buffer mix I should add to a system to restore falling pH (and alkalinity).Or how to make a DIY buffer mix similar to the commercial offered products(which might contain other ingredients such as Borum).This proportion was given by M.Moe as 6 parts bicarb/1 part carb.
Addition of these chemicals do restore low pH and alkalinity to normal values.
But there are two main mechanisms producing a low pH (acidosis).
One is respiratory.In a tank where aeration is not enough for the given bioload,CO2 accumulates faster than it can diffuse into the atmosphere,and pH will fall,due to the reaction shown above by Tom.It only takes to optimize the aeration to reverse this process.
But if aeration is adequate,still another process lowers pH.The animals kept and bacteria produce nitrate (nitric acid) and other organic acids.This is metabolic acidosis.For some time the buffers (alkalinity) neutralize them,but then they start to be lost and as a result,pH falls.This is exactly what we do when we measure alk.with a test;we add acid drop to drop until,all the alk.is lost and pH falls,as evidenced by a shift in reagent´s colour.
It is in this case that the addition of carb/bicarb restores original alkalinity and pH in their original values.
But Jonathan rises a question which is almost philosophical;should we struggle to keep water in the "right" parameters,if deviation from them does not produce any obvious harm?.
Certainly,fish (though not corals and other inverts) seem to live well under conditions very different from the natural water:low salinity,very high No3/Po4 and low alkalinity/pH.
So,why bother to keep chemical/physical parameters within range if fish seem ok?.
I don´t have an answer,but feel that there might be negative effects of these deviations that we still didn´t find.
Frequent water changes or an open system will keep original water conditions.
This not being practical,restoring alkalinity/pH with carb./bicarb.seems advisable.
disc1
-RT * ln(k)
So the answer to that original question is another question. What do you want the properties of this buffer to be?
In reality, you can use either straight carbonate or bicarbonate and after the first hour or two you will be in the same condition either way. These are all equilibrium processes and the CO2 drives them since it is the form that can move in and out of the system.
The reason most folks dose with the carbonate version is simple. Twice the dose for the same number of ml. But it does raise pH a bit, especially with large doses. In systems that already run a high pH or for people who want to make larger single doses bicarbonate may be a better option since it barely budges the pH when you add it. Otherwise the high local pH can cause carbonate precipitation which leads to the runaway 2-part cycle and eventually ends in a thread title "Help I have to add 2 gallons of two part a day and it still doesn't keep up".
Lots of people have kicked around mixing them. When I did the math the way I wanted it, it came out to be 99% bicarb so I said it wasn't worth it. In the end, any pH effect from it will be fleetingly temporary at best so there's really no use in it.
But if you give me a pH that you want this buffer to have, I can certainly work out the buffer ratios or you can consult the buffer tables you've already found. There's absolutely no harm in mixing the two together if that tickles your fancy.
In reality, you can use either straight carbonate or bicarbonate and after the first hour or two you will be in the same condition either way. These are all equilibrium processes and the CO2 drives them since it is the form that can move in and out of the system.
The reason most folks dose with the carbonate version is simple. Twice the dose for the same number of ml. But it does raise pH a bit, especially with large doses. In systems that already run a high pH or for people who want to make larger single doses bicarbonate may be a better option since it barely budges the pH when you add it. Otherwise the high local pH can cause carbonate precipitation which leads to the runaway 2-part cycle and eventually ends in a thread title "Help I have to add 2 gallons of two part a day and it still doesn't keep up".
Lots of people have kicked around mixing them. When I did the math the way I wanted it, it came out to be 99% bicarb so I said it wasn't worth it. In the end, any pH effect from it will be fleetingly temporary at best so there's really no use in it.
But if you give me a pH that you want this buffer to have, I can certainly work out the buffer ratios or you can consult the buffer tables you've already found. There's absolutely no harm in mixing the two together if that tickles your fancy.
Luis:
Simpler version:
BTW, I think your question was answered very early on. It doesn't matter whether you dose bicarbonate or carbonate as far sustained ph is concerned. NSW is about 110ppm bicarbonate and 20ppm bicarbonate if you choose to mimic it when dosing.
If you keep alkalinity in the normal range for a salt water tank ie 7 to 11 dkh, pH will be in the normal ranges 7.8 to 8.5; unless, CO2 levels in the water vary from atmospheric levels( currently reported at 400ppm) .
Surface agitation/aeration helps speed the gas exchange and can help raise pH if the CO2 in the surrounding room air is lower than the tank water ; but, if the air is high in CO2 as it might be in a closed up room or house,it will lower the pH , acidify the water; no matter what buffers you add to increase alkalinity .
The biological acidifying processes you noted add CO2 and H+ ,the sustained levels of which are driven by CO2 in the water which is driven by the gas exchange and air around the tank ;not the alkalinity.
As David notes, many of us including me use baked baking soda(carbonate /soda ash) because it provides 2x as much alkalinity as unbaked baking soda(bicarbonate). I know it's fine to do that since it all equilibrates in a short time. It doesn't matter which form of carbonate alkalinity I dose as far as pH is concerned since CO2 will drive that nor does it matter for calcifiation sincethe calcifying organisms will use bicarbonate whether it's dosed as bicarbonate or as carbonate which shifts to bicarbonate.
Simpler version:
BTW, I think your question was answered very early on. It doesn't matter whether you dose bicarbonate or carbonate as far sustained ph is concerned. NSW is about 110ppm bicarbonate and 20ppm bicarbonate if you choose to mimic it when dosing.
If you keep alkalinity in the normal range for a salt water tank ie 7 to 11 dkh, pH will be in the normal ranges 7.8 to 8.5; unless, CO2 levels in the water vary from atmospheric levels( currently reported at 400ppm) .
Surface agitation/aeration helps speed the gas exchange and can help raise pH if the CO2 in the surrounding room air is lower than the tank water ; but, if the air is high in CO2 as it might be in a closed up room or house,it will lower the pH , acidify the water; no matter what buffers you add to increase alkalinity .
The biological acidifying processes you noted add CO2 and H+ ,the sustained levels of which are driven by CO2 in the water which is driven by the gas exchange and air around the tank ;not the alkalinity.
As David notes, many of us including me use baked baking soda(carbonate /soda ash) because it provides 2x as much alkalinity as unbaked baking soda(bicarbonate). I know it's fine to do that since it all equilibrates in a short time. It doesn't matter which form of carbonate alkalinity I dose as far as pH is concerned since CO2 will drive that nor does it matter for calcifiation sincethe calcifying organisms will use bicarbonate whether it's dosed as bicarbonate or as carbonate which shifts to bicarbonate.
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A few other things mentioned in one o the above posts.. Boron( present as borate and boric acid in salt water) runs around 4ppm in nsw. Increasing it presents toxcity issues: some isopods die at 28ppm and some marine fish in trials have died at a 50% rate at levels around 70ppm.At one point some commercial manufacturers were exceeding those levels because of its pH buffering potential.
Nitric acid mentioned earlier is HNO3 . Nitrate plus an H . It gives up that proton and forms water as a by product in many oxidation reactions with organic materials,for example:
C + 4 HNO<sub>3</sub> → CO<sub>2</sub> + 4 NO<sub>2</sub> + 2 H<sub>2</sub>O
So in that activty you get water nitrite and the pH driver CO2.
Anyway pH is still a measure of total H+ activity not specifc acids wherever it comes from or goes. The total level of hydrogen activity vs neutralizing oxide activity is the pH . Hydrolized CO2 governs the amount of H+ overall and adjust the carbnate eqilibrium accordingly.
Nitric acid mentioned earlier is HNO3 . Nitrate plus an H . It gives up that proton and forms water as a by product in many oxidation reactions with organic materials,for example:
C + 4 HNO<sub>3</sub> → CO<sub>2</sub> + 4 NO<sub>2</sub> + 2 H<sub>2</sub>O
So in that activty you get water nitrite and the pH driver CO2.
Anyway pH is still a measure of total H+ activity not specifc acids wherever it comes from or goes. The total level of hydrogen activity vs neutralizing oxide activity is the pH . Hydrolized CO2 governs the amount of H+ overall and adjust the carbnate eqilibrium accordingly.
Luis A M
Premium Member
Thank you guys for the explanation.It helps to understand more these processes and will be useful for our many readers with similar doubts.
Yes,I got my answer.I can add carbonate and/or bicarb.in any proportion and end with the expected ratio (which depends on pH),all this driven by the CO2 of the water which is (with proper aeration) in equilibrium with surrounding air.
Now,if I understand it right,SW pH is 8.3 and not 7 or 9 because of it´s content of that precise amount of carb/bicarb (+ CO2)buffering system=alkalinity ( plus other ions).
Now,suppose that I start adding an acid to this water.pH will drop but this fall will be reversed by the buffer mechanism.Meaning carb>bicarb>carbonic acid>CO2 which escapes into the air.So alkalinity is reduced,and if I keep dosing acid,it will be exhausted.After that point every H+ added will produce a pH decrease.This process is final,it won´t be spontaneously reversed.
But if I now add alkalinity to the tank (carb/bicarb),pH will start to rise,as it starts to neutralize excess H+.And when it reaches the original SW concentration,pH will be again 8.3.
Don´t you agree?.
Yes,I got my answer.I can add carbonate and/or bicarb.in any proportion and end with the expected ratio (which depends on pH),all this driven by the CO2 of the water which is (with proper aeration) in equilibrium with surrounding air.
Now,if I understand it right,SW pH is 8.3 and not 7 or 9 because of it´s content of that precise amount of carb/bicarb (+ CO2)buffering system=alkalinity ( plus other ions).
Now,suppose that I start adding an acid to this water.pH will drop but this fall will be reversed by the buffer mechanism.Meaning carb>bicarb>carbonic acid>CO2 which escapes into the air.So alkalinity is reduced,and if I keep dosing acid,it will be exhausted.After that point every H+ added will produce a pH decrease.This process is final,it won´t be spontaneously reversed.
But if I now add alkalinity to the tank (carb/bicarb),pH will start to rise,as it starts to neutralize excess H+.And when it reaches the original SW concentration,pH will be again 8.3.
Don´t you agree?.
I do not agree that dosing buffers will manage pH. That will raise alkalinity ,though.Keeping the alkaliity steady at point within the recommended range of 7 to 11dkh and provide adequate gas exchange is my approach
Buffers do not reverse pH drops from high acid content for any sustainable length of time. Maintaining lower CO2 in the water does.
I've tried to explain the processes . This is my last effort at it.
Take HNO3 as an example which you cited in an earlier post and look at the typical reaction. BTW HNO3 has a PKa of less than one, it is a very strong acid acidic. It's good there isn't much that hangs around or our tanks might explode. It disassociates except in very highly acidic conditions.
Watch the H protons closely in this reaction with C ( keep the pH of pure water in mind :
C + 4 HNO<sub>3</sub> → CO<sub>2</sub> + 4 NO<sub>2</sub> + 2 H<sub>2</sub>O
The 4 H protons are now tucked in to newly formed pure water molecules.
The 4 NO2 molecules are nitrite which will oxidize to nitrate and then denitrify to N2 gas via additional bacterial activity and bubble out of the tank.Some may be assimilated by organisms along the way.
That leaves a newly formed CO2 molecule. It can hydrolize to carbonic acid picking off a water molecule and lower pH again or depending on the concentration of CO2 in the tank relative to the surrounding air it can leave the water during gas exchange at the surface.
Buffers do not reverse pH drops from high acid content for any sustainable length of time. Maintaining lower CO2 in the water does.
I've tried to explain the processes . This is my last effort at it.
Take HNO3 as an example which you cited in an earlier post and look at the typical reaction. BTW HNO3 has a PKa of less than one, it is a very strong acid acidic. It's good there isn't much that hangs around or our tanks might explode. It disassociates except in very highly acidic conditions.
Watch the H protons closely in this reaction with C ( keep the pH of pure water in mind :
C + 4 HNO<sub>3</sub> → CO<sub>2</sub> + 4 NO<sub>2</sub> + 2 H<sub>2</sub>O
The 4 H protons are now tucked in to newly formed pure water molecules.
The 4 NO2 molecules are nitrite which will oxidize to nitrate and then denitrify to N2 gas via additional bacterial activity and bubble out of the tank.Some may be assimilated by organisms along the way.
That leaves a newly formed CO2 molecule. It can hydrolize to carbonic acid picking off a water molecule and lower pH again or depending on the concentration of CO2 in the tank relative to the surrounding air it can leave the water during gas exchange at the surface.
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If you dose a mineral acid, it'll destroy alkalinity and lower the pH. An organic acid like vinegar can bind alkalinity only temporarily. When bacteria consume the acid, the alkalinity is released.d
The pH of the ocean is set by the buffers in it and the carbon dioxide level in the air. You can alter the pH of your tank by changing the alkalinity, but the effect is fairly small in the 7-11 dKH range. This article has a chart:
http://reefkeeping.com/issues/2004-09/rhf/index.htm
The pH of the ocean is set by the buffers in it and the carbon dioxide level in the air. You can alter the pH of your tank by changing the alkalinity, but the effect is fairly small in the 7-11 dKH range. This article has a chart:
http://reefkeeping.com/issues/2004-09/rhf/index.htm
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