chriss
05/03/2003, 02:59 PM
I found the following on candybass.com. I have never heard of doing this to raise alkalinity and wanted your opinion. Will this work, is it safe, is it really better than using kalkwasser?
If you can explain it in laymen's terms, it would be greatly appreciated!
Thanks!
Christina
Alkalinity and pH - The Chemistry of the Carbonates
Alkalinity and pH are probably the two most misunderstood concepts in marine aquarium chemistry. Though inter related they are two independent measurements. The term pH refers to the amount of hydrogen or hydroxide molecules available in the system. At pH 7 both hydrogen and hydroxide are in balance and we have a neutral condition. If we add an acid to the water the pH goes downward as hydrogen ions are released. If we added a basic substance the pH goes up as hydroxyl ions are formed. Each increasing or decreasing unit on the pH scale differs from the next unit by a magnitude of ten, so a pH of 5 is ten times as strong an acid as one with a pH of 6. Likewise, a pH of 10 is a hundred times as strong a base as one with a pH of 8.
Seawater has a pH of 8.3 that means it is mildly basic. Most living things tend to have a slightly basic pH. Changes in pH can have a profound effect on the health of any organism.
While pH measures how acid or basic a water is, alkalinity is a measure of the water’s ability to resist a change in pH. It is a measure of buffering capacity. A water with a low alkalinity requires much less acid to lower its pH than a water with high alkalinity.
In our marine environment pH and alkalinity center around carbon dioxide and it’s basic salts, the carbonates. By far the chief carbonate salts are those of calcium and magnesium. If we dissolve carbon dioxide in pure water we form carbonic acid, H2CO3, and the pH drops to just about 4. If this acid solution then comes in contact with limestone, which is comprised of calcium and magnesium carbonate, it will dissolve them forming soluble calcium and magnesium bicarbonate.
H2CO3 + CaCO3 --------> Ca(HCO3)2
And
H2CO3+ MgCO3 ---------> Mg(HCO3)2
Editors Note---Sorry about the chemical equations but any good chemist needs to sneak at least a few into any article. Kinda like a day without sunshine if you get my drift.
If you live in the mid-west, as I do, you may recognize the calcium and magnesium bicarbonates as the two compounds that make the water hard in this part of the country. To understand the nature of the carbonates we need to know a little about their solubility. I use the following table to illustrate:
Calcium
Bicarbonate – HCO3
Carbonate – CO3
Hydroxide - OH
Solubility
Very Soluble
Insoluble
Somewhat Soluble
Magnesium
Bicarbonate – HCO3
Carbonate – CO3
Hydroxide - OH
Solubility
Very soluble
Soluble
Insoluble
You notice I include hydroxides that don’t contain any carbon dioxide. They play a role in the chemistry of carbonates, i.e. Kaukwasser-Ca(OH)2, which I will explain shortly.
There is an interesting fact about the 8.3 pH of seawater. It just so happens that at a pH of 8.3 or below all alkalinity is in the form of bicarbonate. Above 8.3 and alkalinity may be both bicarbonate and carbonate.
Ok you say what is the point? Well if one looks at our table above we see that calcium carbonate is insoluble ( not entirely, at 8.4 pH at 80o F you can dissolve about 60 ppm). This point has a great bearing on how we increase alkalinity (and calcium) in our tanks. We need to explore this and maybe change some ways of thinking.
When I was first studying up on reef systems I of course ran into the chorus of kaukwasser fans. KW the miracle additive! Hey KW is just good old lime water, Ca(OH)2. Claims are that it increases pH and alkalinity while providing beneficial calcium.
Let’s take a closer look remembering that calcium carbonate is insoluble while bicarbonate present in the tank is soluble. Here is the reaction of KW in our tank.
Sorry more equations
Ca(HCO3)2 + Ca(OH)2 ----------->2CaCO3 + 2H2O
Soluble bicarb KW Insoluble carbonate
What???? If I add KW to increase alkalinity I actually bring it down?
And you reduce calcium levels as well.
Clearly KW is overrated. It can only raise alkalinity to the saturation point of calcium carbonate’s solubility. Above that it reduces bicarbonate alkalinity and available calcium as well. Lime is exactly what is added to well water to soften it at municipal water plants. Lime=KW.
Don’t take my word for it. Here is a link to an excellent article by Richard Greenfield who supports my view.
http://www.athiel.com/lib7/greenfield.htm
Thank you Richard, I couldn’t have said it better myself.
So what do we do to increase alkalinity and calcium? Let’s tackle alkalinity first.
The clue is in the table of solubility I provided above. According to that table calcium carbonate is mainly insoluble so it we add something like KW we convert soluble calcium bicarbonate to insoluble calcium carbonate. The result is a pile of precipitated calcium carbonate lying at the bottom of our sump or, worse yet, cementing our substrate together so one’s live sand becomes dead sand. Look in the magnesium listing in the table. It shows that magnesium carbonate is soluble. I’m here to tell you folks calcium and magnesium are interchangeable when it come to buffering capability and magnesium wins out because both the bicarbonate and carbonate are soluble. The solution to raising alkalinity is to add magnesium carbonate to our tanks. Using magnesium carbonate we can hold our tank at any desired alkalinity level without fear of precipitation of carbonate salts.
Take my own tank. I maintain an alkalinity of 280 ppm consistently ( 15dK, 5.3 meg/L) by dosing with magnesium carbonate. Try getting you alkalinity to that level with KW and you will have a mess on you hands. I do run my alkalinity higher than most tanks but I assure you that this level keeps my pH stable in the range of 8.2-8.4 consistently. For the life of me I don’t understand why the purveyors of "magic alkalinity tonics" don’t use magnesium carbonate as the main ingredient in their potions. You can buy magnesium carbonate from test kit suppliers like Hach chemical. It is stable and safe to use (even drug lords don’t have a lot of use for it unless they have heartburn). I make a stock solution by dissolving 50 grams of magnesium carbonate in a liter of RO water (50,000 ppm as magnesium carbonate or 42,150 ppm as calcium carbonate). Dosing this solution allows me to maintain any desired alkalinity while providing magnesium and essential element for reef production (note: normal seawater contains 1300 ppm of magnesium).
Now we are faced with maintaining calcium levels. Open Ocean water has a level of about 400 ppm calcium (1000 ppm as calcium carbonate). This level is easy to maintain by adding non-carbonate calcium in the form of calcium chloride. Calcium chloride is very soluble and invertebrates can use it just as well as carbonate forms of calcium. Fortunately this form of calcium is available in several reef tonics sold by the local fish store. I have heard some people claim that this can cause an excess of chloride in the tank. Seawater has a chloride content of 19,400 ppm. Calcium chloride is just slightly under half chloride so maintaining 400 ppm calcium would only boost chloride levels by a like amount (2% increase in total chlorosity). That is not enough to cause concern.
One final tip on pH and alkalinity. If you pH is dropping and alkalinity too you can use baking soda in an emergency to bring it back up. I don’t recommend using baking soda for routine alkalinity adjustment but if you are out of alkalinity buffer, baking soda comes in handy to correct an alkalinity/pH problem.
If you can explain it in laymen's terms, it would be greatly appreciated!
Thanks!
Christina
Alkalinity and pH - The Chemistry of the Carbonates
Alkalinity and pH are probably the two most misunderstood concepts in marine aquarium chemistry. Though inter related they are two independent measurements. The term pH refers to the amount of hydrogen or hydroxide molecules available in the system. At pH 7 both hydrogen and hydroxide are in balance and we have a neutral condition. If we add an acid to the water the pH goes downward as hydrogen ions are released. If we added a basic substance the pH goes up as hydroxyl ions are formed. Each increasing or decreasing unit on the pH scale differs from the next unit by a magnitude of ten, so a pH of 5 is ten times as strong an acid as one with a pH of 6. Likewise, a pH of 10 is a hundred times as strong a base as one with a pH of 8.
Seawater has a pH of 8.3 that means it is mildly basic. Most living things tend to have a slightly basic pH. Changes in pH can have a profound effect on the health of any organism.
While pH measures how acid or basic a water is, alkalinity is a measure of the water’s ability to resist a change in pH. It is a measure of buffering capacity. A water with a low alkalinity requires much less acid to lower its pH than a water with high alkalinity.
In our marine environment pH and alkalinity center around carbon dioxide and it’s basic salts, the carbonates. By far the chief carbonate salts are those of calcium and magnesium. If we dissolve carbon dioxide in pure water we form carbonic acid, H2CO3, and the pH drops to just about 4. If this acid solution then comes in contact with limestone, which is comprised of calcium and magnesium carbonate, it will dissolve them forming soluble calcium and magnesium bicarbonate.
H2CO3 + CaCO3 --------> Ca(HCO3)2
And
H2CO3+ MgCO3 ---------> Mg(HCO3)2
Editors Note---Sorry about the chemical equations but any good chemist needs to sneak at least a few into any article. Kinda like a day without sunshine if you get my drift.
If you live in the mid-west, as I do, you may recognize the calcium and magnesium bicarbonates as the two compounds that make the water hard in this part of the country. To understand the nature of the carbonates we need to know a little about their solubility. I use the following table to illustrate:
Calcium
Bicarbonate – HCO3
Carbonate – CO3
Hydroxide - OH
Solubility
Very Soluble
Insoluble
Somewhat Soluble
Magnesium
Bicarbonate – HCO3
Carbonate – CO3
Hydroxide - OH
Solubility
Very soluble
Soluble
Insoluble
You notice I include hydroxides that don’t contain any carbon dioxide. They play a role in the chemistry of carbonates, i.e. Kaukwasser-Ca(OH)2, which I will explain shortly.
There is an interesting fact about the 8.3 pH of seawater. It just so happens that at a pH of 8.3 or below all alkalinity is in the form of bicarbonate. Above 8.3 and alkalinity may be both bicarbonate and carbonate.
Ok you say what is the point? Well if one looks at our table above we see that calcium carbonate is insoluble ( not entirely, at 8.4 pH at 80o F you can dissolve about 60 ppm). This point has a great bearing on how we increase alkalinity (and calcium) in our tanks. We need to explore this and maybe change some ways of thinking.
When I was first studying up on reef systems I of course ran into the chorus of kaukwasser fans. KW the miracle additive! Hey KW is just good old lime water, Ca(OH)2. Claims are that it increases pH and alkalinity while providing beneficial calcium.
Let’s take a closer look remembering that calcium carbonate is insoluble while bicarbonate present in the tank is soluble. Here is the reaction of KW in our tank.
Sorry more equations
Ca(HCO3)2 + Ca(OH)2 ----------->2CaCO3 + 2H2O
Soluble bicarb KW Insoluble carbonate
What???? If I add KW to increase alkalinity I actually bring it down?
And you reduce calcium levels as well.
Clearly KW is overrated. It can only raise alkalinity to the saturation point of calcium carbonate’s solubility. Above that it reduces bicarbonate alkalinity and available calcium as well. Lime is exactly what is added to well water to soften it at municipal water plants. Lime=KW.
Don’t take my word for it. Here is a link to an excellent article by Richard Greenfield who supports my view.
http://www.athiel.com/lib7/greenfield.htm
Thank you Richard, I couldn’t have said it better myself.
So what do we do to increase alkalinity and calcium? Let’s tackle alkalinity first.
The clue is in the table of solubility I provided above. According to that table calcium carbonate is mainly insoluble so it we add something like KW we convert soluble calcium bicarbonate to insoluble calcium carbonate. The result is a pile of precipitated calcium carbonate lying at the bottom of our sump or, worse yet, cementing our substrate together so one’s live sand becomes dead sand. Look in the magnesium listing in the table. It shows that magnesium carbonate is soluble. I’m here to tell you folks calcium and magnesium are interchangeable when it come to buffering capability and magnesium wins out because both the bicarbonate and carbonate are soluble. The solution to raising alkalinity is to add magnesium carbonate to our tanks. Using magnesium carbonate we can hold our tank at any desired alkalinity level without fear of precipitation of carbonate salts.
Take my own tank. I maintain an alkalinity of 280 ppm consistently ( 15dK, 5.3 meg/L) by dosing with magnesium carbonate. Try getting you alkalinity to that level with KW and you will have a mess on you hands. I do run my alkalinity higher than most tanks but I assure you that this level keeps my pH stable in the range of 8.2-8.4 consistently. For the life of me I don’t understand why the purveyors of "magic alkalinity tonics" don’t use magnesium carbonate as the main ingredient in their potions. You can buy magnesium carbonate from test kit suppliers like Hach chemical. It is stable and safe to use (even drug lords don’t have a lot of use for it unless they have heartburn). I make a stock solution by dissolving 50 grams of magnesium carbonate in a liter of RO water (50,000 ppm as magnesium carbonate or 42,150 ppm as calcium carbonate). Dosing this solution allows me to maintain any desired alkalinity while providing magnesium and essential element for reef production (note: normal seawater contains 1300 ppm of magnesium).
Now we are faced with maintaining calcium levels. Open Ocean water has a level of about 400 ppm calcium (1000 ppm as calcium carbonate). This level is easy to maintain by adding non-carbonate calcium in the form of calcium chloride. Calcium chloride is very soluble and invertebrates can use it just as well as carbonate forms of calcium. Fortunately this form of calcium is available in several reef tonics sold by the local fish store. I have heard some people claim that this can cause an excess of chloride in the tank. Seawater has a chloride content of 19,400 ppm. Calcium chloride is just slightly under half chloride so maintaining 400 ppm calcium would only boost chloride levels by a like amount (2% increase in total chlorosity). That is not enough to cause concern.
One final tip on pH and alkalinity. If you pH is dropping and alkalinity too you can use baking soda in an emergency to bring it back up. I don’t recommend using baking soda for routine alkalinity adjustment but if you are out of alkalinity buffer, baking soda comes in handy to correct an alkalinity/pH problem.