Why do hydroxides work for alkalinity supplement?

drolmaeye · Jun 1, 2024

Bottom line up front: why do hydroxides work as alkalinity supplements if they don't include the carbonate and bicarbonate stuff corals need to grow?

I know the primary components of a typical 2-part will be calcium chloride and sodium carbonate/bicarbonate. This makes sense to me as there will be building blocks available for calcium carbonate skeletons.

I also know we can use limewater to supplement calcium and alkalinity, and sodium hydroxide to supplement alkalinity, but I don't understand why, as the alkalinity component seems to not include the carbonate part of the necessary building blocks.

It is clear to me that if I add sodium hydroxide my total alkalinity will increase, but I thought it was the carbonate/bicarbonate contributions to total alkalinity that we need, so this sodium hydroxide boost is not helpful to my corals.

What am I missing?

Randy Holmes-Farley · Jun 1, 2024

The concentration of hydroxide in seawater only depends on pH and alk, no matter how you get there.

The hydroxide combines with CO2 to form bicarbonate and carbonate.

Here in the context of kalkwasser/limewater:

Limewater

Limewater (also known by the German term kalkwasser) has been used very successfully by aquarists for decades, and it is the system that I have used exclusively on my aquarium for 19 years. It is comprised of an aqueous solution of calcium and hydroxide ions that can be made by dissolving either quicklime (calcium oxide, CaO) or lime (calcium hydroxide, Ca(OH)2) in fresh water. The only inherent difference between the two is that if you add a molecule of water to quicklime, you get lime, and that a significant amount of heat can be generated when that happens.

CaO + H2O → Ca(OH)2

Quicklime + Water → Lime

Consequently, dissolving quicklime can make the water quite warm, especially if an excess of solids are added. Most hobby companies sell solid calcium hydroxide as “kalkwasser” or some similar name, although the name technically only applies to the solution.

The calcium ions in the solution obviously supply calcium to the tank, and the hydroxide ions supply alkalinity. Hydroxide (OH–) itself provides alkalinity (both by definition and as measured with an alkalinity test), but corals consume alkalinity as bicarbonate, not hydroxide. Fortunately, when limewater is used in a reef tank, it quickly combines with atmospheric and in- tank carbon dioxide (CO2) and bicarbonate (HCO3–) to form bicarbonate and carbonate (CO3—):

OH– + CO2 → HCO3–

OH– + HCO3– → CO3— + H2O

Once in the aquarium at an acceptable pH, there is no concern that the alkalinity provided by limewater is any different than any other carbonate alkalinity supplement. The hydroxide immediately disappears into the bicarbonate/carbonate system. In other words, the amount of hydroxide present in aquarium water is really only a function of pH (regardless of what has been added), and at any pH below 9, it is an insignificant factor in alkalinity tests (much less than 0.1 dKH). Consequently, the fact that alkalinity is initially supplied as hydroxide is not to be viewed as problematic, except as it impacts pH (see below).

The fact that limewater is very basic (the pH is typically above 12) demands that the limewater be added slowly to an aquarium unless very small additions are made. The reason for slow addition is two-fold: to prevent the local pH in the area of the addition from rising too high (slow addition permits more rapid mixing with tank water to reduce the pH), and to prevent the overall tank pH from rising too high (slow addition allows the tank to pull in CO2 from the atmosphere during the slow addition, mitigating the pH rise). Some aquarists advocate rapid addition, and that is acceptable for additions that would add significantly less than 0.5 dKH of alkalinity to the tank, but an addition of 1.4 dKH (0.5 meq/L; the equivalent of adding 1.2% of the tank volume in saturated limewater or 14 grams of solid calcium hydroxide into a 100-gallon tank) drives the pH of the whole tank too high (up by about 0.6 pH units from where ever it started).

Consequently, limewater is most often added slowly, by dripping or slow pumping. Often it is added as the top off water, replacing most or all of the evaporated water. The pumps add cost and complexity to the system, especially if combined with a float valve or switch (I use the latter and a Reef Filler pump).

As mentioned, limewater has a very high pH. This high pH can have significant advantages with respect to impurities present in the lime. Phosphate and many heavy metals will precipitate, either as calcium salts, or as metal oxides and hydroxides. Copper, for example, may accumulate in some aquaria. Copper hydroxide is very insoluble in limewater because of all of the hydroxide present. From an aquarist’s perspective, there will simply be no copper in clear limewater assuming that it has been given a chance to settle out because copper hydroxide is so very insoluble, regardless of whether there is a copper impurity in the calcium hydroxide solid, or in the source water used. Some aquarists get colored residues in limewater systems, and these colors are coming from metal impurities that did not get into the tank.

Another advantage of limewater may be its ability to reduce the phosphate already in the tank water. While it may be as simple as precipitation of calcium phosphate where the high pH, high calcium limewater meets the aquarium water, the mechanism and extent of this effect in typical reef tanks has not been established.

Another important consideration for limewater is the upper limit to the amount that can be added to an aquarium. The solubility limit of calcium hydroxide in fresh water is about 2 level teaspoons per gallon. If an aquarist has a tank near the high end of calcium and alkalinity demand, then replacing all of the evaporated water with saturated limewater may not be adequate to replace the ongoing losses of calcium and alkalinity. There are a couple of tricks to get a little more from the limewater. These are adding fans to increase evaporation, and adding vinegar to increase the solubility of the lime in the limewater (45 mL of vinegar per gallon of limewater will allow three level teaspoons to dissolve instead of just two). Both of these systems have been successfully employed by many aquarists.

Additionally, the use of a small amount of one of the other balanced additive systems (especially the two/three-part additive systems) in conjunction with limewater is often used by aquarists give a little boost to tanks that need a small amount of extra calcium and alkalinity beyond what limewater can supply, without incurring significant capital costs. Likewise, they can be successfully combined with limewater during periods of low evaporation. Unlike some other supplementation schemes, tank salinity will not increase over time through the use of limewater.

The cost of a limewater system can range from very little to quite a lot. If one uses an inexpensive drip system ($20) and bulk sources of lime the cost can be quite low. Bulk calcium hydroxide available to hobbyists sells for less than $2 per pound (maybe much less in a group buy from a large distributor). The cost per thousand milliequivalents (meq) of alkalinity is on the order of $0.15. I realize that this number means nothing to most aquarists, but I’ll use it to permit cost comparisons of very different supplementation schemes, and at the end of the article, I’ll convert it to yearly costs for some typical tanks. Branded hobby and lab grades of calcium hydroxide will be more expensive. A pound of calcium hydroxide from a well-known hobby company costs about $7, or $0.57 per thousand meq of alkalinity.

Of course, dosing pumps can be several hundred dollars, a good float switch can be $50-100, and one needs to get a reservoir as well (often a plastic container like a trash can; I use 44-gallon Rubbermaid Brute trash cans). Depending on the setup, the limewater reservoir can be far from the tank; even in another room or on another floor of the home. A pump like a Reef Filler or Liter Meter pump can be used to send the limewater significant distances, freeing up space around the tank.

Some people use reactors to deliver limewater. These systems automate the delivery of limewater to the tank, and, of course, the costs rise. They consist of a chamber where fresh water enters, is mixed with solid lime, and the fluid limewater exits the system and travels to the tank. They do not permit any additional calcium or alkalinity to be delivered to a tank compared to other limewater delivery methods (assuming that both use saturated limewater), but many claim them to be less hassle than delivery from a still reservoir. Addition of limewater with the simplest drippers may require daily attention, while delivery from a large reservoir may require attention only once every 1-5 weeks, which is about the same as typical limewater reactors. All of the other comments about limewater apply equally well when used with a reactor, a dripper, or a slow pump from a still reservoir (except that the vinegar/limewater combination is technically difficult to use with a limewater reactor).

On the negative side, limewater does have some concerns that don’t apply to most other systems. One is the effect of overdosing. All calcium and alkalinity additives, if added in sufficient overdose, can case abiotic precipitation of calcium carbonate in the tank. Limewater, however, is especially prone to this effect for two reasons. If overdosed, the high pH of the limewater will rapidly convert much of the bicarbonate in the tank to carbonate, increasing the likelihood of precipitating calcium carbonate. Also, addition of solid lime particles can cause local extreme spikes in pH and calcium that nucleate precipitation of calcium carbonate. Consequently, a limewater overdose, and especially the dosing of lime solids, is by far the most frequent cause of “snowstorm” events where calcium carbonate precipitates all through the water column. In some cases, the tank can look like milk. The good news is that this event usually causes no lasting harm to tank inhabitants unless the amount overdosed is exceptionally large, but it is nearly always upsetting to the aquarist. I’ve had it happen numerous times without losing anything.

Another drawback to systems where the limewater dose is tied to evaporation is that the evaporation may change daily or seasonally. I’ve not found that to be problematic in my system, but others who are more concerned about maintaining a very specific alkalinity may have more trouble with this issue. Dosing limewater on a timed pump rather than to match evaporation may eliminate the concern, as long as it doesn’t exceed evaporation rates.

One final note on lime: The high pH of the liquid and the dust hazard of the solid are not to be treated lightly. Inhalation of the dust is to be avoided. Splashing of limewater onto skin is also to be avoided, and should be followed by extensive rinsing with tap water if it happens. Splashing of limewater into the eyes is especially to be avoided, and the use of safety goggles when using large amounts or in situations where exposure is likely is prudent. Extensive and immediate rinsing with tap water, followed by professional help would be advised in the case of eye exposure.

drolmaeye · Jun 1, 2024

Randy Holmes-Farley said:
The concentration of hydroxide in seawater only depends on pH and alk, no matter how you get there.

The hydroxide combines with CO2 to form bicarbonate and carbonate.

Here in the context of kalkwasser/limewater:

Limewater

Limewater (also known by the German term kalkwasser) has been used very successfully by aquarists for decades, and it is the system that I have used exclusively on my aquarium for 19 years. It is comprised of an aqueous solution of calcium and hydroxide ions that can be made by dissolving either quicklime (calcium oxide, CaO) or lime (calcium hydroxide, Ca(OH)2) in fresh water. The only inherent difference between the two is that if you add a molecule of water to quicklime, you get lime, and that a significant amount of heat can be generated when that happens.

CaO + H2O → Ca(OH)2

Quicklime + Water → Lime

Consequently, dissolving quicklime can make the water quite warm, especially if an excess of solids are added. Most hobby companies sell solid calcium hydroxide as “kalkwasser” or some similar name, although the name technically only applies to the solution.

The calcium ions in the solution obviously supply calcium to the tank, and the hydroxide ions supply alkalinity. Hydroxide (OH–) itself provides alkalinity (both by definition and as measured with an alkalinity test), but corals consume alkalinity as bicarbonate, not hydroxide. Fortunately, when limewater is used in a reef tank, it quickly combines with atmospheric and in- tank carbon dioxide (CO2) and bicarbonate (HCO3–) to form bicarbonate and carbonate (CO3—):

OH– + CO2 → HCO3–

OH– + HCO3– → CO3— + H2O

Once in the aquarium at an acceptable pH, there is no concern that the alkalinity provided by limewater is any different than any other carbonate alkalinity supplement. The hydroxide immediately disappears into the bicarbonate/carbonate system. In other words, the amount of hydroxide present in aquarium water is really only a function of pH (regardless of what has been added), and at any pH below 9, it is an insignificant factor in alkalinity tests (much less than 0.1 dKH). Consequently, the fact that alkalinity is initially supplied as hydroxide is not to be viewed as problematic, except as it impacts pH (see below).

The fact that limewater is very basic (the pH is typically above 12) demands that the limewater be added slowly to an aquarium unless very small additions are made. The reason for slow addition is two-fold: to prevent the local pH in the area of the addition from rising too high (slow addition permits more rapid mixing with tank water to reduce the pH), and to prevent the overall tank pH from rising too high (slow addition allows the tank to pull in CO2 from the atmosphere during the slow addition, mitigating the pH rise). Some aquarists advocate rapid addition, and that is acceptable for additions that would add significantly less than 0.5 dKH of alkalinity to the tank, but an addition of 1.4 dKH (0.5 meq/L; the equivalent of adding 1.2% of the tank volume in saturated limewater or 14 grams of solid calcium hydroxide into a 100-gallon tank) drives the pH of the whole tank too high (up by about 0.6 pH units from where ever it started).

Consequently, limewater is most often added slowly, by dripping or slow pumping. Often it is added as the top off water, replacing most or all of the evaporated water. The pumps add cost and complexity to the system, especially if combined with a float valve or switch (I use the latter and a Reef Filler pump).

As mentioned, limewater has a very high pH. This high pH can have significant advantages with respect to impurities present in the lime. Phosphate and many heavy metals will precipitate, either as calcium salts, or as metal oxides and hydroxides. Copper, for example, may accumulate in some aquaria. Copper hydroxide is very insoluble in limewater because of all of the hydroxide present. From an aquarist’s perspective, there will simply be no copper in clear limewater assuming that it has been given a chance to settle out because copper hydroxide is so very insoluble, regardless of whether there is a copper impurity in the calcium hydroxide solid, or in the source water used. Some aquarists get colored residues in limewater systems, and these colors are coming from metal impurities that did not get into the tank.

Another advantage of limewater may be its ability to reduce the phosphate already in the tank water. While it may be as simple as precipitation of calcium phosphate where the high pH, high calcium limewater meets the aquarium water, the mechanism and extent of this effect in typical reef tanks has not been established.

Another important consideration for limewater is the upper limit to the amount that can be added to an aquarium. The solubility limit of calcium hydroxide in fresh water is about 2 level teaspoons per gallon. If an aquarist has a tank near the high end of calcium and alkalinity demand, then replacing all of the evaporated water with saturated limewater may not be adequate to replace the ongoing losses of calcium and alkalinity. There are a couple of tricks to get a little more from the limewater. These are adding fans to increase evaporation, and adding vinegar to increase the solubility of the lime in the limewater (45 mL of vinegar per gallon of limewater will allow three level teaspoons to dissolve instead of just two). Both of these systems have been successfully employed by many aquarists.

Additionally, the use of a small amount of one of the other balanced additive systems (especially the two/three-part additive systems) in conjunction with limewater is often used by aquarists give a little boost to tanks that need a small amount of extra calcium and alkalinity beyond what limewater can supply, without incurring significant capital costs. Likewise, they can be successfully combined with limewater during periods of low evaporation. Unlike some other supplementation schemes, tank salinity will not increase over time through the use of limewater.

The cost of a limewater system can range from very little to quite a lot. If one uses an inexpensive drip system ($20) and bulk sources of lime the cost can be quite low. Bulk calcium hydroxide available to hobbyists sells for less than $2 per pound (maybe much less in a group buy from a large distributor). The cost per thousand milliequivalents (meq) of alkalinity is on the order of $0.15. I realize that this number means nothing to most aquarists, but I’ll use it to permit cost comparisons of very different supplementation schemes, and at the end of the article, I’ll convert it to yearly costs for some typical tanks. Branded hobby and lab grades of calcium hydroxide will be more expensive. A pound of calcium hydroxide from a well-known hobby company costs about $7, or $0.57 per thousand meq of alkalinity.

Of course, dosing pumps can be several hundred dollars, a good float switch can be $50-100, and one needs to get a reservoir as well (often a plastic container like a trash can; I use 44-gallon Rubbermaid Brute trash cans). Depending on the setup, the limewater reservoir can be far from the tank; even in another room or on another floor of the home. A pump like a Reef Filler or Liter Meter pump can be used to send the limewater significant distances, freeing up space around the tank.

Some people use reactors to deliver limewater. These systems automate the delivery of limewater to the tank, and, of course, the costs rise. They consist of a chamber where fresh water enters, is mixed with solid lime, and the fluid limewater exits the system and travels to the tank. They do not permit any additional calcium or alkalinity to be delivered to a tank compared to other limewater delivery methods (assuming that both use saturated limewater), but many claim them to be less hassle than delivery from a still reservoir. Addition of limewater with the simplest drippers may require daily attention, while delivery from a large reservoir may require attention only once every 1-5 weeks, which is about the same as typical limewater reactors. All of the other comments about limewater apply equally well when used with a reactor, a dripper, or a slow pump from a still reservoir (except that the vinegar/limewater combination is technically difficult to use with a limewater reactor).

On the negative side, limewater does have some concerns that don’t apply to most other systems. One is the effect of overdosing. All calcium and alkalinity additives, if added in sufficient overdose, can case abiotic precipitation of calcium carbonate in the tank. Limewater, however, is especially prone to this effect for two reasons. If overdosed, the high pH of the limewater will rapidly convert much of the bicarbonate in the tank to carbonate, increasing the likelihood of precipitating calcium carbonate. Also, addition of solid lime particles can cause local extreme spikes in pH and calcium that nucleate precipitation of calcium carbonate. Consequently, a limewater overdose, and especially the dosing of lime solids, is by far the most frequent cause of “snowstorm” events where calcium carbonate precipitates all through the water column. In some cases, the tank can look like milk. The good news is that this event usually causes no lasting harm to tank inhabitants unless the amount overdosed is exceptionally large, but it is nearly always upsetting to the aquarist. I’ve had it happen numerous times without losing anything.

Another drawback to systems where the limewater dose is tied to evaporation is that the evaporation may change daily or seasonally. I’ve not found that to be problematic in my system, but others who are more concerned about maintaining a very specific alkalinity may have more trouble with this issue. Dosing limewater on a timed pump rather than to match evaporation may eliminate the concern, as long as it doesn’t exceed evaporation rates.

One final note on lime: The high pH of the liquid and the dust hazard of the solid are not to be treated lightly. Inhalation of the dust is to be avoided. Splashing of limewater onto skin is also to be avoided, and should be followed by extensive rinsing with tap water if it happens. Splashing of limewater into the eyes is especially to be avoided, and the use of safety goggles when using large amounts or in situations where exposure is likely is prudent. Extensive and immediate rinsing with tap water, followed by professional help would be advised in the case of eye exposure.

I appreciate this . . . an educational and enjoyable read that answers my question.