Re: He wants to use Sodium Bicarbonate first, then Sodium Ca
In regards to a low pH and low alkalinity (TA) of pool water and how to properly raise both of them, the following is my perspective and how I see this topic.
I think Chem geek was correct the first time (in another post he linked to) that the most effective and most cost efficient way to raise the TA and pH on a very low pH and TA pool situation (as in a Zero Alkalinity Acid Treatment, and a 4.5 pH), is to add soda ash (sodium carbonate) first to re-balance to a pH of around 7.0, and then adding Bicarb (sodium bicarbonate) to finish raising the TA to the proper range of around 80 ppm.
As James suggested in the other linked post, probably the safest way to re-balance the water is to calculate the correct amount of Bicarb to add to achieve an 80 ppm of TA, and then allow the low pH to rise naturally. That program however, requires some time to allow the pH to rise to the proper balanced level.
As was also suggested by James in the other post, it is true that there are potential issues of precipitating copper (if present) and calcium when using soda ash as opposed to adding Bicarb. But as Chem geek also indicated correctly, if draining the pool is not an option, it may be better to precipitate these two metals (by adding Soda ash) and have them filtered out before they adhere to the surface. Of course, vinyl and fiberglass pools would be good candidates for that program.
The bottom line on this issue is that there are different ways to re-balance low pH and low TA water effectively. There can be pros and cons to several ways. And adding and using some borax can be an extra option. And note that Soda ash is 60% stronger than Bicarb in adding alkalinity, and therefore will require a lesser amount to raise the alkalinity (and pH).
The following is my understanding on the issue of carbon dioxide (CO2) and what Bicarb and soda ash does when added to water with a low pH and low TA.
I am going to ignore the involvement of carbonic acid formation for simplicity sake. It is understood that whenever CO2 (carbon dioxide) is formed, there will be a very small fraction of carbonic acid formed establishing an equilibrium (as Chem Geek mentioned) and that it participates in the various reactions discussed below.
When adding sodium bicarbonate (Bicarb) to water having a pH of 4.5 (approximately) and above, will not create or form a “measureable” amount of additional CO2. It simply adds bicarbonate alkalinity and raises the pH of the water. When enough Bicarb is added to raise the pH to 6.4; that will be where there are equal “ppm” amounts (a 1 to 1 ratio) of bicarbonate alkalinity and CO2. For example, at that point (pH 6.4) there may be 10 ppm of CO2 and 10 ppm of bicarbonate alkalinity (TA), or 50 ppm of CO2 and 50 ppm of bicarbonate. When the ratio of bicarbonate to carbon dioxide is 5 to 1, then the pH is 7.0, i.e. 50 ppm of bicarbonate and 10 ppm of CO2, or 100 ppm of bicarbonate and 20 ppm of CO2.
As everyone knows, in time the CO2 out-gasses and is lost to the atmosphere thereby causing the pH to further rise before and after the addition of Bicarb.
When soda ash is added to water with a pH 4.5 and above, it reduces the CO2 content, not create more. It does this by reacting carbonate (from soda ash) with the CO2 in the water to form bicarbonate alkalinity and also increases the pH significantly (more than Bicarb does). Adding more soda ash will continue to reduce the CO2 content until a pH of 8.4 is achieved. At that point, there will not be a “measureable” amount of CO2 in the water.
Those who perform a Zero Alkalinity treatment often add too much acid to the pool and the pH may go as low as 3.0. When the pH goes below 4.5 (about), the TA is considered to be a “negative” amount or is defined as mineral acidity, and contains a more measureable amount of hydrogen ions (acid).
Adding Bicarb or soda ash to water with a pH of 3.0 will cause the formation of some additional CO2, due to the reaction with higher content of hydrogen ions (a strong acid). But once enough Bicarb or soda ash is added and raises the pH to around 4.5 and above, the amount of hydrogen ions becomes negligible and the reactions with Bicarb or Soda ash essentially does not increase the CO2 content, and therefore, can be ignored.
To help put this into perspective, when starting out with 80 ppm of TA (assuming no CYA) and performing a Zero Alkalinity or No-Drain acid bath, adding acid to lower the TA to zero (and pH of 4.5) creates about 70 ppm of CO2. (I hope I did my math right). And again, that excess CO2 is always out-gassing, and therefore is constantly being lost and reduced over time.
Depending on how long the water is allowed to remain in an acidic condition, the content of CO2 may be somewhat high, or could be very low. This can be affected by at least two factors; the natural off-gassing of the CO2, and the fact that the acidic (and aggressive) water will have been dissolving calcium carbonate from the plaster surface (if a plaster pool), which will reduce the CO2 content and add alkalinity (and CH) to the water. So once Bicarb or Soda Ash is added to start the re-balancing program, the pH could be anywhere from 3.0 to 7.0 and the TA may be a negative number, or contain 5 to 10 ppm, or even higher.
A pH meter would be helpful in determining how low the pH is, and along with a TA test, the correct dosages of chemicals could be determined for proper re-balancing.
