Any Advantage To Sodium Bisulfate Versus Muriatic Acid?
Posted 22 November 2009 - 05:49 PM
My 370 gal spa takes a LOT of acid to get from TA 300 down to TA 80. Took 14 oz. today (a combo of some Robard's liquid sodium bisulfate, and 6 oz. of muriatic acid). Both types do the same thing, but the muriatic is much cheaper. (I know that dry sodium bisulfate is cheap too, but I hate having to stand there trying to get it to dissolve in a bucket; it never fully does and then I still have to add it to the spa).
So my question is: Aside from hazardous handling issues of muriatic acid, is there any advantage to using sodium bisulfate (dry or liquid)? Given that it takes a lot for my balancing, is there any danger to my spa shell/plumbling (or my skin when using spa) to using just muriatic acid? Does it affect TA/pH or any other parameters in a different way than sodium bisulfate?
Also, is dosing any different between the two? Poolcalculator.com only shows dosing for muriatic acid. (BTW, I do wish the PoolCalc would add calculation for how much acid it takes to move TA, based on CH, temp, etc.).
Posted 22 November 2009 - 07:25 PM
The pool calculator shows the dosage of muriatic acid and dry acid (sodium bisulfate). The dosage amount depends on your spa size, current TA and borates and how much you want to move pH.
Posted 22 November 2009 - 08:47 PM
I will get some borates to help stabilize. In the meantime I do have a bottle of Leaisuretime "pH Balance" that is supposed to help anchor the pH once I hit it. I assume that it has some borates in it.
One important question I have is this: Since I now know how much total muriatic acid it takes to get a full fresh tub of my source water to my target TA, do I still need to do it in stages--or can I go ahead and do it in one shot? (Maybe I'll use just a little under, say 12 oz.; check and then do the final movement.) What does do you experts think?
Posted 22 November 2009 - 10:42 PM
It is important to lower the TA in stages to avoid a dangerously low pH.
Leisure time pH balance is a phosphate based buffer. It contains Dipotassium phosphate (K2HPO4) and Monosodium phosphate (NaH2PO4).
pKa for phosphoric acid:
pKa1 =2.15, pKa2 = 7.21 and pKa3 =12.34
Phosphoric acid H3PO4.
Based on the pKa values, most of the phosphoric acid will be in the HPO42- form. At a pH of 7.5, 34 % will be H2PO4 and 66 % will be HPO42-.
I would assume that the DiPotassium phosphate (K2HPO4) would make up 66 % of the mix and that Monosodium phosphate (NaH2PO4) would make up the other 34 %.
It should not be used when calcium levels are, or need to be, over 150 ppm because it will combine with calcium to form Calcium Phosphate (CaHPO4) , which will precipitate out and cause scale and cloudiness.
I think that muriatic acid is better to use than dry acid.
Using the calculator, it shows that for a 370-gallon tub, adding 21 ounces of 31.45 % muriatic acid will lower the alkalinity by 222 ppm.
Using the pH part shows that 21 ounces of muriatic acid will lower the pH from 7.8 to 5.1 when the alkalinity is 100 ppm.
You shouldn't allow your pH to drop below 7.1
Posted 23 November 2009 - 10:42 AM
Your (over my head) explanation of Leisure Time "pH Balance" was interesting. And its effect with hard water jives with my one experience with it over a year ago. Actually, Leisure Time makes a product called "pH Balance Plus" which is specifically for use with hard water. Unfortunately, Leisure Time does not seem to post MSD sheets for its products (and even Robert's where you posted the link to the other MSDS does not have one for the "Plus" version for hard water). I wonder what is in that stuff.
No matter really, since I won't be using the bottle of "pH Balance" I have, I can order whatever. Please remind me again of what you think is the best, easiest to use, and most cost effective product for "anchoring" TA/pH once I get the water balanced. (I'll be using Dichlor/Bleach in a tub with an ozonator.)
If you recommend borax for TA/pH stabilizing/buffering, then I will be puzzled once again because I see (on the calculator) that borates raise TA & pH quite a bit. Maybe I am missing something or perhaps the borax is for something else.
All the best,
Posted 23 November 2009 - 04:39 PM
Boric acid is what you want to use to stabilize your pH. Borates are used to raise pH. Borates raise the Total Alkalinity some, but not a lot.
You can think about borates as boric acid without the hydrogen ions attached.
Low pH is caused by an excess of hydrogen ions. You can think of acid as hydrogen ions.
Boric acid = H3BO3 pKa1 =9.14, pKa2 = 12.7
Borate = BO33-
When borates are added to water, they begin to remove hydrogen ions from the water and this raises the pH.
BO33- + 3H+ --> H3BO3
If you use boric acid to add boric acid, you will not have a pH rise because the borates already have the hydrogen ions attached. If you add boric acid by adding borates, you will have to add acid (hydrogen ions) to lower the pH by replacing the missing hydrogen ions.
When the pH rises from an introduced base, some of the hydrogen ions are released back into the water, which lowers the pH. This is what provides the buffer.
Note: This is just a simplified concept to make it easier to understand. Boric acid actually associates or dissociates the hydroxide ion. Here is a different explanation.
pH is a measure of the active hydrogen ions in the water. Its counterpart is pOH, which measures the concentration of hydroxyl ions in the water. The pH + pOH always add up to 14.
When a weak acid like Phosphoric acid H3PO4, carbonic acid (H2CO3) or cyanuric acid (C3H3N3O3) is added to the water, it partially deprotonates ( Some of the hydrogen ions (protons) dissociate (disconnect) from the acid) based on the pH and the pKa of the acid.
Therefore, when carbonic acid is added to water, some of the hydrogen ions dissociate from the acid to form a base called the bicarbonate ion HCO3-or the carbonate ion CO32-.
H2CO3 --> HCO3- + H+ pKa = 6.36
The formula is 10^X/ (1 + 10^X) = Percent of the acid base pair that is acid. X = pKa - pH.
Here are some values for boric acid.
pH = 8.3 Boric acid = 91 % Borate ion = 9 %.
pH = 8.2 Boric acid = 93 % Borate ion = 7 %.
pH = 8.1 Boric acid = 94 % Borate ion = 6 %.
pH = 8.0 Boric acid = 95 % Borate ion = 5 %.
pH = 7.9 Boric acid = 96 % Borate ion = 4 %.
pH = 7.8 Boric acid = 97 % Borate ion = 3 %.
pH = 7.7 Boric acid = 97.5 % Borate ion = 2.5 %.
pH = 7.6 Boric acid = 98 % Borate ion = 2 %.
pH = 7.5 Boric acid = 98.4 % Borate ion = 1.6 %.
Boric acid works in a similar way to cyanuric acid and carbonic acid except that it associates or dissociates the hydroxyl ion instead of the hydrogen ion.
When Borax is added to the water, the borates release hydroxyl ions, which connect to free active hydrogen ions, which raises the pH. Therefore, at a pH of 7.5, 98.4 % of the borates will become boric acid by releasing hydroxyl ions. This also decreases the concentration of hydrogen ions and raises the pH.
Boric acid acts as an acceptor of hydroxyl ions as the pH increases. For example, if the pH goes from 7.6 to 8.0, the amount of boric acid is decreased from 98 % to 95 % and the amount of borate ion goes from 2 % to 5 %, because the boric acid connects with hydroxyl ions to form B(OH)4− (tetrahydroxyborate anion) as the pH increases.
The boric acid removes the hydroxyl ion from the water to leave the hydrogen ion. This lowers the pH.
B(OH)3 + 2 H2O <> B(OH)4− + H+ + H2O
Another way to look at it is that boric acid resists pH rise by connecting with OH- (The hydroxyl ion).
B(OH)3 + OH- <> B(OH)4−
Borax contains Na2[B4O5(OH)4]•8H2O, which becomes
4B(OH)3 + 2Na + 2OH- + 3 H2O
(As Richard has previously shown.)
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