High pH and High Total Alkalinity- use Muriatic acid or not?

[Gassy]

As the topic title states, I have a situation where my TA is very high as is my pH.  The local spa store sold me some "pH down", which is sodium bisulfite.  With a bit of chemistry knowledge and some research, I learned that sodium bisulfite reacts with water to form sulfuric acid and sodium hydroxide, the latter of which is a weak alkali.  So, while this product is good at lowering the pH by virtue of the sulfuric acid, it either does nothing for or increases TA by virtue of the NaOH it adds to the water.

Now, my question is, in the situation of high pH and TA, wouldn't muriatic (hydrochloric) acid, in very small amounts and titrated to achieve slow drops in pH and TA, be better?  I keep hearing/reading about all the caveats associated with MA, but chemically speaking (it dissociates into H+ ion, lowering pH without increasing TA) wouldn't MA make more sense?  Your thoughts?

[Tman122]

Muratic Acid is very good at lowering both PH and TA. However it is also very corrosive and needs to be both handled and added very carefully. Dilute it with water first in a bucket then add it to your spa. Start out slow and take your time.

[Gassy]

Thanks, TMan.  I assume that by corrosive you mean to skin and surrounding structures and not to the internal plumbing and mechanics of the tub, correct?  I will use it sparingly, and thanks for the dilution tip.

[chem geek]

Your chemistry is wrong.  Sodium bisulfate does not create sulfuric acid and sodium hydroxide in water at typical spa pH.  That would only occur in water with a pH near 2 or below since that is the pKa of sodium bisulfate.  In spa water sodium bisulfate dissociates into sodium and sulfate releasing a hydrogen ion (i.e. acid):

NaHSO₄ ---> Na⁺ + H⁺ + SO₄^2-
Sodium Bisulfate ---> Sodium Ion + Hydrogen Ion + Sulfate Ion

You are also wrong about this acid not lowering TA.  It most certainly does as do all strong acids.  In 350 gallons, the TA is lowered by 10 ppm when adding 1.2 ounces weight (around 0.8 fluid ounces volume) of sodium bisulfate (93.2% crystals) or 0.9 fluid ounces volume of full-strength Muriatic Acid (31.45% Hydrochloric Acid).  The pH drops by exactly the same amount with these amounts of acid.  The only advantage to the Muriatic Acid is that it does not build up sulfates, but that is generally only a problem for pools since sulfate can attack plaster surfaces and is bad for splash-out and evaporation with soft stone.  Sulfates along with chloride can increase metal corrosion for stainless steel.  However, it takes significant quantities to become a problem, though for pools I generally recommend Muriatic Acid -- for spas I think you can use either and sodium bisulfate is easier to dose given that it is dry (and it doesn't fume, though you can get half-strength Muriatic Acid that fumes far less).

[Tman122]

Thanks, TMan.  I assume that by corrosive you mean to skin and surrounding structures and not to the internal plumbing and mechanics of the tub, correct?  I will use it sparingly, and thanks for the dilution tip.

Yea don't spill it full strength. Might turn your foot or spa cover to mush.

[goneaway]

LOL.
I once purchased some central heating cleaner from a well know DIY centre in the UK.
This was marketed as central heating cleaner and IIRC was muriatic acid.

This stuff ate it's way through the whole house.
The instructions said to empty the full contents in to the system and leave for up to a week.

Within about 6 hours, i'd spring a small leak, which i thought, "oh well, the radiator must have been on it's last legs, so it's just as well really"
The next day, we woke up to all hell broken loose.

This stuff had eaten it's way through a fibre washer on a stop valve in the roof space, run down the internal wall, removing the paint.
Had found a weak spot in a solder joint so was all over the kitchen floor.
And was starting to show signs of eating it's way out of a radiator valve.

I quickly flushed the whole system out, and kept doing so for about 5 evenings after.

It took about 12 coats of fresh paint before the wall stopped bleeding.

[Gassy]

Chem geek- thank you for the correction and for sharing your knowledge.  It's been more than 30 years since I studied chemistry, and for me it was just a requirement for graduate education.  Now I will feel better using the sodium bisulfate.

[rosewoodsteel]

LOL.
It took about 12 coats of fresh paint before the wall stopped bleeding.

Keat,
If you run into a situation like that again, I recommend using Kilz primer to cover the stain.  You will save yourself a lot of unnecessary painting.