Soda Ash and Muratic Acid

[lborne]

It looks like I am going to do a no drain acid wash to clear up some stains. I'm going to add between 4 and 6 gallons of muratic, brush for a few days, and then counter the acid with soda ash. Pool calculator says I'm going to need more than a 1000 oz of soda ash, so I doubt I'll be adding that much.

Anyway, what happens to the soda ash? Is it nuetralized forever? Does it change any other parameters? Will it change the feel of the water?

 

[Melt In The Sun]

You'll end up with higher TA than when you started. Good idea to not add it all at once!

 

[chem geek]

[EDIT] These calculations are wrong. See this post later on in this thread instead[/url].

In a 12,000 gallon pool starting with a pH of 7.5 and TA of 80 ppm, 5 gallons of full-strength Muriatic Acid (31.45% Hydrochloric Acid) would lower the pH to around 6.6 so not really much of an acid wash per se, but a low pH that could help dissolve stains. It would take about 5.3 pounds (84 ounces weight) of soda ash to restore the pH back to 7.5, not more than 1000 ounces! I don't see how you got 1000 ounces -- how did you use The Pool Calculator to figure that out? If I put in your pool water volume and put in 80 ounces of acid in the "Effects of adding chemicals" it says the pH will drop by 1.6 which if I started from 7.5 would be 5.9 which overestimates what would really happen. If I then put in a starting pH of 5.9 and an ending pH of 7.5 in the pH section I get 261 ounces weight of soda ash required. Again, this is an over-estimation. If I put in 261 ounces of soda ash in the "Effects of adding chemicals" section, it says the pH will rise by 4.72 so clearly it's not balanced and if I put in 84 ounces it says the pH will rise by 1.52 which is about right for this situation.

The pH calculations are not accurate in The Pool Calculator for such wide variations and it says this in the "Effects of adding chemicals" section. If you want accurate calculations, you can use my Pool Equations spreadsheet, but it is not designed for novice users. Perhaps what is needed in The Pool Calculator is a separate section that gives pH neutral equivalents for balancing acid and soda ash or Borax (the latter calculation can be found in the Borate section).

If you were to use the proper amount of soda ash I indicated (i.e. 84 ounces weight), then you would be increasing the Total Alkalinity by around 24 ppm compared to where you started (the salt level would increase by around 30 ppm as well). If you were to use 167 ounces weight (around 2.2 76-ounce boxes) of 20 Mule Team Borax instead, then your TA would only increase by around 1 ppm from where you started though the borates would increase by almost 12 ppm.

Richard

 

[Guest]

We were involved in an in pool acid wash last month, and they took the pH down to zero (It was a PebbleSheen pool) and had several pumps in the pool moving the water around. There was quite a bit of brushing involved, but the finish turned out beautiful (and the homeowner saved about 30,000 gallons of water by not draining and acid washing ).

With plaster, you will not be able to go down that low. How do you plan to keep the water circulating?

 

[lborne]

I was obviously using the poolcalculator wrong. I put in 4 gallons of acid and it said it would lower my pH by 10, so I assumed my pH would then be 0. If I wanted to go from 0 to 7.5, it is some incredible amount of soda ash.

The pool repair guy said that I could use my pump to circulate as long as I bypassed the heater. Should I rent a pump instead?

 

[Melt In The Sun]

do you have a heater besides the solar in your signature? I wouldn't think solar would be any problem with the low pH, but a heat pump/gas heater would.

 

[lborne]

If I put in your pool water volume and put in 80 ounces of acid in the "Effects of adding chemicals" it says the pH will drop by 1.6 which if I started from 7.5 would be 5.9 which overestimates what would really happen.

I put in 512 ounces which is 4 gallons. Why did you use 80? I used 512 because the pool guy said to put in 4 to 6 bottles of acid.

 

[lborne]

do you have a heater besides the solar in your signature? I wouldn't think solar would be any problem with the low pH, but a heat pump/gas heater would.

Yeah, I've got a small electric heater for my spa. It can easily be taken out of the loop. Its a stainless tank, but I think the elements are standard hot water heater elements.

 

[chem geek]

I put in 512 ounces which is 4 gallons. Why did you use 80? I used 512 because the pool guy said to put in 4 to 6 bottles of acid.

Because I made a mistake and multiplied 5 by 16 in the section for fluid ounces instead of the one for cups -- my bad -- when I said the spreadsheet wasn't for novice users, I was including myself . Let me redo the calcs for you again, using the right amount of acid -- I'll use 5 gallons.

So 5 gallons of full-strength Muriatic Acid would lower the pool pH to around 2.6 which sounds much more like an acid wash. To restore the pH using pH Up would take around 42 pounds (672 ounces weight), but that would result in the TA rising by 187 ppm when you were done (in practice, you'd add less pH Up and aerate some, but you'd still be adding a lot of TA -- when you add the acid, a lot of the carbon dioxide will outgas). If you used 83 pounds of 20 Mule Team Borax instead, then the TA rise would only be around 9 ppm, but the borates would increase by around 94 ppm. The most direct way to offset the acid is with a pure base -- lye / caustic soda / sodium hydroxide -- where it would take around 17 pounds to restore the pH with no change in TA and the only side effect being an increase in salt of around 240 ppm, but you would need to add something to increase the TA that was lost from the acid. So pH Up is actually your best bet so long as you don't overdo it.

If I assume that the Muriatic Acid treatment gets rid of most of the carbonates in the water, then adding 28 pounds of pH Up would restore the pH and would overshoot the initial TA by 55 ppm. If you added 22 pounds of pH Up, then you might get back to a pH of 6.7 from which you could aerate the water the rest of the way and end up with the same TA you had when you started. So the rough rule-of-thumb is that every gallon of full-strength Muriatic Acid needs 4.4 pounds of pH Up to restore the TA. If you add enough acid to get the pH below around 3.0 (around 3 gallons of acid per 10,000 gallons of pool water), then aeration can restore the pH. If you are using less acid, then the amounts of pH Up indicated would raise the pH too much (but then this wouldn't be an acid wash).

Sorry for the miscalculation the first time. Even with The Pool Calculator not being correct, you were definitely a lot closer than I was.

 

[lborne]

So the rough rule-of-thumb is that every gallon of full-strength Muriatic Acid needs 4.4 pounds of pH Up to restore the TA.

Thanks. I really appreciate the help. I'll use a combination of pH UP and aeration so that I keep TA within bounds. I'm not doing this until winter when we are not swimming, but I'll be sure to post if I run into trouble.

 

[JasonLion]

Keep in mind that the calcium removed from the pool surface will use up some of the acid, so you aren't going to need the full amount.

 

[chem geek]

Good point. In fact, if one were removing scale, then that amount could be substantial. So adding less pH Up than you think you need would be a wise starting point. A rise of Calcium Hardness (CH) of 10 ppm in 10,000 gallons would be equivalent to 14.1 ounces (somewhat less than a pound) of pH Up in terms of its effect on TA and pH. So if one measured a substantial rise in CH from the acid wash process, then quite a bit less pH Up might be needed. I'm not sure if the CH test works at very low pH (it's not noted in the Taylor interferences, but then again a very low pH during an acid wash isn't a common pool condition -- there's a buffer added in the CH test, but I don't know what it's trying to achieve).

 

[JasonLion]

(((Technical aside: I believe the Taylor CH test is based on the EDTA titration for calcium and/or magnesium. In a typical EDTA titration, a buffer is used to bring the PH up to around 12 to protect against interference from magnesium and other cations.)))

This is a good situation to use a base demand test, if you have one. Otherwise, you can always raise the PH in a sequence of smaller steps, repeating until the PH is in range.

 

[Guest]

The pool repair guy said that I could use my pump to circulate as long as I bypassed the heater. Should I rent a pump instead?

Is that a pool guy that wants to sell you a new pump?!

I would rent a pump (or two) and attach a "floppy" hose to the discharge side to make it agitate the water good. You want a lot of movement in the pool to really mix everything up. I don't know how low you will actually go on pH, but I wouldn't want to run too low of pH through my pump!

 

[Beez]

The pool repair guy said that I could use my pump to circulate as long as I bypassed the heater. Should I rent a pump instead?

Is that a pool guy that wants to sell you a new pump?!

I would rent a pump (or two) and attach a "floppy" hose to the discharge side to make it agitate the water good. You want a lot of movement in the pool to really mix everything up. I don't know how low you will actually go on pH, but I wouldn't want to run too low of pH through my pump!

Dumb question: If this would ruin the main pump, wouldn't it ruin the rented pump(s) as well? :|

 

[lborne]

Actually, I'm going to try and target the stain areas by either pouring acid through a pipe, or filling a bag (think boxed wine bags) and bringing it down to the bottom with me. I don't need to acid wash the entire pool, just the 3% area where the metal dust settled. Hopefully I can get away with only 2 gallons of acid.

I think the pump housing and impeller can take the acid for a short time. The seals may not be able to take it.

 

[Guest]

The pool repair guy said that I could use my pump to circulate as long as I bypassed the heater. Should I rent a pump instead?

Is that a pool guy that wants to sell you a new pump?!

I would rent a pump (or two) and attach a "floppy" hose to the discharge side to make it agitate the water good. You want a lot of movement in the pool to really mix everything up. I don't know how low you will actually go on pH, but I wouldn't want to run too low of pH through my pump!

Dumb question: If this would ruin the main pump, wouldn't it ruin the rented pump(s) as well? :|

Well, I wouldn't call it a dumb question! I think it is a great question (but then I have been called dumb on more than one occasion :-D )

I'm not sure it would have a huge effect on a pump, but I would rather take my chances on a rented pump than my own! I'd also rather contain the water in the pool as opposed to running it all through my system, even if I bypassed certain pieces. I have a trash pump that I would probably not have much of a problem putting in my pool if I was going to do this, but I think I'd try and save my pool pump, if it's "safe" or not!

 

[Beez]

Well, I wouldn't call it a dumb question! I think it is a great question (but then I have been called dumb on more than one occasion :-D )

I'm not sure it would have a huge effect on a pump, but I would rather take my chances on a rented pump than my own! I'd also rather contain the water in the pool as opposed to running it all through my system, even if I bypassed certain pieces. I have a trash pump that I would probably not have much of a problem putting in my pool if I was going to do this, but I think I'd try and save my pool pump, if it's "safe" or not!

:lol: OK, makes sense to me!

 

[JasonLion]

If you can't easily bypass the heater you should rent a pump. The acid won't have much effect on the pump, but it would be very bad for most heaters.