Muriatic / Cyanuric acid: gassing off time required?

[Daf-Tekno]

Hello my fellow Water-Wizards,

Does adding liquid muriatic / cyanuric acid to my hot tub generate gassing off, like when chlorine powder is added?
Would I need to keep my cover open for 45 - 60 minutes to allow for this, like with chlorine powder?

I must say, the Muriatic acid is a rather nasty product!
Loads of health warnings, double-bagged and stinks to high heaven when uncapped!

I am very concerned with its safe storage, not only to protect the wife & I from accidental acid splashes to the face (so it must be stored at floor level), but also to protect our 1 year old daughter, who is just starting to crawl around all the FLOORS!

Garage / outside is not an option. we live in CANADA! Winter gets to -40 C (-40 F).

Am gonna put a lock one of the cupboards, but it still worries me a great deal.
We could experience a life-changing accident with this stuff.

Any thoughts would be of great comfort.

Thanks - D

 

[chem geek]

No, adding muriatic acid to lower pH or cyanuric acid to increase the CYA level do not generate outgassing the way that chlorine reacts with bather waste to produce some volatile products.

The volatility of the muriatic acid is only when it is concentrated. If you find 15-16% strength instead of the 31.45% strength you may find that it fumes a lot less. The problems you mention with its use and storage are one reason why dry acid (sodium bisulfate) is more typically used for spas. For pools, there is the risk of the buildup of sulfates damaging plater surfaces, but for a spa there is less risk since you change the water every so often and spas are usually acrylic not plaster.

As noted in this post, the freezing point of full-strength Muriatic Acid (31.45% Hydrochloric Acid) is -46C (-50.8F) so somewhat below your winter freezing temperatures but possibly a bit risky if you dip significantly lower in temps.

If you get your TA level lower and use 50 ppm Borates, then you should be able to have greater pH stability so use less acid in which case using dry acid should not be a problem.

 

[Daf-Tekno]

Hi ChemGeek,

Thanks for the info.
What's the freezing point of liquid conditioner / Cyanuric acid?

I assume that, once frozen, any of these acids are fundamentally ruined and must be disposed of / replaced..?

Thanks - D

 

[chem geek]

I don't think that freezing the acid ruins it, but I wouldn't risk bursting the container since the acid is hazardous and will corrode metal rather quickly.

I don't know the freezing point of the Instant Pool Water Conditioner. 4 ounces in 10,000 gallons is 1 ppm CYA so that is the same as 1.3352 ounces weight of CYA. Based on the MSDS the specific gravity is 1.16-1.18 so figure that the 4 fluid ounces (118.294 ml) weighs 138.4 grams. So that is 37.85 grams of solute if it were CYA so it's 37.85*(151.05/129.07) = 44.30 grams per 138.4 grams total so 94.1 grams of water. There are 37.85/129.07 = 0.293 moles of sodium cyanurate in 94.1 grams of water so 3.116 moles per kilogram but we need to multiply by 2 since the sodium and cyanurate dissociate so the freezing point depression is 3.116*2*1.86 = 11.6C so the freezing point of liquid conditioner is -11.6C or 11.1F. So not that low.

 

[singingpond]

Since the instant conditioner separates readily on standing (as described in one of Daf-Tekno's other threads), wouldn't that make the bottle even less freeze-proof? In other words, if the upper layer is mainly water (is it?), then that layer would freeze at a higher temperature than the slurry at the bottom, and possible split the container...?

 

[chem geek]

Yes, if it separates then the mostly water section would be more likely to freeze. Sodium cyanurate is somewhat soluble so it won't be pure water, but it is a slurry meaning that the sodium cyanurate is not all dissolved in the water. According to this paper, sodium cyanurate at 20ºC (68ºF) has a solubility of 0.05 mole/liter. The concentration of the slurry is around 3.1 moles/liter so clearly it's mostly not dissolved ions. To be conservative, one should probably treat the product as if it were water.