Bleach And Silver Ion Compatability?

[Nat]

I have a Tiger River Bengal spa, bought a little over a month ago. It is made by Watkins, who makes Hot Springs also. Watkins promotes the "Everfresh Water Care" system, wich consists of a sliver ion cartridge (Ag+ from silver nitrate) in the filter housing, ozonater, and the very light use of dichlor and/or MPS. (specifies 2/3 tsp. dichlor or 1 Tbs. MPS before each use in the 330 gallon Bengal and weekly shocks with 1.5 Tsp. dichlor or 3 Tbs. MPS).

I've been using bleach to sanitize the water for about a week now, after reading this forum.

I have one concern: Is bleach compatable with the silver ion cartridge? Reason I ask is because I read here that one of the bleach by-products is sodium chloride (salt). From chemistry I had in college I understand that sodium chloride (aqueous) will combine very readily with silver nitrate (aqueous) and produce silver chloride as a percipitate that locks the silver (there is no more silver ion) and renders it useless as a sanitizer. (The silver chloride production by mixing salt with a silver nitrate solution was demonstrated in class.) I suppose the spa filter would remove the silver chloride.

The following came from Wikipedea:

"In one of the most famous reactions in chemistry, addition of colorless aqueous silver nitrate to an equally colorless solution of sodium chloride produces an opaque white precipitate of AgCl:

Ag+(aq) + Cl-(aq) → AgCl(s)

This conversion is a common test for the presence of chloride in solution."

If this is the case I, guess I have two options I could consider: 1. just follow the manufacture's water care directions and not use bleach. 2. Don't use the silver ion system (a cartridge costs about $30 and is good for one tub fill--about 4 months) and use dichlor for about a week and then use bleach for the remainder of the tub fill.

Any thoughts? Anyone else have an opinion about the Watkins/Hot Springs "everfresh water care" system?

Thanks.

--Nate

[TinyBubbles]

Very interesting question Nate. I can't wait for the chemistry experts to weigh in.

[B0Darc]

It's a matter of solubility and concentration, plus buffering effects of other ions in solution (your spa). Silver is a metal and therefore low on the solubility scale... like your silver bracelet doesn't just dissolve when you wear it in the spa. However tiny amounts of it do! The silver in the filter add-on is not in pure silver form and has been made to be more soluble... but in more of a time release way. However it would have to be in relatively high concentrations to create a silver chloride "situation"... at the same time I would bet you could test some of that white residue around your spa and it would test positive for silver chloride. Remember we're talking about small concentrations even of chlorine (4 parts per million?!) as being highly effective sanitizers. I wonder how much silver is actually in solution to be an effective anti-microbial (40 parts per billion?)

Your spa water is highly buffered and highly diluted, so the rate reaction you witnessed in Chem class does not strictly apply... for example, if your professor had used water from his spa instead of distilled water for his experiment it may have been less impressive. I would also have to guess the amount of silver and chlorine he used was 10 thousand times the concentration of your 400 gallon spa. I also read that high degrees of silver can cause permanent staining, but that was a thread on another site referring to adding (too much!) colloidal silver designed for pool care to a spa. I'm relatively confident Watkins doesn't make a mineral add-on that cannot be simultaneously used with chlorine or would deliver enough silver to permanently stain someone's spa (or cause a huge silver chloride flocculation)... give 'em a call, and let us know. Good looking out though! The chemistry stuff is fascinating.

[B0Darc]

Oh and BTW I have a MasterSpas EcoPur add-on mineral filter paper pleated cartridge. It allegedly had silver and other magically delicious secret ingredients to lower your need for so much chlorine (we're supposed to stay in the 1-3 PPM range)... my sales rep said they were made in trees by elves. He had my wife rub her hands on the dry filter to see how soft it made her skin feel (implying it would have the same effect in th spa water)... we barely made it back out to our turnip truck before we burst out in laughter. It made a big cloud of dust when she rubbed it and looked like they had sprinkled it with talcum powder... tsk tsk. Luckily we had come with our big shovels... always prepared. Still LOVE my DownEast (MasterSpaz) tub, but I can't really say the mineral filter (EcoPur) is measurably lowering my need for chemicals. I can say that after switching to the bleach+weekly shock my water *is* just like elves come in the night... oh wait that's shoes, nevermind.

[chem geek]

B0Darc is correct. The amount of silver is very low, but improper use can lead to staining from silver precipitation and this happens more noticeably in pools using silver (and copper) products (as B0Darc mentioned -- when too much colloidal silver is added, or added too quickly). However, the solubility product of silver chloride is 1.8x10^(-10) (it's temperature dependent, by the way) so even a saltwater chlorine generator (SWG) pool with 3000 ppm salt (which is a measurement of chloride in units of ppm sodium chloride) is a concentration of 5.13x10(-2) moles/liter chloride so the maximum silver concentration would be 3.5x10(-9) moles/liter which is equivalent to about 0.4 ppb though this calculation ignores ion complexes that can increase actual solubility. [EDIT] This complex ion effect should NOT be ignored -- see below for more details. [END-EDIT] More typical concentrations of salt would be around 1000 ppm or less and that would allow for 1.2 ppb or more before precipitation, but because this is such a small amount, the silver chloride would likely just be suspended in the water or not visible.

[EDIT]

I just looked up some info on complex ion pairs for silver chloride and found the following:

Ag⁺(aq) + 2Cl^-(aq) <--> AgCl₂^-(aq) ...... K = 1x10^6

So again using our example of 3000 ppm salt, this gives a ratio of the silver chloride ion pair to silver ion of 2600 meaning that the vast majority of silver will be in the form of the ion pair and not as free silver ion and therefore not contributing towards its precipitation. The solubility won't be the normally predicted 0.4 ppb but will instead be closer to 1000 ppb or 1 ppm. Even at 500 ppm salt, the ratio is about 70. So at 500 ppm salt, instead of the normally predicted 2.4 ppb maximum, it would actually be closer to 170 ppb or 0.17 ppm if one counts the ion pair as if it were free silver. At 100 ppm salt, instead of the normally predicted 12 ppb it will be closer to 30 ppb though at this lower salt (chloride) concentration the ion pair starts to become less significant. Note how higher salt (chloride) concentrations (above around 50 ppm salt) actually make silver chloride MORE soluble, not less. This is due to the ion pair which forms as the square of the chloride concentration while the silver chloride solid only forms linearly with chloride concentration. When the silver concentrations are quoted for ion systems, they are probably talking about total silver including that in ion pairs. I don't know if such ion pairs have any catalytic disinfecting effect, but I suspect they do not and being negatively charged they would tend to not concentrate as much near negatively charged cell surfaces.

The classroom experiment adding silver nitrate to salt water has relatively high concentrations of silver that overwhelm the effect of the silver chloride ion pair. Even with fully saturated salt water at 36g/100ml, this is 6 moles/liter so the silver chloride ion pair to silver ion ratio would be 3.6x10^7 (ignoring ionic strength effects). The normally predicted precipitation of silver would occur at a silver ion concentration of 3x10(-11) or 0.03 ppb but would actually be around 1200 ppm due to the ion pair. 1200 ppm of silver is 0.01 moles/liter and the silver nitrate solution may have been more concentrated than this (typical reagent solutions are 0.1 moles/liter) or the water may not have been saturated with salt.

[END-EDIT]

Also, ANY source of chlorine will contribute to the chloride level since for every 1 ppm FC this gets converted to 0.8 ppm salt since when chlorine gets used up it converts to chloride. It is true that bleach and chlorinating liquid (and Lithium hypochlorite) all contribute an additional 0.8 ppm of salt due to the manufacturing process so for every 1 ppm FC from these sources you end up with 1.6 ppm salt. If you add 4 ppm FC every day for 3 months, then that would be an increase of 576 ppm salt which is not very much. If you used only Dichlor, then you would add half as much salt.

I'm not so sure the spa filter would remove the silver chloride unless it clumped up (say, with the help of a clarifier). The solution is so dilute that I suspect the particles would just stay suspended -- if they form at all. [EDIT] As noted above when accounting for ion pairs, the silver chloride is unlikely to precipitate at all. So the bottom line is not to worry about the salt level in your spa with regard to the use of silver ions. [END-EDIT]

Richard

[Nat]

Thanks Richard. I was unaware dichlor added sodium chloride (albeit half as much per FC released as bleach) as well. So is it safe to say that the use of bleach (or dichlor) will not render the silver ion cartridge ineffective?

--Nate

[chem geek]

Thanks Richard. I was unaware dichlor added sodium chloride (albeit half as much per FC released as bleach) as well. So is it safe to say that the use of bleach (or dichlor) will not render the silver ion cartridge ineffective?

--Nate

Yes, absolutely you are correct since NO source of chlorine would render the silver ion cartridge ineffective. And remember that ANY source of chlorine adds salt to the pool. In the case of Dichlor (and Trichlor), no "extra" salt is added immediately, but rather when the chlorine later oxidizes an organic or kills a pathogen or breaks down in sunlight, the chlorine is converted to chloride ion (as in sodium chloride -- salt).

What is also true is that at higher salt levels (and the same total silver content) there will be less "free" silver ion available (most of the silver is tied up in the silver chloride ion complex) so the silver might be somewhat less effective, so with twice as much salt there will be roughly 1/4th the amount of free silver ion. However, since silver acts as a catalyst rather than participating in reactions directly (i.e. it is not consumed nor transformed, but speeds up oxidation reactions with dissolved oxygen or chlorine plus other chemical reactions I won't get into here), I don't think this is a big deal -- at least not for comparing Dichlor vs. bleach. For an SWG pool with its much higher salt level, these considerations might be more important and could require higher silver levels to compensate. The silver is synergistic with chlorine, but generally chlorine alone kills pathogens very effectively so between the issue of lower silver ion concentration at higher salt content (using bleach) vs. lower chlorine concentration from higher CYA content (using Dichlor), there is no contest in my opinion. The chlorine level is far more important (and is linearly related to kill times) so Dichlor/Bleach is better than Dichlor alone even with silver systems, though you can probably get away with using Dichlor for longer before switching to bleach and still get reasonable disinfection.

Richard