Sodium vs chloride in salt level measurements

[JRaldon]

Hi everyone, I'm making the switch to a SWG this season, after reading every thread I could find on this forum on the topic! I have one question, however, that hasn't been answered elsewhere.

The Taylor K-1766 salt test, and all salt tests I can find, measure total salt levels which I'm assuming is a combined ppm of both Na+ and Cl- dissolved in the water (though I haven't found verification of that). This would be fine if I had fresh pool water from the tap and I was adding sodium chloride to it, but I haven't drained my pool significantly since I bought it with my house, and in that time I've probably added 200 gallons of bleach to it, which according to pool math will add about 2400 ppm of "salt". But I think the "salt" in this case is only the sodium in bleach.

But if I understand them correctly, the SWG needs chloride (Cl-), and the salinity range the manufacturer recommends is probably based on the assumption that half the "salt" that I measure is indeed Cl. I'm worried that a salt test result might say I'm currently at say 1500 ppm despite having very little chloride, and if I add salt to get up to 3000 ppm suggested by the SWG manufacturer, I might only have half the amount of chloride that the SWG needs. Is there an easy way to measure individual constituents of salt, or am I missing something?

 

[woodyp]

All I can offer is this. Brand new pool...brand new SWG. Added enough lbs. of salt to reach recommended range for the SWG. I calculated I added enough to get salt to 3200 ppm. Just kicked it on first time and the cell itself reads 3,000 ppm. I'll add a little more even thought the K1766 taylor test has a + or - error range of 200.

TEST your water for salt levels before adding anything! I ran it for 3 weeks and salt was already showing 300. The SWG itself should give you a low salt error if you're off too much. Welcome to the forum. Which test kit are you using and what brand of SWG? My pool size is same as yours and I added 320 lbs to get it going. I tested it at 6 bags first so as not to overshoot. Circulate for 24 hours before kicking on the unit.

Please add a signature line to your profile like mine below listing your pool particulars and equipment.

 

[Jimrahbe]

J,

I have no chemistry background at all, but doubt your assumption is correct. Let's see what Matt has to say... Calling @JoyfulNoise

That said our advice is to always test with a salt test kit like the Taylor K-1766 and use that as your starting point before adding any new salt. So, if you want 3600 ppm and your test shows you currently have 2000 ppm, then you only need to add an additional 1600 ppm of salt.

I think you are making this much harder than it needs to be..

Thanks,

Jim R.

 

[JoyfulNoise]

The K-1766 uses Mohr's Method (argentometric titration) to determine the amount of chloride ion in solution. The chemical reaction is such that it is only measuring chloride (Cl-) and nothing else (... unless you happen to have iodide, bromide, or cyanide in your water ....). The silver nitrate titrant has it's concentration adjusted so that it reports values in mg/L (ppm) of sodium chloride (NaCl). This is perfectly fine because an SWG needs chloride ions (Cl-), the counter ion (the cation) is irrelevant. All cell manufacturers rate their cells based on ppm of NaCl. For example, I could add potassium chloride (KCl) or magnesium chloride (MgCl2) or calcium chloride (CaCl2) or rubidium chloride (RbCl) or cesium chloride (CsCl) to the pool and it wouldn't matter, the cell is still getting chloride ions. The cation (the positive ion) doesn't matter to the cell (but it could matter to other things, like your plaster or your health). The only difference is that the weight of what you add will change because all of the chloride compounds have different molar masses.

So, as long as your salt test gets you in the right range for NaCl concentration, then the cell is fine.

 

[JamesW]

But I think the "salt" in this case is only the sodium in bleach.

Sodium hypochlorite adds an equal amount of sodium and chloride.

The chlorine in the hypochlorite becomes chloride.

All forms of chlorine added will add an equal amount of sodium and chloride.

Even if you used chlorine gas, which only has chlorine, which becomes chloride, you would have to add sodium bicarbonate, sodium carbonate or some other sodium based base like sodium hydroxide or sodium tetraborate and that adds an equal amount of sodium as chloride.

 

[JamesW]

Sodium hypochlorite is pH neutral and you can see that it produces an equal amount of sodium as chloride.

2NaOCl + H₂O --> 2Na⁺ + OCl^-+ HOCl + OH^-

OCl^-+ HOCl + OH^- --> O₂ + 2Cl^- + H₂O.

_________________________________________________________________________________________

Trichlor is acidic and requires a base to raise the pH to offset the acid.

2C₃Cl₃N₃O₃ + 6H₂O --> 2C₃H₂N₃O₃^- + 3OCl^- + 3HOCl + 5H⁺

3OCl^- + 3HOCl --> 3O₂ + 6Cl^- + 3H⁺

_________________________________________________________________________________________

Chlorine gas is acidic and requires a base to raise the pH to offset the acid.

2Cl₂ + 2H₂O --> OCl^- + HOCl + 2Cl^- + 3H⁺

OCl^- + HOCl --> O₂ + 2Cl^- + H⁺

 

[JRaldon]

Sodium hypochlorite is pH neutral and you can see that it produces an equal amount of sodium as chloride.

2NaOCl + H₂O --> 2Na⁺ + OCl^-+ HOCl + OH^-

OCl^-+ HOCl + OH^- --> O₂ + 2Cl^- + H₂O.

Okay this equation explains the flaw in my thinking, thank you! I was under the impression that when OCl^- breaks down, eventually the Cl left the pool as Cl₂ or some other molecule. I'm not sure why I thought that, maybe because people say pool chlorine "evaporates" and after reading that for three years my mind just assumed it wasn't there anymore.

Thanks everyone for responding!

 

[Mdragger88]

. Is there an easy way to measure individual constituents of salt, or am I missing something?

If your head is spinning from the chemistry lesson like mine
To put it simply- that’s not necessary.
Use the salinity reading the k1766 gives you & add what you’re lacking to get in range for your new swg. Maybe you won’t need much!

 

[woodyp]

Turning into a Deep End discussion!