Chemical Interaction Throwing Off Taylor Salt Test

[EddieA]

Hi,

I've been using the resources of this forum for quite a while now to understand the water chemistry for our in-ground spa and I have to say that it's an incredibly useful resource. So far I've always been able to find the answers I need. Until now. :grin:

I recently, last week, drained and re-filled the spa. While I was at a local store grabbing my big bag of salt, I noticed that they had some products I hadn't seen before called: MySaltPool, supposedly designed specifically for SWG systems. So I grabbed a bottle of something called "Salt Cell Protector" and added some when refilling the spa, which BTW is about 1,000 gallons. According to the bottle, it contains polymaleic acid and phosphonic acid.

Since then, the salt reading I get using the Taylor test (K-1766) is around 800ppm higher than the Aquapure reading. The Aquapure shows 3,400 and the Taylor test around 4,200. I also took a water sample to my local store and they also had a reading of 3,400. Prior to this, I have used the same store and have always had results within 200ppm of both my Aquapure and my own Taylor testing.

So, I guess my question is which of the testing methods is likely to have been thrown off by this product. I'm fairly certain that it's the Taylor test based on 2 things. Firstly, the amount of salt I added should not have been enough to raise the level above 4,000 based on the charts from the Aquapure manual.

But more ****ing is that if I take a water sample, add 1 drop of this product to that sample and then do the Taylor test, I gave up trying for the colour change after around 40 drops of the R-0718.

I've sent a note to them asking if this product could cause the erroneous test results, which if so, I would have expected a warning as they are (supposed) salt pool experts.

Cheers.

 

[JoyfulNoise]

A cursory search in the inter-webs comes back with phosphonic acid and polymaleic acid (Rohm/Haas trade name OPTIDOSE) being used as boiler scale inhibitors. They are particularly suited for Ca scale inhibition but exert a chelating effect on a wide variety of anions. It would not surprise me in the least that the Ag ions from the silver nitrate in the salt test kit are being chelated by those compounds thus causing an increased silver drop consumption.

The Taylor salt kit uses the argentometric chloride determination method whereby silver nitrate is titrated into a chloride containing solution. The silver nitrate reacts with the chloride ions forming silver chloride which is not very soluble and is what makes the cloudy white precipitate. The test kit uses a potassium chromate (yellow colored) indicator that turns brownish-red (silver chromate) once all of the chloride is used up and the excess silver starts to react with the chromate indicator.

Thus, if the scale inhibitor is sucking up silver ions, you are going to use more drops of the titrating reagent and thus show a false high value.

It may be possible to use a strong acid (like nitric acid) to digest/degrade the chelating agents but that would take quite a bit fiddling around to figure out the right amount of acid to use.

 

[chem geek]

The Taylor website page for the Taylor K-1766 chloride salt test kit lists Potential Interferences as follows:

Orthophosphate at concentrations greater than 25 ppm will precipitate as silver phosphate to cause positive interference. This can be prevented by diluting orthophosphate concentrations below 25 ppm with DI water. Bromide, iodide, and cyanide at all levels titrate as equivalent chloride concentrations. Sulfide, thiosulfate, and sulfite interfere but can be removed by treatment with hydrogen peroxide.

Is the product you are using My Salt Pool Cell Protector? If so, then this MSDS indicates that it contains etidronic acid (otherwise known as HEDP) and 2-Butenedioic acid (2Z)-, homopolymer (aka Maleic acid homopolymer).

Usually these kinds of metal sequestrants bind more strongly to divalent cations (doubly positively charged) such as calcium and magnesium than to monovalent (singly charged) cations like silver. Nevertheless, what you are seeing is consistent with what JoyfulNoise has written. I suspect that it's not so much sequestration but co-precipitation that is occurring since this link shows silver oxalate being somewhat less soluble than silver chloride. The chemical structure of the homopolymer of maleic acid shown here looks very much like a polymeric extension of oxalic acid (maleic acid is used as a starting point due to its double bond, but only a single bond is left in the final polymer). Maleic acid and oxalic acid have a similar pK_a1 so it is plausible that silver is precipitating with the anion (negatively charged salt) of the maleic acid homopolymer.

I think you should contact Taylor about this to ask them what they think about it. They may need to add this to their list of potential interferences (I suspect it's the maleic acid homopolymer, though if HEDP also interferes that would be very useful to know).

 

[JoyfulNoise]

I defer to ChemGeek's more thorough analysis but it makes me wonder - how much scale inhibitor have you added?

If it is co-precipitation and the test is off by as much as you you say (~800ppm) , then assuming a test tolerance of +/- 200ppm, then did you add scale inhibitor up to a concentration of 600ppm? Even if it's like silver oxalate with two silver atoms per oxalate cation, that would something on the order of 300ppm of scale inhibitor.

Is my crude math off or does this sound like an exceptionally high amount of variance?


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[chem geek]

He put part of a bottle of this stuff in a spa of 1000 gallons. According to this link the normal dosage for pools is 1 quart for treating 10,000 gallons. So for a 1000 gallon spa only 3.2 fluid ounces (less than 1/2 cup) should have been used. Eddie, do you remember how much you put in?

The molecular weight for each unit of the polymer (CHCOOH) is around 58 g/mole and the concentration from the MSDS is 10%-30% by weight with a product density of 9.09 pounds per gallon so the weight of polymer in one quart is 103 to 309 grams so 1.8 to 5.3 moles (of a single unit of polymer). 800 ppm or mg/L sodium chloride with a molecular weight of 58.44 g/mole in 3785 liters is 51.8 moles.

So yes, there's a mismatch for either sequestration or precipitation since even one quart of this product should not remove that much silver. Nevertheless, this product is affecting the test in some way.

 

[EddieA]

Jeez. This makes me regret giving up chemistry 45 years ago.

I have to admit I didn't think about checking the Taylor website, or asking for their thoughts.

Is the product you are using My Salt Pool Cell Protector?

Yep, that's the stuff.

He put part of a bottle of this stuff in a spa of 1000 gallons. According to this link the normal dosage for pools is 1 quart for treating 10,000 gallons. So for a 1000 gallon spa only 3.2 fluid ounces (less than 1/2 cup) should have been used. Eddie, do you remember how much you put in?

I didn't measure per se, but as you quote it is one bottle (1 qt) per 10,000 gallons which I did see before starting. Using the level of liquid left in the bottle compared to where it was unopened, I'd be surprised if I actually used even 10% of the bottle. Doing some very rough maths, with a bottle radius of 1.5" and my estimate that I used a depth of 5/8" gives me around 2.5 fl oz. Even allowing for a "heavy pour" still doesn't put me too far out of the normal dosage.

Thank you both for you insights, both way above my league.

Cheers.

 

[Divin Dave]

Hi Eddie,
Welcome to TFP! LOL
We have some pretty good scientific minds as valued members here who are easily capable of dispelling the common notion that TFP way of pool care (formerly known as BBB) are just a bunch of crackpots!

Jeez. This makes me regret giving up chemistry 45 years ago.

I have to admit I didn't think about checking the Taylor website, or asking for their thoughts.

Yep, that's the stuff.

I didn't measure per se, but as you quote it is one bottle (1 qt) per 10,000 gallons which I did see before starting. Using the level of liquid left in the bottle compared to where it was unopened, I'd be surprised if I actually used even 10% of the bottle. Doing some very rough maths, with a bottle radius of 1.5" and my estimate that I used a depth of 5/8" gives me around 2.5 fl oz. Even allowing for a "heavy pour" still doesn't put me too far out of the normal dosage.

Thank you both for you insights, both way above my league.

Cheers.

 

[JoyfulNoise]

I'm at a bit of a loss to explain it (like I should be surprised at that )

However I can say this in my own experience with salt testing, being off by as much as 400ppm is entirely possible. I have used strips and drops. As well, my IC40 is not temperature compensated and it, like most salt generators, is really displaying something more like TDS than pure salt concentration. The strips with the white stripe that climbs up a calibrated scale were always lower than the Taylor drops by ~400ppm. The Taylor drops matched my IC40 display at a water temp of 78F.

So I usually assume that my Taylor drops are the most accurate of all and stick with that. In your case, however, that scale inhibitor is definitely interfering with the drop test.

Oh, and lest I miss an opportunity to rag on the pool stores, they always came in low, like my strips.


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[chem geek]

I called Taylor Technologies on this and they didn't think anything in that product would cause interference. They thought it might be that the Aquapure is using conductivity as a measure and they assume a certain factor to get to implied salt level and that after the water change this was different. Probably the only way to get to the bottom of this is to do a careful experiment with water of a measured salt level (measuring a few times to validate consistency), then add the product, then measure again a few times.

I also called BioGuard and talked with their technical support to find out if there was anything else in the product that could cause interference. They said there were other proprietary chemicals in the product not listed in the MSDS but that none were known to cause any interference with water tests.

Basically, everyone thought it unlikely that only one quart of a product in 1000 gallons would be able to shift the salt reading by 800 ppm. There simply isn't enough product to bind to the silver to cause that much interference. 800 ppm (mg/L) in 1000 gallons is 3 kilograms (6.6 pounds) of salt which even in a brine (saturated salt) solution would be over 2-1/2 quarts.

 

[EddieA]

Sorry, only just spotted this addition.

Not sure I fully understand then either. As I said previously, before re-filling the spa and adding a couple of ounces of the Cell Protector, all 3 readings, Taylor Salt Test, AquaPure, and local pool store all agreed with each other. Well, within 100/200 ppm.

It was only after the re-fill with the Cell Protector that the Taylor test appeared to be way off the mark. Plus, my very unscientific test of adding a small drop, via a toothpick, to a 10ml sample, I was unable to get the sample to change colour.

Just did a repeat Taylor test from the spa. Still taking 21/22 drops to change. The Auapure is still showing around 3,400. Haven't taken a sample back to the pool store since.

I'll try your suggestion of making up a small batch of "salty" water and attempt a controlled test later in the week.

Cheers.

 

[chem geek]

I just bought some My Salt Pool Salt Cell Protector and I will try it out at home since I already have the Taylor K-1766 salt test. If I can reproduce the problem, then I'll call Taylor again and send them this product to try themselves.

 

[JamesW]

The K-1766 measures chloride whereas the AquaPure measures conductivity. Theoretically, the levels should be close but can differ based on the makeup of the water. Both methods assume that most of to all of the tds is from sodium chloride.

The AquaPure can be recalibrated to match the results from the K-1766.

If you want to go further in determining why they're different, you can have the water tested with a recently calibrated salt meter. That should match your AquaPure. You could also get some calibration solution to check the accuracy of your testing.

Also, perhaps the sequestrant is affecting the conductivity of the water making it read lower than it otherwise would. By tying up positive ions, including sodium, maybe the conductivity is reduced. Perhaps it's a combination of both effects causing one test to read high and one to read low.