Taylor CH, Salt tests and Magnesium Chloride

[rlab]

My pool has a high level of Magnesium Chloride in the salt mix (popular in these parts with SWG for the supposed therapeutic benefits of Mg). I was wondering if this might be throwing out a couple of the Taylor tests.

1. CH is reading very high, 900ppm on a new pool with no additives other than the salt and an initial trichlor dose (fill water CH is 75ppm). I recall reading somewhere that despite being a CH specific test, the Taylor test can register Mg in some cases.... but was unsure about this?

2. Salt test is reading 4000ppm. Again my understanding was that the Taylor test is measuring NaCl, so it shouldn't be registering any of the MgCl2. Is that correct?

thanks!

 

[mknauss]

How to differentiate between Magnesium and Calcium on Hardness Test

Apparently, it is difficult to separate out the Calcium from Magnesium hardness levels on the titration tests. I have purchased the Hach and Taylor tests and I think one other titration test for magnesium only. The hardness test should measure the combination of Calcium and Magnesium. The...

www.troublefreepool.com

I did a quick search of the forum entering 'magnesium' in the search box. The above seems to give you some ideas. Lots of threads popped up in the search, appears you Aussies like your Magnesium!!

 

[JoyfulNoise]

If your magnesium levels are too high, it will throw off the calcium hardness test. You can measure magnesium hardness by measuring total hardness (TH) separate from calcium hardness and then subtract the two.

I would argue that you should keep the Epsom salt for bath water and leave it out of the pool. It’s not nearly concentrated enough in a pool to have any therapeutic effect and you’re more likely to run into pool chemistry problems with it present.

 

[JoyfulNoise]

If you add magnesium chloride to your pool, it adds chloride to the water and that is what the Taylor “salt” test measures. There is no way to tell the difference between sodium chloride, potassium chloride or magnesium chloride. They all add chloride. Magnesium chloride is MgCl₂ so on a molar basis it adds twice as much chloride as sodium chloride does.

 

[rlab]

Thanks @JoyfulNoise extremely helpful! Quick follow up Q,
Because some of the chlorides are coming from Mgcl2, does that change the required ppm levels needed for SWG? Or is 4000ppm on the salt test the same from the SWGs point of view regardless of whether it is from Nacl, Mgcl2 or a mix?

 

[JoyfulNoise]

All chloride ions (Cl-) are the same, it doesn’t matter the source so the ppm’s required by manufacturer of the cell does not change. What does change is the amount (weight) of the chemical you add to get the desired effect. If your salt is a mixture of magnesium and sodium chloride, then you’ll have to use your test kit to determine your equivalent sodium chloride level.

 

[JamesW]

Because some of the chlorides are coming from Mgcl2, does that change the required ppm levels needed for SWG? Or is 4000ppm on the salt test the same from the SWGs point of view regardless of whether it is from Nacl, Mgcl2 or a mix

Note that a chloride specific test, such as the K-1766, will be somewhat different from a conductivity test because both assume that sodium chloride was added.

The conductivity test measures the conductivity of the water, which is affected by all charged ions whereas the chloride specific test only measures the chloride ions.

A SWG like the Aquarite uses the performance of the cell, which is based on the chloride concentration.

A SWG like the Intellichlor uses a conductivity sensor.

So, your SWG readout might differ from your K-1766 salt test.

Mostly just make the SWG readout conform to the installation manual requirements.

 

[rlab]

An update on this, I have been messing around trying to get an accurate CH test with no luck.

I thought that if I diluted 1 part pool with 3 parts distilled water, then the Taylor buffer ( R-0010 ) would be able to neutralize the magnesium. Unfortunately it seems like no matter what dilution level I use, I end up with the same extremely high CH reading as a non diluted test (after multiplying the end result to compensate for dilution).

@JoyfulNoise you seem to be the expert on this Any idea why using a diluted sample does not work, given that the whole point of the R-0010 is to precipitate out any Magnesium?

 

[JamesW]

Maybe a fading endpoint due to metal ions like copper or iron.

Try the modified test on a diluted sample.

Also, check the salinity using a K-1766 salt test and a conductivity salt test with the meter set to salt.

 

[JoyfulNoise]

If your magnesium levels are a lot higher than your calcium levels, the test will not work well. I see from your posts that you are adding both sodium chloride and magnesium chloride to your pool. Do you have any idea what the ratios are? What kind of salt are you adding to your pool? Is it pool or water softener salt OR is it some kind of mined road salt?

If you truly wish to get an accurate reading, you may need to have your water tested by an independent lab that can measure the various trace metals independently and accurately. I don’t think your water is unsafe to swim in (as long as you are maintaining proper chlorination levels), but it’s mineral hardness is probably out of balance.

 

[rlab]

This is a proper pool grade salt mix from Zodiac/Jandy: Element Minerals | Salt Pool Chlorinators . Not sold in the US, it seems the Magnesium obsession hasn't reached there yet.

The SDS has:

Ingredient	Conc, %
Sodium chloride	>60
Magnesium chloride hexahydrate	to 100

So I assume that means 40% MgCl2.

I have tried the modified CH test to deal with metal ions as suggested, but it makes no difference to the final result.

K-1766 test shows 4000ppm salt.

 

[JoyfulNoise]

So here's my thought -

If I'm doing my math right, 4000ppm on the overall "chloride ion" test (because the K-1766 is measuring chloride) is equivalent to about 300 lbs of "salt" in your pool and you roughly have 40% of that as magnesium chloride. If you do all the molar mass conversions correctly (and again, I may be off since I'm doing this literally on a napkin), your adding about 1680ppm worth of detectable mineral hardness to your pool water since the CH indicator dye is sensitive to both magnesium and calcium ions. If your tap water (and thus fill water) is as low as you state in calcium hardness (75ppm), then the vast majority of mineral hardness in your water is from the magnesium in your salt, or about 22 times as much magnesium relative to calcium.

So why isn't the CH test working? Thanks for asking, that's a great question ....

The CH test relies on an indicator dye that complexes with Ca2+ ions and changes color (red when calcium is present, blue when there is no calcium). It then uses a stronger complexing agent, EDTA, to strip the calcium away from the dye thus turning the red solution to blue. The concentration of the EDTA titrant (R-0012) is set so that each 40uL drop removes a certain amount fo Ca2+ ions.

Now, there's always a devil in the details. Magnesium ions (Mg2+) look almost exactly the same as calcium ions to the indicator dye and to the titrant. In fact, thermodynamically, there is very little difference. So the indicator/titrant is insensitive to calcium or magnesium and thus magnesium can act as a positive interference. If half of your ionic concentration is from calcium and the other half magnesium, then the indicator dye will react the same.

So how do we tell the difference between calcium hardness and magnesium hardness? Great question again, thanks for asking...

Magnesium is a very soluble ion...except at high pH. Seawater contains lots of sodium but also lots of magnesium (it's the second most abundant element in seawater). Milk of Magnesia (the stuff you drink when you have acid reflux) was discovered by adding lime water (Ca(OH)2) to sea water causing the precipitation of Brucite, or magnesium hydroxide (Mg(OH)2). At a pH below 12.4, calcium compounds are still fairly soluble (unless lots of carbonate is around) and so you can selectively precipitate magnesium before you precipitate calcium. This is the entire basis of the lime softening process that is used to soften hard water.

So, in the Taylor test, there is a chemical called the "Calcium Buffer" (R-0010)....it's kind of misnomer, but it is nothing more than sodium hydroxide, or lye. The sodium hydroxide is used to raise the pH of the sample water and add hydroxide ions (OH-) so that magnesium will precipitate out of solution according to the following reaction -

Mg²⁺ _(aq) + 2 OH^- _(aq) ----> Mg(OH)_{2 (solid)}

So the R-0010 is performing two functions - (1) it is adding hydroxide ions to the water which react with magnesium to form insoluble Brucite, and (2) it is raising the pH of the resulting solution. One thing I did not mention about the calcium hardness test is that it works correctly when the pH is above 10.1 but below 12.4 in water that contains carbonate. The reason being is that the chelation of the metal ions is more rapid at higher pH but you want to stay below a pH of 12.4 so as to not accidentally precipitate calcium carbonate.

With all that said, because your water is so heavily laden with magnesium, the R-0010 simply can not add enough OH- in 10 or 20 drops to sufficiently precipitate all the magnesium and, because the magnesium is using up all the added hydroxide, the pH of the resulting solution after 10 or 20 drops of R-0010 is simply not high enough. Your water is so far out of spec for normal pool water (which assumes magnesium ions are much less concentrated than calcium ions), that the test as directed can not tell you what you need to know. Simply put, you would have to take the water sample into a lab environment and do a lot of calibrations and sensitive pH measurements to know when you've precipitated enough magnesium.

Taylor does sell a general hardness (GH) test that is much simpler than the CH test, but I'm not sure knowing MH + CH is all that useful. As a general opinion, I see no reason why anyone should use salt that has both magnesium and sodium chloride in it. There's not health benefit to do it and, as you can see, it makes a critical component of your pool water testing impossible to do. The good news is that your fill water has very low natural calcium in it so your pool is probably safe from any kind of calcium scaling issues but that is more the exception than the rule. If one lived in a high CH water area, not being able to clearly know your pool's CH value would be a real detriment.

 

[JamesW]

So, this raises the question about what the local pool builders and shops are doing to measure calcium?

Are they not recognizing the problem that they're creating with the magnesium?

Seems that the pools that need a good CSI will be corrosive to plaster and grout.

Tell the people who sell magnesium chloride that your American friends said to knock it off and sell sodium chloride.

 

[rlab]

Thank you for the detailed explanation, that all aligns with my experience trying to test the pool water in various ways, including GH test strips (which just read off the chart as well).

Guh, the fine print they don't put on the salt mix bag when selling you the idea of a Magnesium rich mineral pool. The crazy thing is this idea has really taken off in Australia. In my local pool store I would say about half the salt mixes available are all Magnesium mixes, all under various brand names, with high concentrations of MgCl2. Even the local hardware store sells an Mg pool salt mix. I can't find any mention of what you are meant to do to test CH levels.

I do recall from once owning a saltwater aquarium that there are Mg/Ca aquarium test kits that are meant to give you an accurate reading of Mg versus Ca ions (which are both at high levels in aquariums). I might have to dig up one of those and see if I can get any meaningful results.

 

[JoyfulNoise]

Redseafish.com sells test kits for Ca, Mg and GH. Since your levels are so high, you might get better results with reef aquarium testers.

 

[JamesW]

I wonder how they deal with the interference?

 

[JoyfulNoise]

I wonder how they deal with the interference?

The initial buffer chemical is a much higher concentration of NaOH. If you watch their test videos, they add their buffer drops one at a time with 15secs of swirling between drops. The test solution becomes very milky/cloudy which is the precipitation of MgOH.

If I had the right pH indicators, it’s very easy to show what’s happening. Add a few crystals of Epsom salt to distilled water and then add a high pH indicator. What you’ll see happening is every time you add a drop of R-0010, the color of the solution will flash to a high pH color only to then fade back to the lower pH value. It’s not until you exhaust all the magnesium in solution that the high pH color fixes permanently. You can see it on a pH meter too it’s just less dramatic.

That might be a way to do the test if you had a pH meter. You could take a test sample and then add the R-0010 to it while watching the pH. Once the pH rises above 10.1 and begins to hold, then you’ll know how many drops of R-0010 it takes to exhaust your magnesium load. You can then run the test using the standard procedure but with the new drop count for R-0010. You might run into some trouble with the indicator dye though - it likes to “stick” to magnesium hydroxide particles and so your solution might look like curdled milk. The test may still work at that point though. Hard to know for sure.

 

[rlab]

An interesting update, I had my water tested with a Palintest photometer and it showed a CH of 20, which is about a quarter the fill water CH level. So weirdly it seems the Mg interference pushes the result in the opposite direction for a photometric test.

I have ordered a Red Sea Ca kit, will see what that shows.

 

[rlab]

OK Red Sea Ca kit seems to work perfectly. Fill water, pool water and a 200ppm standard all show expected results. I also added Calcium Chloride to the pool to up my (now confirmed) low CH levels, and the Red Sea kit showed the expected amount of increase in CH levels 24 hours later.

The test itself is pretty fiddly to use compared to the Taylor test, but not too bad considering CH doesn't need to be tested frequently. Also need to remember to multiply the results by 2.5 to get CH as Caco3, as the test result is in ppm Ca ions.

So any other Aussies out there who are using a magnesium salt blend, this is the answer to CH testing.

 

[JoyfulNoise]

Glad you found a work-around!