Magnesium vs NaCl

[xDom]

Someone at work has just installed a pool that's using Magnesium Chloride instead of NaCl.
He was asking about pool care and I was wondering if there are any differences between the level of all the other chemicals compared to the NaCl SWG.
For example, FC/CYA, TA, CH.. can I just suggest the normal TFP recommendations?

 

[JoyfulNoise]

No difference.

You Aussies really LOOOVE your magnesium chloride…

Just note though, high magnesium levels will interfere with the standard Taylor CH test and any other CH test. The indicator dye chelates both types of ions and if the user doesn’t understand this, the CH test results will be way off. If they completely added all their chloride from a magnesium source, then I don’t think their CH levels are going to be accurate.

 

[xDom]

Good to know about the CH test.
In your opinion do you see any benefits to the Mg setup? Is it a gimmick?

 

[JoyfulNoise]

Good to know about the CH test.
I’m your opinion do you see any benefits to the Mg setup? Is it a gimmick?

Just not worth the extra cost and hassle with testing in my opinion. There’s no tangible benefit to using it other than a placebo effect that people get thinking that “bathing is magnesium” is somehow “better” for you.

 

[mgtfp]

Gimmick. About 4-6 times more expensive than NaCl salt.

 

[AUSpool]

You Aussies really LOOOVE your magnesium chloride…

This one doesn’t. The fancy advertising makes me wanna

 

[AUSpool]

The industry hasn’t settled on an ideal Mg level. The stores are using a Lamott 801 disk which is commonly reporting above its range. They will tell customers that it is per the Australian Standard but Mg is not included in the Australian Standard.

Mg salts are not as pure (95-98%) as our regular salt at (99.4%). There is a much greater chance of finding iron as a contaminate in the Mg salts then the regular.

The use of Dead Sea Mg salts is contributing to the environmental decline of the Dead Sea.

Some users with low salt systems have to do partial drain and refills to get back to their starting concentrations.

Scale buildup in the SWG cell has been reported as being more common.

 

[JoyfulNoise]

Scale buildup in the SWG cell has been reported as being more common.

At high pH, which is very common inside the cell, magnesium will form a “mush” of Mg(OH)₂ scale. This will also cause the coprecipitation of calcium carbonate. So it’s not surprising to see that reported.

 

[JamesW]

The use of Dead Sea Mg salts is contributing to the environmental decline of the Dead Sea.

Um...it's already dead, how much worse can it get?

 

[JamesW]

Magnesium chloride will also make it more difficult to manage the salinity of the water as most tests and SWG salinity sensors are calibrated to sodium chloride.

 

[mgtfp]

Magnesium chloride will also make it more difficult to manage the salinity of the water as most tests and SWG salinity sensors are calibrated to sodium chloride.

But doesn't the silver nitrate test pick up chloride only anyway? Shouldn't matter for the test from which source the chloride came from.

I can see how it would affect the reading of meters (including salinity measurements in SWGs) as they assume the chloride to come with sodium and consider that for the calibration.

It shouldn't affect the Australian SWGs that I know, as they don't actually measure or display salinity. They just increase the voltage until the current required for the chosen output has been reached (assuming that each electron at the anode comes from chloride electrolysis, no dirt effects). If that's not possible before the voltage maxes out, they show a "low salt" warning.

 

[JamesW]

If you have an aquarite, it will match the K-1766 regardless of the source of the chloride, but you have a calculator or chart that tells you how much magnesium chloride will give the correct reading.

If you have an Intellichlor, Aquapure, Autopilot or any SWG that uses a conductivity meter, the readings will not match a K-1766 test kit.

It can be managed, but it is more difficult because you have to consider the differences between the two different types of salt.

 

[JamesW]

Chemical formula	MgCl2
Molar mass	95.211 g/mol

Chemical formula	NaCl
Molar mass	58.443 g/mol

It takes 2 moles of sodium chloride to equal 1 mole of magnesium chloride.

So, 1.23 lb of sodium chloride is equal to 1 pound of magnesium chloride for the amount of salt to provide the same amount of chloride ions.

However, the sodium chloride has 4 ions compared to 3 ions for magnesium chloride, which will probably make it more conductive unless the magnesium counts as 2 because it has a double charge.

You would need a chart that shows the conductivity of magnesium chloride and sodium chloride to see the conversion.

 

[JamesW]

For example, below you can see the graph of sodium chloride vs. "442" TDS.

You can see that at a specific conductivity, there are different concentrations of the different types of TDS.

So, a conductivity meter gives a different reading depending on the conversion formula, which depends on the specific makeup of TDS.

 

[JamesW]

If you needed to add 800 lbs of NaCl salt to get to 3,200 ppm, you would add 650 lbs of magnesium chloride and the reading for the Aquarite and the K-1766 should be 3,200 ppm.

The actual concentration of magnesium chloride will be 2,600 ppm.

If you use a conductivity meter that reports in units of sodium chloride, it will not read 3,200 pm or 2,600 ppm.

You would need the conversion chart to see what the conductivity of the salt reading is and then what concentration of magnesium chloride matches the conductivity.

Since you don't know what the concentration of magnesium chloride will give you the correct conductivity, you would have to add some and then check the reading from the cell and keep adding in increments until you get the right reading for the cell to work.

 

[JamesW]

MgCl₂ is 258.82 Scm²mol^-1

106.1 + (2 x 76.36) = 258.82.

Mg is 106.1 Scm²mol^-1

Na is 50.11 Scm²mol^-1

Cl is 76.36 Scm²mol^-1

NaCl is 126.47 Scm²mol^-1

It takes 2 moles of sodium chloride to equal 1 mole of magnesium chloride as far as the number of chloride ions provided, but the magnesium chloride is about 2 times more conductive per mole, so it works out that if you provide the same number of chloride ions, you also provide the same conductivity.

The magnesium is twice as conductive as the sodium and there are 2 chloride ions, so the conductivity is about double.

3,200 ppm/58.443 = 54.754
54.754 x 126.47 = 6,924.76

6,924.76/258.82 = 26.775
26.775 x 95.211 = 2,547.38 ppm.

So, 2,547.38 ppm MgCl₂ should be equal to 3,200 ppm NaCl for conductivity.

So, if you start with 0 salt and want to use magnesium chloride instead of sodium chloride you would use about 80% of the sodium chloride amount for the weight of magnesium chloride to get the same chloride concentration and about the same conductivity.

So, 800 lbs of sodium chloride is equal to about 640 lbs of magnesium chloride.

 

[JamesW]

conductivities of λ∘(Mg2+)=106.1Scm2mol−1 and λ∘(Cl−)=76.3Scm2mol−1​

 

[mgtfp]

However, the sodium chloride has 4 ions compared to 3 ions for magnesium chloride, which will probably make it more conductive unless the magnesium counts as 2 because it has a double charge.

That's where I'm always a bit unsure how much ions really contribute to conductivity, as they actually have to discharge at the electrodes to create a current.

In a SWG cell for example, Na⁺ can't compete with H⁺ in grabbing electrons. Unless you used a Quicksilver kathode and make use of the the higher H⁺ overpotential. I assume that Mg²⁺ would also struggle in competing with H⁺ in the fight for electrons?

How is that in conductivity / salinity / TDS meters? Do ions like Na⁺, Mg²⁺ or Ca²⁺ actually contribute to the conductivity?

 

[JamesW]

For a conductivity meter, all charged ions contribute to the conductivity.

Kohlrausch's law states that the equivalent conductivity of an electrolyte at infinite dilution is equal to the sum of the conductances of the anions and cations.

Calculate the molar conductivity of a solution of MgCl(2) at infinte d

Lambda^(@) (MgCl(2)) = lambda^(@) (Mg^(2+)) + 2 lambda^(@) (Cl^(-)) lambda^(@) (Mg^(2+)) = 106.1 S cm^(2) mol^(-1) lambda^(@) (Cl^(-)) = 76.3 S cm^(2) mol^(-1) therefore " " Lambda^(@) (MgCl(2)) = 106.1 + 2 (76.3) 258.7 S cm^(2) mol^(-1)

www.doubtnut.com

Most monovalent ions in water have limiting molar ionic conductivities in the range of 40–80 S cm2 mol−1. For example:

Cation λ, S cm2 mol−1 Li+ 38.6 Na+ 50.1 K+ 73.5 Ag+ 61.9

Anion λ, S cm2 mol−1 F− 55.4 Cl− 76.4 Br− 78.1 CH3COO− 40.9

Molar conductivity - Wikipedia

en.m.wikipedia.org

Conductivity (electrolytic) - Wikipedia

en.wikipedia.org

 

[JamesW]

Conductivity is completely different than what happens in the cell.

In the cell, chloride loses an electron and hydrogen gains an electron.

This is called electrolysis and it produces chlorine gas and hydrogen gas.

Electrolysis uses DC voltage.

For conductivity, the ions carry the charge, but they do not gain or lose electrons.

So, all ions carry charge more or less equally.

You can look up the conductivity of many different ions or solutions.

Conductivity meters use an AC voltage to prevent electrolysis and to prevent a buildup of charge on the electrodes.

Because you are alternating the charge of the electrodes, the charged ions keep moving back and forth based on the frequency of the AC voltage.

The electrons in the wires also move back and forth.

So, you have an AC current that is carried by the movement of electrons in the wires and the movement of charged ions in the water.

The movement creates magnetic fields that keep changing.