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Thread: How to differentiate between Magnesium and Calcium on Hardness Test

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    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 Calcium Test "should" measure just the calcium, and the Magnesium test should measure just the magnesium. I am not complaining about the tests these companies have made, as they do produce excellent tests kits in general. Anyway, I used all of the tests and could not reconcile the results. I ended up sending out a pool water sample to an agricultural analytical lab that uses something called Inductively Coupled Plasma Mass Spectroscopy. The results that I got back pretty much suggest to me that magnesium really interferes with the calcium titration test results. The calcium test seems to really show calcium plus a lot of the magnesium too. Since magnesium actually accounts for much of my hardness, it would be really great to have a reliable calcium-only test, as it it the calcium in water that really contributes to scaling. So, my question is, is there an inexpensive sort of test that can be used to get a good calcium test result at home?



    Thanks in advance for any comments.

    Del

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    JoyfulNoise's Avatar
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    Re: How to differentiate between Magnesium and Calcium on Hardness Test

    The Taylor CH test gives you the calcium hardness. It removes the Mg interference by adding the 10drops of R-0010 which is sodium hydroxide. The R-0010 causes any Mg to precipitate out as magnesium hydroxide. What is left is only the calcium portion of the hardness.

    Now, if your sample water has very high levels of Mg (which pool water should not have) then you may need more R-0010 to compensate.

    Taylor sells both a total hardness test (Mg + Ca hardness) as well as a specific CH test. You can use the TH test to see how high your Mg levels might be.

    Magnesium does not readily scale out. Magnesium carbonate is 5-8 times more soluble in water than calcium carbonate.
    Matt
    16k IG PebbleTec pool, 650gal spa, spillway and waterfall, 3HP IntelliFlo VS / 1.5HP WhisperFlo, Pentair QuadDE-100 filter, IC40 SWCG, MasterTemp 400k BTU/hr NG heater, KreepyKrauly suction-side cleaner Dolphin S300i robot, EasyTouch controls, city water, K-1001, K-2006 and K-1766 test kits, Mannitol test for borates

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    Re: How to differentiate between Magnesium and Calcium on Hardness Test

    Also, since you have ICP-MS results, you could call up Taylor Technologies (1-800-TEST-KIT) and ask to speak with a member of their technical support team. If you give them your magnesium and calcium numbers, then they might be able to suggest a way to modify their procedures to make the testing more accurate. For the CH test, it may be nothing more than just adding in more drops of the R-0010 buffer to reduce the magnesium level.

    By the way, why is your pool water so high in magnesium? That is very unusual to have magnesium hardness levels higher than calcium levels.
    Matt
    16k IG PebbleTec pool, 650gal spa, spillway and waterfall, 3HP IntelliFlo VS / 1.5HP WhisperFlo, Pentair QuadDE-100 filter, IC40 SWCG, MasterTemp 400k BTU/hr NG heater, KreepyKrauly suction-side cleaner Dolphin S300i robot, EasyTouch controls, city water, K-1001, K-2006 and K-1766 test kits, Mannitol test for borates

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    Re: How to differentiate between Magnesium and Calcium on Hardness Test

    I live in San Diego, and the tap water is hard. The pool water evaporates quickely, so I have lots of concentrated Ca and Mg. I do have high levels of both Magnesium and Calcium. The hardness values for both are expressed as ppm Calcium Carbonate, which obscures comparisons a bit, as conversion factors are used. My Calcium levels are higher than Magnesium. I will repeat the tests with your suggestion to use more NaOH reagent to drop out the magnesium first. I wonder if the NaOH might also drop out CaCO3 alkalinty, confusing matters more. Anyway, I will repeat some tests to see if I can make the drop tests work. I don't really want to bother the Taylor people too much, but I may give them a call.

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    Re: How to differentiate between Magnesium and Calcium on Hardness Test

    What are you using to test your water? How old are your test reagents? Can you post up a full set of numbers for your pool water?

    Also, what problems are you having with the CH test? If you can describe them, perhaps we can help sort it out.

    One way to check the testing methodology is to simply test your tap water. If you water supply is from a municipal source, the CH is likely to be in a very reasonable range. Believe me, I live in the desert southwest (Tucson) and I can almost guarantee that my water hardness is at least as calcium rich as yours, if not more so.

    And, by the way, you are not "bothering" the Taylor people. I can guarantee that if you call them, they will be very willing to offer advice and assistance. As a customer of theirs (you buy their products), they want to help you. It's how they stay in business....
    Matt
    16k IG PebbleTec pool, 650gal spa, spillway and waterfall, 3HP IntelliFlo VS / 1.5HP WhisperFlo, Pentair QuadDE-100 filter, IC40 SWCG, MasterTemp 400k BTU/hr NG heater, KreepyKrauly suction-side cleaner Dolphin S300i robot, EasyTouch controls, city water, K-1001, K-2006 and K-1766 test kits, Mannitol test for borates

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    Re: How to differentiate between Magnesium and Calcium on Hardness Test

    Joyfulnoise,

    Thanks for the reply. All of our reagents are about a couple of months old, purchased from Taylor. Again, I have lots of confidence in Taylor.

    My pool water, without me doing anything to change it had the following characteristics:


    Drop Test / Analytic Lab Test

    Total Alk by Taylor Test Kit 70 By Analytical Lab 74

    Calcium hardness by Taylor Test Kit 1960 (as CaCO3) Test not done by outside lab

    Calcium 770 by outside test lab

    magnesium 180 by Red Sea drop test 120 by outside test lab



    Keep in mind the conversion factors for getting from ppm of the ions to ppm as CaCO3. For Mg the conversion factor is is 4.118 For Ca, it is 2.497. I do not see any real discrepancy with the above test results. So, based only on this I have confidence in all of the test methods.

    However, I tried to soften the water to see if I could do so and to see what would result. After my attempt at softening, I got:

    Alk 90 by drop test 91 by outside lab
    Ca hardness by Taylor drop test 460 Not done by outside lab
    Ca only by outside lab 33
    Mg only by outside lab 100


    My comment here is that the Taylor drop test kits seems to show total hardness, not just Ca hardness. This is the reason I posted in the first place. So, please understand that the much higher relative Mg level only comes about after I have removed Ca by softening.

    Del

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    Re: How to differentiate between Magnesium and Calcium on Hardness Test

    I guess I'm confused. In the first test, if you multiply the calcium ion concentration from the external lab (770) by the conversion factor for Ca-to-CaCO3 conversion (2.497), you get a number very close to what you measured using the Taylor kit (1960ppm versus 1922ppm).

    You then you tried to "soften the water" (what did you do to "soften" it??) and now your numbers don't match....so what did you do?

    The Taylor test for calcium absolutely removes magnesium. First of all, the R-0010 calcium buffer causes free magnesium to for magnesium hydroxide which is a solid precipitate. Also, the R-0010 raises the pH up above 12. In doing that, the indicator dye reacts more strongly with calcium ions than any other divalent metal. The R-0012 titrant then removes the calcium from the dye complex and that causes the indicator color to change. The only other interferences in this process can come from other divalent transition metals, most notable iron or copper. HOWEVER, the R-0012 is formulated with at least two chelating agents - one for calcium and one to strongly bind up other metals. So the test most definitely does not measure total hardness. As I said previously, if your Mg levels are very high then you might need to add more R-0010.

    Here's the issue I think you might be running into - fading endpoint. When the CH level is very high (above 1000ppm), you have to use A LOT of drops of the R-0012 and, in fact, you likely should add an extra drop or two of the indicator dye. The problem I have noticed with very high CH is that, as you get close to the endpoint, the color transition from the intermediate purple shade to the final end point of blue can fade back and forth very quickly. So, you can actually add too many drops of R-0012 because it will transition from purple to blue and then back to purple very quickly. There are two ways to get around this -

    1. Add the R-0010 and then, before adding any indicator dye, add half the number of drops you'll think you need of the R-0012. By doing this, you'll lock up some of the calcium ions BEFORE the dye can bind to them and it makes the transition point more crisp. You must count ALL of the R-0012 drops you add as your total amount of titrant - the ones before the dye and after.

    2. Dilute your sample water 1:1 with distilled water. While this will technically make the accuracy of the test lower, it will cut the amount of calcium in solution by half. Doing this, along with step 1 above, can make the end point much more crisp. You will sacrifice some accuracy in the process.

    Also, you should always report hardness (calcium or magnesium) in units of calcium carbonate concentration (ppm of [CaCO3]). It's how most of the titrants are calibrated against AND, when calculating saturation indices, the formulae for that always expect calcium hardness and TA in those units. I realize the ICP-MS lab reports will show them in ion units but that's because of the way they measure the ions in solution and how the equipment analyzes them. For aqueous water chemistry, everything should be converted to CaCO3 units.
    Matt
    16k IG PebbleTec pool, 650gal spa, spillway and waterfall, 3HP IntelliFlo VS / 1.5HP WhisperFlo, Pentair QuadDE-100 filter, IC40 SWCG, MasterTemp 400k BTU/hr NG heater, KreepyKrauly suction-side cleaner Dolphin S300i robot, EasyTouch controls, city water, K-1001, K-2006 and K-1766 test kits, Mannitol test for borates

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    Re: How to differentiate between Magnesium and Calcium on Hardness Test

    Hi JoyfulNoise,

    Thanks for all of this insight. I promise to report future numbers as ppm CaCO3. I was forced to report the lab tests otherwise, as that is what I was presented with. Your point about pH going over 12 is appreciated, as this could be an issue. You are correct in that the non-softened water results make complete sense, and I see no issues with the non-softened resuts. I am a chemical engineer by training (not a water professional) and am trying to come up with a way to remove Ca only from pool water, which is why the Mg goes proportionately higher after the "softening", which is really Ca ion removal. I suppose that I am really not softening in the traditional sense, in that I am only taking out Ca, which is the real demon with respect to scaling. I believe that having lots of Mg in the pool does not harm, but let me know if you disagree. I will do more testing tomorrow using your recommendations, and will report back what I learn.

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    Re: How to differentiate between Magnesium and Calcium on Hardness Test

    JoyfulNoise,

    I tried your suggestions. What I learned is that at pH 13 the magnesium will precipitate out OK. The pH 12 did not work out. Anyway, I thank you again for your insights as they were most helpful.

    Del

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