Magnesium in water supply

[Sithdad]

My local water report doesn't have Mg listed and according to the water dept. they are required to test for it. Is there any way of calculating it from values that I do have or should I just send a sample out and have it tested.

 

[Nateo]

Do you have the total hardness? About 30% of the total hardness is Mg, the rest is Ca. The ratio varies a bit in different waters, but 30% will get you close.

 

[ajdelange]

If you have the total hardness and the calcium hardness you can just subtract the calcium hardness from the total hardness. The difference is the magnesium hardness. If you have a calcium mg/L number convert it to calcium hardness by dividing by 20 and multiplying by 50. For example 30 mg/L calcium is 30*50/20 = 75 ppm as CaCO3.

If you have a complete water report except for magnesium you can estimate magnesium by calculating the amount of cation required to balance the profile. This is very iffy as a complete water report would list magnesium, all other ions must be measured and roundoff and other measurement errors will be interpreted as more or less magnesium. Best to send a sample off.

 

[Sithdad]

Directly from the water report: Alkalinity - 136 mg/L, Ca Hardness - 78 mg/L,

 

[ajdelange]

I can do a very gross calculation based on alkalinity. You have 78/50 mEq/L calcium and 136/50 mEq/L bicarbonate so if there is nothing else in the water you would have (136-78)/50 mEq/L magnesium. But I'm sure there is lots of other stuff in there as well like sodium, potassium, sulfate and chloride. I need the total hardness to do the magnesium calculation as anything better than a total WAG.

 

[Sithdad]

That's the problem. I called them and they don't have the total hardness either. Just the alkalinity.

Sodium - 1.9
Sulfate - 10.0
Chloride - 12.0

 

[ajdelange]

It would take 19.4 mg/L Mg++ to balance the other ions you have listed but I wouldn't put a lot of faith in that estimate. $25 to Ward Labs answers your question much better than I can.

 

[Vance71975]

It would take 19.4 mg/L Mg++ to balance the other ions you have listed but I wouldn't put a lot of faith in that estimate. $25 to Ward Labs answers your question much better than I can.

+1 get it tested!