Effect of Temperature on Calcium Hardness Reading

TomU · Mar 3, 2009

Hi all.

I was doing some testing of my tap water tonight, and noticed some strange behavior. Here are my results (I was only testing CH - looking to get a new water softener):

Test 1 - Cold Water (60 degrees) - 380 ppm
Test 2 - Hot water (120 degrees) - 150 ppm
Test 3 - Cold water heated in the microwave (130 degrees) - 160 ppm
Test 4 - Cold water allowed to come to room temp (70 degrees) - 400 ppm

What gives? I've checked Taylor, and here, and I'm not seeing any documented link between temperature and calcium hardness results. I initially thought maybe my water heater was somehow removing hardness from the water, but the microwave test disproved that. Is there some compensation factor that needs to be applied to the calcium hardness test based on water temperature? Thanks!

CaOCl2 · Mar 3, 2009

I think it may be pH related, pH is temperature dependent.

I did my own little experiment this morning, nothing scientific at all:

25 mL sample at 14,4 Â°C has a pH of 9,1, when I added 20 drops of buffer pH then reads 12,5
25 mL sample at 54,0 Â°C has a pH of 11, wnen I added 20 drops of buffer pH then reads 11

The Standard Methods mention that for the EDTA calcium test the pH has to be raised high enough to precipitate magnesium, they say 12 to 13.

But then again you couldn't be reading total hardness at that temperature, the difference you measured is just too important.

So I don't know. Good question though!

Aquaman95 · Mar 3, 2009

I can't answer the testing part of the question but I can tell you that calcium is more soluble in cold water than hot.

Taylor has a good tech line at 1-800-TEST-KIT that I'm sure could answer the question fully.

chem geek · Mar 5, 2009

I E-mailed Taylor about this and they said that you should not be using the Calcium Hardness test (or any other test) above 100F. The tests are designed and calibrated for room temperature.

Richard

PaulR · Mar 5, 2009

I believe JasonLion has said pH fluctuates with temperature so that test should be done at the sample's original temperature?

JasonLion · Mar 5, 2009

The PH of pool water will change with temperature, but that isn't necessarily the reason you should do the water testing at room temperature. Various chemical reactions will behave differently at different temperatures. I am not sure exactly why the CH test behaves strangely at high temperatures, but PH shifts are only one of several possible causes.

TomU · Mar 5, 2009

With the amount of variation I'm seeing, I'm concerned that I may not be getting an accurate (or even close) reading unless I bring the water closer to a optimal temperature. Tonight when I get home I'll do some more testing. I'll test for everything, not just CH, at multiple temperatures.

This makes me wonder if I can trust the results I get when testing hot tub water, or very cold water (like right now).

duraleigh · Mar 5, 2009

Well, the lesson to be learned here is not to let your pool get to 160 degrees because your CH testing will be off!! :shock:

Just kidding, Tom. But, I do think it's important that an accurate test of your pool water ought to be done in normal pool water temps. That may get a little frustrating as we all want to get an early jump in the Spring but I think that any pool water less than 70 degrees or so should be brought inside and warmed to 80 and then tested.