Help with Calcium test

[AlexPowell]

Hi everyone,
I could do with some help with the calcium test please

Logs: PoolMath Logs

My calcium was 125, I added 8kg and that increased it to 175. I added another 8kg however it still tested 175.
I changed from the 25ppm testing method to the 10ppm testing method and this time it tested 250 which was more in my expectation.

Any ideas what I have done wrong here? I have included pictures of the final tests. There is a lot of precipitate in there which makes it difficult and I am also colour blind which obviously makes it harder again. I confirmed my testing methods with the 200ppm standard that came with the kit - but I am obviously doing something wrong because the 25ppm and 10ppm methods produce different results with my pool water (but not the standard)

 

[Texas Splash]

Your 10ml sample is definitely a better color for test completion. The 25 ml sample appears to still have a slight purple look to it as if it needed a bit more reagent to fully change. I always use the 10ml sample size myself.

 

[trivetman]

I also have gotten a lot of precipitate when testing ch. the precipitate would stay pink as the solution turned blue making it very hard to read the end point.

@JoyfulNoise told me to mix the pool water 50/50 with distilled water and do the test. This resulted in much less precipitate. then count each drop as twice the ppm as the test would be would be at full strength. This got consistent results for me.

 

[JoyfulNoise]

By chance have you used any magnesium chloride or magnesium sulfate in your pool? Do you happen to know the total hardness (Ca + Mg hardness ) of your water? Australian pools/owners tend to utilize magnesium chloride and Epsom salt (magnesium sulfate) in their pools. Not sure why that is so popular but high magnesium levels can cause the floating precipitates you see. I suggested you follow the extended test instructions where you add 5 drops of R-0012 first, then add the R-0010, R-0011L and finally complete the titration with R-0012. You’ll include the 5 initial drops of R-0012 in your final count. You can do that with the suggested dilution above as well. Just make sure to dilute with distilled water.

 

[AlexPowell]

By chance have you used any magnesium chloride or magnesium sulfate in your pool? Do you happen to know the total hardness (Ca + Mg hardness ) of your water? Australian pools/owners tend to utilize magnesium chloride and Epsom salt (magnesium sulfate) in their pools. Not sure why that is so popular but high magnesium levels can cause the floating precipitates you see. I suggested you follow the extended test instructions where you add 5 drops of R-0012 first, then add the R-0010, R-0011L and finally complete the titration with R-0012. You’ll include the 5 initial drops of R-0012 in your final count. You can do that with the suggested dilution above as well. Just make sure to dilute with distilled water.

There isn't any magnesium in my pool (to my knowledge)

I will try these two ways of doing the test - thank you both

 

[AlexPowell]

Pool is actually looking great

 

[JamesW]

Copper can also cause interference.

 

[mgtfp]

A while ago there was another Aussie member who knew to have Magnesium in the pool. Back then, Matt had explained that the Taylor/CCL buffer is not concentrated enough to suppress the Magnesium interference in this case.

What worked for them was to use an aquarium CH-test that is designed to cope with higher Magnesium levels. I think the Red Sea CH-test did the trick. The test is a bit fiddly because the titration is done with a syringe rather than just dropped, but it seemed to work.

There are also aquarium Magnesium tests to test that directly to rule this out as the root cause.

 

[AlexPowell]

I will test for magnesium. I will be very disappointed with Poolwerx who supplied the salt if this is the case.
I am fairly confident there is no copper as I have not used any algaecide. However I am not expecting there to be magnesium either so I will test for copper.

 

[mgtfp]

I will test for magnesium. I will be very disappointed with Poolwerx who supplied the salt if this is the case.
I am fairly confident there is no copper as I have not used any algaecide. However I am not expecting there to be magnesium either so I will test for copper.

The standard Poolwerx salt doesn't contain Magnesium (apart from potential contamination). But they also sell "Mineral" salt, that seems to be quite popular in Australia. I guess, anything that can be sold at a higher price is "popular" here...

 

[AlexPowell]

The standard Poolwerx salt doesn't contain Magnesium (apart from potential contamination). But they also sell "Mineral" salt, that seems to be quite popular in Australia. I guess, anything that can be sold at a higher price is "popular" here...

Yeah I deliberately did not want a magnesium/mineral pool

 

[trivetman]

I remember Joyful saying my issues were likely from magnesium interference as well. I took that to be that it could just be present jn the fill water, not a result of chemical additive

 

[JoyfulNoise]

If you measure total hardness it should be greater than calcium hardness. Total hardness counts both magnesium and calcium ions. So, TH — CH = MH. Magnesium can be found in any water supply. It’s typically lower in concentration than calcium.

 

[AlexPowell]

Total hardness is 98 (mg/L) as per the report from SAWater. Magnesium is 10.32 mg/L

 

[mgtfp]

Is that both in mg CaCO3 per L?

If Mg is shown as mg Mg per L, then this would contribute with 42.3 mg CaCO3 per L to a TH test, if I got my maths right (factor 4.1 between the molecular weights).

 

[AlexPowell]

Yes that's close enough to the SA Water Report

 

[AlexPowell]

Hi everyone,
I have now figured this out. The test is precipitating and the precipitate appears to be a calcium salt or similar. So the test is showing as complete prematurely because the calcium is coming out of solution partially and then the reagent is showing the solution as no further calcium present

A good shake takes the precipitate back into solution and you can continue the test as normal

I just did the 10ml test and it came up with an expected result of 275ppm. Thanks everyone for their help. If anyone ever gets this problem, what I exactly do is do the drop, then it's lid on and a violent single shake, then the next drop. Previously I had been swirling and this was what caused the precipitate to fall out of solution

 

[mgtfp]

Interesting. Never heard about that. Curious if Matt or James have something to add there.

Do you have speed stir? Continuous stirring might help.

 

[AlexPowell]

I do not have a speed stir yet but it would be interesting to see if that made a difference

 

[JamesW]

Definitely interesting about the shaking.

Maybe a new technique to avoid interference?