Calcium hardness and metal ions

[AndrewFH]

In doing a first test for CH using a TF100, I was getting a "fading endpoint". According to the extended instructions, this is caused by interfernce from metal ions in the water. I was able to complete the test by adding five drops of R-0012 at the beginning per the instructions (result was 90).
Are the metal ions something I should be concerned about or somehow test for?
Is the likely source of the metal from my fill water (which is city water)?
When I do my next drain/refill should I use one of the fill filters (such as the Pre Fresh) that are supposed to remove (at least some) metals from the water?

 

[Richard320]

Most likely the metal is Magnesium, which is in the tap water. Copper and Silver get introduced by "mineral systems" or possibly extremely acidic water etching the heater. They'll start staining. Iron will show up by coloring the water.

If the water looks good and everything else is balanced, don't worry. I've been doing the test with the R-0012 first for years.

 

[chem geek]

It's not magnesium since that is precipitated out when you add the "calcium buffer" which is sodium hydroxide so precipitates magnesium hydroxide. The fading endpoint is from trace metals such as iron, copper or manganese that interfere with the titration that is done and uses EDTA (a metal sequestrant). By adding titrant (EDTA) first, you bind up such trace metals whereas if you don't do that then they can displace calcium that is bound to EDTA later on during the test. This displacement happens more slowly because the metal gets precipitated at the same time the calcium buffer is added but can then bind to EDTA later by slowly dissolving again. So if you add titrant first, then you bind up the metal ions right away. The difference is as follows:

Add Calcium Buffer: Magnesium + Sodium Hydroxide ---> Magnesium Hydroxide solid
and if metal ions are present: Metal Ions + Sodium Hydroxide ---> Metal Hydroxide solid (or ion pairs)
Add Indicator Dye: Calcium + Blue-Dye ---> Calcium-bound-to-dye-making-it-red
Add Titrant Drops: Calcium-bound-to-dye-making-it-red + EDTA ---> Calcium-bound-to-EDTA + Blue-Dye
Fading Endpoint: Metal Hydroxide solid (or ion pairs) + Calcium-bound-to-EDTA ---> Metal-bound-to-EDTA + Calcium
and then: Calcium + Blue-Dye ---> Calcium-bound-to-dye-making-it-red

If you add some titrant drops first, you have the following:

Add Titrant Drops: Metal Ions + EDTA ---> Metal-bound-to-EDTA
Add Calcium Buffer: Magnesium + Sodium Hydroxide ---> Magnesium Hydroxide solid
Add Indicator Dye: Calcium + Blue-Dye ---> Calcium-bound-to-dye-making-it-red
Add Titrant Drops: Calcium-bound-to-dye-making-it-red + EDTA ---> Calcium-bound-to-EDTA + Blue-Dye

 

[AndrewFH]

Since I obviously have metal in the water, should I be using a sequesterent?

 

[chem geek]

I would say not unless you find the metals to be a problem such as seeing staining or colored water. A small amount of metals isn't going to cause a problem unless the pH got very high and using a metal sequestrant is not without its own issues. At a minimum, metal sequestrants increase chlorine demand since they slowly breakdown, though the HEDP we recommend breaks down more slowly than other metal sequestrants such as EDTA.

Generally speaking, we don't add extra things to the pool that aren't really necessary, especially if they have other side effects. You can certainly add it if you are worried about it, but personally I'd wait to see if it's really an issue that matters in your pool.