Imbalanced two-part dosing

[rmurken]

Imponderable: if you dose only one part of a two-part for an extended period—for instance, dosing only NaCO3 but not CaCl (or MgCl/SO4)—what balances the Na+ ions that remain after the CO3/resulting carbonate alkalinity species is consumed by calcification?

Conversely, same question for the Cl- ions after Ca is consumed if no alkalinity is added?

What effect would this have on the water chemistry?

 

[Randy Holmes-Farley Verified Community Expert]

Imponderable: if you dose only one part of a two-part for an extended period—for instance, dosing only NaCO3 but not CaCl (or MgCl/SO4)—what balances the Na+ ions that remain after the CO3/resulting carbonate alkalinity species is consumed by calcification?

Conversely, same question for the Cl- ions after Ca is consumed if no alkalinity is added?

What effect would this have on the water chemistry?

Well, it isn't really possible to dose only one for a long time since the other will get so depleted that the other cannot be used.

In the short term, there is no imbalance of ions.

If the alk is being used to form calcium carbonate, you have:


Na2CO3 + Ca++ --> 2Na+ + CaCO3

In that case, you swapped sodium for calcium

 

[rmurken]

I see, and in that example where Na2CO3 is being added without a Ca supplement, once Ca is depleted to where organisms cannot calcify, you’ll just start accumulating sodium and alkalinity with no further drop in Ca. Is that right?

 

[Randy Holmes-Farley Verified Community Expert]

I see, and in that example where Na2CO3 is being added without a Ca supplement, once Ca is depleted to where organisms cannot calcify, you’ll just start accumulating sodium and alkalinity with no further drop in Ca. Is that right?

yes.

Same would happen adding just calcium chloride, where you are swapping chloride for carbonate until the process can no longer happen.

CaCl2 + CO3-- --> CaCO3 --> 2 Cl-