Dkh/ph

[jeff williams]

was planning on running hyposalinity in my dt to correct a fluke problem. Was going to add ro/di to drop sg.
I need to add bicarbonate/carbonate to buffer the ro/di. I’m planning on using two part to do this, my question is will ph test kit show a difference so I know how much to add ? Or will I measure the corrrect out using an alkalinity test kit?
How do I know how much to add?

 

[Randy Holmes-Farley Verified Community Expert]

I'm not sure pH in hypo is desirable to "control" ( I wouldn't), but a pH kit or pH meter will work in hypo.

 

[jeff williams]

I'm not sure pH in hypo is desirable to "control" ( I wouldn't), but a pH kit or pH meter will work in hypo.

The thing I’m worried about by lowering sg with ro/di is massive ph swings. Or as option 2 I guess instead of ro/di I could use tap water treated with prime this would definitely control ph swings. Tap water acting as a buffer.

 

[Randy Holmes-Farley Verified Community Expert]

RO/dI won’t swing the pH much, and it’s not even clear what direction it will go, or whether it matters, but you can rely on kits or meters if that concerns you.

I see no advantage to using tap water.

 

[jeff williams]

RO/dI won’t swing the pH much, and it’s not even clear what direction it will go, or whether it matters, but you can rely on kits or meters if that concerns you.

I see no advantage to using tap water.

Ok I’m confused. Ro/di water is pure it has no alkalinity if added to saltwater In a large enough volume to lower sg to 1.009 how could this not affect buffering there would be such a low amount of kh that ph would drop like a rock correct? Or am I missing the point ?

 

[Randy Holmes-Farley Verified Community Expert]

Ok I’m confused. Ro/di water is pure it has no alkalinity if added to saltwater In a large enough volume to lower sg to 1.009 how could this not affect buffering there would be such a low amount of kh that ph would drop like a rock correct? Or am I missing the point ?

The alkalinity will drop. The pH may or may not drop. I can't see it dropping like a rock, but it may decline. The effect of dilution on pH of a buffered solution is complicated, especially if the pKa of the acid involved changes on dilution.

What I am questioning is the assumed importance of either alk or pH in hypo, but adding alk is certainly fine.

 

[jeff williams]

The alkalinity will drop. The pH may or may not drop. I can't see it dropping like a rock, but it may decline. The effect of dilution on pH of a buffered solution is complicated, especially if the pKa of the acid involved changes on dilution.

What I am questioning is the assumed importance of either alk or pH in hypo, but adding alk is certainly fine.

If there’s no worries of large ph fluctuations then my ? Is mute. I was under the assumption from reading other articles onR2R about hypo that there could be large ph swings and I was assuming due to no bicarbonate or carbonate in the low sg water here’s the link I think it’s second paragraph
https://www.reef2reef.com/threads/hyposalinity.247596/

 

[Randy Holmes-Farley Verified Community Expert]

If there’s no worries of large ph fluctuations then my ? Is mute. I was under the assumption from reading other articles onR2R about hypo that there could be large ph swings and I was assuming due to no bicarbonate or carbonate in the low sg water here’s the link I think it’s second paragraph
https://www.reef2reef.com/threads/hyposalinity.247596/

The up and down swings could certainly be larger with lower alkalinity, and maintaining the alkalinity and/or pH at some target value is a fine plan. Alk kits, pH meters, and pH kits (although not my favorite in general) are all acceptable for this purpose.

If the hypo tank has rising nitrate, that will continually deplete alk and lower pH, and you may want or need to dose buffer for that reason, even if simple dilution of new salt water would have no such problem.

This is probably not the place for a detailed debate on the effects of salinity and alk on pH since I expect you have no actual interest in that topic, but note that it is a complicated topic that does not necessarily lend itself to simple answers. 7 dKH hyposaline water likely will, for example, have a higher pH than 7 dKH full strength seawater equilibrated with the same air (because bicarbonate is a stronger acid at higher salinity). I doubt it is enough higher to be any concern, but as I mentioned, it is not simple and I expect very few reefers explore the details.