Low PH with High Calcium and Alkalinity

[Hans-Werner]

But I do not believe there is any plausible reason to suggest that higher alkalinity leads to lower pH, unless one is supplying alkalinity back to the system in a way that itself lowers pH.

I think when precipitation of aragonite starts, the alkalinity in fact is dropping, CO2 is released and the pH is dropping too.

Take a saltwater with let's say 450 ppm calcium and then slowly add sodium bicarbonate (most alkalinity in saltwater is bicarbonate at pH 8.2) until precipitation starts. Watch pH.

 

[Randy Holmes-Farley Verified Community Expert]

I think when precipitation of aragonite starts, the alkalinity in fact is dropping, CO2 is released and the pH is dropping too.

Take a saltwater with let's say 450 ppm calcium and then slowly add sodium bicarbonate (most alkalinity in saltwater is bicarbonate at pH 8.2) until precipitation starts. Watch pH.

Certainly true. You are doing exactly what I suggested above: using a pH lowering supplement (bicarbonate) to replace the lost carbonate.

if you instead choose carbonate to replace the lost carbonate, the effect must be pH neutral. Carbonate in and carbonate out, with no possible impact to pH.

If you used hydroxide (and carbon dioxide from the air) to replace the lost carbonate, the effect is rising pH.

I also certainly do not dispute the fact that calcium carbonate formation by itself tends to lower alkalinity and pH in seawater. But that is not the same as saying that high alkalinity is resulting in lower pH. precipitation of calcium carbonate and replacement of the carbonate with bicarbonate will lower pH.

 

[Hans-Werner]

Yes, of course. Since the contribution to alkalinity by the seawater carbonate-system is 90 % bicarbonate, 6.7 % carbonate and 3 % borate you can add bicarbonate or a buffer that is exactly reproducing the seawater carbonate system. Otherwise, when adding only carbonate for example, you will shift the pH by increasing the proportion of carbonate alone. This has nothing to do with the alkalinity we titrate.

 

[Hans-Werner]

But that is not the same as saying that high alkalinity is resulting in lower pH. precipitation of calcium carbonate and replacement of the carbonate with bicarbonate will lower pH.

I think I didn't say exactly this or at least I didn't mean exactly this. I said when alkalinity is such high that precipitates can form it may actually lower pH. Aragonite precipitates may form in many tanks, localized, on heaters, pumps in the gravel, and finally calcification of corals and coralline algae is just a localized formation of calcium carbonate, taking calcium carbonate out of the water and leaving more bicarbonate(?) in proportion.

 

[Hans-Werner]

Me once again ...
I have made a trial to test if there is a simple relationship between pH and titratable alkalinity in exactly the same buffer system.
For this I made 2 liters fresh saltwater with our Classic Sea Salt with 60 g/l which is ca. 52.5 PSU salinity. Then I diluted this concentrated saltwater 670 : 330 ml and 330 : 670 ml to get saltwater with ca. 35 PSU salinity and 17.5 PSU salinity, one liter each. Now I had three saltwaters with exactly the same buffer systems but different salinities and different alkalinities in the ratios 1 : 2 : 3. I expected that all 3 saltwaters would be quite exactly the same pH but to my surprise they were not. Do you know what came out?

 

[Randy Holmes-Farley Verified Community Expert]

I think I didn't say exactly this or at least I didn't mean exactly this. I said when alkalinity is such high that precipitates can form it may actually lower pH. Aragonite precipitates may form in many tanks, localized, on heaters, pumps in the gravel, and finally calcification of corals and coralline algae is just a localized formation of calcium carbonate, taking calcium carbonate out of the water and leaving more bicarbonate(?) in proportion.

I agree with that as far as formation of calcium carbonate goes. Taking out carbonate lowers pH.

But the whole process considered, from supplement to solid CaCO3 does not, if one adds back exactly what was taken out; carbonate.

 

[Randy Holmes-Farley Verified Community Expert]

Do you know what came out?

Absolutely, and not what you think I think.

A classic experiment that is much more complicated than the alkalinity change. It is the basis of one of my posted reef questions of the day.

Adding totally pure fresh water at pH 7 to seawater at pH 8.2 will instantly raise the pH.

Why? Because bicarbonate is a stronger acid at higher salinity and weaker at lower salinity (established fact), and so adding fresh water at pH 7 shifts some of the carbonate to bicarbonate, taking up H+ and raising pH.

That is not an alkalinity effect.

To test alkalinity alone, you need to change just the alkalinity, and then equilibrate with the atmosphere (if that is part of the purpose of the experiment).

I validated that alk effect experimentally in the article I posed above (I added in the final alkalinity attained in each sample for this purpose:

Supplement	Initial pH	24 hour pH	120 hour pH
none (alk = 2.25 meq/L)	8.10	8.11	8.21
0.5 meq/L HCO3- (alk = 2.75)	8.06	8.15	8.33
0.5 meq/L CO3– (alk = 2.75)	8.44	8.28	8.34
0.5 meq/L OH- (alk = 2.75)	8.76	8.47	8.33
0.5 meq/L H+ (alk = 1.75 meq/L)	6.91	7.91	8.15

 

[Hans-Werner]

But the whole process considered, from supplement to solid CaCO3 does not, if one adds back exactly what was taken out; carbonate.

Funny chemist vs. biologist discussion

Again I have a "but" - but the leading theory says that CO2 (as bicarbonate and free associated CO2) keeps the calcium bicarbonate in solution and the removal of CO2 precipitates calcium carbonate in the calcification process. So CO2 is essential to calcification of corals and coralline algae and indeed it is essential as nutrient "inorganic carbon" to the algae or the algal partner.

I think the role of urea and glutamine hydrolysis with resulting ammonia release and the role of phosphate in calcification process usually is grossly underestimated.

 

[Hans-Werner]

Adding totally pure fresh water at pH 7 to seawater at pH 8.2 will instantly raise the pH.

Why? Because bicarbonate is a stronger acid at higher salinity and weaker at lower salinity (established fact), and so adding fresh water at pH 7 shifts some of the carbonate to bicarbonate, taking up H+ and raising pH.

That is not an alkalinity effect.

To test alkalinity alone, you need to change just the alkalinity, and then equilibrate with the atmosphere (if that is part of the purpose of the experiment).

Yes, I already thought this, taking up protons when I lower salinity and increasing pH in this way.

"Equilibrating it with the atmosphere" - that is the key to many pH problems, CO2 produced in the tank, i. e. by organic carbon dosing, and low phosphate concentrations to most of the remaining.

Low phosphate concentrations just because calcium carbonate precipitation and calcification take place at lower pH at low phosphate concentrations.

 

[b0bburg3r]

If you run a skimmer you can add a diy Co2 scrubber to the airline filled with soda lime.
Since I installed this on my 32G Biocube my PH at the peak of the cycle has been a steady 8.1 - 8.2.
It has made a big difference.
If your interested I can give you the details.

I know this is old but hit me up with the details if you feel so inclined. I would really appreciate it.

 

[karley]

If you run a skimmer you can add a diy Co2 scrubber to the airline filled with soda lime.
Since I installed this on my 32G Biocube my PH at the peak of the cycle has been a steady 8.1 - 8.2.
It has made a big difference.
If your interested I can give you the details.

I'm super interested in this! Please do share.