Does pH impact the alkalinity level?

[jlemoine2]

Hello,

I've been trying to digest this article by Randy Holmes-Farley:
http://reefkeeping.com/issues/2005-07/rhf/index.php

In the section on Calcium Carbonate Precipitation, there is a line that reads "Consequently, as the pH is raised at a fixed alkalinity, the concentration of carbonate rises, thereby increasing the supersaturation of calcium carbonate."

So, as the concentration of carbonate goes up (because of higher pH), does this translate to a higher reading in alkalinity?

I believe my tank is going through some sort of precipitation event, the water has been cloudy for a few days. I've been jiggering the calcium and alkalinity levels (mostly alk) so I can get to a good point to start regular dosing of 2-part via dosing pumps.

To get my alkalinity reading, I've been testing whenever it suited me, day or night... to check to see if my alkalinity was at the desired level. Depending on the time of day, the pH could be at 8.11 or 8.32... I did not give much thought to pH in relation to alkalinity.

Since I've had the cloudy water, I'm wondering if I've dosed too much alkalinity because alk measured low during a low pH time of day.

Perhaps I dosed too much alk too fast with calcium dosed to quickly right behind it... but in any event, I'd like to understand the relationship between pH and testing alkalinity, if any.

Thanks in advance,

-James

[2thdeekay]

I've been trying to digest this article by Randy Holmes-Farley:
http://reefkeeping.com/issues/2005-07/rhf/index.php

In the section on Calcium Carbonate Precipitation, there is a line that reads "Consequently, as the pH is raised at a fixed alkalinity, the concentration of carbonate rises, thereby increasing the supersaturation of calcium carbonate."

So, as the concentration of carbonate goes up (because of higher pH), does this translate to a higher reading in alkalinity?



No, it doesn't. In that context, Randy was pointing out high pH drives the carbonate equilibrium towards CO3--, as opposed to HCO3-.

HCO3- + OH- ---> CO3-- + H2O

Is it possible your system has experienced elevated pH? pH doesn't directly affect alk, but can increase supersaturation of CO3-, and cause ppt events @ elevated pH.

Hope everything is under control, LMK if you need anything. :)

[jlemoine2]

Thanks Mohri!

Great, it's nice to know that I can measure alk without taking into consideration the pH. Although, when I get into a regular dosing schedule, I will perform maintenance alk measurements at the same time each day. I stopped reading the equations after I passed high school chemistry 17 years ago. I prefer someone to describe it verbally ;)

The only pH spikes are when dosing the alkalinity. I am using recipe 1. However, the spike was short lived. I would go from 8.1-ish to 8.45 or so immediately after the alk dose, which I would dump into a high flow area in the sump. The pH would drop down to 8.2 over the next couple of hours.

I'm installing my dosing pumps today. Dosing will start as soon as the cloudiness goes away. Rather than doing large doses of Calcium and Alk (mostly alk) to get the numbers where I like, I'll just up the time on the dosing pumps and raise it ever so slowly until the numbers are to my liking. Hopefully this will help me avoid any more of these precipitation events.

Thanks again...

-James

[Randy Holmes-Farley]

In sort, alkalinity does not change with daily pH changes. What is changing is the amount of CO2 available in the water. :)

What exactly did you add?

[2thdeekay]

meant supersaturation of CO3-- and Ca++, not just CO3

it forms calcium carbonate precipitate (ppt)

[2thdeekay]

Thanks Randy. I don't know how much James added or how fast, but almost certain he's been using your improved diy 2 part recipe #1, with baked baking soda & anhydrous CaCl2, with the reef chem calculator.

Lots of hobbyists here have benefitted from your articles/info. Not many are using purple-up or Eco-aqualizers anymore, including LFS's :D

[jlemoine2]

Hello Randy and Mohri,

Yes, I was dosing the improved DIY 2-part recipe 1. I don't have the exact numbers in front of me at the moment, but here are the general numbers:

My intial alk level was less than 5 dKH (tested using the Hanna Colorimeter). I don't recall how much total alk was required to boost my levels (it seemed like a lot), but I was dosing 200ml of alk at a time in 177 gallons of total system water, every 8 to 12 hours. It took a few days to get me up to 7.5 dKH or so. I only dosed the alk when the pH was less than 8.25.

Initially, I did not add calcium, as it was in the 430 neighborhood (salifert). After a couple of days of dosing alk it dropped to around 380... which makes sense based on the many excellent articles by Randy (BTW, thanks Randy!). I then started dosing Calicium in equal amounts to what I was dosing in Alk (by hand). I would dump in 200ml in a time span of 5 seconds or so, into the skimmer section of my sump. Calcium would be 30-60 seconds or so after the Alk in the same section of the sump.

At the time, my thought to dosing in the skimmer section was that it would mix in with water, go through the bubble trap, and mix further in the return section, then go through the return manifold which feeds 3 tanks in the system (120g display, 40g frag, 20g refugium) where the mixture of alk and calcium would be diluted further. Now that I think about it, I believe I dosed too fast with the alk and calcium in the same confined section of the sump (about a 12 gallon section).

I am not yet sure of my daily consumption of alkalinity or calicum. I stopped checking alk and calcium levels once the precipitation event started, as suggested by one of Randy's articles. Once the precip goes away, I expect my alk and calcium levels to be low again.

I'm not dosing anything now as I am waiting for the cloudiness to clear. Which by the way, is worse in the evenings than in the mornings (tank time), I'm not sure what that is about. As I mentioned previously, I plan on using my dosing pumps for these additives and simply run them longer than needed until my levels get to where they need to be.

Additional thoughts?

Thanks in advance for your insights!

-James

[Randy Holmes-Farley]

I'm not sure why you have cloudy water since you boosted alk reasonably slowly, unless the alk kit was off.

[jlemoine2]

Interesting. So dosing 200ml of Alk and then 200ml of Calcium (recipe 1) about a minute later, into the same 12 gallon skimmer section of the sump, is considered reasonably slow?

Most of the articles are great at describing how to make calcium and alkalinity and how much to dose, but they are not very good at saying HOW to dose 2 part (at least I have not found such an article). Pouring 200ml in within 5 seconds seems like a lot to me compared to a 1.1ml/hr dosing pump. Reef calculaters suggest spreading the dose (which could be 750 ml) out and not to raise more than 1.5 dKH per day. Nothing I can find says it's safe to dose 200ml per hour per 200 gallons (as for example), or if it is okay or not to dose calcium and alkalinity at the same time.

When I get home, I'll have to look through my log to see if there might have been something else I did in relation to dosing that might have caused the cloudiness... but I'm not sure what that would be.

Thanks again!

[Randy Holmes-Farley]

As long as the first one mixes in before the second one is added is all that is needed in terms of timing separation.

Generally, a dosing pump is a convenience, not a need, although with a big dose it can be nice to spread out the alk and pH boosts one gets. :)