Total Alkalinity hobby level test?

Robthorn · Mar 2, 2016

Does anyone know of a test kit or checker type meter that measures total alkalinity?
I currently have the Hanna checker and a salifert kit which both read very close to each other but I can't find anywhere that says total alkalinity.
Is there even a way to test for total alkalinity without testing the companents separately?
Is anyone sure what the 2 tests I own actually test?

Just curious for future purchase.

Thanks

Randy Holmes-Farley · Mar 2, 2016

All titration methods (kits) measure total alkalinity. Measuring carbonate alkalinity alone is much more complicated and usually requires more than one measurement and then a subtraction of borate alkalinity from total alk to get (mostly) carbonate alkalinity.

I expect the Hanna is also reporting total alk, but they do not explicitly say so. The number difference is minor, and they'd have to be subtracting a "typical" value for borate alk if they were to be reporting carbonate alkalinity, and that seems very unlikely to be the case without telling users.

The only method I know of that produces carbonate alkalinity results in the Mindstream which uses pH and CO2 measurements to get to carbonate alkalinity.

Robthorn · Mar 2, 2016

Awesome and thank you Randy. The Reefgrow buffer/Alk supplement I use has added borate is why I am wondering. If the tests are for total I can up my alk a little to try to get a little closer to natural sea water.

Robthorn · Mar 2, 2016

On another note there is some confusion at least in my head about the actual total alkalinity in natural seawater near reefs.
I have seen around 125ppm up to around 170ppm. What is your take on this?
Thanks again

Randy Holmes-Farley · Mar 2, 2016

Robthorn said:
On another note there is some confusion at least in my head about the actual total alkalinity in natural seawater near reefs.
I have seen around 125ppm up to around 170ppm. What is your take on this?
Thanks again

It is not 170 ppm.

Total alkalinity in 35 ppt seawater is about 2.3 meq/l, which translates about 115 ppm calcium carbonate equivalents. Many people round that to 2.5 meq/L, or 125 ppm calcium carbonate equivalents.

Robthorn · Mar 3, 2016

Is that just carbonates or borate and bicarbonates included? I don't pretend to understand chemistry. I am just trying to hack my way through.

Thanks again.

Randy Holmes-Farley · Mar 3, 2016

Robthorn said:
Is that just carbonates or borate and bicarbonates included? I don't pretend to understand chemistry. I am just trying to hack my way through.

Thanks again.

Total alkalinity is bicarbonate plus two times the carbonate plus borate plus some other minor contributors:

TA = [HCO3-] + 2[CO3--] + [B(OH)4-] + [OH-] + [Si(OH)3O-] + [MgOH+] + [HPO4--] + 2[PO4---] - [H+]

while carbonate alkalinity is just the first two terms:

CA = [HCO3-] + 2[CO3--]

In normal natural seawater at pH 8.0, CA is more than 96% of the total alkalinity, borate is about 2.7%, everything else together is about 0.5%.

Robthorn · Mar 3, 2016

Thank you again and please understand I am not trying to argue, just understand which I.believe I may have a little better understanding this evening after talking to my buddy and looking at a chart he sent me from his text book and then I ran across it again this evening researching more.
The 2nd table down is what we have talked about.
http://www.seafriends.org.nz/oceano/seawater.htm

According to that table showing the exact numbers as his text book, bicarbonate alkalinity is 145ppm and then adding 27ppm of borate you should have a total alkalinity of 172ppm not including the other minor elements you mentioned. I believe these numbers are found at 35ppt.

Is this just a difference in ph maybe? I did see a graph that showed a higher alkalinity at higher ph .

It seems weird to me that the normal number you see when searching total alkalinity is 125ppm if text books are teaching 172ish.

Thanks again.

Randy Holmes-Farley · Mar 4, 2016

Ah, I see your confusion. There are two issues that make your interpretation of those linked numbers as alkalinity incorrect.

Most importantly, those are ppm of the specific chemicals, while when reporting alkalinity in ppm, it is referring to ppm calcium carbonate equivalents, not ppm of the ions involved.

The reason that is the case is because alkalinity is a direct measure of the amount of ability to resist a large pH drop (to the low 4's), and that relates to the number of ions present, not the weight concentration of them (which is what ppm is). So, for example, you want one bicarbonate ion to be "counted" the same as one borate ion when thinking of alkalinity. But they weigh very different amounts per ion. So one has to convert them to a unit that measures numbers of ions, such as meq/L, dKH, or ppm calcium carbonate equivalents.

In that sense, 145 ppm bicarbonate = 2.3 mmol/kg (they show that in the table; that unit is a measure of the number of ions).

2.3 mmol/kg = 2.36 mmole/L (converting from number per kg to number per liter since a liter of seawater weighs just over a liter) .

To convert mmole/L bicarbonate that to alkalinity, one can use mmol/L directly for bicarbonate (it equals meq/L since bicarbonate counts once in alkalinity (not twice like carbonate does; in the case of carbonate, that mmol/L would be multipled by 2 to get to meq/L)). The meq/l value can be multiplied by 50 to get ppm calcium carbonate equivalents, = 118 ppm calcium carbonate equivalents.

I can explain the exact reason for 50 ppm calcium carbonate equivalents = 1 meq/L alkalintiy if you want, but it is the standard conversion.

As to the borate, there is a second problem using that number. Yes, you have to convert it to number of ions and not weight, but moreover, most of the boron in seawater is actually present as boric acid. Only a small fraction of it is really present as borate, and only the borate form contributes to alkalinity. So its contribution is much less than it would seem from the concentration in that link.

Hope that clarifies things.