Stable ph calibration fluids?

[elysics]

Currently gathering materials for a diy alkalinity auto tester.

From asking around about all the different commercial models, it seems that the largest annoyance and source of wrong results is pH probe calibration or the lack thereof.

Now my idea was, why not automate that too? Exact volume of calibration solution isn't all that important, so the cost/calibration of the extra 2 or 3 pumps (ph 4, ph7, maybe RODI for rinsing) is negligible.

But is that even feasible? Are there (reefsafe in trace amounts) pH reference solutions that would keep stable +- 0.05 ph for at least a month or two in a dosing container attached to a pump, maybe with a one way air valve to prevent constant gas exchange?

The alkalinity testers only need ph 4 and 7 calibration, which I heard are less unstable regarding air contact than ph 10?

Or is this just a stupid idea that can't work/is too toxic?

 

[Ron Reefman]

bump for thoughts and ideas?

 

[elysics]

Some initial googleing produced some ingredients for ph buffers that don't sound super toxic, but then again i have no idea how stable they are:

ph 4:

"Buffer solutions contain citric acid 1 H₂O (11,768 g); sodium hydroxide 50% (6 ml); hydrochlorid acid 35% (3,96 ml); preservative 1% (5 ml) and water (1 l)"

Leaving out the preservative, that sounds fine, right? I don't know about citric acid, but we are only talking about the traces left over after rinsing with RODI anyway.

for ph 7 i am finding all kinds of mixtures of sodium and potassium (hydrogen-)phosphates, people are actually dosing that stuff to raise phosphate, so those should be fine too.

Most of those recipes (or rather ingredient lists) claim an accuracy of ±0.02 pH, using a 0.001g scale and factoring in tiny errors in DIYing it and some degradation from storage in a dosing container, would that be close enough to accurately hit the titration endpoint? I guess the packets sold in hobby stores aren't much better either, but then again i'm not exactly planning on storing them properly.

And if trusting those recipes isn't realiable enough and using a pH meter calibrated to a higher precision/accuracy beyond initial tinkering is neccesary on an ongoing basis, then that would kind of defeat the purpose too.

 

[Randy Holmes-Farley Verified Community Expert]

I'm not sure of the reason for DIY buffers. If the goal is an alk titration, then certainly, pH 4 and 7, or any set of 2 or more buffers spanning the low 4's is good.

There are lots of commercial and recipe buffers that work fine. Stability should not be a concern at pH 7 or less, though I would not leave them open to the air long term.

You might find this discussion useful:

https://assets.thermofisher.com/TFS-Assets/LPD/Product-Information/Automated-titrator-best-practices-OrionStar-ebook-EBPHTITR-EN.pdf

 

[NeonRabbit221B]

I asked a similar question in this thread

Ph Calibration Powder Solution Storage

I wasn't able to find anything online about this besides that the powder packets for pH calibration are typically good for two years. How about once they are mixed? I stored them in old solution bottles and use about 100 ml in a graduated cylinder to calibrate but was wondering if the mixed...

www.reef2reef.com

Randy gave me this link http://reefkeeping.com/issues/2005-02/rhf/index.htm which was an interesting read at the time.

 

[elysics]

I'm not sure of the reason for DIY buffers. If the goal is an alk titration, then certainly, pH 4 and 7, or any set of 2 or more buffers spanning the low 4's is good.

There are lots of commercial and recipe buffers that work fine. Stability should not be a concern at pH 7 or less, though I would not leave them open to the air long term.

You might find this discussion useful:

https://assets.thermofisher.com/TFS-Assets/LPD/Product-Information/Automated-titrator-best-practices-OrionStar-ebook-EBPHTITR-EN.pdf

Well the reason for diy buffer would be cost mostly if going through ~50ml (the entire point is not having to remove the probe from the titration vessel) of each buffer every calibration, with weekly calibration/test against the buffers, or more often, and having to dump and replace the leftovers after some interval to prevent too much drifting. The PDF you linked (very helpful btw) says to calibrate daily.

 

[elysics]

I asked a similar question in this thread

Ph Calibration Powder Solution Storage

I wasn't able to find anything online about this besides that the powder packets for pH calibration are typically good for two years. How about once they are mixed? I stored them in old solution bottles and use about 100 ml in a graduated cylinder to calibrate but was wondering if the mixed...

www.reef2reef.com

Randy gave me this link http://reefkeeping.com/issues/2005-02/rhf/index.htm which was an interesting read at the time.

Yeah that article inspired me to ask about diy, buying buckeloads of those packages seemed less appropriate after reading that. And just the thought of regularly buying liters if proper lab grade buffers hurts in the wallet.

 

[elysics]

Just a thought, is CO2 entering the only problem with air getting to the buffers? If so, putting a small amount of CO2 absorbent media like people use for skimmers on the air intake of the dosing containers would prevent/considerableyslow that right? Air throughput would be minimal, only equalizing the pressure from buffer being taken out by dosing pump

 

[Randy Holmes-Farley Verified Community Expert]

Just a thought, is CO2 entering the only problem with air getting to the buffers? If so, putting a small amount of CO2 absorbent media like people use for skimmers on the air intake of the dosing containers would prevent/considerableyslow that right? Air throughput would be minimal, only equalizing the pressure from buffer being taken out by dosing pump

That's a fine plan for air pressure equalization, as would simply a substantial length of thin air line tubing. CO2 will not travel the length of a few feet to any great extent.